Q12. The structure of the major product Q of the following reaction is
Grignard reagents react with epoxides under dry conditions followed by hydrolysis to yield primary alcohols via ring-opening at the less substituted carbon. The starting material is 2-bromo-2-methylpropane ((CH₃)₃CBr), a tertiary alkyl bromide, which forms a Grignard reagent that attacks the epoxide (likely ethylene oxide based on option structures).
Reaction Mechanism
The first step involves forming the Grignard reagent: (CH₃)₃CBr + Mg → (CH₃)₃CMgBr (in dry ether). This tertiary Grignard acts as a strong nucleophile, attacking the less substituted (terminal) carbon of the epoxide ring in an Sₙ2 manner, opening it to form (CH₃)₃C-CH₂CH₂OMgBr. Hydrolysis with D₂O (noted in the scheme) protonates (deuterates) the alkoxide to give (CH₃)₃C-CH₂CH₂OD as the major product Q, a primary alcohol with deuterium labeling.
Option Analysis
Option (A): (CH₃)₂C=CH₂ (isobutene)
This elimination product might occur if the tertiary halide underwent E2 during Grignard formation due to steric hindrance, but standard conditions favor Grignard formation over elimination. Incorrect, as epoxide reaction wouldn’t yield this.
Option (B): CH₃-CH(OMgD)-CH₃
Suggests isopropyl-derived product, but the halide is tertiary (not isopropyl). Mismatch in carbon skeleton; incorrect.
Option (C): (CH₃)₂C(OMgD)-CH₃
Implies attack yielding secondary/tertiary alcohol, violating epoxide regiochemistry (Grignards attack less substituted carbon). Incorrect.
Option (D): (CH₃)₃C-CH₂CH₂OMgD
Matches the expected ring-opened primary alkoxide from t-butyl Grignard + ethylene oxide, prior to final hydrolysis (though D₂O gives OD). Correct major product Q.
Introduction to Grignard Reaction with Epoxide
The Grignard reaction with epoxide forms primary alcohols by ring-opening, crucial for CSIR NET Life Sciences and organic chemistry exams. In this query, 2-bromo-2-methylpropane reacts with Mg in dry ether, then an epoxide, followed by D₂O hydrolysis to give major product Q. Keywords like “Grignard epoxide product” highlight its utility in chain extension.
Step-by-Step Mechanism Breakdown
- Grignard Formation: Tertiary bromide (CH₃)₃CBr inserts Mg to yield (CH₃)₃CMgBr, despite steric challenges (tertiary Grignards form but may be less stable).
- Nucleophilic Attack: The carbanion attacks epoxide’s CH₂ (less hindered), yielding (CH₃)₃C-CH₂CH₂OMgBr via backside Sₙ2.
- Hydrolysis: D₂O gives (CH₃)₃C-CH₂CH₂OD, matching option D’s structure (pre-hydrolysis form shown). This extends the chain by two carbons.
Why Each Option Fails (Detailed Comparison)
| Option | Structure | Reason Incorrect |
|---|---|---|
| A | (CH₃)₂C=CH₂ | Elimination byproduct, not epoxide reaction product |
| B | CH₃-CH(OMgD)-CH₃ | Wrong carbon skeleton (isopropyl, not t-butyl) |
| C | (CH₃)₂C(OMgD)-CH₃ | Violates regiochemistry; attack should be at primary carbon |
| D | (CH₃)₃C-CH₂CH₂OMgD | Correct: t-Bu adds to ethylene oxide’s terminal carbon |
CSIR NET Exam Tips
Practice Grignard epoxide regiochemistry—always less substituted carbon with strong nucleophiles. Tertiary halides work but watch for elimination side reactions. Use for synthesis questions extending alcohol chains.
