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? (a) CC1COC1C (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

Compound (b) will give a tertiary alcohol.

Step 1: Identify the Reactants and Product

The question asks which compound will give a tertiary alcohol on reaction with methyl magnesium bromide (\(\mathrm{CH}_{3}\mathrm{MgBr}\)). Tertiary alcohols are formed when a ketone or an ester reacts with a Grignard reagent.

Step 2: Evaluate Compound (a)

Compound (a) is \(\mathrm{CCOOC}\), a cyclic ether, not a ketone or ester. Reaction with \(\mathrm{CH}_{3}\mathrm{MgBr}\) will not yield a tertiary alcohol.

Step 3: Evaluate Compound (b)

Compound (b) is an ester \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{CO}_{2} \mathrm{CH}_{3}\). Reaction with \(\mathrm{CH}_{3}\mathrm{MgBr}\) can produce a tertiary alcohol by reacting with two moles of the Grignard reagent to form a tertiary alcohol after hydrolysis of the formed \(\beta\)-keto intermediate.

Step 4: Evaluate Compound (c)

Compound (c) is benzaldehyde \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO}\), an aldehyde which will form a secondary alcohol, not a tertiary one, upon reaction with \(\mathrm{CH}_{3}\mathrm{MgBr}\).

Step 5: Evaluate Compound (d)

Compound (d) is acetic acid \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{COOH}\). Carboxylic acids do not react effectively with Grignard reagents to form the desired alcohol products; instead, they are protonated which uses up the Grignard reagent inefficiently and doesn’t yield tertiary alcohol.

Step 6: Determine the Correct Answer

Based on the analysis, compound (b), which is an ester, is the only compound that can produce a tertiary alcohol upon reaction with \(\mathrm{CH}_{3}\mathrm{MgBr}\) followed by hydrolysis.

Key Concepts

Tertiary Alcohol Formation

Tertiary alcohols are special types of alcohols where the hydroxyl group (-OH) is attached to a carbon atom bonded to three other carbon atoms. Forming a tertiary alcohol generally involves the use of Grignard reagents. Grignard reagents are organometallic compounds that contain a carbon-metal bond, usually a magnesium-halogen bond. They are highly reactive and useful in forming different types of alcohols from various carbonyl-containing compounds. In order to produce a tertiary alcohol, a Grignard reagent reacts with a ketone or an ester. When reacting with an ester, it typically takes two moles of the Grignard reagent to first form a ketone intermediate, which reacts further to yield a tertiary alcohol after hydrolysis. It's the unique interaction between the organometallic reagent and these carbonyl compounds that leads to the formation of a tertiary alcohol.

Reaction with Esters

Esters are a type of organic compound that contain a carbonyl group adjacent to an ether linkage (R-CO-O-R'). They play a significant role in organic chemistry reactions, especially with organometallic compounds like Grignard reagents. When an ester reacts with a Grignard reagent such as methyl magnesium bromide (\(\mathrm{\text{CH}_3\text{MgBr}}\)), it typically involves a two-step process. Initially, the ester reacts to form a ketone, an intermediate compound. This is followed by a second addition of the Grignard reagent, leading to the formation of an alcohol after hydrolysis. This procedure effectively replaces the original alkoxy group with the Grignard group. The final result is a tertiary alcohol, given that at least three different alkyl groups are attached to the carbon hosting the hydroxyl group.

Organometallic Compounds

Organometallic compounds, such as Grignard reagents, consist of metals directly bonded to carbon atoms from organic groups. Typically involving metals like magnesium or lithium, these compounds are integral to many synthetic processes in organic chemistry. Grignard reagents, in particular, are formed by reacting an alkyl halide with magnesium metal. In general notation, they are written as R-Mg-X, where R is an alkyl or aryl group, and X is a halogen. The carbon in the Grignard reagent is highly nucleophilic, allowing it to attack the electrophilic carbon in carbonyl compounds to forge new carbon-carbon bonds, a primary step in the synthesis of alcohols through reactions with aldehydes, ketones, and esters.

Chemical Reactions in Organic Chemistry

In the fascinating field of organic chemistry, chemical reactions are the heart of synthetic transformations. These processes involve making new bonds, breaking old ones, and rearranging atomic structures to create novel compounds with desired properties. Grignard reactions are a perfect example of how specific chemical reactions can be manipulated to produce valuable organic compounds. The versatility and reactivity of Grignard reagents allow chemists to synthesize a variety of alcohols and other substances with precision by choosing appropriate starting materials like ketones, esters, or aldehydes. In the realm of organic synthesis, such reactions provide robust tools for crafting complex molecules useful in pharmaceuticals, materials science, and industrial applications.