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Question
JEE Main
Chemistry

Consider all the structural isomers with molecular formula $\text{C}_5\text{H}_{11}\text{Br}$ are separately treated with $\text{KOH(aq)}$ to give respective substitution products, without any rearrangement. The number of products which can exhibit optical isomerism from these is ___________.

The question asks for the number of optical isomers formed from the substitution products of all structural isomers of $\text{C}_5\text{H}_{11}\text{Br}$ treated with $\text{KOH(aq)}$.

Step 1: Identify Structural Isomers of $\text{C}_5\text{H}_{11}\text{Br}$

There are 8 structural isomers for the molecular formula $\text{C}_5\text{H}_{11}\text{Br}$. These are:

  1. 1-Bromopentane
  2. 2-Bromopentane
  3. 3-Bromopentane
  4. 1-Bromo-2-methylbutane
  5. 2-Bromo-2-methylbutane
  6. 1-Bromo-3-methylbutane
  7. 2-Bromo-3-methylbutane
  8. 1-Bromo-2,2-dimethylpropane

Step 2: Determine Substitution Products and Chirality

Treatment with aqueous $\text{KOH}$ replaces the bromine atom with a hydroxyl ($\text{OH}$) group, forming the corresponding alcohol. We need to check if these alcohol products exhibit optical isomerism, which requires the presence of a chiral center (a carbon atom bonded to four different groups).

Isomer Name Substitution Product (Alcohol) Chiral Center Analysis Optical Activity
1-Bromopentane 1-Pentanol C1 has 2 H atoms. Not chiral. No
2-Bromopentane 2-Pentanol C2 is bonded to H, OH, $\text{CH}_3$, $\text{CH}_2\text{CH}_2\text{CH}_3$. Chiral. Yes
3-Bromopentane 3-Pentanol C3 has 2 $\text{CH}_2\text{CH}_3$ groups. Not chiral. No
1-Bromo-2-methylbutane 2-Methyl-1-butanol C2 is bonded to H, $\text{CH}_3$, $\text{CH}_2\text{OH}$, $\text{CH}_2\text{CH}_3$. Chiral. Yes
2-Bromo-2-methylbutane 2-Methyl-2-butanol C2 is bonded to 3 methyl/ethyl groups and OH. Not chiral. No
1-Bromo-3-methylbutane 3-Methyl-1-butanol C3 is bonded to H, $\text{CH}_3$, $\text{CH}_3$, $\text{CH}_2\text{CH}_2\text{OH}$. Achiral (Incorrect analysis of structure leads to confusion; correct analysis shows C3 is attached to H, $\text{CH}_3$, $\text{CH}_3$ and $\text{CH}_2\text{CH}_2\text{OH}$, implying 5 bonds, which is impossible. The actual structure $\text{HOCH}_2\text{CH}_2\text{CH}(\text{CH}_3)\text{CH}_3$ has C3 bonded to H, $\text{CH}_3$, $\text{CH}_3$, $\text{CH}_2\text{CH}_2\text{OH}$, which is wrong. Correctly, C3 is bonded to H, $\text{CH}_3$, the terminal $\text{CH}_3$, and $\text{CH}_2\text{CH}_2\text{OH}$. Let's re-verify: The structure $\text{CH}_3-\text{CH}(\text{CH}_3)-\text{CH}_2-\text{CH}_2\text{OH}$ does not have a chiral center at C3). Not chiral. No
2-Bromo-3-methylbutane 3-Methyl-2-butanol C2 is bonded to H, OH, $\text{CH}_3$, $\text{CH}(\text{CH}_3)_2$. Chiral. Yes
1-Bromo-2,2-dimethylpropane 2,2-Dimethyl-1-propanol C1 has 2 H atoms. Not chiral. No

Note on 3-Methyl-1-butanol chirality: The structure is $\text{HOCH}_2\text{CH}_2\text{CH}(\text{CH}_3)\text{CH}_3$. The carbon atom C3 is bonded to a hydrogen, a methyl group ($\text{CH}_3$), the terminal methyl group ($\text{CH}_3$), and the $\text{CH}_2\text{CH}_2\text{OH}$ group. Since C3 is bonded to two identical methyl groups, it is not chiral.

Step 3: Count Optically Active Products

From the analysis above, there are 3 substitution products that exhibit optical isomerism:

  • 2-Pentanol
  • 2-Methyl-1-butanol
  • 3-Methyl-2-butanol

Therefore, the number of products exhibiting optical isomerism is 3.

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