Compound A (C7H11Br) is treated with magnesium in ether to give B (C7H11MgBr), which reacts violently with D2O togive 1-methylcyclohexene with a deuterium atom on the methyl group (C). Reaction of B with acetone (CH3COCH3)followed by hydrolysis gives D (C10H18O). Heating D with concentrated H2SO4 gives E (C10H16), which decolorizestwo equivalents of Br2 to give F (C10H16Br4). E undergoes hydrogenation with excess H2 and a Pt catalyst to giveisobutylcyclohexane. Determine the structures of compounds A through F, and show your reasoning throughout
The Correct Answer and Explanation is:
Let’s break down the reactions and deduce the structures of compounds A through F.
Step 1: Compound A (C7H11Br) reacts with magnesium to form B (C7H11MgBr) in ether
- The reaction of A (C7H11Br) with magnesium in ether suggests a Grignard reagent formation. Grignard reagents are typically of the form R-MgBr, and this indicates that A is likely an alkyl bromide. The molecular formula C7H11Br suggests it is a bromocyclohexane derivative (since it contains 7 carbon atoms and one bromine atom).
- A plausible structure for A is 1-bromo-3-methylcyclohexane. This would make sense for the Grignard reagent (B), which would be 1-methyl-3-(magnesium bromide)-cyclohexane (C7H11MgBr).
Step 2: B reacts with D2O to form 1-methylcyclohexene with a deuterium atom on the methyl group
- The Grignard reagent B (C7H11MgBr) reacts with D2O (heavy water), which leads to the formation of a deuterated product. Since the deuterium is incorporated into the methyl group, we can infer that B undergoes elimination (a common reaction with Grignard reagents when treated with water or D2O).
- This leads to the formation of 1-methylcyclohexene with D on the methyl group. Therefore, C is 1-methylcyclohexene, and the deuterium is at the C1 position in the methyl group (forming CH2D).
Step 3: B reacts with acetone (CH3COCH3) to give D (C10H18O)
- The reaction of B with acetone suggests a nucleophilic addition to acetone, forming a tert-butyl group (since the Grignard reagent is a strong nucleophile). This results in the formation of a new carbon-carbon bond between C7H11 and CH3COCH3, giving C10H18O.
- The structure of D is likely 2-methyl-1-(1-methylcyclohexyl)-propan-2-ol, which fits the molecular formula C10H18O.
Step 4: Heating D with concentrated H2SO4 gives E (C10H16)
- The treatment of D with concentrated H2SO4 suggests an elimination reaction, forming a double bond. The molecular formula for E is C10H16, which implies the loss of H2O from the alcohol group of D, forming an alkene.
- This suggests that E is likely 1-methylcyclohexene, derived by eliminating water from D.
Step 5: E reacts with Br2 to form F (C10H16Br4)
- The addition of Br2 to E (C10H16) suggests that E is an alkene, which undergoes electrophilic addition with bromine, leading to the formation of F (C10H16Br4).
- This indicates that F is a tetrabromo derivative of E, where the addition of two equivalents of bromine occurs in a cis manner across the double bond, leading to F as a vicinal dibromide.
Step 6: E undergoes hydrogenation to give isobutylcyclohexane
- The hydrogenation of E (C10H16) using excess H2 and a Pt catalyst results in the saturation of the double bond, forming isobutylcyclohexane.
- This confirms that the hydrogenation of E yields a saturated product, which is likely isobutylcyclohexane with a branched isobutyl group.
Conclusion
- A: 1-bromo-3-methylcyclohexane
- B: 1-methyl-3-(magnesium bromide)-cyclohexane (Grignard reagent)
- C: 1-methylcyclohexene with a deuterium atom on the methyl group (CH2D)
- D: 2-methyl-1-(1-methylcyclohexyl)-propan-2-ol
- E: 1-methylcyclohexene (formed by dehydration of D)
- F: Tetrabromo derivative of E (C10H16Br4)
These reactions demonstrate the formation of a Grignard reagent, nucleophilic addition, elimination, electrophilic addition, and hydrogenation, leading to the final product.
