3.7: Saturated Hydrocarbons

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Approximately one-third of the compounds produced industrially are organic compounds. All living organisms are composed of organic compounds, as are most foods, medicines, clothing fibers, and plastics. The detection of organic compounds is useful in many fields. In one recently developed application, scientists have devised a new method called “material degradomics” to monitor the degradation of old books and historical documents. As paper ages, it produces a familiar “old book smell” from the release of organic compounds in gaseous form. The composition of the gas depends on the original type of paper used, a book’s binding, and the applied media. By analyzing these organic gases and isolating the individual components, preservationists are better able to determine the condition of an object and those books and documents most in need of immediate protection.

The simplest class of organic compounds is the hydrocarbons, which consist entirely of carbon and hydrogen. Petroleum and natural gas are complex, naturally occurring mixtures of many different hydrocarbons that furnish raw materials for the chemical industry. The four major classes of hydrocarbons are the following: the alkanes, which contain only carbon–hydrogen and carbon–carbon single bonds; the alkenes, which contain at least one carbon–carbon double bond; the alkynes, which contain at least one carbon–carbon triple bond; and the aromatic hydrocarbons, which usually contain rings of six carbon atoms that can be drawn with alternating single and double bonds. Alkanes are also called saturated hydrocarbons, whereas hydrocarbons that contain multiple bonds (alkenes, alkynes, and aromatics) are unsaturated.

Alkanes

The simplest alkane is methane (CH₄), a colorless, odorless gas that is the major component of natural gas. In larger alkanes whose carbon atoms are joined in an unbranched chain (straight-chain alkanes), each carbon atom is bonded to at most two other carbon atoms. The structures of two simple alkanes are shown in Figure \(\PageIndex{1}\), and the names and condensed structural formulas for the first 10 straight-chain alkanes are in Table \(\PageIndex{1}\). The names of all alkanes end in -ane, and their boiling points increase as the number of carbon atoms increases.

Table \(\PageIndex{1}\): The First 10 Straight-Chain Alkanes
Name	Number of Carbon Atoms	Molecular Formula	Condensed Structural Formula	Boiling Point (°C)	Uses
methane	1	CH₄	CH₄	−162	natural gas constituent
ethane	2	C₂H₆	CH₃CH₃	−89	natural gas constituent
propane	3	C₃H₈	CH₃CH₂CH₃	−42	bottled gas
butane	4	C₄H₁₀	CH₃CH₂CH₂CH₃ or CH₃(CH₂)₂CH₃	0	lighters, bottled gas
pentane	5	C₅H₁₂	CH₃(CH₂)₃CH₃	36	solvent, gasoline
hexane	6	C₆H₁₄	CH₃(CH₂)₄CH₃	69	solvent, gasoline
heptane	7	C₇H₁₆	CH₃(CH₂)₅CH₃	98	solvent, gasoline
octane	8	C₈H₁₈	CH₃(CH₂)₆CH₃	126	gasoline
nonane	9	C₉H₂₀	CH₃(CH₂)₇CH₃	151	gasoline
decane	10	C₁₀H₂₂	CH₃(CH₂)₈CH₃	174	kerosene

Alkanes with four or more carbon atoms can have more than one arrangement of atoms. The carbon atoms can form a single unbranched chain, or the primary chain of carbon atoms can have one or more shorter chains that form branches. For example, butane (C₄H₁₀) has two possible structures. Normal butane (usually called n-butane) is CH₃CH₂CH₂CH₃, in which the carbon atoms form a single unbranched chain. In contrast, the condensed structural formula for isobutane is (CH₃)₂CHCH₃, in which the primary chain of three carbon atoms has a one-carbon chain branching at the central carbon. Three-dimensional representations of both structures are as follows:

The systematic names for branched hydrocarbons use the lowest possible number to indicate the position of the branch along the longest straight carbon chain in the structure. Thus the systematic name for isobutane is 2-methylpropane, which indicates that a methyl group (a branch consisting of –CH₃) is attached to the second carbon of a propane molecule. Similarly, Section 2.6 states that one of the major components of gasoline is commonly called isooctane; its structure is as follows:

The compound has a chain of five carbon atoms, so it is a derivative of pentane. There are two methyl group branches at one carbon atom and one methyl group at another. Using the lowest possible numbers for the branches gives 2,2,4-trimethylpentane for the systematic name of this compound.

Alkenes

The simplest alkenes are ethylene, C₂H₄ or CH₂=CH₂, and propylene, C₃H₆ or CH₃CH=CH₂ (part (a) in Figure \(\PageIndex{2}\)). The names of alkenes that have more than three carbon atoms use the same stems as the names of the alkanes (Table \(\PageIndex{1}\) "The First 10 Straight-Chain Alkanes") but end in -ene instead of -ane.

As with alkanes, more than one structure is possible for alkenes with four or more carbon atoms. For example, an alkene with four carbon atoms has three possible structures. One is CH₂=CHCH₂CH₃ (1-butene), which has the double bond between the first and second carbon atoms in the chain. The other two structures have the double bond between the second and third carbon atoms and are forms of CH₃CH=CHCH₃ (2-butene). All four carbon atoms in 2-butene lie in the same plane, so there are two possible structures (part (a) in Figure \(\PageIndex{2}\)). If the two methyl groups are on the same side of the double bond, the compound is cis-2-butene (from the Latin cis, meaning “on the same side”). If the two methyl groups are on opposite sides of the double bond, the compound is trans-2-butene (from the Latin trans, meaning “across”). These are distinctly different molecules: cis-2-butene melts at −138.9°C, whereas trans-2-butene melts at −105.5°C.

Figure \(\PageIndex{2}\): Some Simple (a) Alkenes, (b) Alkynes, and (c) Cyclic Hydrocarbons. The positions of the carbon atoms in the chain are indicated by C₁ or C₂.

Just as a number indicates the positions of branches in an alkane, the number in the name of an alkene specifies the position of the first carbon atom of the double bond. The name is based on the lowest possible number starting from either end of the carbon chain, so CH₃CH₂CH=CH₂ is called 1-butene, not 3-butene. Note that CH₂=CHCH₂CH₃ and CH₃CH₂CH=CH₂ are different ways of writing the same molecule (1-butene) in two different orientations.

The name of a compound does not depend on its orientation. As illustrated for 1-butene, both condensed structural formulas and molecular models show different orientations of the same molecule. It is important to be able to recognize the same structure no matter what its orientation.

Note

The positions of groups or multiple bonds are always indicated by the lowest number possible.

Alkynes

The simplest alkyne is acetylene, C₂H₂ or HC≡CH (part (b) in Figure \(\PageIndex{2}\)). Because a mixture of acetylene and oxygen burns with a flame that is hot enough (>3000°C) to cut metals such as hardened steel, acetylene is widely used in cutting and welding torches. The names of other alkynes are similar to those of the corresponding alkanes but end in -yne. For example, HC≡CCH₃ is propyne, and CH₃C≡CCH₃ is 2-butyne because the multiple bond begins on the second carbon atom.

Note

The number of bonds between carbon atoms in a hydrocarbon is indicated in the suffix:

alkane: only carbon–carbon single bonds
alkene: at least one carbon–carbon double bond
alkyne: at least one carbon–carbon triple bond