Which of these is most easily dehydrohalogenated?


a. 5-Chlorocyclohexa-1,3-diene (1)
b. 3-Chlorocyclohexa-1,4-diene (2)
c. 1-Chlorocyclohexa-1,4-diene (3)
d. Both 1 and 3

I encountered this question a while ago. The given answer is option (b) (compound 2). But the dehydrohalogenation of compound 1 produces benzene which has high heat of formation. So why isn't that the answer?

  • 1
    $\begingroup$ They both form benzene. Does that change your analysis? $\endgroup$
    – jerepierre
    Nov 18 '15 at 16:15
  • 3
    $\begingroup$ @jerepierre Can you elaborate a bit? $\endgroup$
    – Soham
    Jun 10 '18 at 7:36
  • 1
    $\begingroup$ Can anyone please explain? $\endgroup$
    – Iceberry
    Jan 25 '19 at 15:53

That's a fair question,

Assuming that you use a strong base for dehydrohalogenation you will be deprotonating the most acidic $\ce{H}$ resulting in formation of a carbanion which has the highest stability in compound number 2

You can see that the carbanion formed in compound numer 1 will have resonance with only one of the double bond and hence will be less stable then the carbanion in the compound 2 which has resonance from both the double bond.

Mechanism can be as follow :-



Reaction Intermediate

  • $\begingroup$ I thought that extended conjugation is a better stabilizing effect than cross conjugation? $\endgroup$ Jan 26 '19 at 13:53

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