[mandy]

I'm doing a lab and we had to make cyclohexene from the dehydration of cyclohenanol (E1 reaction).  For 1 part of the lab we had to do test reactions.  The first one was with an alkane and Br2 in CH2CL2.  We mixed these two in two different test tubes and then put one by a light source (a light bulb) and the second one in a dark drawer.  The one in the light changed colour and the one in the dark had no reaction.  Similaryly, the secon test reaction was the same except using cyclohexene instead of an alkane. 

I can't find anything on why the light affects the reaction.
Any ideas?  Hints?
Thanks

[xiankai]

alkanes are normally unreactive, and light provides an excellent catalyst with its high energy photons.

its something similar for cyclohexane, but why? 

[Yggdrasil]

Whenever you see light as a catalyst, the reaction most likely will proceed through a radical mechanism.