SN2 and SN1 Reactions

SN2 Rxn

  • A substituted rxn
  • Single step (concerted rxn) with an intermediate transition state
  • Nucleophile (Nu) attacks the substrate (RX, X=F, Cl, Br, I) at 180° away from leaving group (X)
  • Rate determining step is bi-molecular
  • Stereochemistry: inversion of configuration will occur if substitution takes place at an asymmetric center (chiral) →Walden Inversion
  • Rate= k[nucleophile][alkyl halide]   (k=rate constant)
  • Relative rates:

Methyl halide (CH3-X)>1° alkyl halide (RCH2-X)>2° alkyl halide (R2CH-X)

3° alkyl halide (R3C-X) don’t undergo SN2 rxn

  • Common solvents: acetone, DMSO, DMF

Screen Shot 2016-11-15 at 9.03.37 AM.png

SN2 Rxn Trends

  1. As temperature increases, the faster the rxn.
  2. Rate= k[nucleophile][alkyl halide]

Since rate is directly proportional to concentration of alkyl halide, as [Nu] increases, the faster the reaction.

  1. Since rate is directly proportional to concentration of alkyl halide, as [alkyl halide] increases, the faster the rxn.
  2. As steric hindrince increases, the slower the rxn (due to less accessibility of the electrophile carbon atom).
  3. The weaker the base, the better the leaving group, the faster the reaction (RI> RBr> RCl> RF).

SN1 Rxn

  • A substitution rxn
  • Takes place in a stepwise fashion
  • Carbocations are involved and rearrangements may occur
  • Rate-determining step is uni-molecular
  • Requires polar, protic solvent
  • Stereochemistry: (either complete or partial) will occur if substitution takes place at an asymmetric center
    • Complete racemization: the conversion of an enantiomer to equal amounts of two enantiomeric products
    • Partial racemization: the conversion of an enantiomer to unequal amounts of two enantiomeric products
  • Rate= k[alkyl halide]
  • Relative rates:

3° alkyl halide > 2°

1° alkyl halide and methyl halide don’t undergo SN1 rnx

screen-shot-2016-11-17-at-10-32-02-am

SN1 Rxn Trends

  1. As temperature increases, the faster the rxn
  2. Rate= k[alkyl halide]

Since rate is NOT proportional to the concentration of the Nu, the [Nu] does not affect the rate of reaction

  1. Since rate is proportional to the concentration of the alkyl halide, as [alkyl halide] increases, the faster the rxn
  2. As steric hindrance increases, the faster the rxn (due to stability of carbocation intermediate)
  3. The weaker the base, the better the leaving group, the faster the rxn (RI> RBr> RCl> RF)

SN2 Rxn vs. SN1 Rxn

Alkyl Halide

SN2 Rxn

SN1 Rxn

Methyl halides

Yes

No

Primary alkyl halides

Yes

No

Secondary alkyl halides

Yes

Yes

Tertiary alkyl halides

No

Yes

Comparing the SN1 and the SN2 reactions

 

SN1 Rxn

SN2 Rxn

Rate Law

Unimolecular (substrate only)

Bimolecular ( substrate and nucleophile)

*Big Barrier*

Carbocation stability

Steric Hindrance

Alkyl halide (electrophile)

3°>2°>>1°

1°>2°>>3°

Nucleophile

Weak (generally neutral)

Strong (generally bearing a negative charge)

Solvent

Polar protic (e.g. alcohols)

Polar aprotic (e.g. DMSO, acetone)

Stereochemistry

Mix of retention and inversion

Inversion

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