Branched Chains

Look at the butane and isobutane formulas in Figure 5. Both compounds have a molecular formula of C₄H₁₀, but their structural formulas are different. In isobutane, there is a branch at the point where a carbon atom bonds to three other carbon atoms.

Compounds with the same molecular formula but different structural formulas are isomers. Differences in structure affect some properties of isomers. Butane boils at −0.5°C, but isobutane boils at −11.7°C. The number of possible isomers increases rapidly each time an additional carbon atom is added to the chain. For example, octane (C₈H₁₈) has 18 isomers, while decane (C₁₀H₂₂) has 75.

Rings

Figure 5 also shows the structural formula for cyclobutane. The carbon atoms in cyclobutane are linked in a four-carbon ring. Because each carbon atom forms bonds with two other carbon atoms, it can bond with only two hydrogen atoms. So cyclobutane (C₄H₈) molecules have two fewer hydrogen atoms than butane (C₄H₁₀) molecules. Most ring alkanes, or cyclic hydrocarbons, have rings with five or six carbons.

Figure 5 Butane, isobutane, and cyclobutane represent three ways that carbon atoms are arranged in an alkane—in a straight chain, in a branched chain, and in a ring. Classifying Explain why butane and cyclobutane are not isomers.

d