Lec_10_22a

Lec_10_22a - SN vs E Predict the reaction: Molecularity? SN...

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1 1 © Kay Sandberg Predict the reaction: H C H 3 C C Br H H H SN vs E Molecularity? 2 o 2 alkyl “partial pumps” carbocation can form unimolecular rxns albeit slower than tertiary! 2 © Kay Sandberg Predict the reaction: H C H 3 C C Br H H H SN E Molecularity? 2 o 1 “skinny” hydrogen Nu can get to “backside” bimolecular S N 2 albeit slower than primary! 3 © Kay Sandberg Predict the reaction: SN E Nu Substitution or elimination ? H C H 3 C C Br H H H Strong bases Weak bases small nucleophilic atoms big “squishy” nucleophilic atoms S N 1 S N 2 4 © Kay Sandberg Temperature effect SN E Hint: Δ S (change in entropy) Increase in temperature increases the rates of both substitution and elimination reactions, but guess which reaction rate increases the fastest? SN: 2 reactants 2 products E: 2 reactants 3 products Elimination dominates at high temperatures Δ G = Δ H - T Δ S 5 © Kay Sandberg S N 2 S N 1 E1 E2 The predominate pathway is mainly determined by •Classification of the substrate •Basicity of nucleophile Alkyl halides can undergo E2 E1 Favored by a high concentration of a good nucleophile or a strong base Favored by a poor nucleophile that is a weak base 6 © Kay Sandberg Section 9.3 Br 3 o S N 2 S N 1 “squishy” nucleophile “squishy” nucleophile = polarizable weak base “non-squishy” weak base heat E1 “non-squishy” weak base = solvent (alcohol) E2 strong base strong base = “non-squishy” negative ions (like alkoxide ions) carbocation intermediate means loss of enantiomeric purity thus use only sticks in structure to indicate racematization Alkene formation results in achiral product*, thus only sticks are in structure *assuming there are no other specified chiral carbons in the molecule. “squishy” means polarizable (large atoms/ions are polariable) no go “non-squishy” weak base
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2 7 © Kay Sandberg Section 9.3 Br 1 o S N 2 S N 1 E1 E2 “bulky” strong base “bulky” strong base = hindered (like 3 o alkoxide ions) “squishy” nucleophile or “skinny” strong base “skinny” strong base = unhindered (like 1 o alkoxide ion) no go no go 8 © Kay Sandberg Section 9.3 Br 2 o S N 2 S N 1 E1 E2 strong base “squishy” nucleophile non-basic nucleophile “non-squishy” weak base heat stereospecific If α carbon is chiral, then inversion of configuration* is observed *provided nucleophile has the same weight priority as leaving group
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This note was uploaded on 10/31/2008 for the course CH 221 taught by Professor Melander during the Fall '07 term at N.C. State.

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Lec_10_22a - SN vs E Predict the reaction: Molecularity? SN...

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