Question

Acetyl bromide reacts with excess of \(\mathrm{CH}_{3} \mathrm{MgI}\) followed by treatment with a saturated solution of \(\mathrm{NH}_{4} \mathrm{Cl}\) gives [2004] (a) acetone (b) acetamide (c) 2-methyl-2-propanol (d) acetyl iodide

Short answer

The product is 2-methyl-2-propanol (option c).

Step-by-step solution

Understanding the reaction components

We begin by examining the components involved in the reaction. Acetyl bromide is an acyl halide with the formula \( \mathrm{CH}_3\mathrm{COBr} \). \( \mathrm{CH}_3 \mathrm{MgI} \) is a Grignard reagent, which is used to create alcohols by reacting with carbonyl compounds. The final component is a saturated solution of \( \mathrm{NH}_4 \mathrm{Cl} \), which is typically used to quench the Grignard reaction.

Reaction of acetyl bromide with Grignard reagent

When acetyl bromide (\( \mathrm{CH}_3\mathrm{COBr} \)) reacts with \( \mathrm{CH}_3 \mathrm{MgI} \), the carbonyl carbon of acetyl bromide is attacked by the carbanion from the Grignard reagent. This results in the formation of a ketone intermediate (\( \mathrm{CH}_3\mathrm{C}(\mathrm{CH}_3)=\mathrm{O} \)), specifically acetone. However, since there is an excess of \( \mathrm{CH}_3 \mathrm{MgI} \), further reaction occurs.

Formation of tertiary alcohol

The intermediate product, acetone (\( \mathrm{CH}_3\mathrm{C}(\mathrm{CH}_3)=\mathrm{O} \)), reacts further with another mole of \( \mathrm{CH}_3 \mathrm{MgI} \) to form a tertiary alcohol. This additional reaction adds another methyl group to the carbonyl carbon, resulting in \( \mathrm{2-methyl-2-propanol} \) (\( \mathrm{(CH_3)_3COH} \)).

Quenching the reaction

Finally, the reaction mixture is treated with a saturated \( \mathrm{NH}_4 \mathrm{Cl} \) solution, which helps to quench the reaction by neutralizing any remaining Grignard reagent and replacing magnesium halides with hydroxy groups. This step completes the formation of the desired alcohol, which is \( \mathrm{2-methyl-2-propanol} \).

Conclusion

After following the steps of the reaction, the final product formed from the reaction of acetyl bromide with excess \( \mathrm{CH}_3 \mathrm{MgI} \) followed by treatment with a saturated solution of \( \mathrm{NH}_4 \mathrm{Cl} \) is \( \mathrm{2-methyl-2-propanol} \).

Key concepts

Synthetic Organic Chemistry

Synthetic organic chemistry is a fascinating field that focuses on constructing complex molecules from simpler entities. It is like building a structure, where each chemical reaction is a step adding a part to the final structure. Grignard reactions are an essential tool in this domain, allowing chemists to synthesize alcohols by reacting organometallic compounds with carbonyl compounds.
The primary aim in synthetic organic chemistry is to create target molecules, often found in pharmaceuticals, agrochemicals, and materials. The process generally involves multiple steps starting from readily available starting materials. Techniques and methods from synthetic organic chemistry pave the way to manipulate molecules with precision, offering the tools to:

• Create new materials
• Develop new pharmaceuticals and other chemicals
• Understand biochemical routes in biological systems

Grignard reactions specifically help in the construction of carbon-carbon bonds, a critical part of many synthetic strategies.

Tertiary Alcohol Formation

One of the exciting outcomes of using Grignard reagents is the formation of tertiary alcohols. These alcohols have the general structure R₃COH, where R represents an alkyl group. Tertiary alcohols are more complex molecules with industrial and pharmaceutical significance. For instance, they can be used as solvents or intermediates in further chemical synthesis.
In the reaction of acetyl bromide with an excess of \( \mathrm{CH}_3 \mathrm{MgI} \), the first product formed is the ketone acetone. However, this is not the end of the story. When excess Grignard reagent is present, as seen in this reaction, acetone can react further to form the tertiary alcohol 2-methyl-2-propanol, also known as tert-butanol. This additional step involves adding another methyl group to the carbonyl carbon, transforming the molecule from a ketone into a more complex alcohol.
This process highlights a crucial principle in organic synthesis: control over reaction conditions can direct the pathway towards creating more complex structures.

Carbonyl Compound Reactions

Carbonyl compounds are indispensable in organic chemistry due to their reactivity. They contain a carbon-oxygen double bond (C=O), where the carbon is electrophilic. This makes it susceptible to attacks from nucleophiles such as Grignard reagents.
In the presented exercise, the carbonyl group in acetyl bromide reacts with \( \mathrm{CH}_3 \mathrm{MgI} \). Initially, this interaction forms acetone by adding a methyl group to the carbonyl carbon. Yet, when there is an excess of the Grignard reagent, the initial product can further react, introducing another methyl group to result in a tertiary alcohol.
Carbonyl reactions, like the one seen here, allow chemists to build complex organic molecules by forming new bonds. These reactions are fundamental in understanding how to synthesize various chemical compounds while controlling the reaction conditions to achieve desired outcomes. Mastering the reactivity of carbonyl compounds can unlock numerous possibilities in synthetic strategy design, making them a backbone in the field of synthetic organic chemistry.