Problem 98 - A Complete Resource Book in Chemistry for JEE Main

Question

Formaldehyde gives an additive product with meth ylmagnesium iodide which on aqueous hydrolysis gives (a) methyl alcohol (b) isopropyl alcohol (c) propyl alcohol (d) ethyl alcohol

Step-by-Step Solution

Answer

(d) ethyl alcohol

Step 1: Understanding the Reaction

Formaldehyde ( ext{HCHO}) reacts with methylmagnesium iodide ( ext{CH}_3 ext{MgI}), a Grignard reagent, in a nucleophilic addition reaction. This reaction is generally known for forming primary alcohols.

Step 2: Chemical Equation Setup

The Grignard reagent ext{CH}_3 ext{MgI} will add to the carbonyl carbon of formaldehyde ( ext{HCHO}). The general reaction can be summarized as follows: \[ ext{HCHO} + ext{CH}_3 ext{MgI} ightarrow ext{CH}_3 ext{CH}_2 ext{OMgI} \]

Step 3: Aqueous Hydrolysis

The intermediate ext{CH}_3 ext{CH}_2 ext{OMgI} undergoes hydrolysis in the presence of water to replace the MgI group with a hydrogen atom, forming an alcohol:\[ ext{CH}_3 ext{CH}_2 ext{OMgI} + ext{H}_2 ext{O} ightarrow ext{CH}_3 ext{CH}_2 ext{OH} + ext{Mg(OH)I} \]

Step 4: Identifying the Product

The resulting compound from the hydrolysis is ext{CH}_3 ext{CH}_2 ext{OH}, which is ethanol, also known as ethyl alcohol.

Key Concepts

Formaldehyde

Formaldehyde, with its chemical formula \( \text{HCHO} \), is one of the simplest and most reactive aldehydes. It is a colorless gas with a pungent odor, often found as an aqueous solution called formalin. In organic chemistry, formaldehyde acts as a powerful electrophile due to the carbonyl group, where the carbon atom is electron-deficient, making it highly susceptible to nucleophilic attack. This property makes formaldehyde an ideal compound for forming various chemical bonds, particularly in Grignard reactions. During the reaction with Grignard reagents, such as methylmagnesium iodide, formaldehyde serves as the starting material for synthesizing primary alcohols through a nucleophilic addition mechanism.

Aqueous Hydrolysis

Aqueous hydrolysis is a reaction involving water, often seen in organic chemistry to break down compounds. In the context of the Grignard reaction, aqueous hydrolysis occurs after the initial formation of a magnesium-containing intermediate. Once the Grignard reagent \( \text{CH}_3\text{MgI} \) attacks the formaldehyde, forming \( \text{CH}_3\text{CH}_2\text{OMgI} \), it undergoes hydrolysis. This process replaces the \( \text{OMgI} \) group with a hydrogen atom, resulting in the formation of an alcohol. The hydrolysis can be indicated by the general equation:

• \( \text{Intermediate} + \text{H}_2\text{O} \rightarrow \text{Alcohol} + \text{Mg(OH)I} \)

This step is crucial because it transforms the organometallic intermediate into a stable organic molecule, completing the synthesis of the alcohol product.

Primary Alcohols

Primary alcohols are a class of alcohols where the hydroxyl group \( (-\text{OH}) \) is attached to a carbon atom that is only bonded to one other carbon atom. In the Grignard reaction involving formaldehyde, the alcohol product is ethanol. The reaction starts with formaldehyde, which reacts with methylmagnesium iodide, leading to the formation of an ethyl magnesium iodide intermediate. After aqueous hydrolysis, this intermediate yields ethanol. Ethanol, \( \text{CH}_3\text{CH}_2\text{OH} \), is the simplest form of a primary alcohol, and its formation in this reaction underscores the utility of using formaldehyde and Grignard reagents to synthesize primary alcohols efficiently. Primary alcohols like ethanol are widely used in chemical synthesis and as solvents in laboratory settings. Understanding their formation helps in grasping more complex organic synthesis processes.