Question

Which of the following is not a cumulated diene?
A. Hexa-1, 2-diene
B. Hexa-2, 3-diene
C. Penta-2, 3-diene
D. Penta-1, 3-diene

Answer 1

Hint: Cumulated diene has two continuous double bonds $C = C = C$ structure.

Complete answer:
 Given:
\[
  Hexa - 1,{\text{ }}2 - diene \to C{H_2} = C = CH - C{H_2} - C{H_2} - C{H_3} \\
  Hexa - 2,{\text{ }}3 - diene \to C{H_3} - CH = C = CH - C{H_2} - C{H_3} \\
  Penta - 2,{\text{ }}3 - diene \to C{H_3} - CH = C = CH - C{H_3} \\
  Penta - 1,{\text{ }}3 - diene \to C{H_2} = CH - CH = CH - C{H_3} \\
 \]
We know that cumulated diene has two continuous double bonds $C = C = C$ structure.
Therefore only penta-1, 3-diene is not a cumulated diene. It does not have continuous double bonds around the carbon atoms.

Hence option ‘C’ is the correct answer.

Note:Cumulated dienes are the two double bonds connected to similar carbon atoms. Cumulated dienes are less stable than the other alkenes. Conjugated dienes are two double bonds separated by a single bond. Non conjugated (isolated) dienes are two double bonds separated by more than one single bond.