[spoons]

It's driving me crazy that i couldn't figure this out. The starting materials were toluene and HBR and I had to devise a mechanism to create the attached image.

I got as far as brominating the vinyl group on a styrene via a secondary carbocation, but had no idea where to go from there. Any ideas?

[AlphaScent]

There has to be more than just Toluene and HBr in this reaction.  Are you stating that there was also styrene?

Please list all starting materials as given on the exam.

[spoons]

Oops i meant styrene!!! NOT TOLUENE!

[spirochete]

H-Br will first act as a Bronsted acid and protonate styrene to make the more stable of two possible carbocations. Another molecule of styrene can then act as a nucleophile and attack the resulting carbocation, again making the more stable of two possible cations. Then something in solution can act as a base (probably Bromide) and do an E1 elimination to form the product.

[AlphaScent]

That is the only feasible way I see this reaction proceeding.  But I do think there is going to be many side products.

[spoons]

I guess that makes sense. I don't get why he made this the one big mechanistic question when we had to learn a crap ton of aromatic and organometalic mechanisms. I still don't quite understand why the double bond on the styrene will so willingly break when it comes into contact with the other carbocation though! How was i just supposed to assume that styrene would be nucleophilic enough for this reaction to happen? He always said we shouldn't "make up" mechanisms, but i dont understand how anyone could've gotten this without making assumptions/guesses. The textbook never mentions anything like this. I hate my life.

[orgopete]

I guess that makes sense. I don't get why he made this the one big mechanistic question when we had to learn a crap ton of aromatic and organometalic mechanisms. I still don't quite understand why the double bond on the styrene will so willingly break when it comes into contact with the other carbocation though! How was i just supposed to assume that styrene would be nucleophilic enough for this reaction to happen...

Two lessons here. I always listed the reactions covered in my course so when I wrote an exam, I would reasonably represent them so students wouldn't feel their studying was wasted. Second lesson was I always thought I shouldn't expect students to answer problems they hadn't practiced for.

I wasn't in class, didn't read the book, so I can't say whether this is fair or not. It could have been an experiment by your professor as to whether students could advance the basic concepts in a more abstract way. I suspect the exams are curved. If that is so, then scores are relative to whether others in class answered it correctly.

[spoons]

Thanks so much for you input. The tests actually aren't curved, but an 85s an A, which helps slightly.

[Dan]

I still don't quite understand why the double bond on the styrene will so willingly break when it comes into contact with the other carbocation though! How was i just supposed to assume that styrene would be nucleophilic enough for this reaction to happen?

You are comfortable with the double bond breaking in the presence of H⁺, this is a conceptually similar mode of reactivity - just with C⁺ instead of H⁺.