[nocrown]

Hey guys, I have a bit of a silly simple question and maybe I'm just a bit cloudy right now but I can't seem to quite figure it out.

Here it states that A is harder to protonate than B. I just always assumed isolated dienes were naturally easier to undergo protonation because they were less stable than something with single bonded carbons only - but this is not the case/sufficient enough and now I'm very confused. Can anyone point me in the right direction?

[discodermolide]

What are the differences between the two molecules?
Which one would be more liable to react with acid and why?

[nocrown]

What are the differences between the two molecules?
Which one would be more liable to react with acid and why?

Well compound B has an alkene double bond replacing the C-C single bonds.

I think Compound B is more liable to react with acid? It can undergo an electrophilic attack on the diene ..? I think you can attack the carbonyl in compound B using either thermodynamic or kinetic product formations. Can the double bond in compound B "break" to accept whatever is attached to the H+ (like Br) so theres a single bond formed at Br and the H+ would go to the -O while in compound A there is no double bond to "break" and therefore you have to force protonation and have a resulting Br- left over?

[discodermolide]

The key here is the double bond, as you stated. Also don't forget the ring strain plays a part as well.

[nocrown]

Thanks a ton, I get it now

[Dan]

I would have expected protonation on O for both A and B, and then thought about what Erich Hückel might have reasoned...

[nocrown]

I would have expected protonation on O for both A and B, and then thought about what Erich Hückel might have reasoned...

Yes I believe both molecules can be protonated on the O but the focus for this section was on dienes and double bonds. I am not familiar with Erich Hückel other than the 4n+2 rule. Are you saying B is easier to protonate because of aromatic properties?

[Dan]

Are you saying B is easier to protonate because of aromatic properties?

Draw the protonated cyclopropenone as its carbocation resonance form - i.e. Hydroxycyclopropenium ion. Do you see what I'm getting at?