Q.67 - Organic Chemistry

Question

Although dehydrohalogenation occurs with anti periplanar geometry, some eliminations have syn periplanar geometry. Examine the starting material and product of each elimination, and state whether the elimination occurs with syn or anti periplanar geometry.

a.

b.

Step-by-Step Solution

Answer

The stepwise mechanism of the elimination reactions are shown below:-

Consideration of planarity for undergoing elimination reaction

In most cases, anti-periplanar geometry is considered where the $\mathbf{\text{β}}$proton and the leaving groups lie in a trans orientation.

 

But for reaction a., synperiplanar geometry is considered where the $\text{β}$ proton and the leaving groups lie in cis orientation. On the other hand, reaction b. follows an anti-periplanar mechanistic pathway.

Seleno-Cope reaction

The reaction a. is a Seleno-Cope reaction where the elimination occurs via a syn pathway. The double bonded Se-O gets ionized into  $\stackrel{+}{\text{Se-}}\stackrel{-}{\text{O}}$. Both the $\stackrel{+}{\text{Se-}}\stackrel{-}{\text{O}}$ and H lie in the wedge position a. As a result, the negative charge over the oxygen atom abstracts a proton from its same position, and the simultaneous removal of the seleno group occurs. 

 

The elimination product that is formed contains deuterium, which indicates that the mechanism follows a syn-mechanism pathway.

Representation of the elimination reaction scheme for a

Use of Zn metal

The use of Zn metal in reaction b. is to form a Zn-Br bond. The bond C-Zn-Br is an organometallic bond where C carries the negative charge (polung-umpolung). 

 

The negative charge over carbon donates its negative charge in its C-C bond, and the removal of Br takes place. This follows an anti-periplanar mechanistic pathway, and to do that, bond rotation is required.

Representation of the elimination reaction scheme for b