random ochem question

[cloak25]

---

Members don't see this ad.

If the following compound were treated with strong acid, at which carbon in the pi-network is it most likely to gain H+?

http://i.imgur.com/xXVyr.jpg

at carbon a, b, c or d?

I understand that carbon "a" is secondary while carbon "d" is tertiary so it is easier to protonate the less hindered carbon. Answer is carbon "a". What's the reason for eliminating b and c?

 

[kasho11]

I can't give a definitive answer but when dealing with pi systems the answer almost always has to do with resonance. It would seem that adding a hydrogen to carbon a would disrupt the pi electron system the least because it is at the end of it (whereas d would less favorable because of hindrance?). Adding an H+ to B or C would disrupt the pi system more unfavorably? This seems like the right line of thinking, but I am not sure.

 

[cloak25]

I can't give a definitive answer but when dealing with pi systems the answer almost always has to do with resonance. It would seem that adding a hydrogen to carbon a would disrupt the pi electron system the least because it is at the end of it (whereas d would less favorable because of hindrance?). Adding an H+ to B or C would disrupt the pi system more unfavorably? This seems like the right line of thinking, but I am not sure.

I think you're right. The explanation says that "a conjugated diene can be protonated at either terminal carbon of the pi-network, because the carbocation that results is resonance stabilized. Therefore, b and c are eliminated. So for conjugated dienes, H+ can only add at the terminal carbons? hmm..didn't know that

 

[jsp132]

I think you're right. The explanation says that "a conjugated diene can be protonated at either terminal carbon of the pi-network, because the carbocation that results is resonance stabilized. Therefore, b and c are eliminated. So for conjugated dienes, H+ can only add at the terminal carbons? hmm..didn't know that

try adding a H+ to carbon b and c

can you push it anywhere else without violating the octet rule?

doesn't seem like it that's probably why you can only add it to the terminal carbons because they other part of the structure is resonance stabilized