SN1 reactions of cyclic ethers

[baldbrain]

The Attempt at a Solution

Firstly, since -Br is the better leaving group among the three, it's either (a) or (b). Further, since -Br is present at α position in (b), which also has an acidic α hydrogen to the same carbon, it would react better with a weak nucleophile (favouring S_N1) than in (a). So, the answer's (b). But is my line of action correct?

 

[TeethWhitener]

But is my line of action correct?

Not quite. If I had to guess, this question is really asking about carbocation stability. Your leaving group analysis is fine. So out of a and b, which carbocation is more stable?

 

[baldbrain]

Not quite. If I had to guess, this question is really asking about carbocation stability. Your leaving group analysis is fine. So out of a and b, which carbocation is more stable?

Well, I can't really decide.
Is molecule (a) more stable because of separation of charges, or, is (b) more stable due to the highly polar C-O bond?

 

[TeethWhitener]

Why is a tertiary carbocation more stable than a primary one? (ignoring steric considerations)

 

[baldbrain]

Hyperconjugation

 

[TeethWhitener]

Hyperconjugation

Which is?

 

[baldbrain]

Which is?

Interaction of σ electrons in an adjacent empty π orbital.

 

[baldbrain]

Interaction of σ electrons in an adjacent empty π orbital.

Oh, so you mean the σ electrrons of the oxygen would interact with the π orbital of the carbocation, hence leading to more stability?

 

[TeethWhitener]

Oh, so you mean the σ electrrons of the oxygen would interact with the π orbital of the carbocation?

Yes.

 

[baldbrain]

Yes.

Ok. Thanks

 

[baldbrain]

@TeethWhitener you didn't reply to my last question on 'How can any of these give the same S_N1 & S_N2 product'. I'd asked your advice on something

 

[TeethWhitener]

What question?

 

[baldbrain]

All my concepts are quite clear, but I still **** up during exams (probably due to haste). I also solve a lot of problems, and I can get them quicker when there's no time limit. That ticking clock makes me feel like a guillotine will behead me any moment.
Sometimes, even the opposite happens. I get the good problems, and **** up the easy ones. Can you give me some advice regarding this?

 

[TeethWhitener]

I don't know if I’m the best one to give advice. Practice is the most important thing. People make mistakes occasionally. If you really understand a concept, then the number of mistakes will drop over time and become a kind of random noise. If you make an undue number of mistakes with particular concepts, it means you don’t quite understand it and you should seek outside help.

 

[baldbrain]

I don't know if I’m the best one to give advice. Practice is the most important thing. People make mistakes occasionally. If you really understand a concept, then the number of mistakes will drop over time and become a kind of random noise. If you make an undue number of mistakes with particular concepts, it means you don’t quite understand it and you should seek outside help.

All Right. Thank you again