Carbocations formed on carbons located in allylic positions are called allylic cations.
Allylic carbocations have superior stability to equally substituted but non-allylic carbocations.
[2] Tertiary and allylic carbocation.
[4] Tertiary carbocation.
It is experimentally observed that the substrate loses chlorine 100 times faster than the substrate. This speed difference is due to the greater stability of the carbocation formed from the substrate. As the chlorine is located in an allylic position, the carbocation formed is stabilized by distributing the charge over two carbons. The carbocation formed from the substrate has a positive charge located on a single carbon, and is therefore less stable.
Resonance stabilized allylic cation.
An allylic carbocation is all the more stable the more resonant structures it has. If there is a tie, the one with the positive charges on the most substituted carbons wins in stability.
Both carbocations have two structures, but on the left the positive charges are located on primary and tertiary carbons, while on the right they are primary, secondary carbons. In conclusion the left carbocation is more stable than the right.