Hydrocarbons

The largest database¹ of organic compounds lists about 10 million substances, which include compounds originating from living organisms and those synthesized by chemists. The number of potential organic compounds has been estimated² at 10⁶⁰—an astronomically high number. The existence of so many organic molecules is a consequence of the ability of carbon atoms to form up to four strong bonds to other carbon atoms, resulting in chains and rings of many different sizes, shapes, and complexities.

The simplest organic compounds contain only the elements carbon and hydrogen, and are called hydrocarbons. Even though they are composed of only two types of atoms, there is a wide variety of hydrocarbons because they may consist of varying lengths of chains, branched chains, and rings of carbon atoms, or combinations of these structures. In addition, hydrocarbons may differ in the types of carbon-carbon bonds present in their molecules. Many hydrocarbons are found in plants, animals, and their fossils; other hydrocarbons have been prepared in the laboratory. We use hydrocarbons every day, mainly as fuels, such as natural gas, acetylene, propane, butane, and the principal components of gasoline, diesel fuel, and heating oil. The familiar plastics polyethylene, polypropylene, and polystyrene are also hydrocarbons. We can distinguish several types of hydrocarbons by differences in the bonding between carbon atoms. This leads to differences in geometries and in the hybridization of the carbon orbitals.

Alkanes

Alkanes, or saturated hydrocarbons, contain only single covalent bonds between carbon atoms. Each of the carbon atoms in an alkane has sp³ hybrid orbitals and is bonded to four other atoms, each of which is either carbon or hydrogen. The Lewis structures and models of methane, ethane, and pentane are illustrated in [link]. Carbon chains are usually drawn as straight lines in Lewis structures, but one has to remember that Lewis structures are not intended to indicate the geometry of molecules. Notice that the carbon atoms in the structural models (the ball-and-stick and space-filling models) of the pentane molecule do not lie in a straight line. Because of the sp³ hybridization, the bond angles in carbon chains are close to 109.5°, giving such chains in an alkane a zigzag shape.

The structures of alkanes and other organic molecules may also be represented in a less detailed manner by condensed structural formulas (or simply, condensed formulas). Instead of the usual format for chemical formulas in which each element symbol appears just once, a condensed formula is written to suggest the bonding in the molecule. These formulas have the appearance of a Lewis structure from which most or all of the bond symbols have been removed. Condensed structural formulas for ethane and pentane are shown at the bottom of [link], and several additional examples are provided in the exercises at the end of this chapter.

A common method used by organic chemists to simplify the drawings of larger molecules is to use a skeletal structure (also called a line-angle structure). In this type of structure, carbon atoms are not symbolized with a C, but represented by each end of a line or bend in a line. Hydrogen atoms are not drawn if they are attached to a carbon. Other atoms besides carbon and hydrogen are represented by their elemental symbols. [link] shows three different ways to draw the same structure.

Drawing Skeletal Structures Draw the skeletal structures for these two molecules:

Solution Each carbon atom is converted into the end of a line or the place where lines intersect. All hydrogen atoms attached to the carbon atoms are left out of the structure (although we still need to recognize they are there):

Figure a shows a branched skeleton structure that looks like a plus sign with line segments extending up and to the right and down and to the left of the rightmost point of the plus sign. Figure b appears in a zig zag pattern made with six line segments. The segments rise, fall, rise, fall, rise, and fall moving left to right across the figure. Check Your Learning Draw the skeletal structures for these two molecules:

Figure a shows five C H subscript 2 groups and one C H group bonded in a hexagonal ring. A C H subscript 3 group appears above and to the right of the ring, bonded to the ring on the C H group appearing at the upper right portion of the ring. In b, a straight chain molecule composed of C H subscript 3 C H subscript 2 C H subscript 2 C H subscript 2 C H subscript 3 is shown.

Answer:

In a, a hexagon with a vertex at the top is shown. The vertex just to the right has a line segment attached that extends up and to the right. In b, a zig zag pattern is shown in which line segments rise, fall, rise, fall, and rise moving left to right.

Interpreting Skeletal Structures Identify the chemical formula of the molecule represented here:

This figure shows a pentagon with a vertex pointing right, from which a line segment extends that has two line segments attached at its right end, one extending up and to the right, and the other extending down and to the right. Solution There are eight places where lines intersect or end, meaning that there are eight carbon atoms in the molecule. Since we know that carbon atoms tend to make four bonds, each carbon atom will have the number of hydrogen atoms that are required for four bonds. This compound contains 16 hydrogen atoms for a molecular formula of C₈H₁₆.

Location of the hydrogen atoms:

In this figure a ring composed of four C H subscript 2 groups and one C H group in a pentagonal shape is shown. From the C H group, which is at the right side of the pentagon, a C H is bonded. From this C H, a C H subscript 3 group is bonded above and to the right and a second is bonded below and to the right. Check Your Learning Identify the chemical formula of the molecule represented here:

A skeleton model is shown with a zig zag pattern that rises, falls, rises, and falls again left to right through the center of the molecule. From the two risen points, line segments extend both up and down, creating four branches.

Answer:

C₉H₂₀

All alkanes are composed of carbon and hydrogen atoms, and have similar bonds, structures, and formulas; noncyclic alkanes all have a formula of C_nH_2n+2. The number of carbon atoms present in an alkane has no limit. Greater numbers of atoms in the molecules will lead to stronger intermolecular attractions (dispersion forces) and correspondingly different physical properties of the molecules. Properties such as melting point and boiling point ([link]) usually change smoothly and predictably as the number of carbon and hydrogen atoms in the molecules change.

Properties of Some Alkanes³
methane	CH₄	–182.5	–161.5	gas	1
ethane	C₂H₆	–183.3	–88.6	gas	1
propane	C₃H₈	–187.7	–42.1	gas	1
butane	C₄H₁₀	–138.3	–0.5	gas	2
pentane	C₅H₁₂	–129.7	36.1	liquid	3
hexane	C₆H₁₄	–95.3	68.7	liquid	5
heptane	C₇H₁₆	–90.6	98.4	liquid	9
octane	C₈H₁₈	–56.8	125.7	liquid	18
nonane	C₉H₂₀	–53.6	150.8	liquid	35
decane	C₁₀H₂₂	–29.7	174.0	liquid	75
tetradecane	C₁₄H₃₀	5.9	253.5	solid	1858
octadecane	C₁₈H₃₈	28.2	316.1	solid	60,523

Hydrocarbons with the same formula, including alkanes, can have different structures. For example, two alkanes have the formula C₄H₁₀: They are called n-butane and 2-methylpropane (or isobutane), and have the following Lewis structures:

The figure illustrates three ways to represent molecules of n dash butane and 2 dash methlylpropane. In the first row of the figure, Lewis structural formulas show element symbols and bonds between atoms. The n dash butane molecule shows 4 carbon atoms represented by the letter C bonded in a straight horizontal chain with hydrogen atoms represented by the letter H bonded above and below all carbon atoms. H atoms are bonded at the ends to the left and right of the left-most and right-most C atoms. In the second row, ball-and-stick models are shown. In these representations, bonds are represented with sticks, and elements are represented with balls. Carbon atoms are black and hydrogen atoms are white in this image. In the third row, space-filling models are shown. In these models, atoms are enlarged and pushed together, without sticks to represent bonds. The molecule names are provided in the fourth row.

The compounds n-butane and 2-methylpropane are structural isomers (the term constitutional isomers is also commonly used). Constitutional isomers have the same molecular formula but different spatial arrangements of the atoms in their molecules. The n-butane molecule contains an unbranched chain, meaning that no carbon atom is bonded to more than two other carbon atoms. We use the term normal, or the prefix n, to refer to a chain of carbon atoms without branching. The compound 2–methylpropane has a branched chain (the carbon atom in the center of the Lewis structure is bonded to three other carbon atoms)

Identifying isomers from Lewis structures is not as easy as it looks. Lewis structures that look different may actually represent the same isomers. For example, the three structures in [link] all represent the same molecule, n-butane, and hence are not different isomers. They are identical because each contains an unbranched chain of four carbon atoms.

The Basics of Organic Nomenclature: Naming Alkanes

The International Union of Pure and Applied Chemistry (IUPAC) has devised a system of nomenclature that begins with the names of the alkanes and can be adjusted from there to account for more complicated structures. The nomenclature for alkanes is based on two rules:

This figure shows structural formulas for propane, 2 dash chloropropane, 2 dash methylpropane, 2 comma 4 dash difluorohexane, and 1 dash bromo dash 3 dash chlorohexane. In each of the structures, the carbon atoms are in a row with bonded halogen atoms and a methyl group bonded below the figures. Propane is listed as simply C H subscript 3 C H subscript 2 C H subscript 3, with the numbers 1, 2, and 3 appearing above the carbon atoms from left to right. 2 dash chloropropane similarly shows C H subscript 3 C H C H subscript 3, with the numbers 1, 2, and 3 appearing above the carbon atoms from left to right. A C l atom is bonded below carbon 2. The C l atom is red. 2 dash methylpropane similarly shows C H subscript 3 C H C H subscript 3, with the numbers 3, 2, and 1 appearing above the carbon atoms from left to right. A C H subscript 3 group is bonded beneath carbon 2 and is red. 2 comma 4 dash difluorohexane similarly shows C H subscript 3 C H subscript 2 C H C H subscript 2 C H C H subscript 3, with the numbers 6, 5, 4, 3, 2, and 1 appearing above the carbon atoms from left to right. F atoms are bonded to carbons 4 and 2 at the bottom of the structure and are red. 1 dash bromo dash 3 dash chlorohexane similarly shows C H subscript 2 C H subscript 2 C H C H subscript 2 C H subscript 2 C H subscript 3, with numbers 1, 2, 3, 4, 5, and 6 appearing above the carbon atoms from left to right. B r is bonded below carbon 1 and C l is bonded below carbon 3. Both B r and C l are red.

When more than one substituent is present, either on the same carbon atom or on different carbon atoms, the substituents are listed alphabetically. Because the carbon atom numbering begins at the end closest to a substituent, the longest chain of carbon atoms is numbered in such a way as to produce the lowest number for the substituents. The ending -o replaces -ide at the end of the name of an electronegative substituent (in ionic compounds, the negatively charged ion ends with -ide like chloride; in organic compounds, such atoms are treated as substituents and the -o ending is used). The number of substituents of the same type is indicated by the prefixes di- (two), tri- (three), tetra- (four), and so on (for example, difluoro- indicates two fluoride substituents).

Naming Halogen-substituted Alkanes Name the molecule whose structure is shown here:

This structure shows a C atom bonded to the H atoms and another C atom. This second C atom is bonded to two H atoms and another C atom. This third C atom is bonded to a B r atom and another C atom. This fourth C atom is bonded to two H atoms and a C l atom. Solution

This structure shows a C atom bonded to the H atoms and another C atom. This second C atom is bonded to two H atoms and another C atom. This third C atom is bonded to an H atom, a B r atom, and another C atom. This fourth C atom is bonded to two H atoms and a C l atom. The C atoms are numbered 4, 3, 2, and 1 from left to right. The four-carbon chain is numbered from the end with the chlorine atom. This puts the substituents on positions 1 and 2 (numbering from the other end would put the substituents on positions 3 and 4). Four carbon atoms means that the base name of this compound will be butane. The bromine at position 2 will be described by adding 2-bromo-; this will come at the beginning of the name, since bromo- comes before chloro- alphabetically. The chlorine at position 1 will be described by adding 1-chloro-, resulting in the name of the molecule being 2-bromo-1-chlorobutane.

Check Your Learning Name the following molecule:

This figure shows a C atom bonded to three H atoms and another C atom. This second C atom is bonded to two H atoms and a third C atom. The third C atom is bonded to two B r atoms and a fourth C atom. This C atom is bonded to an H atom, and I atom, and a fifth C atom. This last C atom is bonded to three H atoms.

Answer:

3,3-dibromo-2-iodopentane

We call a substituent that contains one less hydrogen than the corresponding alkane an alkyl group. The name of an alkyl group is obtained by dropping the suffix -ane of the alkane name and adding -yl:

In this figure, methane is named and represented as C with four H atoms bonded above, below, to the left, and to the right of the C. The methyl group is shown, which appears like methane without the right most H. A dash remains at the location where the H was formerly bonded. Ethane is named and represented with two centrally bonded C atoms to which six H atoms are bonded; two above and below each of the two C atoms and to the left and right ends of the linked C atoms. The ethyl group appears as a similar structure with the right-most H atom removed. A dash remains at the location where the H atom was formerly bonded.

The open bonds in the methyl and ethyl groups indicate that these alkyl groups are bonded to another atom.

Naming Substituted Alkanes Name the molecule whose structure is shown here:

A chain of six carbon atoms, numbered 6, 5, 4, 3, 2, and 1 is shown. Bonded above carbon 3, a chain of two carbons is shown, numbered 1 and 2 moving upward. H atoms are present directly above, below, left and right of all carbon atoms in positions not already taken up in bonding to other carbon atoms. SolutionThe longest carbon chain runs horizontally across the page and contains six carbon atoms (this makes the base of the name hexane, but we will also need to incorporate the name of the branch). In this case, we want to number from right to left (as shown by the blue numbers) so the branch is connected to carbon 3 (imagine the numbers from left to right—this would put the branch on carbon 4, violating our rules). The branch attached to position 3 of our chain contains two carbon atoms (numbered in red)—so we take our name for two carbons eth- and attach -yl at the end to signify we are describing a branch. Putting all the pieces together, this molecule is 3-ethylhexane.

Check Your Learning Name the following molecule:

Answer:

4-propyloctane

Some hydrocarbons can form more than one type of alkyl group when the hydrogen atoms that would be removed have different “environments” in the molecule. This diversity of possible alkyl groups can be identified in the following way: The four hydrogen atoms in a methane molecule are equivalent; they all have the same environment. They are equivalent because each is bonded to a carbon atom (the same carbon atom) that is bonded to three hydrogen atoms. (It may be easier to see the equivalency in the ball and stick models in [link]. Removal of any one of the four hydrogen atoms from methane forms a methyl group. Likewise, the six hydrogen atoms in ethane are equivalent ([link]) and removing any one of these hydrogen atoms produces an ethyl group. Each of the six hydrogen atoms is bonded to a carbon atom that is bonded to two other hydrogen atoms and a carbon atom. However, in both propane and 2–methylpropane, there are hydrogen atoms in two different environments, distinguished by the adjacent atoms or groups of atoms:

In this figure, propane is shown as a chain of three bonded C atoms. Eight H atoms are shown with three bonded to the first C atom, two to the middle C atom, and three to the third C atom. The H atoms bonded to the middle C atom are purple. 2 dash methylpropane is also shown, which similarly has a chain of three bonded C atoms. In this structure, A C atom is bonded beneath the middle C atom of the chain. Ten H atoms are shown with three bonded to the first C atom, one to the middle C atom, three to the third C atom, and three to the C atom also bonded to the middle C atom. The H atom bonded to the middle C atom is green.

Each of the six equivalent hydrogen atoms of the first type in propane and each of the nine equivalent hydrogen atoms of that type in 2-methylpropane (all shown in black) are bonded to a carbon atom that is bonded to only one other carbon atom. The two purple hydrogen atoms in propane are of a second type. They differ from the six hydrogen atoms of the first type in that they are bonded to a carbon atom bonded to two other carbon atoms. The green hydrogen atom in 2-methylpropane differs from the other nine hydrogen atoms in that molecule and from the purple hydrogen atoms in propane. The green hydrogen atom in 2-methylpropane is bonded to a carbon atom bonded to three other carbon atoms. Two different alkyl groups can be formed from each of these molecules, depending on which hydrogen atom is removed. The names and structures of these and several other alkyl groups are listed in [link].

Note that alkyl groups do not exist as stable independent entities. They are always a part of some larger molecule. The location of an alkyl group on a hydrocarbon chain is indicated in the same way as any other substituent:

This figure shows structures of 3 dash ethylheptane, 2 comma 2 comma 4 dash trimethylpentane, and 4 dash isopropylheptane. The 3 dash ethylheptane structure shows C H subscript 3 C H subscript 2 C H subscript 2 C H subscript 2 C H C H subscript 2 C H subscript 3. Under the C atom labeled 3, is a bond to C H subscript 2 C H subscript 3 which appears in red. The C atoms are labeled 7, 6, 5, 4, 3, 2, and 1 from left to right. The 2 comma 2 comma 4 dash trimethylpentane structure shows C H subscript 3 C bonded to C H subscript 2 C H C H subscript 3. The C atoms are labeled 1, 2, 3, 4, and 5 from left to right. The C atom labeled 2 has a C H subscript 3 bonded above it and below it. The C H subscript 3 groups both appear in red. The C atom labeled 4 has a bond above it to C H subscript 3. The C H subscript 3 group appears in red. The 4 dash isopropylheptane structure shows C H subscript 3 C H subscript 2 C H subscript 2 C H C H subscript 2 C H subscript 2 C H subscript 3. From the fourth C counting from left to right, there is a C H group bonded above. Bonded up and to the right and and up to the left of this C H group are C H subscript 3 groups.

Alkanes are relatively stable molecules, but heat or light will activate reactions that involve the breaking of C–H or C–C single bonds. Combustion is one such reaction:

Alkanes burn in the presence of oxygen, a highly exothermic oxidation-reduction reaction that produces carbon dioxide and water. As a consequence, alkanes are excellent fuels. For example, methane, CH₄, is the principal component of natural gas. Butane, C₄H₁₀, used in camping stoves and lighters is an alkane. Gasoline is a liquid mixture of continuous- and branched-chain alkanes, each containing from five to nine carbon atoms, plus various additives to improve its performance as a fuel. Kerosene, diesel oil, and fuel oil are primarily mixtures of alkanes with higher molecular masses. The main source of these liquid alkane fuels is crude oil, a complex mixture that is separated by fractional distillation. Fractional distillation takes advantage of differences in the boiling points of the components of the mixture (see [link]). You may recall that boiling point is a function of intermolecular interactions, which was discussed in the chapter on solutions and colloids.

$This figure contains a photo of a refinery, showing large columnar structures. A diagram of a fractional distillation column is also shown. Near the bottom of the column, an arrow pointing into the column from the left shows a point of entry for heated crude oil. The column contains several layers at which different components are removed. At the very bottom, residue materials are removed through a pipe as indicated by an arrow out of the column. At each successive level, different materials are removed through pipes proceeding from the bottom to the top of the column. In order from bottom to top, these materials are fuel oil, followed by diesel oil, kerosene, naptha, gasoline, and refinery gas at the very top. To the right of the column diagram, a double sided arrow is shown that is blue at the top and gradually changes color to red moving downward. The blue top of the arrow is labeled, “Small molecules: low boiling point, very volatile, flows easily, ignites easily.” The red bottom of the arrow is labeled, “Large molecules: high boiling point, not very volatile, does not flow easily, does not ignite easily.”$

In a substitution reaction, another typical reaction of alkanes, one or more of the alkane’s hydrogen atoms is replaced with a different atom or group of atoms. No carbon-carbon bonds are broken in these reactions, and the hybridization of the carbon atoms does not change. For example, the reaction between ethane and molecular chlorine depicted here is a substitution reaction:

This diagram illustrates the reaction of ethane and C l subscript 2 to form chloroethane. In this reaction, the structural formula of ethane is shown with two C atoms bonded together and three H atoms bonded to each C atom. The H atom on the far right is red. Ethane is added to C l bonded to C l, followed by an arrow that points right. The arrow is labeled, “Heat or light.” To the right, the chloroethane molecule is shown with two C atoms bonded together. The left C atom has three H atoms bonded to it, but the right C atom has two H atoms bonded above and below it along with a C l atom. The C l atom appears in red with 3 pairs of electron dots at the right end of the molecule. This is followed by a plus sign, which in turn is followed in red by H bonded to C l. Three pairs of electron dots are present above, to the right, and below the C l.

The C–Cl portion of the chloroethane molecule is an example of a functional group, the part or moiety of a molecule that imparts a specific chemical reactivity. The types of functional groups present in an organic molecule are major determinants of its chemical properties and are used as a means of classifying organic compounds as detailed in the remaining sections of this chapter.

Want more practice naming alkanes? Watch this brief video tutorial to review the nomenclature process.

Alkenes

Organic compounds that contain one or more double or triple bonds between carbon atoms are described as unsaturated. You have likely heard of unsaturated fats. These are complex organic molecules with long chains of carbon atoms, which contain at least one double bond between carbon atoms. Unsaturated hydrocarbon molecules that contain one or more double bonds are called alkenes. Carbon atoms linked by a double bond are bound together by two bonds, one σ bond and one π bond. Double and triple bonds give rise to a different geometry around the carbon atom that participates in them, leading to important differences in molecular shape and properties. The differing geometries are responsible for the different properties of unsaturated versus saturated fats.

Ethene, C₂H₄, is the simplest alkene. Each carbon atom in ethene, commonly called ethylene, has a trigonal planar structure. The second member of the series is propene (propylene) ([link]); the butene isomers follow in the series. Four carbon atoms in the chain of butene allows for the formation of isomers based on the position of the double bond, as well as a new form of isomerism.

Ethylene (the common industrial name for ethene) is a basic raw material in the production of polyethylene and other important compounds. Over 135 million tons of ethylene were produced worldwide in 2010 for use in the polymer, petrochemical, and plastic industries. Ethylene is produced industrially in a process called cracking, in which the long hydrocarbon chains in a petroleum mixture are broken into smaller molecules.

Recycling Plastics

Polymers (from Greek words poly meaning “many” and mer meaning “parts”) are large molecules made up of repeating units, referred to as monomers. Polymers can be natural (starch is a polymer of sugar residues and proteins are polymers of amino acids) or synthetic [like polyethylene, polyvinyl chloride (PVC), and polystyrene]. The variety of structures of polymers translates into a broad range of properties and uses that make them integral parts of our everyday lives. Adding functional groups to the structure of a polymer can result in significantly different properties (see the discussion about Kevlar later in this chapter).

An example of a polymerization reaction is shown in [link]. The monomer ethylene (C₂H₄) is a gas at room temperature, but when polymerized, using a transition metal catalyst, it is transformed into a solid material made up of long chains of –CH₂– units called polyethylene. Polyethylene is a commodity plastic used primarily for packaging (bags and films).

Polyethylene is a member of one subset of synthetic polymers classified as plastics. Plastics are synthetic organic solids that can be molded; they are typically organic polymers with high molecular masses. Most of the monomers that go into common plastics (ethylene, propylene, vinyl chloride, styrene, and ethylene terephthalate) are derived from petrochemicals and are not very biodegradable, making them candidate materials for recycling. Recycling plastics helps minimize the need for using more of the petrochemical supplies and also minimizes the environmental damage caused by throwing away these nonbiodegradable materials.

Plastic recycling is the process of recovering waste, scrap, or used plastics, and reprocessing the material into useful products. For example, polyethylene terephthalate (soft drink bottles) can be melted down and used for plastic furniture, in carpets, or for other applications. Other plastics, like polyethylene (bags) and polypropylene (cups, plastic food containers), can be recycled or reprocessed to be used again. Many areas of the country have recycling programs that focus on one or more of the commodity plastics that have been assigned a recycling code (see [link]). These operations have been in effect since the 1970s and have made the production of some plastics among the most efficient industrial operations today.

The name of an alkene is derived from the name of the alkane with the same number of carbon atoms. The presence of the double bond is signified by replacing the suffix -ane with the suffix -ene.** The location of the double bond** is identified by naming the smaller of the numbers of the carbon atoms participating in the double bond:

Molecules of 1-butene and 2-butene are structural isomers; the arrangement of the atoms in these two molecules differs. As an example of arrangement differences, the first carbon atom in 1-butene is bonded to two hydrogen atoms; the first carbon atom in 2-butene is bonded to three hydrogen atoms.

The compound 2-butene and some other alkenes also form a second type of isomer called a geometric isomer. In a set of geometric isomers, the same types of atoms are attached to each other in the same order, but the geometries of the two molecules differ. Geometric isomers of alkenes differ in the orientation of the groups on either side of a

C = C

Carbon atoms are free to rotate around a single bond but not around a double bond; a double bond is rigid. This makes it possible to have two isomers of 2-butene, one with both methyl groups on the same side of the double bond and one with the methyl groups on opposite sides. When structures of butene are drawn with 120° bond angles around the sp²-hybridized carbon atoms participating in the double bond, the isomers are apparent. The 2-butene isomer in which the two methyl groups are on the same side is called a cis-isomer; the one in which the two methyl groups are on opposite sides is called a trans-isomer ([link]). The different geometries produce different physical properties, such as boiling point, that may make separation of the isomers possible:

moiety is a reactive functional group. A π bond, being a weaker bond, is disrupted much more easily than a σ bond. Thus, alkenes undergo a characteristic reaction in which the π bond is broken and replaced by two σ bonds. This reaction is called an addition reaction. The hybridization of the carbon atoms in the double bond in an alkene changes from sp² to sp³ during an addition reaction. For example, halogens add to the double bond in an alkene instead of replacing hydrogen, as occurs in an alkane:

This diagram illustrates the reaction of ethene and C l subscript 2 to form 1 comma 2 dash dichloroethane. In this reaction, the structural formula of ethane is shown. It has a double bond between the two C atoms with two H atoms bonded to each C atom plus C l bonded to C l. This is shown on to the left of an arrow. The two C atoms and the double bond between them are shown in red. To the right of the arrow, the 1 comma 2 dash dichloroethane molecule is shown. It has only single bonds and each C atom has a C l with three pairs of electron dots bonded beneath it. The C and C l atoms, single bond between them, and electron pairs are shown in red. Each C atom also has two H atoms bonded to it.

<div data-type="example" markdown="1"> Alkene Reactivity and Naming Provide the IUPAC names for the reactant and product of the halogenation reaction shown here:

The left side of a reaction and arrow are shown with an empty product side. On the left, C H subscript 3 is bonded down and to the right to C H which has a double bond to another C H. The second C H is bonded up and to the right to C H subscript 2 which is also bonded to C H subscript 3. A plus sign is shown with a C l atom bonded to a C l atom following it. This is also followed by a reaction arrow.

Solution The reactant is a five-carbon chain that contains a carbon-carbon double bond, so the base name will be pentene. We begin counting at the end of the chain closest to the double bond—in this case, from the left—the double bond spans carbons 2 and 3, so the name becomes 2-pentene. Since there are two carbon-containing groups attached to the two carbon atoms in the double bond—and they are on the same side of the double bond—this molecule is the cis-isomer, making the name of the starting alkene cis-2-pentene. The product of the halogenation reaction will have two chlorine atoms attached to the carbon atoms that were a part of the carbon-carbon double bond:

C H subscript 3 is bonded down and to the right to C H which is bonded down and to the left to C l. C H is also bonded to another C H which is bonded down and to the right to C l and up and to the right to C H subscript 2. C H subscript 2 is also bonded to C H subscript 3.

This molecule is now a substituted alkane and will be named as such. The base of the name will be pentane. We will count from the end that numbers the carbon atoms where the chlorine atoms are attached as 2 and 3, making the name of the product 2,3-dichloropentane.

Check Your Learning Provide names for the reactant and product of the reaction shown:

Alkynes

Hydrocarbon molecules with one or more triple bonds are called alkynes; they make up another series of unsaturated hydrocarbons. Two carbon atoms joined by a triple bond are bound together by one σ bond and two π bonds. The sp-hybridized carbons involved in the triple bond have bond angles of 180°, giving these types of bonds a linear, rod-like shape.

The simplest member of the alkyne series is ethyne, C₂H₂, commonly called acetylene. The Lewis structure for ethyne, a linear molecule, is:

The structural formula and name for ethyne, also known as acetylene, are shown. In red, two C atoms are shown with a triple bond illustrated by three horizontal line segments between them. Shown in black at each end of the structure, a single H atom is bonded.

The IUPAC nomenclature for alkynes is similar to that for alkenes except that the suffix -yne is used to indicate a triple bond in the chain. For example,

{CH}_{3} {CH}_{2} C \equiv CH

Structure of Alkynes Describe the geometry and hybridization of the carbon atoms in the following molecule:

A structural formula is shown with C H subscript 3 bonded to a C atom which is triple bonded to another C atom which is bonded to C H subscript 3. Each C atom is labeled 1, 2, 3, and 4 from left to right. Solution Carbon atoms 1 and 4 have four single bonds and are thus tetrahedral with sp³ hybridization. Carbon atoms 2 and 3 are involved in the triple bond, so they have linear geometries and would be classified as sp hybrids.

Check Your Learning Identify the hybridization and bond angles at the carbon atoms in the molecule shown:

A structural formula is shown with an H atom bonded to a C atom. The C atom has a triple bond with another C atom which is also bonded to C H. The C H has a double bond with another C H which is also bonded up and to the right to C H subscript 3. Each C atom is labeled 1, 2, 3, 4, or 5 from left to right.

Answer:

carbon 1: sp, 180°; carbon 2: sp, 180°; carbon 3: sp², 120°; carbon 4: sp², 120°; carbon 5: sp³, 109.5°

functional group has two π bonds, alkynes typically react even more readily, and react with twice as much reagent in addition reactions. The reaction of acetylene with bromine is a typical example:

This diagram illustrates the reaction of ethyne and two molecules of B r subscript 2 to form 1 comma 1 comma 2 comma 2 dash tetrabromoethane. In this reaction, the structural formula of ethyne, an H atom bonded to a red C atom with a red triple bond to another red C atom bonded to a black H atom, plus B r bonded to B r plus B r bonded to B r is shown to the left of an arrow. On the right, the form 1 comma 1 comma 2 comma 2 dash tetrabromoethane molecule is shown. It has an H atom bonded to a C atom which is bonded to another C atom which is bonded to an H atom. Each C atom is bonded above and below to a B r atom. Each B r atom has three pairs of electron dots. The C and B r atoms, single bond between them, and electron pairs are shown in red.

Acetylene and the other alkynes also burn readily. An acetylene torch takes advantage of the high heat of combustion for acetylene.

Aromatic Hydrocarbons

Benzene, C₆H₆, is the simplest member of a large family of hydrocarbons, called aromatic hydrocarbons. These compounds contain ring structures and exhibit bonding that must be described using the resonance hybrid concept of valence bond theory or the delocalization concept of molecular orbital theory. (To review these concepts, refer to the earlier chapters on chemical bonding). The resonance structures for benzene, C₆H₆, are:

This structural formula shows a six carbon hydrocarbon ring. On the left side there are six C atoms. The C atom on top and to the left forms a single bond to the C atom on the top and to the right. The C atom has a double bond to another C atom which has a single bond to a C atom. That C atom has a double bond to another C atom which has a single bond to a C atom. That C atom forms a double bond with another C atom. Each C atom has a single bond to an H atom. There is a double sided arrow and the structure on the right is almost identical to the structure on the left. The structure on the right shows double bonds where the structure on the left showed single bonds. The structure on the right shows single bonds where the stucture on the left showed double bonds.

Valence bond theory describes the benzene molecule and other planar aromatic hydrocarbon molecules as hexagonal rings of sp²-hybridized carbon atoms with the unhybridized p orbital of each carbon atom perpendicular to the plane of the ring. Three valence electrons in the sp² hybrid orbitals of each carbon atom and the valence electron of each hydrogen atom form the framework of σ bonds in the benzene molecule. The fourth valence electron of each carbon atom is shared with an adjacent carbon atom in their unhybridized p orbitals to yield the π bonds. Benzene does not, however, exhibit the characteristics typical of an alkene. Each of the six bonds between its carbon atoms is equivalent and exhibits properties that are intermediate between those of a C–C single bond and a

C = C

double bond. To represent this unique bonding, structural formulas for benzene and its derivatives are typically drawn with single bonds between the carbon atoms and a circle within the ring as shown in [link].

There are many derivatives of benzene. The hydrogen atoms can be replaced by many different substituents. Aromatic compounds more readily undergo substitution reactions than addition reactions; replacement of one of the hydrogen atoms with another substituent will leave the delocalized double bonds intact. The following are typical examples of substituted benzene derivatives:

Three structural formulas are shown. The first is labeled toluene. This molecule has a six carbon hydrocarbon ring in which five of the C atoms are each bonded to only one H atom. At the upper right of the ring, the C atom that does not have a bonded H atom has a red C H subscript 3 group attached. A circle is at the center of the ring. The second is labeled xylene. This molecule has a six carbon hydrocarbon ring in which four of the C atoms are each bonded to only one H atom. At the upper right and right of the ring, the two C atoms that do not have bonded H atoms have C H subscript 3 groups attached. These C H subscript 3 groups appear in red. A circle is at the center of the ring. The third is labeled styrene. This molecule has a six carbon hydrocarbon ring in which five of the carbon atoms are each bonded to only one H atom. At the upper right of the ring, the carbon that does not have a bonded H atom has a red C H double bond C H subscript 2 group attached. A circle is at the center of the ring.

Toluene and xylene are important solvents and raw materials in the chemical industry. Styrene is used to produce the polymer polystyrene.

Structure of Aromatic Hydrocarbons One possible isomer created by a substitution reaction that replaces a hydrogen atom attached to the aromatic ring of toluene with a chlorine atom is shown here. Draw two other possible isomers in which the chlorine atom replaces a different hydrogen atom attached to the aromatic ring:

Solution Since the six-carbon ring with alternating double bonds is necessary for the molecule to be classified as aromatic, appropriate isomers can be produced only by changing the positions of the chloro-substituent relative to the methyl-substituent:

Check Your Learning Draw three isomers of a six-membered aromatic ring compound substituted with two bromines.

Answer:

![Three pairs of structural formulas are shown. The first has a six carbon hydrocarbon ring in which four of the C atoms are each bonded to only one H atom. At the upper right and right of the ring, the two C atoms that do not have bonded H atoms have one B r atom bonded each. A circle is at the center of the ring. Beneath this structure, a similar structure is shown which has a hexagon with a circle inside. From vertices of the hexagon at the upper right and right single B r atoms are attached. The second has a six carbon hydrocarbon ring in which four of the C atoms are each bonded to only one H atom. At the upper right and lower right of the ring, the two C atoms that do not have bonded H atoms have a single B r atom bonded each. A circle is at the center of the ring. Beneath this structure, a similar structure is shown which has a hexagon with a circle inside. From vertices of the hexagon at the upper right and lower right single B r atoms are attached. The third has a six carbon hydrocarbon ring in which four of the C atoms are each bonded to only one H atom. At the upper right and lower left of the ring, the two C atoms that do not have bonded H atoms have B r atoms bonded. A circle is at the center of the ring. Beneath this structure, a similar structure is shown which has a hexagon with a circle inside. From vertices of the hexagon at the upper right and lower left, single B r atoms are attached.](/chemistry-book/resources/CNX_Chem_20_01_AromatIso3_img.jpg)

Key Concepts and Summary

Strong, stable bonds between carbon atoms produce complex molecules containing chains, branches, and rings. The chemistry of these compounds is called organic chemistry. Hydrocarbons are organic compounds composed of only carbon and hydrogen. The alkanes are saturated hydrocarbons—that is, hydrocarbons that contain only single bonds. Alkenes contain one or more carbon-carbon double bonds. Alkynes contain one or more carbon-carbon triple bonds. Aromatic hydrocarbons contain ring structures with delocalized π electron systems.

Chemistry End of Chapter Exercises

Write the chemical formula and Lewis structure of the following, each of which contains five carbon atoms:

(a) an alkane

(b) an alkene

There are several sets of answers; one is:* * *

(a) C₅H₁₂* * *

A chain of five C atoms with single bonds is shown. Each C atom has an H atom bonded above and below it. The C atoms on the end of the chain have a third H atom bonded to them each. ;* * *

(b) C₅H₁₀* * *

;* * *

On a microscopic level, how does the reaction of bromine with a saturated hydrocarbon differ from its reaction with an unsaturated hydrocarbon? How are they similar?

Both reactions result in bromine being incorporated into the structure of the product. The difference is the way in which that incorporation takes place. In the saturated hydrocarbon, an existing C–H bond is broken, and a bond between the C and the Br can then be formed. In the unsaturated hydrocarbon, the only bond broken in the hydrocarbon is the π bond whose electrons can be used to form a bond to one of the bromine atoms in Br₂ (the electrons from the Br–Br bond form the other C–Br bond on the other carbon that was part of the π bond in the starting unsaturated hydrocarbon).

Explain why unbranched alkenes can form geometric isomers while unbranched alkanes cannot. Does this explanation involve the macroscopic domain or the microscopic domain?

Unbranched alkanes have free rotation about the C–C bonds, yielding all orientations of the substituents about these bonds equivalent, interchangeable by rotation. In the unbranched alkenes, the inability to rotate about the $C = C$

bond results in fixed (unchanging) substituent orientations, thus permitting different isomers. Since these concepts pertain to phenomena at the molecular level, this explanation involves the microscopic domain.

Write the Lewis structure and molecular formula for each of the following hydrocarbons:

(a) hexane

(b) 3-methylpentane

(d) 4-methyl-1-pentene

(e) 3-hexyne

(f) 4-methyl-2-pentyne

(a) C₆H₁₄* * *

This figure shows a horizontal hydrocarbon chain consisting of six singly bonded carbon atoms. Each C atom has an H atom bonded above and below it. The two C atoms on either end of the chain each of a third H atom bonded to it. ;* * *

(b) C₆H₁₄* * *

;* * *

(d) C₆H₁₂* * *

;* * *

(e) C₆H₁₀* * *

;* * *

(f) C₆H₁₀* * *

Write the chemical formula, condensed formula, and Lewis structure for each of the following hydrocarbons:

(a) heptane

(b) 3-methylhexane

(d) 4-methyl-1-hexene

(e) 2-heptyne

(f) 3,4-dimethyl-1-pentyne

Give the complete IUPAC name for each of the following compounds:

(a) CH₃CH₂CBr₂CH₃

(b) (CH₃)₃CCl

(c)* * *

This structure shows a hydrocarbon chain composed of C H subscript 3 C H C H subscript 2 C H subscript 3 with a C H subscript 3 group attached beneath the second C atom counting left to right. (d) ${CH}_{3} {CH}_{2} C \equiv {CH CH}_{3} {CH}_{2} C \equiv CH$

(e)* * *

This structure shows a horizontal chain composed of C H subscript 3 C F C H subscript 2 C H subscript 2 C H subscript 2 C H subscript 3 with a C H subscript 2 C H triple bond C H group attached beneath the second C atom counting left to right. (f)* * *

This structure shows two double bounded C atoms with C l attached to the upper left, C H subscript 3 attached to the lower right, and H atoms attached to the upper right and lower left in the structure. (g) ${({CH}_{3})}_{2} {CHCH}_{2} CH = {CH}_{2}$

(a) 2,2-dibromobutane; (b) 2-chloro-2-methylpropane; (c) 2-methylbutane; (d) 1-butyne; (e) 4-fluoro-4-methyl-1-octyne; (f) trans-1-chloropropene; (g) 5-methyl-1-pentene

Give the complete IUPAC name for each of the following compounds:

(a) (CH₃)₂CHF

(b) CH₃CHClCHClCH₃

(c)* * *

This structure shows a hydrocarbon chain composed of C H subscript 3 C H C H subscript 3 with a C H subscript 2 C H subscript 3 group attached beneath the second C atom counting left to right. (d) ${CH}_{3} {CH}_{2} CH = {CHCH}_{3}$

(e)* * *

This structure shows a hydrocarbon chain composed of C H subscript 3 C H subscript 2 C H subscript 2 C H B r C H subscript 2 C H subscript 3 with a C H subscript 2 C H double bond C H subscript 2 group attached beneath the second C atom counting left to right. (f) ${({CH}_{3})}_{3} {CCH}_{2} C \equiv CH$

Butane is used as a fuel in disposable lighters. Write the Lewis structure for each isomer of butane.

![Two structures are shown. The first includes a chain of four singly bonded C atoms. Each C atom has two H atoms bonded above and below it. The two C atoms at either end of the chain each have a third H atom bonded to it. The molecule is named n dash butane. The second includes a chain of three singly bonded C atoms with a C atom bonded above the middle C atom in the chain. The first C atom (from left to right) has three H atoms bonded to it. The second C atom has one H atom bonded below it and a C atom bonded above it. The C atom bonded above the middle C atom has three H atoms bonded to it. The third C atom in the chain has three H atoms bonded to it. This molecule is named 2 dash methylpropane.](/chemistry-book/resources/CNX_Chem_20_01_ex1_14_img.jpg)

Write Lewis structures for the cis–trans isomers of ${CH}_{3} CH = CHCl.$

![This figure includes two structural formulas. The first structure shows two double bounded C atoms with C l bonded to the upper right, C H subscript 3 bonded to the upper left, and H atoms attached to the lower right and lower left in the structure. This structure is labeled cis dash. The second structure shows two double bounded carbon atoms with C l attached to the lower right, C H subscript 3 attached to the upper left, and H atoms attached to the upper right and lower left in the structure. This structure is labeled trans dash.](/chemistry-book/resources/CNX_Chem_20_01_ex1_16_img.jpg)

Isooctane is the common name of the isomer of C₈H₁₈ used as the standard of 100 for the gasoline octane rating:* * *

The hydrocarbon molecular structure shown includes C H subscript 3 C H C H subscript 2 C C H subscript 3. There is a C H subscript 3 group bonded to the second C atom in the chain (from left to right). There are two C H subscript 3 groups bonded above and below the fourth C atom in the chain. (a) What is the IUPAC name for the compound?

(b) Name the other isomers that contain a five-carbon chain with three methyl substituents.

(a) 2,2,4-trimethylpentane; (b) 2,2,3-trimethylpentane, 2,3,4-trimethylpentane, and 2,3,3-trimethylpentane:* * *

Write Lewis structures and IUPAC names for all isomers of C₄H₉Cl.

![Four structural formulas are provided. The first has a hydrocarbon chain with a length of four C atom. All bonds are single. Nine H atoms are attached and a single C l atom is attached at the far right end of the structure which is labeled 1 dash chlorobutane. The second has a hydrocarbon chain with a length four C atoms. All bonds are single. Nine H atoms are attached and a single C l atom is attached above the second carbon counting left to right. This structure is labeled 2 dash chlorobutane. The third has a hydrocarbon chain with a length of three C atoms. All bonds are single. A single C l atom is bonded beneath the middle C atom and a C H subscript 3 group is also bonded above the middle C atom. Six H atoms are attached, and the structure is labeled 2 dash chloro dash 2 dash methylpropane. The fourth structure has a hydrocarbon chain with a length of three C atoms. All bonds are single. A single C l atom is bonded beneath the first C atom (from left to right) and a C H subscript 3 group is bonded above the middle C atom. Six H atoms are attached, and the structure is labeled 1 dash chloro dash 2 dash methylpropane.](/chemistry-book/resources/CNX_Chem_20_01_ex1_20_img.jpg)

Benzene is one of the compounds used as an octane enhancer in unleaded gasoline. It is manufactured by the catalytic conversion of acetylene to benzene:* * *

3 C_{2} H_{2} ⟶ C_{6} H_{6}

Draw Lewis structures for these compounds, with resonance structures as appropriate, and determine the hybridization of the carbon atoms in each.

Two structural formulas are shown. The first shows two C atoms with a triple bond between them. At each end of the structure, a single H atom is bonded. The second structure involves a hydrocarbon ring of 6 C atoms with a circle at the center. There are alternating double bonds between C atoms. Each C atom is bonded to a single H atom. In acetylene, the bonding uses sp hybrids on carbon atoms and s orbitals on hydrogen atoms. In benzene, the carbon atoms are sp² hybridized.

Write two complete, balanced equations for each of the following reactions, one using condensed formulas and one using Lewis structures.

(a) 1 mol of 1-butyne reacts with 2 mol of iodine.

(b) Pentane is burned in air.

(a) $CH \equiv {CCH}_{2} {CH}_{3} + 2 I_{2} ⟶ {CHI}_{2} {CI}_{2} {CH}_{2} {CH}_{3}$

;* * *

(b) ${CH}_{3} {CH}_{2} {CH}_{2} {CH}_{2} {CH}_{3} + 8 O_{2} ⟶ 5 {CO}_{2} + {6H}_{2} O$

Acetylene is a very weak acid; however, it will react with moist silver(I) oxide and form water and a compound composed of silver and carbon. Addition of a solution of HCl to a 0.2352-g sample of the compound of silver and carbon produced acetylene and 0.2822 g of AgCl.

(a) What is the empirical formula of the compound of silver and carbon?

(b) The production of acetylene on addition of HCl to the compound of silver and carbon suggests that the carbon is present as the acetylide ion, $C_{2}^{2−}$

. Write the formula of the compound showing the acetylide ion.

Glossary

addition reaction: reaction in which a double carbon-carbon bond forms a single carbon-carbon bond by the addition of a reactant. Typical reaction for an alkene.

alkane: molecule consisting of only carbon and hydrogen atoms connected by single (σ) bonds

alkene: molecule consisting of carbon and hydrogen containing at least one carbon-carbon double bond

alkyl group: substituent, consisting of an alkane missing one hydrogen atom, attached to a larger structure

alkyne: molecule consisting of carbon and hydrogen containing at least one carbon-carbon triple bond

aromatic hydrocarbon: cyclic molecule consisting of carbon and hydrogen with delocalized alternating carbon-carbon single and double bonds, resulting in enhanced stability

functional group: part of an organic molecule that imparts a specific chemical reactivity to the molecule

organic compound: natural or synthetic compound that contains carbon

saturated hydrocarbon: molecule containing carbon and hydrogen that has only single bonds between carbon atoms

skeletal structure: shorthand method of drawing organic molecules in which carbon atoms are represented by the ends of lines and bends in between lines, and hydrogen atoms attached to the carbon atoms are not shown (but are understood to be present by the context of the structure)

substituent: branch or functional group that replaces hydrogen atoms in a larger hydrocarbon chain

substitution reaction: reaction in which one atom replaces another in a molecule

Properties of Some Alkanes³
Alkane	Molecular Formula	Melting Point (°C)	Boiling Point (°C)	Phase at STP⁴	Number of Structural Isomers
methane	CH₄	–182.5	–161.5	gas	1
ethane	C₂H₆	–183.3	–88.6	gas	1
propane	C₃H₈	–187.7	–42.1	gas	1
butane	C₄H₁₀	–138.3	–0.5	gas	2
pentane	C₅H₁₂	–129.7	36.1	liquid	3
hexane	C₆H₁₄	–95.3	68.7	liquid	5
heptane	C₇H₁₆	–90.6	98.4	liquid	9
octane	C₈H₁₈	–56.8	125.7	liquid	18
nonane	C₉H₂₀	–53.6	150.8	liquid	35
decane	C₁₀H₂₂	–29.7	174.0	liquid	75
tetradecane	C₁₄H₃₀	5.9	253.5	solid	1858
octadecane	C₁₈H₃₈	28.2	316.1	solid	60,523

Alkanes

The Basics of Organic Nomenclature: Naming Alkanes

Alkenes

Alkynes

Aromatic Hydrocarbons

Key Concepts and Summary

Chemistry End of Chapter Exercises

Footnotes

Glossary