Which of the following alkenes are stable, and which ones are unstable? Support your answer with a suitable reason.

a. trans-3,4-dibromocyclopent-1-ene
b. 1,2-dibromocyclopent-1-ene
c. trans-cyclooctene
d. trans-1,2-dibromocyclopent-1-ene

• A. (a). Stable—A double bond can exist in a 5-membered ring
  (b). Stable—A double bond can exist in a 5-membered ring
  (c). Unstable—A trans double bond can not exist in an 8-membered ring
  (d). Highly unstable—A trans double bond can not exist in a 5-membered ring.
• B. (a). Stable—A double bond can exist in a 5-membered ring
  (b). Stable—A double bond can exist in a 5-membered ring
  (c). Stable—A trans double bond can exist in an 8-membered ring
  (d). Highly unstable—A trans double bond can not exist in a 5-membered ring.
• C. (a). Unstable—A double bond can not exist in a 5-membered ring
  (b). Unstable—A double bond can not exist in a 5-membered ring
  (c). Stable—A trans double bond can exist in an 8-membered ring
  (d). Highly unstable—A trans double bond can not exist in a 5-membered ring.
• D. (a). Stable—A double bond can exist in a 5-membered ring
  (b). Stable—A double bond can exist in a 5-membered ring
  (c). Stable—A trans double bond can exist in an 8-membered ring
  (d). Stable—A trans double bond can exist in a 5-membered ring.

Answer: (B)