Which statement best defines an unsaturated hydrocarbon?

Which statement best defines an unsaturated hydrocarbon?

• A. A hydrocarbon in which all bonds between carbon atoms are single bonds
• B. A hydrocarbon that contains only saturated carbon-hydrogen bonds
• C. A hydrocarbon with rings only and no double bonds
• D. A hydrocarbon in which one or more bonds between carbon atoms is double or triple

Answer: (D)

Unsaturated hydrocarbons are defined by the presence of one or more carbon–carbon multiple bonds, such as a double bond (C=C) or a triple bond (C≡C). These pi bonds mean the carbon framework cannot hold as many hydrogens as a fully single-bonded, or saturated, structure. In other words, when a molecule has a C=C or C≡C bond, it has fewer hydrogen atoms than the corresponding alkane with the same number of carbon atoms. This is why a hydrocarbon with only single C–C bonds is described as saturated—each carbon is bonded to the maximum number of hydrogens possible. A ring with no double bonds is a saturated cycloalkane, not unsaturated, since it lacks a pi bond. And a hydrocarbon that supposedly contains only saturated C–H bonds describes a saturated system as well. Thus, the statement that captures the essence of unsaturation is the one noting a double or triple bond between carbon atoms.

Unsaturated hydrocarbons are defined by the presence of one or more carbon–carbon multiple bonds, such as a double bond (C=C) or a triple bond (C≡C). These pi bonds mean the carbon framework cannot hold as many hydrogens as a fully single-bonded, or saturated, structure. In other words, when a molecule has a C=C or C≡C bond, it has fewer hydrogen atoms than the corresponding alkane with the same number of carbon atoms.

This is why a hydrocarbon with only single C–C bonds is described as saturated—each carbon is bonded to the maximum number of hydrogens possible. A ring with no double bonds is a saturated cycloalkane, not unsaturated, since it lacks a pi bond. And a hydrocarbon that supposedly contains only saturated C–H bonds describes a saturated system as well.

Thus, the statement that captures the essence of unsaturation is the one noting a double or triple bond between carbon atoms.