Problem 85 - A Complete Resource Book in Chemistry for JEE Main

Question

When one of the following compounds will react with Grignard reagent to yield a primary alcohol containing two more carbons? (a) \(\mathrm{CH}_{3} \mathrm{CHO}\) (b) C1COC1 (c) HCHO (d) \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\)

Step-by-Step Solution

Answer

Formaldehyde (c) HCHO will react to yield a primary alcohol with two more carbons.

Step 1: Understand the Grignard Reaction

Grignard reagents react with carbonyl compounds to form alcohols. When a Grignard reagent reacts with an aldehyde (except formaldehyde), it forms a secondary alcohol. However, if it reacts with formaldehyde, a primary alcohol is produced.

Step 2: Identify the Compounds

The compounds listed are: (a) acetaldehyde (\(\mathrm{CH}_3\mathrm{CHO}\)), (b) ethylene oxide, (c) formaldehyde (HCHO), and (d) acetone (\(\mathrm{CH}_3\mathrm{COCH}_3\)).

Step 3: Recognize Formaldehyde as Special Case

In the context of Grignard reactions, only formaldehyde (HCHO) will yield a primary alcohol because it is the simplest aldehyde with only one carbon atom. When it reacts with a Grignard reagent, it will add one carbon and transform into a primary alcohol.

Step 4: Identify the Reaction

When formaldehyde reacts with a Grignard reagent (like \(\mathrm{R-MgX}\)), the reaction is as follows: \[\mathrm{R-MgX} + \mathrm{HCHO} \rightarrow \mathrm{RCH}_{2}\mathrm{OH}\] This reaction adds a total of two carbon atoms to the alcohol, producing a primary alcohol with two more carbons than the original Grignard reagent.

Key Concepts

Primary Alcohol Production

In the world of organic chemistry, producing primary alcohols is a fundamental reaction that involves the addition of a Grignard reagent to formaldehyde. Primary alcohols have the chemical formula \(-CH_2OH\). They are distinguished because the hydroxyl group (\(-OH\)) is attached to a primary carbon atom that is only linked to one other carbon atom.

Grignard reagents, written as \(R-MgX\) where \(R\) is an organic group and \(X\) is a halogen, are powerful in extending carbon chains. When they react with formaldehyde (HCHO), the unique formation of a primary alcohol occurs. This interaction is specific because formaldehyde is the simplest aldehyde.

The process can be outlined as:

• The Grignard reagent adds to the carbonyl group of the formaldehyde.
• It then forms an intermediate complex.
• With the help of acid workup, the intermediate finally converts into a primary alcohol.

This reaction is valuable for extending carbon chains, especially in synthetic chemistry, where creating longer carbon chains with specific alcohol groups is desired.

Formaldehyde Reaction

Formaldehyde plays a special role in reactions with Grignard reagents due to its simplicity as an aldehyde. It has the composition \(HCHO\), which provides only one carbon atom in its structure, making it crucial for the formation of primary alcohols when reacting with Grignard reagents.

The uniqueness of formaldehyde lies in its ability to react with the Grignard reagent to add a single carbon atom:

• This forms an alcohol where the hydroxyl group is linked to what becomes the new primary carbon.
• The reaction involves a simple nucleophilic addition of the Grignard reagent to the carbonyl carbon.
• Upon completion, an acid workup results in the hydrolysis of the intermediate to yield a primary alcohol.

This transformation underscores the versatility of formaldehyde in organic synthesis, as it provides an efficient route to produce simple alcohols from basic building blocks.

Carbonyl Compounds

Carbonyl compounds contain a carbon-oxygen double bond (-C=O). This functional group is highly reactive, making it a central feature in numerous organic reactions. Grignard reactions with carbonyl compounds are instrumental for alcohol production.

The general process involves the Grignard reagent attacking the carbon atom in the carbonyl group:

• This step breaks the carbon-oxygen double bond.
• The addition of the Grignard reagent forms an alkoxide intermediate, which after acidic treatment, converts into an alcohol.

Grignard reagents will yield different alcohol types depending on the type of carbonyl compound used:

• Formaldehyde yields primary alcohols.
• Other aldehydes produce secondary alcohols.
• Ketones typically result in tertiary alcohols.

Carbonyl compounds' versatility enhances their significance in organic synthesis, facilitating the creation of a wide range of alcohols by varying the carbon chain lengths and types of alcohol produced.