Question

This section contains multiple choice questions. Each question has 4 choices (a), (b), (c) and (d), out of which ONLY ONE is correct. A compound (A) reacts with phenyl hydrazine to give a crystalline yellow precipitate. When (A) is added to a solution of rosaniline-hydrochloride decolourized by \(\mathrm{SO}_{2}\) gas, the solution remains colourless. (A) is reduced with LiAlH to give (B). (B) is a (a) phenol (b) primary alcohol (c) aromatic polyhydric alcohol (d) secondary alcohol

Short answer

Answer: (d) secondary alcohol

Step-by-step solution

Reaction of compound (A) with phenyl hydrazine

It is given that compound (A) reacts with phenyl hydrazine to form a crystalline yellow precipitate. This reaction is characteristic of carbonyl groups in aldehydes and ketones. The formation of phenylhydrazone (yellow precipitate) indicates that compound (A) contains a carbonyl group.

Reaction of compound (A) with rosaniline-hydrochloride solution decolorized by SO2 gas

The exercise tells us that when compound (A) is added to the decolorized rosaniline-hydrochloride solution (decolorized by SO2 gas), the solution remains colorless. Such behavior is exhibited by ketones that do not have an alpha-hydrogen. Aldehydes with alpha-hydrogens would cause the solution to regain its color due to the reaction between SO2, aldehyde, and rosaniline. Hence, we can conclude that compound (A) is a ketone with no alpha-hydrogens.

Reduction of compound (A) by LiAlH4 to give compound (B)

It is mentioned that compound (A) is reduced with LiAlH4 to give compound (B). LiAlH4 is a strong reducing agent that is known to reduce carbonyl groups in aldehydes and ketones, converting them to alcohols. Since compound (A) is a ketone with no alpha-hydrogens, its reduction would give a secondary alcohol. Hence, compound (B) is a secondary alcohol. Thus, the correct answer to this multiple-choice question is: (d) secondary alcohol

Key concepts

carbonyl compounds

Carbonyl compounds are a key class of organic compounds characterized by a carbon-oxygen double bond, called the carbonyl group ( abla C=O abla). These compounds primarily include aldehydes and ketones. Though similar in structure, aldehydes have the carbonyl group at the end of the carbon chain, making them more reactive, while ketones have it in the middle.

• Aldehydes: React with more ease due to the hydrogen atom attached to the carbonyl group.
• Ketones: Typically less reactive because the carbonyl group is sandwiched between carbon atoms. 

In reactions, the presence of the carbonyl group significantly influences the chemical properties of these compounds. It makes these compounds act as electrophiles, attracting electrons, which allows them to participate in various chemical reactions, such as the phenyl hydrazine reaction that produces a yellow precipitate, indicating a carbonyl compound.

phenyl hydrazine reaction

The phenyl hydrazine reaction is particularly significant in identifying carbonyl compounds, like aldehydes and ketones. When phenyl hydrazine reacts with a compound containing a carbonyl group, it forms a solid, crystalline phenylhydrazone product. This product is often yellow, serving as a visual indication of the carbonyl group. Here's what happens during the reaction: - Phenyl hydrazine ( abla C_6H_5NHNH_2 abla) attacks the carbon in the carbonyl group. - The oxygen in the carbonyl group is replaced, leading to the formation of the phenylhydrazone. - This reaction is a hallmark test for carbonyl groups as it's particularly noticeable due to the colored precipitate. This simple reaction is integral in differentiating aldehydes and ketones from other compounds, even though it doesn't distinguish between the two types.

LiAlH4 reduction

LiAlH4, or Lithium Aluminium Hydride, is an extremely powerful reducing agent used widely in organic chemistry. Its main function is to convert carbonyl groups into alcohols, a transformation critical in synthesizing various organic substances. When we react LiAlH4 with a carbonyl compound: - It donates hydrides (H⁻ ions) to the carbon atom of the carbonyl group. - This process effectively reduces the double-bonded oxygen, converting it into an -OH group. - The transformation creates an alcohol from an aldehyde or a ketone. - For ketones, like the compound (A) in the exercise, the result is a secondary alcohol due to the initial structure bearing no alpha-hydrogens. LiAlH4 is highly reactive and typically requires controlled conditions for safe and effective reductions.

secondary alcohols

Secondary alcohols are a type of alcohol where the hydroxyl group (-OH) is attached to a carbon atom bonded to two other carbon atoms. This configuration is crucial as it dictates the reactivity and properties of the alcohol. Key features of secondary alcohols include:

• Structural insight: Their typical formula is R₂CHOH, where R represents carbon-containing groups.
• Reactivity: Secondary alcohols can undergo various reactions such as oxidation, where they turn into ketones, an important conversion in synthetic pathways.
• Formation: As seen from the exercise, ketones reduced by LiAlH4 often result in secondary alcohols due to the nature of the parent carbonyl compound.

Understanding secondary alcohols is integral in organic chemistry as they serve as intermediates in pharmaceuticals and other chemical manufacturing processes.