Problem 98 - A Complete Resource Book in Chemistry for JEE Main

Question

Formaldehyde gives an additive product with meth ylmagnesium iodide which on aqueous hydrolysis gives (a) methyl alcohol (b) isopropyl alcohol (c) propyl alcohol (d) ethyl alcohol

Step-by-Step Solution

Answer

The product is methyl alcohol (methanol).

Step 1: Understanding the Reactants

The reactants are formaldehyde and methylmagnesium iodide. Formaldehyde is the simplest aldehyde with the formula \( ext{HCHO}\), and methylmagnesium iodide \( ext{CH}_3 ext{MgI}\) is a Grignard reagent.

Step 2: Writing the Mechanism of the Reaction

In the reaction, the methyl group \( ext{CH}_3^-\) from methylmagnesium iodide attacks the electrophilic carbon atom of formaldehyde, forming a new carbon-carbon bond. This generates an alkoxide intermediate.

Step 3: Hydrolysis Step

The alkoxide intermediate formed in the previous step undergoes aqueous hydrolysis. This process involves adding water to break the bonds created with magnesium iodide, producing an alcohol.

Step 4: Determine the Alcohol Product

After hydrolysis, the major product is methanol ( ext{CH}_3 ext{OH}). This occurs as the addition of a single methyl group to formaldehyde followed by protonation from water results in formation of methanol.

Key Concepts

Formaldehyde

Formaldehyde, with the chemical formula \( \text{HCHO} \), is the simplest form of aldehyde. It's a colorless gas with a distinct, pungent smell that is typically encountered in solution form. It is an essential building block in organic chemistry because it readily participates in various chemical reactions such as polymerization and oxidation.
One key aspect of formaldehyde is its electrophilic nature. The carbon atom in formaldehyde is slightly positive due to the presence of an electronegative oxygen atom. This makes it susceptible to attacks by nucleophiles, chemical species that provide a pair of electrons.
In the Grignard reaction, formaldehyde acts as an electrophile. The lone pair on the oxygen creates a polar double bond. The positive character on the carbon can be attractive to compounds like Grignard reagents, which carry nucleophilic properties.

Methylmagnesium Iodide

Methylmagnesium iodide \( \text{CH}_3\text{MgI} \) is part of a well-known class of organometallic compounds called Grignard reagents. These compounds are formed by reacting magnesium with an alkyl halide, and they are important for forming carbon-carbon bonds.
Grignard reagents are highly reactive, primarily due to the polar carbon-magnesium bond. The carbon atom is even more electronegative compared to magnesium, which makes it nucleophilic. This nucleophilic behavior is central to its role in the Grignard reaction, as it interacts with electrophiles, like the carbon in formaldehyde.
During the Grignard reaction, methylmagnesium iodide donates its methyl group \( \text{CH}_3^- \) to the electrophilic carbon, facilitating the formation of a carbon-carbon bond. This step is a critical part of extending carbon chains in organic synthesis.

Methanol Synthesis

The synthesis of methanol from formaldehyde and methylmagnesium iodide is a classic example of the utility of the Grignard reaction. This method involves combining an aldehyde (formaldehyde) and the nucleophilic agent (methylmagnesium iodide).
Here's how the process unfolds:

• The methyl group from the Grignard reagent attacks the electrophilic carbon in formaldehyde, leading to the creation of an alkoxide intermediate.
• Aqueous hydrolysis of this intermediate follows, wherein the compound is treated with water. This step effectively cleaves the bond with magnesium iodide, replacing it with a hydroxyl group.
• The end product is methanol \( \text{CH}_3\text{OH} \), a simple alcohol formed from the addition of the methyl group to the base aldehyde structure.

This synthesis highlights the versatility of Grignard reagents in forming alcohols from simpler organic compounds, demonstrating their importance in extending molecular complexity in organic chemistry.