Question

Choose the incorrect option(s). $$ \mathrm{CH}_{3} \mathrm{CN} \stackrel{' \mathrm{X}^{\prime}}{\longrightarrow}(\mathrm{P}) \stackrel{\mathrm{H}_{3} \mathrm{O}^{+}}{\longrightarrow}(\mathrm{Q}) $$ (A) If ' \(X\) ' is stannous chloride in presence of hydrochloric acid, then formation of \((Q)\) is known as 'Stephens reduction. (B) If \(\mathrm{X}\) is \(\mathrm{DIBAL}-\mathrm{H}\), then \(\mathrm{Q}\) is ethanal: (C) If \(\mathrm{X}\) is \(\mathrm{CH}_{3} \mathrm{MgI}\) then \((\mathrm{Q})\) gives (-ve) Tollen's as well as Fehling test. (D) If \(\mathrm{X}\) is \(\mathrm{LiAlH}_{4}\), then \((\mathrm{Q})\) can directly obtain by reacting \(\mathrm{CH}_{3} \mathrm{CN}\) with \(\mathrm{H}_{3} \mathrm{O}^{\oplus:}\)

Short answer

Option D is incorrect, as LiAlH₄ cannot directly react with CH₃CN and H₃O⁺ to form Q. Instead, it would produce CH₃CH₂NH₂ (ethylamine) and subsequent reaction with H₃O⁺ would lead to the protonation of the amine to produce CH₃CH₂NH₃⁺, not Q.

Step-by-step solution

Option A - Stannous chloride in the presence of hydrochloric acid

Stephens reduction involves the reduction of nitriles to imines with stannous chloride (SnCl₂) in the presence of hydrochloric acid (HCl), followed by hydrolysis with aqueous acid to form aldehydes. Since the given option states that Q is formed using this method and X is SnCl₂ in the presence of HCl, this option is correct.

Option B - DIBAL-H

DIBAL-H (Diisobutylaluminium hydride) is a reducing agent that selectively reduces nitriles to aldehydes with gentle reaction conditions. If X is DIBAL-H, then the given product Q is ethanal, which can be represented as CH₃CHO. Hence, this option is correct.

Option C - CH₃MgI

If X is CH₃MgI (methylmagnesium iodide), it acts as a Grignard reagent. This would lead to the formation of a tertiary alcohol, not an aldehyde. Tollen's and Fehling tests react with aldehydes to produce a positive test result. Since Q is not an aldehyde, it would give a negative Tollen's and Fehling test. Thus, this option is correct.

Option D - LiAlH₄

LiAlH₄ (Lithium aluminum hydride) is a powerful reducing agent capable of reducing nitriles to primary amines. However, it cannot directly react with CH₃CN and H₃O⁺ to form Q. Instead, the reaction with LiAlH₄ would produce CH₃CH₂NH₂ (ethylamine), and subsequent reaction with H₃O⁺ would lead to the protonation of the amine to produce CH₃CH₂NH₃⁺, not Q. This option is therefore incorrect. In conclusion, Option D is the incorrect option.

Key concepts

Stephens reduction

Stephens reduction is a chemical reaction used in organic chemistry to convert nitriles into imines. This process involves the use of stannous chloride (SnCl₂) in the presence of hydrochloric acid (HCl). The imine is then hydrolyzed to form an aldehyde.

The reduction starts by breaking the triple bond in the nitrile, transforming it into a double bond present in the imine. Hydrolysis with aqueous acid then leads to the formation of an aldehyde, a less complex molecule. This method is widely used due to its ability to facilitate controlled transformations, making it an important tool in synthetic chemistry.

DIBAL-H reduction

DIBAL-H, or diisobutylaluminium hydride, is a selective reducing agent particularly favored for its mild reaction conditions. It reduces nitriles to aldehydes without affecting other potentially reactive groups.

When DIBAL-H is applied to a nitrile, it interrupts the triple bond, forming an intermediate aluminate complex, which upon hydrolysis, yields the corresponding aldehyde. Its selective nature makes it suitable for complex organic synthesis where only specific bonds should be targeted, especially when forming compounds like ethanal.

Grignard reagent

Grignard reagents are organomagnesium compounds, such as methylmagnesium iodide (CH₃MgI), used in various chemical reactions for forming carbon-carbon bonds.

These reagents are commonly involved in creating alcohols. When a Grignard reagent reacts with a nitrile, a new carbon-carbon bond is formed, typically leading to a tertiary alcohol after hydrolysis. Grignard reagents are essential in synthesizing complex organic molecules, facilitating transformations that add to the carbon skeleton of these compounds.

LiAlH₄ reduction

Lithium aluminum hydride (LiAlH₄) is a robust reducing agent widely used in organic chemistry for reducing a range of functional groups. In the context of nitriles, LiAlH₄ reduces them to primary amines.

This reduction is straightforward: the powerful LiAlH₄ breaks the nitrile's triple bond, converting it to a primary amine (e.g., changing CH₃CN to CH₃CH₂NH₂). Such reactions are instrumental when synthesizing basic amines, which are vital building blocks in various industrial and pharmaceutical applications.

Tollen's test

Tollen's test is a qualitative analysis used to identify aldehydes via their oxidation properties. This test involves a reagent containing silver nitrate in ammonia, known as Tollen's reagent.

An aldehyde will react by reducing the silver ions to metallic silver, often producing a distinct silver mirror on the test tube’s surface. This test is particularly specific for aldehydes and typically yields negative results for ketones and alcohols. It’s a handy diagnostic tool in laboratories to confirm the presence of aldehyde functional groups in unknown compounds.