Substitution-elimination-decision

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Summary of Substitution/Elimination Features 
SN2: 
Stereochemistry: 100% inversion 
Substrate: CH3 > 1° > 2° >> 3° 
Nucleophile: Need strong nucleophile, usually anionic (see below) 
Leaving group: X– or –OTos 
Solvent: Polar aprotic (DMF, DMSO, HMPA, CH3CN) 
SN1: 
Stereochemistry: racemization 
Substrate: 3° > 2° >> 1°. Allylic or benzylic work, even if 1° 
Nucleophile: generally weak (H2O, ROH, HX) 
Leaving group: X– or –OTos. OH can be a good leaving group under acidic conditions 
Solvent: Polar protic 
E2: 
Stereochemistry: C-H and C-X bonds must be anti 
Substrate: 1°, 2°, or 3°. For 1° need hindered base 
Base: Need strong base (NaOR, NaNR2) 
Leaving group: X– or –OTos 
Solvent: Polar protic or aprotic 
E1: 
Stereochemistry: mixture of E and Z 
Substrate: 3° > 2° >> 1°. Allylic or benzylic work, even if 1° 
Base: generally weak (H2O, ROH, H2SO4) 
Leaving group: X– or –OTos. OH can be a good leaving group under acidic conditions 
Solvent: Polar protic 
Nucleophiles (counterion can be Li or K; R = any carbon group): 
Strong and basic nucleophiles: NaOH, NaOR, NaNH2, NaNR2, NaCCR 
Strong and non-basic nucleophiles: NaSH, NaSR, NaCN, NaN3, NaO2CR, NH3, NR3, X– 
Weak and non-basic nucleophiles: H2O, ROH, RCO2H 
SN1, SN2, E1, E2 Decision Flow Chart 
 
Yes 
Is the nucleophile/base 
neutral or acidic (H2O, 
ROH, RCO2H, H2SO4)? 
Is the alkyl 
halide 
secondary? 
Is the 
nucleophile 
an anion? 
Is the substrate an 
alcohol and the 
reagent H2SO4? 
Is the alkyl 
halide tertiary? Is the reagent a base? 
Is the alkyl 
halide primary? 
Is it benzylic 
or allylic? 
Is the nucleophile a 
hindered, strong base 
(tert-butoxide, LDA)? 
No 
E2 
Yes 
SN2 
No 
Yes 
No 
SN1 or E1. SN1 
favored unless an 
alcohol with H2SO4, 
then E1. 
Yes 
No 
Yes 
SN2 
Is the nucleophile a 
strong base (NaOR, 
NaNR2, NaCCR)? 
Yes 
E2 
Yes 
No 
No 
SN1 
No 
E1 
Yes 
No 
No 
Yes 
E2 
Yes