Substitution-elimination-decision PDF

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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 E2 Yes Yes

Is it benzylic or allylic?

Is the alkyl halide primary?

No

Is the nucleophile a hindered, strong base (tert-butoxide, LDA)?

No SN2

Yes

Is the nucleophile/base neutral or acidic (H2O, ROH, RCO2H, H2SO4)?

No

No

SN1 or E1. SN1 favored unless an alcohol with H2SO4, then E1.

Yes

Is the alkyl halide secondary?

Yes

Is the nucleophile an anion?

Yes

Is the nucleophile a strong base (NaOR, NaNR2, NaCCR)? No

No

No

Yes E1

Is the substrate an alcohol and the reagent H2SO4?

SN2

No

SN1

No Is the alkyl halide tertiary?

Yes

Is the reagent a base?

Yes E2

Yes E2...