[Pranav]

Clearly B is correct as double bonds cannot be present on a small bridged system. But how would I check the stability for A,C and D. Also, both the structures in A look the same, why they have used cis-trans when they are same?
 
I am clueless here. Any help is appreciated. Thanks!

[TheHitokiri]

Rule of thumb: Trans is usually always more stable than cis. Like-wise, E is usually always more stable than Z.
http://en.wikipedia.org/wiki/Cis%E2%80%93trans_isomerism

Unless you have dipole-moment affecting the stability or intramolecular hydrogen-bonding.

[Pranav]

Rule of thumb: Trans is usually always more stable than cis. Like-wise, E is usually always more stable than Z.
http://en.wikipedia.org/wiki/Cis%E2%80%93trans_isomerism

Unless you have dipole-moment affecting the stability or intramolecular hydrogen-bonding.

Then, it would mean that A is incorrect but the key states that it is correct.

[TheHitokiri]

Try drawing the H about the double bond for A.

Transannular interaction destabilises it.

[betterbesafehero]

For C : No of α-H is considered or the number of substituent attached to doubly bonded carbon.


So, it's incorrect..!

For D: Again cis-trans concept, incorrect..!

[Pranav]

I am extremely sorry for the delay in reply.

@TheHitokiri: I am not sure but when I tried to draw the structure for the trans compound, the hydrogen atoms were forced to move into the interior of ring, does that makes it unstable?

@betterbesafehero: Thanks, I totally forgot about the effect on stability of the doubly bonded carbon.

[Wingpin]

I had a similar problem like you for question (a) for my first exam. If you have the textbook Organic Chemistry, 8th ed., by Francis Carey et. al, he explains cis vs. trans. stability in cycloalkenes on page 198.

In part A, the compound is a cyclodecene, and 10 is less than 12, so cis is more stable than trans in this case. Don't know if there's a deeper logic behind it like TheHitokiri suggested, but hope it helped a little.

Edit: Sorry, was not aware of issues for copying text directly from book. Don't know if this is a better way, but here's a McGraw-Hill link available to all instead of direct text:
http://dev6.mhhe.com/textflowdev/genhtml/0073402613/5.7.htm
Last paragraph has the same exact information.

[Pranav]

I had a similar problem like you for question (a) for my first exam. If you have the textbook Organic Chemistry, 8th ed., by Francis Carey et. al, he explain cis vs. trans. stability in cycloalkenes on page 198.
Here is his argument:
(text from book)

In part A, the compound is a cyclodecene, and 10 is less than 12, so cis is more stable than trans in this case. Don't know if there's a deeper logic behind it like TheHitokiri suggested, but hope it helped a little.

Thanks Wingpin!

Its not a good idea to post the part of the text. You could have posted a link to google books where I could see it. I think this book is available for preview there.

[camptzak]

B