Question

Which of the following are benzylic alcohols? (A) \(\mathrm{Ph}-\mathrm{CH}_{2}-\mathrm{CH}_{2} \mathrm{OH}\) (B) \(\mathrm{Ph}-\mathrm{CH}_{2}-\mathrm{OH}\) (C) CC(O)C(C)(C)C (D) \(\mathrm{C}_{6} \mathrm{H}_{11}-\mathrm{CH}_{2}-\mathrm{OH}\)

Short answer

The only compound that meets the criteria for a benzylic alcohol is (B) \(\mathrm{Ph}-\mathrm{CH}_{2}-\mathrm{OH}\), as it has a hydroxyl group (OH) attached to a carbon atom directly bonded to a benzene ring (Ph).

Step-by-step solution

Identify the benzene ring in each compound

A benzene ring can be represented by 'Ph' or it can be drawn as a hexagonal structure with alternating single and double bonds. This is an aromatic compound, and it is essential for the formation of a benzylic alcohol.

Identify the hydroxyl group in each compound

A hydroxyl group (OH) is necessary for a molecule to be classified as an alcohol. Once we find the hydroxyl group, we can check if it is attached to the appropriate carbon atom.

Examine each compound to determine if it meets the criteria for benzylic alcohols

We will now examine each compound to see if it has a hydroxyl group attached to a carbon atom that is directly bonded to a benzene ring. (A) \(\mathrm{Ph}-\mathrm{CH}_{2}-\mathrm{CH}_{2} \mathrm{OH}\): There is a benzene ring (Ph) and a hydroxyl group (OH) in this compound. However, the hydroxyl group is bonded to a carbon atom that is not directly bonded to the benzene ring (there is an additional CH2 group between them), so it is not a benzylic alcohol. (B) \(\mathrm{Ph}-\mathrm{CH}_{2}-\mathrm{OH}\): Here, we have a benzene ring (Ph) and a hydroxyl group (OH). The hydroxyl group is also attached to the carbon atom, which is directly bonded to the benzene ring. Therefore, this compound meets the criteria for a benzylic alcohol. (C) \CC(O)C(C)(C)C\ : This compound does not contain a benzene ring. Thus, it cannot be a benzylic alcohol. (D) \(\mathrm{C}_{6} \mathrm{H}_{11}-\mathrm{CH}_{2}-\mathrm{OH}\): In this compound, there is a hydroxyl group and a six-membered ring, but the ring is not an aromatic benzene ring. Therefore, this compound does not meet the criteria for a benzylic alcohol.

Conclusion

Among the given compounds, only option (B) \(\mathrm{Ph}-\mathrm{CH}_{2}-\mathrm{OH}\) meets the criteria for a benzylic alcohol.

Key concepts

Aromatic Compounds

Aromatic compounds are a class of molecules that include one or more benzene rings—structures composed of six carbon atoms bonded together in a hexagonal shape, with alternating single and double bonds. These compounds are known for their high stability and distinctive aromaticity, a concept in organic chemistry describing the cyclic, planar structure with delocalized pi electrons above and below the plane of the molecule.

Aromaticity gives these compounds unique chemical properties, such as resistance to addition reactions, preference for substitution reactions, and significant resonance stabilization. Understanding the nature of aromatic compounds is crucial for students preparing for competitive exams like JEE Organic Chemistry, as these concepts frequently form the basis of various reaction mechanisms and synthesis problems.

Hydroxyl Group Identification

The hydroxyl group, represented as -OH, is a functional group consisting of an oxygen atom bonded to a hydrogen atom. When present in an organic compound, this group is indicative of an alcohol. Identifying the hydroxyl group is fundamental for classifying various alcohols, including benzylic alcohols. In benzylic alcohols, the hydroxyl group is directly attached to a carbon atom that is itself directly connected to a benzene ring. The correct identification of the hydroxyl group's position relative to other structural features, such as aromatic rings, is not only important in nomenclature but also influences the physical and chemical properties of the compound. Students who master this concept can adeptly navigate through complex organic chemistry topics and should ace questions in exams like JEE on such matters.

JEE Organic Chemistry

Organic chemistry is a significant and challenging area in the Joint Entrance Examination (JEE), one of the most competitive exams for admission to engineering courses in India. JEE Organic Chemistry covers various topics, including nomenclature, reaction mechanisms, stereochemistry, and identification of functional groups.

For success in JEE Organic Chemistry, a meticulous grasp of concepts like the structure and reactivity of aromatic compounds and the recognition of functional groups, such as hydroxyl groups in alcohols, is essential. Effective learning strategies include problem-solving practice, understanding the underlying principles rather than rote memorization, and applying these concepts to real-world scenarios. Clarity in topics such as benzylic alcohols directly influences a student's ability to deconstruct and analyze complex organic structures and reactions, a skill that is highly tested in the JEE.