Which of the following is a secondary benzylic alcohol?
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Logic Tip: A simple counting rule for alcohols: If the $-OH$ bearing carbon has $CH_2$, it's primary. If it has $CH$, it's secondary. If it has just $C$ (no hydrogens), it's tertiary. "Benzylic" just means the nearest neighbor is a benzene ring.
Concept:
Alcohols are classified based on the hybridization and environment of the carbon atom to which the hydroxyl ($-OH$) group is attached.
Benzylic alcohol: The $-OH$ group is attached to an $sp^3$-hybridized carbon atom that is directly attached to an aromatic ring (benzene ring).
Primary/Secondary/Tertiary: This classification depends on how many other carbon atoms are directly attached to the carbon holding the $-OH$ group. A secondary ($2^\circ$) alcohol has exactly two carbon atoms attached to the hydroxyl-bearing carbon.
Step 1: Analyze the structure of a secondary benzylic alcohol.
For a compound to be a secondary benzylic alcohol, the central carbon must be attached to:
1. The $-OH$ group.
2. A benzene ring (phenyl group, $C_6H_5-$).
3. One other alkyl group (e.g., a methyl group, $-CH_3$).
4. One hydrogen atom.
This yields the general formula: $Ar-CH(OH)-R$.
Step 2: Evaluate the given options.
Let's classify each of the chemical structures provided:
(A) $C_6H_5-CH_2-OH$ (Benzyl alcohol): The carbon with the $-OH$ is attached only to the benzene ring. This is a primary ($1^\circ$) benzylic alcohol.
(B) $C_6H_5-CH(OH)-CH_3$ (1-Phenylethanol): The carbon with the $-OH$ is attached to the benzene ring and one methyl group. This is a secondary ($2^\circ$) benzylic alcohol.
(C) $C_6H_5-C(CH_3)_2-OH$ (2-Phenylpropan-2-ol): The carbon with the $-OH$ is attached to the benzene ring and two methyl groups. This is a tertiary ($3^\circ$) benzylic alcohol.
(D) $C_6H_5-OH$ (Phenol): The $-OH$ group is attached directly to the $sp^2$-hybridized carbon of the aromatic ring. This is a phenol, not a benzylic alcohol.
Option (B) perfectly matches the criteria for a secondary benzylic alcohol.
