25.3 Alkanes

Table 25.1 lists several of the simplest alkanes. Many of these substances are familiar because of their widespread use. Methane is a major component of natural gas and is used for home heating and in gas stoves and hot-water heaters. Propane is the major component of bottled gas used for home heating and cooking in areas where natural gas is not available. Butane is used in disposable lighters and in fuel canisters for gas camping stoves and lanterns. Alkanes with from 5 to 12 carbon atoms per molecule are found in gasoline.

The formulas for the alkanes given in Table 25.1 are written in a notation called condensed structural formulas. This notation reveals the way in which atoms are bonded to one another but does not require drawing in all the bonds. For example, the Lewis structure and the condensed structural formulas for butane, C4H10, are

AABAAQT0

We will frequently use either Lewis structures or condensed structural formulas to represent organic compounds. Notice that each carbon atom in an alkane has four single bonds, whereas each hydrogen atom forms one single bond. Notice also that each succeeding compound in the series listed in Table 25.1 has an additional CH2 unit.

Structures of Alkanes

The Lewis structures and condensed structural formulas for alkanes do not tell us anything about the three-dimensional structures of these substances. As we would predict from the VSEPR model, the geometry about each carbon atom in an alkane is tetrahedral; that is, the four groups attached to each carbon are located at the vertices of a tetrahedron. The three-dimensional structures can be represented as shown for methane in Figure 25.2. The bonding may be described as involving sp3 hybridized orbitals on the carbon.

Figure 25.2 Representations of the three-dimensional arrangement of bonds about carbon in methane.

Rotation about a carbon-carbon single bond is relatively easy, and it occurs very rapidly at room temperature. To visualize such rotation, imagine grasping the top left methyl group in Figure 25.3, which shows the structure of propane, and spinning it relative to the rest of the structure. Because motion of this sort occurs very rapidly in alkanes, a long-chain alkane molecule is constantly undergoing motions that cause it to change its shape, something like a length of chain that is being shaken.

Figure 25.3 Three-dimensional models for propane, C3H8, showing rotations about the carbon-carbon single bonds.

Structural Isomers

The alkanes listed in Table 25.1 are called straight-chain hydrocarbons because all the carbon atoms are joined in a continuous chain. Alkanes consisting of four or more carbon atoms can also form branched chains; hydrocarbons with branched chains are called branched-chain hydrocarbons. Figure 25.4 shows the condensed formulas and space-filling models for all the possible structures of alkanes containing four and five carbon atoms. Notice that there are two ways that four carbon atoms can be joined to give C4H10: as a straight chain (top left) or a branched chain (top right). For alkanes with five carbon atoms, C5H12, there are three different arrangements.

Figure 25.4 Possible structures, names, and melting and boiling points of alkanes of formula C4H10 and C5H12.

Compounds with the same molecular formula but with different bonding arrangements (and hence different structures) are called structural isomers. The structural isomers of a given alkane differ slightly from one another in physical properties. Note the melting and boiling points of the isomers of butane and pentane, given in Figure 25.4. The number of possible structural isomers increases rapidly with the number of carbon atoms in the alkane. For example, there are 18 possible isomers having the molecular formula, C8H18, and 75 possible isomers with the molecular formula, C10H22.

Nomenclature of Alkanes

The first names given to the structural isomers shown in Figure 25.4 are the so-called common names. The isomer in which one CH3 group is branched off the major chain is labeled the iso-isomer, for example, isobutane. However, as the number of isomers grows, it becomes impossible to find a suitable prefix to denote each isomer. The need for a systematic means of naming organic compounds was recognized early in the history of organic chemistry. In 1892 an organization called the International Union of Chemistry met in Geneva, Switzerland, to formulate rules for systematic naming of organic substances. Since that time the task of updating the rules for naming compounds has fallen to the International Union of Pure and Applied Chemistry (IUPAC). Chemists everywhere, regardless of their nationality or political affiliation, subscribe to a common system for naming compounds.

The IUPAC names for the isomers of butane and pentane are the ones given in parentheses for each compound in Figure 25.4. The following steps summarize the procedures used to arrive at these names and the names of other alkanes. We use a similar approach to write the names of other organic compounds.

  1. Find the longest continuous chain of carbon atoms, and use the name of this chain as the base name of the compound. The longest chain may not always be written in a straight line, as seen in the following example:

    AABAAQV0

    Because this compound has a chain of six C atoms, it is named as a substituted hexane. Groups attached to the main chain are called substituents because they are substituted in place of an H atom on the main chain.

  2. Number the carbon atoms in the longest chain, beginning with the end of the chain that is nearest to a substituent. In our example we number the C atoms from the upper right because that places the CH3 substituent on the second C atom of the chain; if we number from the lower right, the CH3 would be on the fifth C atom. The chain is numbered from the end that gives the lowest number for the substituent position.

  3. Name and give the location of each substituent group. A substituent group that is formed by removing an H atom from an alkane is called an alkyl group. Alkyl groups are named by replacing the -ane ending of the alkane name with -yl. For example, the methyl group, CH3, is derived from methane, CH4. Likewise, the ethyl group, C2H5, is derived from ethane, C2H6. Table 25.2 lists several common alkyl groups. The name 2-methylhexane indicates the presence of a methyl, CH3, group on the second carbon atom of a hexane (six carbon) chain.

  4. When two or more substituents are present, list them in alphabetical order. When there are two or more of the same substituent, the number of substituents of that type is indicated by a prefix: di- (two), tri- (three), tetra- (four), penta- (five), and so forth. Notice how the following example is named:

AABAAQX0

SAMPLE EXERCISE 25.1

Name the following alkane:

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SOLUTION To name this compound properly, you must first find the longest continuous chain of carbon atoms. This chain, extending from the upper left CH3 group to the lower left CH3 group, is seven carbon atoms long:

AABAAQZ0

The compound is thus named as a derivative of heptane. We might number the carbon atoms starting from either end. However, IUPAC rules state that the numbering should be done so that the numbers of carbons bearing side chains are as low as possible. This means that we should start numbering with the upper left carbon. There is a methyl group on carbon 3, and one on carbon 4. The compound is thus called 3,4-dimethylheptane.

PRACTICE EXERCISE

Name the following alkane:

AABAARA0

Answer: 2,4-dimethylpentane

SAMPLE EXERCISE 25.2

Write the condensed structural formula for 3-ethyl-2-methylpentane.

SOLUTION The longest continuous chain of carbon atoms in this compound is five. We can therefore begin by writing out a string of five C atoms:

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We next place a methyl group on the second carbon, and an ethyl group on the third carbon atom of the chain. Hydrogens are then added to all the other carbon atoms to make the four bonds to each carbon. Thus, the structural formula is

AABAARB0

The formula can be written more concisely as CH3CH(CH3)CH(C2H5)CH2CH3.

PRACTICE EXERCISE

Write the condensed structural formula for 2,3-dimethylhexane.

Answer:

AABAARC0

Cycloalkanes

Alkanes can form not only branched chains, but also rings or cycles. Alkanes with this form of structure are called cycloalkanes. Figure 25.5 illustrates a few examples of cycloalkanes. Cycloalkane structures are sometimes drawn as simple polygons in which each corner of the polygon represents a CH2 group. This method of representation is similar to that used for benzene rings. In the case of aromatic structures each corner represents a CH group.

Figure 25.5 Condensed structural formulas for three cycloalkanes.

Carbon rings containing fewer than five carbon atoms are strained because the C C C bond angle in the smaller rings must be less than the 109.5° tetrahedral angle. The amount of strain increases as the rings get smaller. In cyclopropane, which has the shape of an equilateral triangle, the angle is only 60°; this molecule is therefore much more reactive than its straight-chain analog, propane.

Cycloalkanes, particularly the small ring compounds, sometimes behave chemically like unsaurated hydrocarbons, which we will disuss shortly. Note that the general formula for cycloalkanes, CnH2n, differs from the straight chain alkanes, for which the general formula is CnH2n+ 2.

Reactions of Alkanes

Most alkanes are relatively unreactive. For example, at room temperature they do not react with acids, bases, or strong oxidizing agents, and they are not even attacked by boiling nitric acid. One reason for their low chemical reactivity is the strength of C C and C H bonds.

Alkanes are not completely inert, however. One of their most commercially important reactions is combustion in air, which is the basis of their use as fuels. For example, the complete combustion of ethane proceeds as follows:

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In the following sections we will see two ways in which hydrocarbons can be modified to impart greater reactivity: the introduction of unsaturation into the carbon-carbon framework and the attachment of other reactive groups to the hydrocarbon backbone.