Question

A confused student was doing an isomer problem and listed the following six names as different structural isomers of \(\mathrm{C}_{7} \mathrm{H}_{16}\) a. 1 -sec-butylpropane b. 4 -methylhexane c. 2 -ethylpentane d. 1 -ethyl-1-methylbutane e. 3 -methylhexane f. 4 -ethylpentane How many different structural isomers are actually present in these six names?

Short answer

There are 5 different structural isomers of \(\mathrm{C}_{7} \mathrm{H}_{16}\) present in the given six names.

Step-by-step solution

Understanding structural isomers

Structural isomers are molecules with the same molecular formula but have a different arrangement of atoms. They have the same number of atoms of each element but are connected in a different order. In this problem, all the molecules have the molecular formula of \(\mathrm{C}_{7} \mathrm{H}_{16}\), and we need to figure out if they have different structures.

Drawing the structures of the given hydrocarbons

Before we begin, let's familiarize ourselves with the prefixes used in naming hydrocarbons: Methyl - CH3 Ethyl - CH2CH3 Propyl - CH2CH2CH3 Butyl - CH2CH2CH2CH3 Pentyl - CH2CH2CH2CH2CH3 Now, let's draw the structures of the given hydrocarbons: a. 1-sec-butylpropane - sec-butyl: (CH3)2CHCH2 - propane: CH3CH2CH3 b. 4-methylhexane - methyl: CH3 - hexane: CH3(CH2)4CH3 c. 2-ethylpentane - ethyl: CH2CH3 - pentane: CH3(CH2)3CH3 d. 1-ethyl-1-methylbutane - ethyl: CH2CH3 - methyl: CH3 - butane: CH3(CH2)2CH3 e. 3-methylhexane - methyl: CH3 - hexane: CH3(CH2)4CH3 f. 4-ethylpentane - ethyl: CH2CH3 - pentane: CH3(CH2)3CH3

Identifying and counting different structural isomers

Now that we have drawn the structures of the given hydrocarbons, let's identify which of them are different structural isomers: a. 1-sec-butylpropane: - Structure: (CH3)2CHCH2CH2CH2CH3 , a different structure. b. 4-methylhexane: - Structure: CH3CH2CH2CH(CH3)CH2CH3, a different structure. c. 2-ethylpentane: - Structure: CH3CH(CH2CH3)CH2CH2CH3, a different structure. d. 1-ethyl-1-methylbutane: - Structure: CH3CH2CH(CH3)CH2CH3, this structure is the same as 2-ethylpentane (c). e. 3-methylhexane: - Structure: CH3CH2CH(CH3)CH2CH2CH3, a different structure. f. 4-ethylpentane: - Structure: CH3CH2CH2CH(CH2CH3)CH3, a different structure. Looking at the structures, we can see that there are 5 different structural isomers present in these six names (excluding the repeat of 1-ethyl-1-methylbutane, which is the same as 2-ethylpentane).