Problem 29 - A Complete Resource Book in Chemistry for JEE Main

Question

The reaction of Grignard reagent with formaldehyde followed by acidification gives (a) an aldehyde (b) a ketone (c) a carboxylic acid (d) a primary alcohol

Step-by-Step Solution

Answer

The reaction yields a primary alcohol.

Step 1: Understand the Problem

We are told that a Grignard reagent reacts with formaldehyde and is followed by acidification. We need to determine the type of product formed from this reaction. Options given include an aldehyde, a ketone, a carboxylic acid, and a primary alcohol.

Step 2: Recall Grignard Reagent Reactions

Grignard reagents are organomagnesium compounds often written as R-MgX. They are nucleophilic and react with carbonyl compounds to form alcohols upon acidification. When reacted with aldehydes, formaldehyde is the simplest aldehyde.

Step 3: React Grignard Reagent with Formaldehyde

The Grignard reagent adds to the carbon atom of the formaldehyde's carbonyl group. This step creates an intermediate alkoxide (R-CH2-O⁻MgX⁺). The structure so far is an alkoxide with an additional carbon bonded from the Grignard reagent.

Step 4: Acidification Stage

The intermediate alkoxide is then protonated by an acid (usually dilute acid like HCl) to give a primary alcohol. The protonation process results in the final product, which has one more carbon than the original Grignard reagent.

Step 5: Identify the Product

Considering the steps above, the product is a primary alcohol due to the addition of one methylene group (CH2) from formaldehyde, forming R-CH2-OH. Therefore, option (d) is the correct answer.

Key Concepts

Grignard Reagents

Grignard reagents are essential players in organic chemistry known for their wide utility in creating carbon-carbon bonds. These reagents are organometallic compounds denoted as R-MgX, where R represents an organic group such as an alkyl or aryl, and X is a halogen, typically chloride, bromide, or iodide.

One of the interesting characteristics of Grignard reagents is their nucleophilic nature, meaning they can donate a pair of electrons to other compounds in reactions. This makes them highly reactive, especially with carbonyl groups like aldehydes, ketones, and formaldehyde.

It's crucial to note that Grignard reagents are sensitive to moisture. Hence, they must be handled with care in a dry environment to prevent their reaction with water, which would render them ineffective. Their fascinating chemical behavior and reactivity have made them vital tools for chemists, especially in forming alcohols and extending carbon chains.

Formaldehyde Reaction

When a Grignard reagent reacts with formaldehyde, a specific type of carbonyl reaction occurs that results in the formation of an intermediate structure known as an alkoxide. Let's break it down.

Formaldehyde is the simplest aldehyde with the chemical formula HCHO. It has a carbonyl group (C=O) that readily accepts the nucleophilic attack from the Grignard reagent. During the reaction, the organic group attached to the Grignard reagent (R) is added to the carbon atom of the formaldehyde, forming an alkoxide intermediate:\[ R-MgX + HCHO \rightarrow R-CH_2-O^-MgX^+ \]

This interaction is fundamental because it sets the stage for further transformations to achieve final products. The carbon-carbon bond formation here is a key step in extending the carbon structure by integrating the methylene (-CH2-) group from formaldehyde. This initial addition reaction forms the basis for producing the end product: a primary alcohol.

Primary Alcohol Formation

The culmination of the Grignard reaction with formaldehyde is the formation of a primary alcohol. After the initial attack of the Grignard reagent on formaldehyde, the resulting alkoxide needs to be protonated to become a stable alcohol.

The process of adding an acidic solution, such as dilute hydrochloric acid (HCl), causes a protonation of the intermediate alkoxide to produce the primary alcohol:\[ R-CH_2-O^-MgX^+ + HCl \rightarrow R-CH_2-OH + MgXCl \]

In this step, the hydrocarbon group (R) from the Grignard reagent is now bonded to the new -OH group from water resulting in the formation of the primary alcohol R-CH2-OH. This sequence showcases how a Grignard reagent can extend carbon chains and incorporates them into alcohol compounds.

Thus, in the context of solving the exercise, we find that reacting a Grignard reagent with formaldehyde, followed by acidification, specifically forms a primary alcohol, confirmed as the right product from the given options.