Question

1-Chloro-4-isopropylcyclohexane exists as two stereoisomers: one cis and one trans. Treatment of either isomer with sodium ethoxide in ethanol gives 4-isopropylcyclohexene by an E2 reaction. CC(C)C1CC=CCC1 1-Chloro-4- 4-Isopropylcyclohexene isopropylcyclohexane The cis isomer undergoes E2 reaction several orders of magnitude faster than the trans isomer. How do you account for this experimental observation?

Short answer

Answer: The difference in reaction rates between the cis and trans 1-chloro-4-isopropylcyclohexane can be attributed to their conformations. The cis isomer's stable chair conformation allows for an efficient E2 reaction due to the anti-coplanar geometry (hydrogen and chloride ion in axial positions), while the trans isomer needs to adopt a higher energy, less stable conformation for the E2 reaction to occur. This significant conformational difference results in a much faster reaction rate for the cis isomer compared to the trans isomer.

Step-by-step solution

Review: E2 Mechanism

E2 (elimination bimolecular) reactions occur when a base removes a proton nearest to the leaving group, causing the leaving group to leave at the same time and leading to the formation of a double bond. In E2 reactions, the hydrogen atom and the leaving group must be in an anti-coplanar geometry (180 degrees dihedral angle) for the reaction to occur most effectively. Step 2: Analyze the conformations of cis and trans 1-chloro-4-isopropylcyclohexane

Cis and Trans Conformations

The cis isomer has the chlorine atom and isopropyl group on the same side of the cyclohexane ring, while the trans isomer has them on opposite sides. When examining the conformations of these isomers, it is important to consider their chair conformations. In the chair conformations, hydrogen atoms on axial positions are likely to participate in E2 reactions. Step 3: Identify the most stable chair conformation for each isomer

Stable Chair Conformations

In the case of cis 1-chloro-4-isopropylcyclohexane, the most stable chair conformation has the isopropyl group and chloride ion in equatorial positions, while for the trans isomer, the most stable conformation has the chloride ion in the equatorial position and the isopropyl group in the axial position in order to minimize steric strain. Step 4: Compare the E2 reaction pathways for both isomers

E2 Reaction Pathways Comparison

For the cis isomer, the most stable chair conformation already has the chloride ion and hydrogen atom in the axial position allowing for an efficient E2 reaction. Whereas, for the trans isomer, the most stable chair conformation would need to adopt a less stable conformation having both chloride and the hydrogen atom in axial positions (higher energy state) for the reaction to occur. Step 5: Explain the rate difference

Rate Difference Explanation

The difference in reaction rates between the cis and trans 1-chloro-4-isopropylcyclohexane can be attributed to their conformations. The cis isomer's anti-coplanar geometry (hydrogen and chloride ion in axial positions) in its stable chair conformation allows for a faster and more efficient E2 reaction. On the other hand, the trans isomer has to adopt a higher energy, less stable conformation for the E2 reaction to occur, which significantly reduces its reaction rate compared to the cis isomer.