Question

Which alkene in each pair is more stable?

a.

b.

c.

Short answer

a. In the case of a., the stable structure is:

b. In the case of b., the stable structure is:

c. In the case of c., the stable structure is:

Step-by-step solution

Identification of the double-bonded carbons

The double bonds are identified first and marked as blue.

Representation of the double bond in the structure

For pair (a), both the structures are disubstituted

For pair (a), both the structures are disubstituted

Pair a

In the case of (b), the first structure is trisubstituted, whereas the second one is disubstituted.

Pair b

For (c), both are disubstituted.

Pair c

Stability based on trans-orientation and electron-donating groups

Stability on the double bond depends upon two factors:

• When the high priority groups are in a trans orientation, there will be no repulsion among the groups, and as a result, the alkene gets stable.

• The introduction of the alkyl groups donates electrons through the +I effect. So when more alkyl groups are added, the +I effect too increases, and stability increases too.

For pair (a), the second structure is more stable due to the presence of two alkyl groups which through its +I effect imparts stability to the double bond.

More stable structure in pair a

For pair (b), the second structure is more stable due to the presence of three alkyl groups which through its +I effect imparts stability to the double bond. The structure is trisubstituted, unlike the first pair, which is disubstituted.

More stable structure in pair b

For pair (c), the second structure is more stable due to its trans orientation, which imparts less repulsion among the alkyl groups.

More stable structure in pair c