If one hydrogen in a hydrocarbon is replaced by a halogen atom, the number of isomers that exist for the substituted compound depends on the number of types of hydrogen in the original hydrocarbon. Thus there is only one form of chloroethane (all hydrogens in ethane are equivalent), but there are two isomers of propane that arise from the substitution of a methyl hydrogen or a methylene hydrogen. How many isomers can be obtained when one hydrogen in each of the compounds named below is replaced by a chlorine atom? $$ \begin{array}{ll}{\text { a. } n \text { -pentane }} & {\text { c. } 2,4 \text { -dimethylpentane }} \\ {\text { b. } 2 \text { -methylbutane }} & {\text { d. methylcyclobutane }}\end{array} $$

Start by drawing the Lewis structure of each compound (a, b, c, and d) to visualize the location of hydrogen atoms.

Categorize each hydrogen atom by its position on the carbon chain (primary, secondary, tertiary) and adjacency to functional groups.

Note the number of distinct types of hydrogen atoms in each compound, as these represent the different isomers that can be formed upon substitution with a chlorine atom. a. n-pentane (\(CH_3CH_2CH_2CH_2CH_3\)) 1. Primary hydrogens (\(3x\) on terminal carbons) 2. Secondary hydrogens (\(2x\) on the inner carbons) The number of distinct types of hydrogen atoms in n-pentane is 2. b. 2-methylbutane (\(CH_3CH(CH_3)CH_2CH_3\)) 1. Primary hydrogens (\(3x\) on terminal carbons) 2. Secondary hydrogens (\(2x\) on the inner carbon) 3. Tertiary hydrogens (\(1x\) on the branched carbon) The number of distinct types of hydrogen atoms in 2-methylbutane is 3. c. 2,4-dimethylpentane (\(CH_3C(CH_3)CHCH(CH_3)CH_3\)) 1. Primary hydrogens (\(3x\) on terminal carbons) 2. Secondary hydrogens (\(2x\) on inner carbons) 3. Tertiary hydrogens (\(2x\) on branched carbons) The number of distinct types of hydrogen atoms in 2,4-dimethylpentane is 3. d. methylcyclobutane (\(CH_2CH_2CH_2C(CH_3)H\)) 1. Primary hydrogens (\(1x\) on terminal carbon) 2. Secondary hydrogens (\(6x\) on the cyclic carbons) The number of distinct types of hydrogen atoms in methylcyclobutane is 2.

The number of isomers that can be obtained when one hydrogen is replaced by a chlorine atom in each compound is equal to the number of distinct types of hydrogen atoms: a. n-pentane: 2 isomers b. 2-methylbutane: 3 isomers c. 2,4-dimethylpentane: 3 isomers d. methylcyclobutane: 2 isomers