Question

\(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHBr} \stackrel{\mathrm{Mg} / \mathrm{ether}}{\longrightarrow} \mathrm{A} \stackrel{\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CO}-\mathrm{CH}_{3}}{\longrightarrow} \mathrm{B}\) The product \(B\) in the above reaction is (a) 2 -phenyl- 3 -methyl butan- 2 -ol (b) 2 -phenylpentan- 2 -ol (c) 3 -phenyl butan-2-ol (d) propan-2-ol and ethyl benzene

Short answer

Answer: The product B formed in the given reaction is 2-phenyl-3-methylbutan-2-ol.

Step-by-step solution

Formation of Grignard reagent (A)

In the first step, \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHBr}\) reacts with \(\mathrm{Mg}\) to form a Grignard reagent. The magnesium inserts itself between the carbon and bromine atoms. The Grignard reagent (compound A) formed is: \((\mathrm{CH}_{3})_{2} \mathrm{CH}\mathrm{MgBr}\).

Nucleophilic addition of Grignard reagent to a carbonyl compound

In the second step, Grignard reagent (A) \((\mathrm{CH}_{3})_{2} \mathrm{CH}\mathrm{MgBr}\) undergoes a nucleophilic addition reaction with a carbonyl compound, which is \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CO}-\mathrm{CH}_{3}\). The Grignard reagent acts as the nucleophile and attacks the electrophilic carbonyl carbon, leading to the formation of an alkoxide anion.

Protonation of alkoxide anion to form alcohol (B)

Finally, the alkoxide intermediate is protonated to form compound B, an alcohol. The hydrogen atom in this step comes from the reaction between Grignard reagent and a carbonyl compound. The alcohol product (B) is generated: \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CO}(\mathrm{CH}_{3})_{2} \mathrm{CH}\mathrm{OH}\).

Identifying the correct structure and name for product B

We now need to identify the correct structure and name for product B from the given options. The structure found in Step 3 is \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CO}(\mathrm{CH}_{3})_{2} \mathrm{CH}\mathrm{OH}\). This corresponds to the structure: \(\mathrm{CH}_{3}\mathrm{C}(\mathrm{CH}_{3})\mathrm{CH}\mathrm{C}( \mathrm{OH})\mathrm{C}_{6} \mathrm{H}_{5}\), which has the IUPAC name 2-phenyl-3-methylbutan-2-ol. Thus, option (a) is the correct answer. In conclusion, the product B in the given reaction is 2 -phenyl- 3 -methyl butan- 2 -ol.

Key concepts

Nucleophilic Addition

Nucleophilic addition is a key reaction in organic chemistry where a nucleophile attacks an electrophile, typically found in carbonyl compounds. In this context, the Grignard reagent

• acts as the nucleophile, meaning it has extra electrons and is ready to donate them.
• The carbonyl compound, which is often a ketone or aldehyde, features a carbon atom double-bonded to oxygen, called a carbonyl carbon.

Grignard reagents like ((CH_3)_2CHMgBr) have a high affinity for the partially positive carbonyl carbon.
This attack breaks the double bond, resulting in an alkoxide ion ((a negatively charged oxygen)), which is a temporary stage before forming the final product.
The reaction is powerful because it allows the formation of carbon-carbon bonds, enabling the synthesis of more complex molecules.

Alcohol Formation

Alcohol formation is an essential transformation that occurs after nucleophilic addition. Once the Grignard reagent has attacked the carbonyl group, the resulting alkoxide ion is converted into an alcohol.
This happens through protonation, which involves adding a proton to neutralize the negative charge on the oxygen.

• The added proton typically comes from water or a weak acid, which donates a hydrogen atom required for this step.
• This results in the formation of a stable alcohol group ((R-OH)), where 'R' stands for any alkyl or aromatic group.

The creation of alcohols is significant as these compounds serve as foundational building blocks for many biological and synthetic processes.
In the exercise, this transformation led to the final alcohol product: 2-phenyl-3-methylbutan-2-ol.

Carbonyl Compounds

Carbonyl compounds are organic molecules characterized by a carbon atom double-bonded to oxygen ((C=O)). This functional group is highly reactive, especially towards nucleophiles.

• Common carbonyl compounds include aldehydes ((RCHO)) and ketones ((RCOR')).
• The carbonyl carbon is electrophilic because the oxygen pulls electron density away, making it an ideal target for nucleophilic attack.

This reactivity is central to many organic reactions, including those involving Grignard reagents.
In this exercise, the carbonyl compound used was (C_6H_5-CO-CH_3), also known as acetophenone.
Understanding carbonyl chemistry helps predict reaction outcomes and design synthetic strategies for creating various complex organic molecules.