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

Question

Fomaldehyde when reacted with methyl magnesium bromide followed by hydrolysis gives (a) 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

Option (c) \\( \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH} \\\) is the product.

Step 1: Understanding the Reaction

Formaldehyde ( ext{HCHO}) reacts with methyl magnesium bromide ( ext{CH}_3 ext{MgBr}) through a Grignard reaction. Grignard reagents add to the carbonyl carbon in aldehydes, resulting in an alcohol.

Step 2: Initial Addition Step

The methyl group from ext{CH}_3 ext{MgBr} adds to the carbonyl carbon in formaldehyde, forming an intermediate compound ext{CH}_3 ext{CH}_2 ext{OMgBr} (an alkoxide).

Step 3: Hydrolysis Step

The intermediate alkoxide is then protonated through hydrolysis, resulting in ethanol ( ext{C}_2 ext{H}_5 ext{OH}). This occurs because the ext{OMgBr} group is replaced by an -OH group after hydrolysis.

Key Concepts

Formaldehyde

Formaldehyde, chemically denoted as \( ext{HCHO}\), is the simplest form of aldehyde. It is one of the most basic building blocks in organic chemistry. Formaldehyde consists of a single carbon atom doubly bonded to oxygen (the carbonyl group) and a single hydrogen atom bonded to that same carbon atom. This structure makes it highly reactive, particularly in addition reactions.

In Grignard reactions, the carbonyl group acts as a nucleophile. The electron-rich nature of the carbonyl oxygen allows it to attract electron-poor species such as Grignard reagents. This ability to form strong new carbon-to-carbon bonds makes formaldehyde extraordinarily useful as a starting material for alcohols when reacted with organometallic compounds like Grignard reagents.

Methyl Magnesium Bromide

Methyl magnesium bromide, with the chemical formula \( ext{CH}_3 ext{MgBr}\), is an organometallic compound commonly referred to as a Grignard reagent. These reagents are a cornerstone in organic synthetic chemistry. They are formed by the reaction of alkyl halides with magnesium in an ether solvent.

Methyl magnesium bromide acts as a strong nucleophile and can effectively transfer its methyl group in chemical reactions. In its interaction with formaldehyde, it attacks the electron-deficient carbon of the carbonyl group. This process forms a new carbon-carbon bond, leading to the additional formation of an intermediate alkoxide. Its role in extending carbon chains is pivotal in organic synthesis applications, particularly when creating alcohols from aldehydes and ketones.

Hydrolysis

Hydrolysis is a chemical process that involves breaking down a compound by reacting with water. In the context of Grignard reactions, hydrolysis is the step that follows the initial addition of the Grignard reagent to the carbonyl carbon.

After methyl magnesium bromide reacts with formaldehyde, hydrolysis plays a crucial role in transforming the intermediate alkoxide (\( ext{CH}_3 ext{CH}_2 ext{OMgBr}\)) into the desired alcohol. Water (or another protic solvent) donates a proton to replace the \( ext{OMgBr}\) group in the intermediate with an -OH group. This protonation creates ethanol (\( ext{C}_2 ext{H}_5 ext{OH}\)), completing the transformation from the aldehyde to alcohol. Hydrolysis is thus an essential step in realizing the potential of organometallic reactions in synthesizing various organic compounds.

Aldehydes

Aldehydes are a class of organic compounds containing a carbonyl group (a carbon-oxygen double bond), where the carbon is also bonded to a hydrogen atom and an R group, which can be a hydrogen or an alkyl group. This structure makes aldehydes distinct from ketones, which have carbonyl carbon bonded to two R groups.

The reactive carbonyl carbon in aldehydes makes them very susceptible to nucleophilic addition reactions. In Grignard reactions, like the one involving formaldehyde, the carbonyl carbon is attacked by a nucleophile—often an organometallic compound. This interaction produces an alcohol after further steps, such as hydrolysis.

• Formaldehyde itself is an aldehyde with an interesting property—it generates primary alcohols like ethanol after reacting with Grignard reagents.
• Understanding the behavior of aldehydes in such reactions provides insight into the creation of complex organic molecules from simple starting materials.