Organic Chemistry

Check Also in Elimination Reactions:

Practice

1.

Identify the products and draw the curved-arrow mechanism for each of the following stereospecific E2 reaction: Drawing Newman projections to achieve the correct anti-periplanar conformation will be helpful.

Check your answers

Solution

a) The substrate has two β positions but only one β Hydrogen and that is why the E2 reaction is stereospecific thus only one stereoisomeric product will be formed. The β positions and the β Hydrogen are shown in blue:

In order to determine which stereoisomer will be formed, we can draw the Newman projection and also the sawhorse representation which helps us to visualize the arrangement of all the groups on the carbon with the bromide as the leaving group and the β Hydrogen.
Remember that the elimination reaction will only occur in a conformation where the β Hydrogen and the leaving group are positioned in an anti-periplanar orientation to one another.

In this case, the initially shown conformation is optimal for the E2 mechanism and therefore the reaction proceeds via this conformation producing the E isomer as the only product.

 

b) The substrate has two β positions but only one β Hydrogen and that is why the E2 reaction is stereospecific thus only one stereoisomeric product will be formed.
When we draw the Newman projection from the indicated angle, we can see that the β Hydrogen and the leaving group are not anti-periplanar to one another. However, this confirmation is possible to achieve by a 120o rotation:

The structure of the alkene can be predicted from this Newman projection where the phenyl group is cis to the methyl and the ethyl group is cis to the Hydrogen:

 

c) The substrate has two β positions but only one β Hydrogen and that is why the E2 reaction is stereospecific thus only one stereoisomeric product will be formed.
When we draw the Newman projection from the indicated angle, we can see that the β Hydrogen and the leaving group are not anti-periplanar to one another. However, this confirmation is possible to achieve by a 120o rotation:

The structure of the alkene can be predicted from this Newman projection where the cyclic group is cis to the methyl and the isopropyl group is cis to the Hydrogen:

 

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