Question

Which of the following pairs of compounds will have same number of geometrical isomers?

• A. and
  
• B. and
  
• C. and
  
• D. None of the above

Answer 1

The correct option is C and


For a two double bond system like
$R-CH=CH-CH=CH-R$
the number of possible geometrical isomers are 3 if the terminal groups are same due to symmetrical in molecule.
If the terminal groups are different, they have four possible geometrical isomers.

For a two triple bond system like
$R-CH=CH-CH=CH-CH=CH-R$
the number of possible geometrical isomers are 6 if the terminal groups are same due to symmetrical in molecule.
If the terminal groups are different, they have 8 possible geometrical isomers.

In (c), both the compounds are differ by the presence of double bond in the six member ring system. In cyclohexene ring, cis/trans isomers does not exist about the C-C pi bond. Hence, presence of double bond in the ring does not affect the number of geometrical isomers.
In (c), the ring in both the compound shows cis trans isomers due to restricted rotation of C-C single bond.
Therefore, there are 8 possible geometrical isomers.