Question

Indicate the major product of the following reaction:

\

• A. Fig A
• B. Fig B
• C. Fig C
• D. Fig D

Hint:

For \( \text{E2} \) elimination in cyclohexanes, stereochemical control (anti-periplanar requirement) completely overrides thermodynamic control (Zaitsev's rule). Always draw the substituents in a chair conformation to verify which H (or D) is trans-diaxial to the leaving group.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The reaction involves the E2 (Elimination Bimolecular) mechanism on a substituted cyclohexane ring.
The E2 mechanism is a concerted process where a base abstracts a proton (or deuteron) simultaneously as the leaving group ($Br$) departs.
In cyclohexane systems, the spatial arrangement of the atoms is critical. The leaving group and the hydrogen/deuterium atom to be eliminated must be in an anti-periplanar (trans-diaxial) relationship.
If the required atoms cannot achieve a trans-diaxial orientation in any chair conformation, the E2 reaction cannot occur at that position.
Step 2: Key Formula or Approach:
1. Draw the chair conformation of the starting material.
2. Identify the substituents: A wedged Br at C1, a hashed D at C2, and a wedged Methyl at C2.
3. Check the axial/equatorial positions. Only a substituent in the axial position can be anti-periplanar to another axial substituent on the adjacent carbon.
Step 3: Detailed Explanation:
Let's analyze the starting material. At C1, we have a Bromine atom (wedged/UP). At C2, we have a Deuterium atom (hashed/DOWN) and a Methyl group (wedged/UP).
In the chair conformation where the bulky Methyl group is in the stable equatorial-up position, the adjacent Deuterium atom must be in the axial-down position.
In this same chair, the UP Bromine at C1 will be in the axial-up position.
Now, look at the relative orientation: The UP Bromine is axial, and the adjacent DOWN Deuterium is also axial. They are trans-diaxial (anti-periplanar).
On the other side of the Bromine (let's say C6), there are only Hydrogen atoms. To be eliminated, a Hydrogen at C6 would also need to be axial-down.
However, the base ($CH_3O^-$) reacts with the most accessible anti-periplanar atom. The presence of the Deuterium in a perfect trans-diaxial alignment makes its removal favorable.
The base abstracts the axial Deuterium atom. The electrons from the $C-D$ bond shift to form a $\pi$-bond between C1 and C2, and the axial Bromine atom leaves as a bromide ion ($Br^-$).
This process eliminates $DBr$, not $HBr$.
The final product is an alkene (cyclohexene derivative) where the double bond is between C1 and C2, the Methyl group remains at its original position, and the Deuterium has been removed.
This specific structural outcome is represented by Figure C.
Step 4: Final Answer:
Due to the strict stereochemical requirement of a trans-diaxial transition state, the Deuterium is eliminated. The resulting alkene is shown in option (C).