NucSubstSummary

NucSubstSummary - Nucleophilic Substitution OVERVIEW...

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Nucleophilic Substitution OVERVIEW Characteristics S N 2 S N 1 1) kinetics 2 nd order (Nu dependent) 1 st order (Nu independent) 2) stereochemistry inversion racemization 3) solvent polar aprotic (no OH groups) activate nucleophile (major role) stabilize transition state (minor role) polar protic (w/ OH or NH group) stabilize carbocation intermediate 4) nucleophile big rate effect (more below) no rate effect 5) leaving group same trends for both S N 2 and S N 1 reactions TsO I Br Cl F HO , RO , H 60,000 30,000 10,000 200 1 0 >> > > > 2 N GOOD leaving groups POOR leaving groups 6) substrate (R-X) 1°>2°>3° reflects steric hindrance in transition state 3°>2°>1° reflects stability of carbocation intermediate DETAILS Solvents CH 3 OH CH 3 CH 2 OH CH 3 CO 2 H (CH 3 ) 2 S=O CH 3 CH 2 OCH 2 CH DMF (CH 3 ) 2 NCH=O (CH 3 ) 2 C=O CH 2 Cl 2 CH 3 (CH 2 ) 4 CH 3 THF H 2 O HCO 2 H water Solvent Structure formic acid methanol 3 79 Dielectric Constant 59 33 Protic Solvents ethanol acetic acid DMSO acetonitrile 24 6 47 38 37 Polar Aprotic Solvents acetone CH 3 C 21 dichloromethane diethyl ether hexane 9 4.3 1.9 Nonpolar Aprotic Solvents N 7.6 O HMPA [(CH 3 ) 2 N] 3 P=O 30 I NC CH 3 O Br PhO NH 3 Cl PhS CH 3 CO 2 SO 4 –2 F H 2 O Benzenethiolate > 2 x 10 8 Species Name Nucleophilicity Iodide 631,000 Cyanide 126,000 The two most important properties of the solvent in either an S N 2 or S N 1 reaction are: polarity (polar or nonpolar) hydrogen-bonding ability (protic or aprotic) Solvents can be either protic (usually polar protic), polar aprotic, or nonpolar aprotic. The best solvents for most S N 2 reactions will be polar aprotic . These solvents activate the nucleophile by solvating only it’s counterion. The best solvents for most S N 1 reactions will be polar protic (indeed, these are the ONLY solvents for S N 1 reactions). These solvents solvate both the carbocation intermediate and it’s counterion (leaving group). Note that polar protic solvents also solvate the nucleophile which deactivates it. This slows down S N 2 reactions but has no effect on the rate of S N 1 reactions.
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This note was uploaded on 04/21/2010 for the course CHEM 2261 taught by Professor Crowe during the Fall '08 term at LSU.

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NucSubstSummary - Nucleophilic Substitution OVERVIEW...

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