Question

Reaction of 2-methylcyclohepta-1,4-diene and HBr??? There are two double bonds. Which one do I break????

Answer 1

the first step in the mechanism is protonation, so protonate both and compare the stability of the resulting carbocation (higher stability -> preferred)

Answer 2

you want the carbocation to be on the most substituted carbon

https://en.wikipedia.org/wiki/Markovnikov%27s_rule

Answer 3

unless you see hv (light), then it follows anti markovnikov

Answer 4

I don't think follows anti-makovnikov in UV light unless there is peroxide.

Answer 5

If you see HBr and hv you assume there is a radical initiator, this is a specific case though that I'm mentioning.

Answer 6

This relates to the discovery Morris Kharasch made in 1933, the alkenes he was using contained peroxide contaminants. He showed by adding free radical traps he could prevent anti-marcovnikov addition to alkenes

Answer 7

@Australopithecus is correct. The UV light should indicate radical reaction.