Which one of the following does not lose water even in conc. \(\text{H}_{2}\text{SO}_{4}\)?
Show Hint
Dehydration reactions that lead to antiaromatic products (like cyclobutadiene, which has $4\pi$ electrons) are thermodynamically and kinetically highly unfavorable. Conversely, those leading to aromatic products (like the tropylium cation or benzene) occur with remarkable ease.
Step 1: Understanding the Question:
The question asks us to identify which of the given cyclic alcohols does not undergo dehydration (loss of water) when treated with concentrated sulfuric acid ($\text{H}_2\text{SO}_4$).
Step 2: Detailed Explanation:
Dehydration of alcohols under acidic conditions typically proceeds via a carbocation intermediate (E1 mechanism) to form a stable alkene. Let us analyze each compound:
- Compound 1: Cyclopentanol can easily lose a molecule of water to form cyclopentene, which is a highly stable, non-strained cyclic alkene.
- Compound 3: Cycloheptatrienol (tropyl alcohol) loses water extremely easily under acidic conditions because the resulting carbocation is the tropylium cation. This carbocation is planar, fully conjugated, contains $6\pi$ electrons, and is exceptionally stable due to its aromatic character.
- Compound 4: Cyclohexa-1,3-dien-5-ol (or a similar cyclohexadienol isomer) undergoes dehydration very rapidly because the loss of water generates a highly stable, aromatic benzene ring.
- Compound 2: This is a cyclobutenol derivative. If this compound were to undergo dehydration, it would require the elimination of water to form cyclobutadiene. Cyclobutadiene is a planar cyclic conjugated system with $4\pi$ electrons, making it highly antiaromatic and extremely unstable. Due to the high energy barrier associated with forming an antiaromatic product, this compound does not undergo dehydration even under harsh acidic conditions.
Therefore, compound 2 does not lose water.
