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

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

Step 1: Understand the Reactants

The reaction starts with formaldehyde (HCHO) reacting with methyl magnesium bromide (CH3MgBr). This belongs to a class of reactions known as Grignard reactions, where organomagnesium reagents are used to form alcohols.

Step 2: Identify the Grignard Reagent Reaction

The Grignard reagent CH3MgBr will attack the carbonyl carbon of formaldehyde because it's slightly positive. This leads to the formation of an alkoxide intermediate. The reaction can be written as: \[\text{HCHO + CH}_3\text{MgBr} \rightarrow \text{CH}_3\text{CH}_2\text{OMgBr}\]

Step 3: Hydrolysis of the Alkoxide

The alkoxide intermediate formed by the reaction of formaldehyde and methyl magnesium bromide is then subjected to hydrolysis. During hydrolysis, water is used to convert the alkoxide to an alcohol. The hydrolysis reaction can be written as: \[\text{CH}_3\text{CH}_2\text{OMgBr} + \text{H}_2\text{O} \rightarrow \text{CH}_3\text{CH}_2\text{OH} + \text{Mg(OH)Br}\]

Step 4: Identify the Final Product

The reaction ends with ethyl alcohol (ethanol, \(\text{C}_2\text{H}_5\text{OH}\)) being formed as the final product. This is formed from the initial reaction of formaldehyde with the Grignard reagent followed by hydrolysis.

Key Concepts

Formaldehyde

Formaldehyde is a simple aldehyde with the chemical formula HCHO. It consists of one carbon atom bonded to two hydrogen atoms and a highly reactive carbonyl group. This compound is known for its pungent odor and for being a fundamental building block in organic chemistry.
In the context of Grignard reactions, formaldehyde is an essential compound because it can readily engage with Grignard reagents. Its carbonyl carbon is electrophilic, meaning it can attract nucleophiles such as those provided by Grignard reagents. When the Grignard reagent attacks formaldehyde, it adds an alkyl group to the carbonyl carbon, transforming the aldehyde into an alcohol once hydrolyzed. This reactivity makes formaldehyde a key player in producing alcohols through Grignard reactions.

Methyl Magnesium Bromide

Methyl magnesium bromide, with the formula CH₃MgBr, belongs to a category of chemicals known as Grignard reagents. These reagents are powerful tools in organic synthesis, especially for forming carbon-carbon bonds.
The methyl group in this reagent behaves like a nucleophilic carbon center. This happens because the bond between magnesium and carbon is highly polar.

• The magnesium atom holds a partial positive charge.
• The carbon atom carries a partial negative charge.

This polarity allows the carbon atom in CH₃MgBr to attack electrophilic centers, such as the carbonyl carbon in formaldehyde. This attack results in the formation of a new C-C bond, forming an alkoxide intermediate.

Alkoxide

The term "alkoxide" refers to a compound containing an alkyl group attached to an oxygen atom via a negatively charged ion. Alkoxides are significant intermediates formed during Grignard reactions.
When methyl magnesium bromide reacts with formaldehyde, it attaches a CH₃ group to the carbonyl carbon, resulting in an intermediate that is not yet a stable alcohol. This intermediate is an alkoxide, specifically, ethoxide in the context of this reaction. The reaction can be shown as:

• ext{HCHO + CH}_3 ext{MgBr} → ext{CH}_3 ext{CH}_2 ext{OMgBr}

The alkoxide ion is stabilized by the remaining MgBr⁺ part of the Grignard reagent. It's a crucial step towards the final formation of the alcohol, as it sets the stage for the last transformation through hydrolysis.

Hydrolysis

Hydrolysis is a chemical reaction that involves water breaking down a compound. In the case of Grignard reactions, hydrolysis follows the formation of an alkoxide, converting it into an alcohol. During a Grignard reaction, after the alkoxide is formed, water is added to the reaction mixture. This water reacts with the alkoxide to produce an alcohol by breaking the bond between the oxygen and the metal component of the alkoxide. The specific reaction for converting the ethoxide formed from formaldehyde and methyl magnesium bromide is:

• ext{CH}_3 ext{CH}_2 ext{OMgBr} + ext{H}_2 ext{O} → ext{CH}_3 ext{CH}_2 ext{OH} + ext{Mg(OH)Br}

This step is crucial for producing the final alcohol product, ethanol, in this case, which is a common outcome from such Grignard reaction procedures.