Question

Crotonyl alcohol is an example of

• A. benzylic alcohol
• B. polyhydric alcohol
• C. allylic alcohol
• D. vinylic alcohol

Hint:

To quickly classify unsaturated alcohols, look for the sequence: $\mathrm{C=C-C-OH}$ represents an allylic alcohol, whereas $\mathrm{C=C-OH}$ represents a vinylic alcohol.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The problem requires us to properly classify crotonyl alcohol based on its structural functional groups and the chemical environment of its hydroxyl ($\mathrm{-OH}$) group.

Step 2: Key Formula or Approach:
We evaluate the structural formula of crotonyl alcohol and match the position of the $\mathrm{-OH}$ group with standard definitions of alcohol classes:

• Allylic alcohol: The $\mathrm{-OH}$ group is attached to an $\mathrm{sp^3}$ hybridized carbon atom adjacent to a carbon-carbon double bond ($\mathrm{C=C}$).

• Vinylic alcohol: The $\mathrm{-OH}$ group is bonded directly to an $\mathrm{sp^2}$ carbon of a double bond.

• Benzylic alcohol: The $\mathrm{-OH}$ group is attached to an $\mathrm{sp^3}$ carbon directly bonded to an aromatic ring.

Step 3: Detailed Explanation:
The chemical structure of crotonyl alcohol (also known as (2E)-but-2-en-1-ol) is: $$\mathrm{CH_3-CH=CH-CH_2OH}$$ In this molecule, the hydroxyl group ($\mathrm{-OH}$) is covalently bound to the $\mathrm{-CH_2-}$ group, which is an $\mathrm{sp^3}$ hybridized carbon atom.
This specific $\mathrm{sp^3}$ carbon is directly attached to the neighboring $\mathrm{-CH=}$ carbon involved in the double bond.
By definition, a position next to a carbon-carbon double bond is called an allylic position. Therefore, crotonyl alcohol is classified as an allylic alcohol.
Let's check why other options are incorrect:

• It is not benzylic because there is no aromatic benzene ring present.

• It is not polyhydric because it contains only one $\mathrm{-OH}$ group (monohydric).

• It is not vinylic because the $\mathrm{-OH}$ group is not directly on the double-bonded carbon.

Step 4: Final Answer:
Crotonyl alcohol is an allylic alcohol, which corresponds to option (C).