Problem 107 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following compounds on reaction with \(\mathrm{CH}_{3} \mathrm{MgBr}\) will give a tertiary alcohol? CC(C)(C)C 11 (a) (b) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{CO}_{2} \mathrm{CH}_{3}\) (c) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO}\) (d) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{COOH}\)

Step-by-Step Solution

Answer

(b) \( \text{C}_2\text{H}_5\text{CO}_2\text{CH}_3 \) forms a tertiary alcohol with \( \text{CH}_3\text{MgBr} \).

Step 1: Understand Grignard Reagent Reaction

Grignard reagents, such as \( \text{CH}_3\text{MgBr} \), are used to form carbon-carbon bonds. When they react with carbonyl compounds, they form alcohols. The type of alcohol formed depends on the type of carbonyl compound:\(\text{Aldehydes} \) typically form secondary alcohols, \(\text{ketones} \) form tertiary alcohols, and \(\text{esters} \) require two equivalents of Grignard reagent to form tertiary alcohols.

Step 2: Identify Compound Types

Examine each compound's structure:- (a) is likely a hydrocarbon, meaning it does not react with Grignard reagents to form an alcohol.- (b) \( \text{C}_2\text{H}_5\text{CO}_2\text{CH}_3 \) is an ester, which can form a tertiary alcohol with Grignard reagent.- (c) \( \text{C}_6\text{H}_5\text{CHO} \) is an aldehyde, forming a secondary alcohol.- (d) \( \text{C}_2\text{H}_5\text{COOH} \) is a carboxylic acid, which typically does not react to form alcohols with Grignard reagents.

Step 3: Choose Esters for Tertiary Alcohol Formation

Among the provided compounds, esters react with two equivalents of Grignard reagent to form tertiary alcohols. In this case, (b) \( \text{C}_2\text{H}_5\text{CO}_2\text{CH}_3 \). The ester will react with two moles of \( \text{CH}_3\text{MgBr} \) to ultimately yield a tertiary alcohol.

Step 4: Confirm by Reaction Mechanism

The reaction of the ester \( \text{C}_2\text{H}_5\text{CO}_2\text{CH}_3 \) with \( \text{CH}_3\text{MgBr} \) follows these steps:1. First equivalent of \( \text{CH}_3\text{MgBr} \) attacks the carbonyl carbon, leading to a ketone.2. Second equivalent attacks the ketone to form the tertiary alcohol.

Key Concepts

Tertiary Alcohol Formation

Tertiary alcohols are a specific class of alcohols characterized by the presence of a hydroxyl group (OH) bonded to a carbon atom, which is itself connected to three other carbon atoms. This structure gives tertiary alcohols distinct properties and reactivity compared to primary and secondary alcohols.
In the context of forming tertiary alcohols, Grignard reagents are quite essential. These reagents are known for their ability to convert carbonyl-containing compounds into alcohols, using their power to form new carbon-carbon bonds. Specifically, when a Grignard reagent reacts with an ester, a tertiary alcohol is formed. This reaction is pivotal for chemists aiming to create branched and complex molecules within a laboratory setting or on an industrial scale.
To recognize tertiary alcohol formation in reaction equations, look for the carbon chain complexity around the hydroxyl-bearing carbon and the integration of the Grignard reagent into the compound's structure.

Ester Reaction with Grignard Reagent

Ester compounds have a characteristic structure: a carbonyl group (C=O) attached to an -O-R group, where R represents an alkyl group. When esters react with Grignard reagents, two equivalents of the reagent are necessary to convert the ester into a tertiary alcohol.

Initially, the Grignard reagent donates an alkyl group to the carbonyl carbon of the ester, forming a ketone intermediate.

In the second stage, a second equivalent of the Grignard reagent adds again to the carbonyl carbon of the ketone, producing the tertiary alcohol.

This multi-step reaction is a classic example of a nucleophilic addition reaction, highlighting how Grignard reagents can dramatically alter molecular structures. Through this, new compounds are synthesized, exhibiting the versatility and utility of Grignard chemistry in organic synthesis.

Carbon-Carbon Bond Formation

The Grignard reaction is renowned for its ability to facilitate the formation of carbon-carbon bonds. This characteristic is crucial in organic chemistry for building complex molecules from simpler ones through the addition of carbon chains.
Grignard reagents are organomagnesium compounds, usually represented as \( \text{R-MgX} \) where R is the alkyl or aryl group, and X is the halogen. The hallmark of Grignard reactions in carbon-carbon bond formation is that they not only attack carbonyl groups in aldehydes and ketones but can also react with esters and acyl chlorides to forge new bonds.
In the context of our specific exercise, when engaging esters, the Grignard reagent, such as \( \text{CH}_3\text{MgBr} \), evidently replaces the ester linkage with new carbon chains, thus extending the backbone of the molecule. This process is fundamental because it exemplifies core organic reactions that allow chemists to craft higher-order structures with desired properties from simpler, basic ones.

Alcohol Synthesis

The synthesis of alcohols using Grignard reagents is a foundational reaction in organic chemistry. Alcohols can be formed by reacting organometallic Grignard reagents with various carbonyl-containing compounds.
During synthesis:

• Primary alcohols are generally formed by reactions with formaldehyde,
• Secondary alcohols result from reactions with aldehydes,
• Tertiary alcohols are synthesized primarily through reactions with ketones and the mentioned two-equivalent reaction with esters.

The ability to gauge what type of alcohol will be formed can be a strategic advantage in designing synthetic routes. For instance, when a tertiary alcohol is desired, opting for an ester coupled with a Grignard reagent usually guarantees that result, provided the right conditions are met.
This practice of alcohol synthesis underscores the versatility and strategic importance of Grignard reagents in creating complex organic molecules in laboratories worldwide.