Identifying a Diene

The four isomeric C9H14 dienes listed below all give 1,1-dimethylcycloheptane on exhaustive catalytic hydrogenation.

 
  A   5,5-dimethyl-1,3-cycloheptadiene
  B   6,6-dimethyl-1,3-cycloheptadiene
  C   6,6-dimethyl-1,4-cycloheptadiene
  D   3,3-dimethyl-1,4-cycloheptadiene

Because of their symmetry, two of these isomers have five structurally distinct carbon atoms; the other two have eight such carbons.
One of the symmetrical dienes (X) has a uv absorption maximum near 250 nm.
The other symmetrical diene (Y) does not absorb uv light above 200 nm in wavelength.

  1. Which isomer is X?   A   B   C   D
  2. Which isomer is Y?   A   B   C   D

           
This script written by William Reusch, Dept. of Chemistry, Michigan State University.
Please send comments and corrections to whreusch@pilot.msu.edu.