I faced a question like this:

The total number of alkene products possible from the dehydrobromination of 3-bromo-3-cyclopentylhexane is:

The answer is given 5. But I only found 3.

  1. 3-cyclopentylhex-2-ene
  2. 3-cyclopentylhex-3-ene
  3. (hexan-3-ylidene)cyclopentane

I can't find the other 2. Any hint?

  • 11
    $\begingroup$ Stereoisomers are counted apparently. Both 1 and 2 have (E)- and (Z)-isomers. $\endgroup$
    – orthocresol
    May 23 '17 at 1:44
  • $\begingroup$ I agree with Orthocresol, do consider stereoisomers,sometimes the question doesn't mention it. $\endgroup$ May 23 '17 at 3:27

solution re

The structures (i) and (ii) have two isomers each,namely E and Z isomers.

For the structure (iii) there is a plane of symmetry (marked in the diagram) so it is not a geometrical isomer.

Thus Number of isomers=2 (of structure i) +2 (of structure ii) + 1(of structure iii)= 5 isomers

  • 6
    $\begingroup$ Umm, plane of symmetry should not be considered here, as we are looking for geometrical isomerism. Instead you can say that two groups attached to the double bond are identical. $\endgroup$
    – Sawarnik
    May 24 '17 at 3:36
  • 1
    $\begingroup$ In fact, the molecule does not have a plane of symmetry there, since an ethyl residue (chain left-hand side) and a propyl residue (chain, right-hand side) are not identical. What you should be discussing are the identical substituents on the other side of the double bond. $\endgroup$
    – Jan
    Aug 11 '17 at 1:17

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