Q11-74E - Organic Chemistry (Mcmurry)

Question

Although anti-periplanar geometry is preferred for E2 reactions, it isn’t absolutely necessary. The deuterated Bromo compound shown here reacts with a strong base to yield an undeuterated alkene. Clearly, a syn elimination has occurred. Make a molecular model of the reactant, and explain the result.

Step-by-Step Solution

Answer

The deuterated compound gave the product containing no deuterium in this case due to the Exo Br group cannot achieve a dihedral angle of  $180°$ with the endo hydrogen because of the rigid structure of the molecule. The dihedral angle here is about $120°$. So, these leaving groups prefer syn elimination with a dihedral angle of about $0°$ to anti-elimination with an angle of about  $120°$.

Step 1: Bimolecular elimination reaction

The ${\mathbf{E}}_2$ reaction (for elimination, bimolecular) occurs when an alkyl halide is treated with a strong base, such as a hydroxide ion or alkoxide ion $\mathbf{\left(}{\mathbf{RO}}_2\mathbf{\right)}$. It is the most commonly occurring pathway for elimination.

Step 2: Explanation

The deuterated compound gave the product containing no deuterium in this case due to the Exo Br group cannot achieve a dihedral angle of $180°$ with the endo hydrogen because of the rigid structure of the molecule. The dihedral angle here is about $120°$. So, these leaving groups prefer syn elimination with a dihedral angle of about $0°$ to anti-elimination with an angle of about $120°$.