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Posts tagged chemistry.
When people suggest to me I should become a organic synthetic chemist I half laugh and half cry because named reactions and mechanisms. #nothanks
6 months ago on November 13, 2012 at 08:33pm
Heya! You’re always welcome to ask questions :) And yes, there is definitely a difference. In my mind I like to group E1/E2/Sn1/Sn2 together because for a lot of conditions, any combinations of those could be in competition.
So just like in SN reactions you have to consider factors like the substrate, the nucleophile/base, and the leaving group. First, the substrate. In E1, you always get a carbocation - so ANYTHING that will stabilise this intermediate will favour the E1 mechanism (conjugation, substitution, benzene rings, etc). This doesn’t matter so much in E2. For both cases, the leaving group just has to be decent enough (usually a halide), like in all the four types of reactions. You often get E1 and Sn1 competing with each other because if there is a decent enough nucleophile around, there isn’t much to stop it from simply attacking the cation (this can be avoided by doing the reaction at higher temp.
In E2, the nucleophile matters more. If you have a really good nucleophile in addition to a good leaving group, chances are it’s in competition with an Sn2 reaction. BUT if you have a really shitty nucleophile but an amazing base, then it will take a proton instead and the molecule will eliminate. IMPORTANTLY, the hydrogen being taken off and the leaving group HAVE to be in anti-peri planar conformation so that the orbitals will align and electrons can actually “move” to form the pi bond.
(Also there are solvent preferences for E1/Sn1 - polar protic ones are better at stabilising the charged intermediate).
Basically, if you take all these factors into account, usually one mechanism will appear to be favoured above the others. But in real life they are generally often in competition and it’s about optimising the conditions to get the product you want. If you want to read more, my previous organic lecturer wrote some great wiki pages on this as course notes and he’s FANTASTIC at explaining these things, so I highly recommend you have a quick glance (he also has great diagrams and orbital pictures): http://openwetware.org/wiki/Todd:Chem3x11_ToddL11. I hope all that helped!
(Sorry for posting this entire essay but easier to type like this than in the fan mail response thing).
6 months ago on October 27, 2012 at 08:40pm
Little Willie, and so and so,
Put H2S in our H2O.
Now this was not so bad at all,
For we drink nought but alcohol.
Wendell M. Latimer
7 months ago on October 18, 2012 at 07:54pm
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