Compounds X and Y both have the formula C7H14. Both X and Y react with one molar equivalent of hydrogen in the presence of a palladium catalyst to form 2-methylhexane. The heat of hydrogenation of X is greater than that of Y. Both X and Y react with HCl to give the same single C7H15Cl compound as the major product. What is the structure of X?

Alkenes undergo hydrogenation to give the corresponding alkanes. Where the structure of the original alkene is unknown, we can deduce the structure of the alkene from the structure of the products obtained when it undergoes various chemical reactions.

Now, the fact that we obtained 2-methylhexane upon hydrogenation and the two compounds had different heats of hydrogenation means that the two compounds were geometric isomers. The original compounds must have been cis-2-methyl-3-hexene and trans-2-methyl-3-hexene.

When reacted with HCl, the same compound C7H15Cl is formed because the stereo chemistry is removed.

However, we know that the trans isomer is more stable than the cis isomer hence the cis isomer always has a higher heat of hydrogenation than the trans isomer. Thus X is cis-2-methyl-3-hexene.