[orgo814]

I am having trouble understanding the cause of the alpha effect. I know that the HOMO increases but why? Why does this happen and why does this make it more reactive as a nucleophile?

[orgo814]

My professor was talking about how SN1 at an alpha carbon to a carbonyl is usually decelerated (something about resonance I believe). He said SN2 would be accelerated due to proper orbital symmetry. He then mentioned how SN1 could occur at a primary carbon next to an ether oxygen due to resonance and that this allows the SN1 to occur there. Doesn't this contradict what we discussed about the carbonyl situation for SN1 since resonance prevented SN1 from occurring at that carbon supposedly? And what do we mean by proper orbital symmetry accelerates SN2 at that carbon adjacent to a carbonyl.

Thanks.

[mjc123]

It's not a contradiction because resonance can have either an electron-withdrawing or an electron-donating effect. The carbonyl resonance is electron withdrawing and so destabilises an adjacent carbocation, while the ether resonance is electron donating and stabilises it. Try drawing a carbocation next to (i) a carbonyl and (ii) an ether oxygen. What resonance structures can you draw? Can you see how they stabilise or destabilise the cation?

[orgo814]

Okay thanks. And would you have an idea why SN2 would be accelerated adjacent to carbonyl (something to do with MO orbital symmetry)

[mjc123]

Frankly, no.

[Babcock_Hall]

Do you mean that the energy of the HOMO increases?

[Babcock_Hall]

I would think that S_N1 reactions would be disfavored at the alpha position because of induction, as opposed to resonance.  On the other hand the carbonyl group may accelerate S_N2 processes by the same logic as why the allylic position is activated.

[Dan]

I have merged your threads because you're asking the same question about Sn2/alpha effect.

I was taught that the carbonyl π* stabilises the lone pair of the incoming nucleophile in the transition state.

Something like this (bottom of page 4):

http://www.columbia.edu/~crg2133/Files/CambridgeIA/Chemistry/NucleophilicSubstitution.pdf

[Babcock_Hall]

Hi Dan,

Maybe I was under a misapprehension, but I thought that the alpha-effect referred to nucleophiles with a pair of electrons next to (one atom over from) the pair of electrons engaged in the nucleophilic attack.  EDT  Thanks for the link.

[orgo814]

And why is the allylic accelerated?

[Babcock_Hall]

Someone more versed in MO theory should probably answer, but from Dan's link, I would say that the LUMO is lower in energy, due to the pi bond.