Question

Which of the following is the most stable?
(A) $1 - butene$
(B) $2 - butene$
(C) $1 - pentene$
(D) $2 - pentene$

Answer 1

The stability of different alkenes can be determined on the basis of Saytzeff’s rule. Draw the structures of each alkene and then compare the stabilities by applying the Saytzeff’s rule. The number written before the name of each alkene indicates the position of the double bond in the compound.

Complete Step By Step Answer:
Saytzeff's rule states that the most stable alkene is the one which is highly substituted. A substituted alkene contains double bonded carbon atoms that are linked to more alkyl groups and not hydrogen atoms.
The presence of directly linked alkyl groups adjacent to a double bond enhances its stability by the phenomenon of hyperconjugation. Each hydrogen atom that is present in the alkyl group acts as an $\alpha$ hydrogen and gives one hyperconjugative structure. Greater the number of hyperconjugative structures, higher is the stability of the alkene.
Thus, the alkene containing the maximum number of $\alpha$ hydrogens is the most stable alkene.
The number of $\alpha$ hydrogens is maximum in the case of $2 - butene$ with formula $C{H_3} - CH = CH - C{H_3}$ where both the double bonded carbon atoms contain three $\alpha$ hydrogens each.
$\Rightarrow$ Thus, $2 - butene$ is the most stable alkene out of all given alkenes.
Option (B) is correct.

Note:
$2 - pentene$ contains a higher number of carbon atoms but the double bonded carbon atoms contain five $\alpha$ hydrogens only and is therefore less stable than $2 - butene$ . The substitution alone cannot decide the stability, the number of hydrogens attached to the carbon atom directly attached to double bonded carbon control the stability.