substitution-elimination-decision

substitution-elimination-decision - Summary of...

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Unformatted text preview: 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 Is the alkyl halide primary? Yes Is it benzylic or allylic? No Is the nucleophile a hindered, strong base (tert-butoxide, LDA)? No SN 2 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? SN 2 No SN 1 No Is the alkyl halide tertiary? Yes Is the reagent a base? Yes E2 Yes E2 ...
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This note was uploaded on 12/22/2011 for the course CHEM 3053 taught by Professor Laurashirtcliff during the Fall '11 term at Oklahoma State.

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