Problem 16 - A Complete Resource Book in Chemistry for JEE Main

Question

Fomaldehyde when reacted with methyl magnesium bromide followed by hydrolysis gives (a) \(\mathrm{HCHO}\) (b) \(\mathrm{CH}_{3} \mathrm{COOH}\) (c) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) (d) \(\mathrm{CH}_{3} \mathrm{CHO}\)

Step-by-Step Solution

Answer

The product of the reaction is ethanol (\(\text{C}_2\text{H}_5\text{OH}\)).

Step 1: Understanding the Reaction

Formaldehyde, with the formula \( ext{HCHO}\), reacts with methyl magnesium bromide, \( ext{CH}_3 ext{MgBr}\), a Grignard reagent. Grignard reagents are known for their ability to attack the electrophilic center in carbonyl compounds.

Step 2: Nucleophilic Addition Process

The carbon in the Grignard reagent (\( ext{CH}_3^-\) in \( ext{CH}_3 ext{MgBr}\)) acts as a nucleophile and attacks the carbonyl carbon in formaldehyde, forming an alkoxide intermediate. The reaction converts \( ext{HCHO}\) to \( ext{CH}_3 ext{CH}_2 ext{O}^-\).

Step 3: Hydrolysis of Alkoxide

Once the alkoxide is formed, hydrolysis is performed by adding water (\( ext{H}_2 ext{O}\)). This step provides a proton to the alkoxide ion, converting \( ext{CH}_3 ext{CH}_2 ext{O}^-\) to \( ext{C}_2 ext{H}_5 ext{OH}\), which is also known as ethanol.

Key Concepts

Nucleophilic Addition

The Grignard reaction begins with the key step of nucleophilic addition. In the context of this reaction, "nucleophile" refers to a chemical species that donates an electron pair to form a chemical bond. In the case of formaldehyde reacting with methyl magnesium bromide, the common Grignard reagent, it's the methyl group (\( \text{CH}_3^- \)) that acts as a nucleophile.

This nucleophilic addition involves the methyl group from \( \text{CH}_3\text{MgBr} \) attacking the electrophilic carbon atom in the formaldehyde molecule. Formaldehyde (\(\text{HCHO}\)) contains a carbonyl group which has a significantly polarized carbon-oxygen double bond. The carbon atom bears a partial positive charge, making it an ideal target for the nucleophilic methyl group to attack.

As the methyl group forms a bond with the carbonyl carbon, the double bond between carbon and oxygen shifts electrons to oxygen, resulting in the formation of a new intermediate called an "alkoxide." This step is foundational for progressing further to complete the Grignard reaction.

Alkoxide Formation

Once the nucleophilic addition step concludes, an intermediate species known as an "alkoxide" is formed. This stage is crucial because it establishes the primary reaction intermediate transitioning towards the final product.

In our specific reaction, when the methyl group bonds with the carbon in formaldehyde forming \( \text{CH}_3 \text{CH}_2 \text{O}^- \), an alkoxide ion is created. This alkoxide is an anion, having a negatively charged oxygen due to the electron-rich environment resulting from the disrupted carbon-oxygen bond in formaldehyde.

The alkoxide represents a stable yet reactive species ready to undergo further transformation through the next step, hydrolysis. Alkoxides are typically seen as intermediates in many organic reactions due to their high reactivity and ability to easily transform into alcohols.

Hydrolysis Reaction

Following the formation of the alkoxide, the subsequent step in the Grignard reaction is the hydrolysis reaction. Hydrolysis literally means "to break apart with water," and this is indeed what happens in this phase.

In this reaction, water is added to the alkoxide ion. The hydrogen atom from a water molecule (\(\text{H}_2\text{O}\)) donates a proton (\(\text{H}^+\)) to the negatively charged oxygen in the alkoxide, thereby neutralizing its negative charge.

This protonation step transforms the alkoxide ion into an alcohol. Specifically, the product in this scenario is ethanol (\(\text{C}_2\text{H}_5\text{OH} \)).

• Water acts as both a solvent and a proton source.
• This step ensures completion by yielding a stable alcohol.

The conversion from alkoxide to alcohol is fundamental to producing the desired alcohol product in the Grignard synthesis.