L21-skeletal-6

L21-skeletal-6 - Leaving group reactivity If 4 equiv of _...

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1 1 © Kay Sandberg If 4 equiv of _________ are added to 1 equivalent of this molecule, predict the dominate mechanism at each reacting site. OTs TsO OTs TsO A 1 2 3 4 6 75 8 9 10 11 12 BC D If 4 equiv of ____are added to 1 equivalent of this molecule, predict the dominate mechanism at each reacting site. W) E1; X) E2; Y) S N 1; Z) S N 2 W) E1; X) E2; Y) S N 1; Z) S N 2 2 Section 8.15 © Kay Sandberg Leaving group reactivity Increasing rate of substitution by nucleophiles R F << R Cl < R Br ~ R OH 2 + < R I < R OTs < R OSO 2 CF 3 100,000,000 100,000 100 10 10 1 0.00001 Poorest leaving group F - Best leaving group - OSO 2 CF 3 Triflate Tosylate protonated alcohol Only leaving group that does not “pop off” with a negative charge. (neutral water) R F R Cl R Br R I R OTs R OH 2 + R OSO 2 CF 3 3 © Kay Sandberg Section 9.3 Br 3 o S N 2 S N 1 polarizable nucleophile weak base heat E1 molecules containing small partially negative atoms of the 2 nd period (ex – water, alcohols) E2 strong base ions containing small fully negative atoms of the 2 nd period with conjugate acid p K a greater than 15.7 (example – 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. large atoms/ions are polarizable atoms in 3 rd period & beyond no go non-polarizable weak base 4 © 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) Polarizable nucleophile or “skinny” strong base “skinny” strong base = unhindered (like 1 o alkoxide ion) no go no go 5 © Kay Sandberg Section 9.3 Br 2 o S N 2 S N 1 E1 E2 strong base polarizable nucleophile nonpolarizable nucleophile that is a weak base 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 carbocation formation watch out for rearrangements loss of enantiomeric purity 6 OTs DMSO NaI © Kay Sandberg ( S )-tosylate Classification of alkyl tosylate?
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This note was uploaded on 03/30/2009 for the course CH ORGANIC CH taught by Professor Kaysandberg during the Spring '09 term at N.C. State.

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L21-skeletal-6 - Leaving group reactivity If 4 equiv of _...

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