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Unformatted text preview: Sn2 Less substituted is better, why? Sterics We learned that Sn2 must occur via "back attack." Therefore, the more carbons that are in the `back' the harder time it will have to actually move in and interact with the biggest lobe ( ) of the electrophile. Here when we compare a 3 Alkyl halide (on the left) with a 1 Alkyl halide we see why its so hard for the Nucleophile OH to interact with it. Sterics are to blame in this case and they seem to be the overriding factor. Therefore we can conclude, generally, that the more substituted the electrophile carbon is the less favorable Sn2 becomes. sigma*CBR
Br H H H H H C C H H C H H D H Br sigma*CBR H C H H OH O-sp3
H O- O-sp3 Sn1 Unlike in Sn2, an electronic effect (instead of a steric one) seems to be the overriding factor in Sn1. Another way of saying this is that the added stabilization of the extra methyl groups seem to be the most important factor. This means that the more substituted the electrophile carbon is, the more favorable and Sn1 mechanism becomes. Below, we see a diagram that shows one of the three methyl groups stabilizing the extremely unstable carbocation intermediate. Methyl groups are able to stabilize the empty C2p by donating some of the electron density in their C - H molecular orbital's. The more methyl groups there are the more C - H bonds there are to donate some of their electrons. empty C-2p
Br H H H H H C C H H C H H 1st Sn1 step H H H C H C electron donation is overall stabilizing! sigmaC-H
H slow H H C H H The second step of Sn1 is where we see the racemate being formed. When the Nucleophile is attacking the empty C2p it has the option of attacking either side (the top or the bottom) and it usually will attack both sides. empty C-2p
H H H C C H H C H H H 1 2nd Sn2 Step OH2 H Osp3 weak nucleophile is able to attack 2 ways 2 Summary Chart How substitutedness affects reaction rate Nucleophilicity of attacking molecule Sn2 Methyl>1>2>3 Sn1 3>2>1>Methyl Strong Nucleophiles Weak Nucleophiles ...
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This note was uploaded on 06/19/2008 for the course CHEM 221 taught by Professor Weinschenk during the Summer '08 term at Emory.
- Summer '08
- Organic chemistry