Question

Hydrolysis of an alkyl bromide X (C\(_4\)H\(_9\)Br) follows first-order kinetics. Reaction of X with Mg in dry ether followed by treatment of D\(_2\)O gave Y. What is Y?

• A.
• B.
• C.
• D.

Hint:

Grignard reagents react with D\(_2\)O to replace MgBr with D, forming a deuterated hydrocarbon.

Answer 1

The Correct Option is D

Step 1: Identifying the Type of Alkyl Bromide
- The hydrolysis follows first-order kinetics, which suggests an S\(_N1\) mechanism.
- S\(_N1\) reactions occur faster for tertiary > secondary > primary alkyl halides.
- Hence, X must be a tertiary alkyl bromide. Step 2: Formation of Grignard Reagent
- When X (tert-butyl bromide) is reacted with Mg in dry ether, a Grignard reagent is formed: \[ (CH_3)_3CBr + Mg \rightarrow (CH_3)_3CMgBr \] Step 3: Reaction with D\(_2\)O
- Grignard reagents act as strong nucleophiles and react with D\(_2\)O to give a deuterated product: \[ (CH_3)_3CMgBr + D_2O \rightarrow (CH_3)_3CD + Mg(OD)Br \] Step 4: Identifying the Product (Y)
- The final product (Y) is tert-butyl deuteride.
- The correct option corresponds to the tert-butyl group with deuterium (D) replacing hydrogen.
Thus, the correct answer is option (4). \bigskip

Answer 2

The given image shows a compound formed after the reaction of an alkyl bromide with Mg (to form a Grignard reagent) followed by treatment with D₂O (heavy water).

Step 1: Since the hydrolysis of the alkyl bromide follows first-order kinetics, this indicates an SN1 reaction, which typically occurs with tertiary alkyl halides.
Hence, the starting compound X must be tert-butyl bromide (2-bromo-2-methylpropane).

Step 2: Reaction with Mg in dry ether forms the Grignard reagent:
(CH₃)₃CBr → (CH₃)₃C-MgBr

Step 3: On treatment with heavy water (D₂O), the Grignard reagent reacts to form:
(CH₃)₃C-D
That is, the hydrogen (proton) normally added during quenching is replaced with deuterium (D).

Step 4: The final product Y is tert-butyl-d₁ (deuterated tert-butane). The structure shown in the image corresponds to:
CH₃-CH₂-C(D)(CH₃)₂

So, the correct identity of Y is tert-butane with one D atom (instead of H) at the central carbon.