nucleophilic rxns

nucleophilic rxns - enantiomers Carbocation Rearrangement...

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Ranking Leaving Groups: Best LGs are the weakest basess determine relative acidities Recognizing Substrates LG must be bonded to a sp3 carbon! Classify Substrates 0 ° ,1 ° , 2 ° , 3 ° , allylic, benzylic Recognizing Nucleophiles Must have lone pair electrons Ranking Nucleophiles: Best are strongest bases determine relative acidities Exceptions: big-atom nucleophiles are better than small-atom nucleophiles crowding diminishes nucleophilic ability Concentration and Rates SN2: rate = k[sub][nuc] SN1: rate = k[sub] Ranking SN2 substrates Best offer the least steric resistance (0 ° , 1 ° , 2 ° ) Ranking SN1 substrates Best have most stable carbocation (2 ° , 3 ° ) (3 ° benzylic > 2 ° benzylic >3 ° allylic > 1 ° benzylic > 3 ° > 2 ° allylic > 2 ° > 1 ° allylic > 1 ° > methyl) SN2 vs. SN1 nucleophiles: SN2 – strong bases, big-atom nucs, neg charge SN1 – weak bases, neutral Prediciting Stereochemical outcome SN2 – backside attack – inversion (R/S) can happen SN1 – back and front attack – can produce racemic mixtures of diasteromers and
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Unformatted text preview: enantiomers Carbocation Rearrangement Possible in SN1 reactions generates a unique product occurs when the product is a more stable carbocation (3 benzylic > 2 benzylic >3 allylic > 1 benzylic > 3 > 2 allylic > 2 > 1 allylic > 1 > methyl) Eliminations Nucleophile acts as a base strong bases E2 occurs in SN2 rxns alkene forms b/w neighboring carbon, removed proton must anti from the LG nuc picks up a proton E1 occurs in SN1 rxns alkene forms b/w neighboring carbon, proton doesnt have to be anti can lead to many products nuc picks up a proton Major product = most highly substituated Energy Diagrams SN2 one step, one transition state SN 1 the steps, three transition states (LG leaving, nuc attaching, H leaving the nuc)...
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This note was uploaded on 04/08/2008 for the course CH 203 taught by Professor Rubio during the Fall '07 term at BU.

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