Problem 94 - A Complete Resource Book in Chemistry for JEE Main

Question

To prepare 3 -ethylpentan-3-ol, the reagents needed are (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{MgBr}+\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COCH}_{2} \mathrm{CH}_{3}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{MgBr}+\mathrm{CH}_{3} \mathrm{COCH}_{2} \mathrm{CH}_{3}\) (c) \(\mathrm{CH}_{3} \mathrm{MgBr}+\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2}^{3} \mathrm{COCH}_{2} \mathrm{CH}_{3}^{3}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{MgBr}+\mathrm{CH}_{3} \mathrm{COCH}_{2} \mathrm{CH}_{3}\)

Step-by-Step Solution

Answer

The correct reagents are option (d) CH₃CH₂MgBr and CH₃COCH₂CH₃.

Step 1: Identify the Target Structure

The compound we wish to synthesize is 3-ethylpentan-3-ol, which has the molecular structure C-N-C where the central carbon (C) is connected to an ethyl (CH₃CH₂−) group and two other carbon chains.

Step 2: Analyze Possible Routes

To form 3-ethylpentan-3-ol, the starting material must allow for an ethyl group and an additional methyl group to be attached to a carbonyl carbon (initially), which will then be reduced to an alcohol group.

Step 3: Consider Reaction of Carbonyl with Organometallic Reagent

The Grignard reagent (R-MgBr) will add to a ketone (or aldehyde), converting the carbonyl group into an alcohol after protonation. Here, we need a reaction where a ketone with three carbon backbone (such as CH₃COCH₂CH₃ - pentan-2-one) can react with an organometallic reagent carrying an ethyl group.

Step 4: Choose Suitable Reagents

To synthesize 3-ethylpentan-3-ol, the required approach involves reacting an ethyl Grignard reagent (CH₃CH₂MgBr) with pentan-2-one (CH₃COCH₂CH₃). Based on the options provided, select the set of reagents that facilitates this reaction.

Step 5: Verify the Choice

Looking through the provided choices, choice (d) CH₃CH₂MgBr and CH₃COCH₂CH₃ matches the required reagents needed to form the desired alcohol, 3-ethylpentan-3-ol, through nucleophilic addition and subsequent protonation steps.

Key Concepts

ethyl Grignard reagent

The ethyl Grignard reagent is a type of organomagnesium compound represented as \( \text{CH}_3\text{CH}_2\text{MgBr} \). It is known for its role in creating carbon-carbon bonds through reactions with ketones and aldehydes.
Forming this reagent involves reacting ethyl bromide with magnesium in an ether solvent. The role of the ether is essential since it helps stabilize the Grignard reagent.
The Grignard reagent acts as a strong nucleophile due to the presence of carbon bonded to magnesium, which carries a partial negative charge. This makes it highly reactive and an excellent choice for attaching carbon chains to form complex organic structures.

synthesis of alcohols

Grignard reagents are pivotal in the synthesis of alcohols. When they react with carbonyl compounds, such as ketones or aldehydes, they add an alkyl group to the carbonyl carbon.
The typical reaction leads to an alcohol through the following steps:

• A Grignard reagent approaches a carbonyl carbon.
• It forms a new carbon-carbon bond, transforming the structure into an alkoxide intermediate.
• The ensuing step, protonation with an acid, converts the alkoxide into an alcohol.

This versatile reaction allows for the creation of various alcohols simply by altering the R group on the Grignard reagent or choosing different carbonyl substrates.

carbonyl compounds

Carbonyl compounds feature a carbon atom double-bonded to an oxygen atom and are common in organic chemistry as a functional group.
Ketones and aldehydes are the main carbonyl compounds involved in Grignard reactions. Ketones are characterized by two carbon groups attached to the carbonyl carbon, while aldehydes have at least one hydrogen atom connected to this carbon.
These structures act as electrophiles because the electron-rich oxygen draws electrons away from the carbon, making the carbonyl carbon susceptible to attack by nucleophiles like the Grignard reagent.
In Grignard reactions, the electrophilic nature of the carbonyl is essential for the nucleophilic addition pathway, resulting in the transformation into an alcohol.

nucleophilic addition

Nucleophilic addition is a critical mechanism in transforming carbonyl compounds into alcohols using Grignard reagents.
In this reaction, the nucleophile, which in this context is the Grignard reagent, attacks the electrophilic carbon of the carbonyl group.
This leads to the formation of a new carbon-carbon bond, followed by the conversion of the oxygen into an alkoxide ion. The reaction proceeds through these core steps:

• The nucleophile attaches to the carbonyl carbon.
• This disrupts the double bond, creating an alkoxide ion.
• An acid is then used to protonate the alkoxide, forming an alcohol.

Nucleophilic addition is fundamental for creating new organic compounds and is widely utilized for synthesizing a diverse array of alcohols.