Which Reaction Pathway?

Summary Table:
Strong Nucleophile
Weak Nucleophile
Strong Base
Weak Base
Strong Base
Weak Base
Methyl Halide
SN2
SN2
v. slow SN2 rxn.
v. slow SN2 rxn.
1° Alkyl Halide
mostly SN2
SN2
E2
slow rxn.
2° Alkyl Halide
E2 and SN2
mostly SN2
E1* or E2
SN1* and E1*
3° Alkyl Halide
E2
SN1* and E1*
E1* or E2
SN1* and E1*
*in polar, protic solvents

This can be broken down into separate factors:

1) The substrate:

Alkyl Halide Substitution (1º, 2º, 3º) indicates unimolecular (SN1 and E1) or bimolecular (SN2 and E2).
Substrate Possible Mechanisms
CH3X Bimolecular (only SN2 is possible)
1° Alkyl Halide* Bimolecular
2° Alkyl Halide Unimolecular and Bimolecular
3° Alkyl Halide Unimolecular (and E2)
*Allylic and benzylic alkyl halides enable carbocation formation (i.e., unimolecular rxns).

2) The Nucleophile/Base.

Most nucleophiles are also bases and most bases are also nucleophiles. There are certain cicumstances in which one reaction will prevail over the other. For example, if one desires an elimination product, a bulky base is used. The reactants shown in the table below are grouped according to their preference for subtitution reactions, elimination reactions, or both.

SUBSTITUTION is favored by strong Nucleophiles that are not also strong bases
BOTH Substitution and Elimination will be observed for molecules that are equally good as Nucleophiles and Bases
ELIMINATION is favored by strong Bases that are not also strong nucleophiles
I-, Br-, RS- (on large atoms) OH- ,RO- (CH3)3CO- & LDA
RCO2- (delocalized negative charge) H2O, ROH, NH3 (the more basic and the more bulky the more elimination) (these are too bulky to be good nucleophiles; if the alkyl halide is also bulky this may not give the Zaitsev product)
CN- (stabilized by hybridization and inductive effect)

3) Other Factors.

Alkyl halides may
react via two or more competing mechanisms but sometimes there are factors that favor one of these mechanisms over the others. These factors include:
  • carbocation stability (other factors, like resonance conjugation, that stabilize a carbocation will speed the unimolecular mechanisms).
  • solvent (polar protic solvents enhance carbocation formation, therefore, usually favoring unimolecular mechanisms but polar aprotic solvents may speed up SN2)
  • Leaving Group ability (better leaving groups tend to speed up unimolecular carbocation formation more than they speed up bimolecular mechanisms)
  • Nucleophile/Base strength (stronger nucleophiles speed up SN2 and stronger bases speed up E2)
  • Nucleophile/Base Concentration (high concentration favors bimolecular reactions)
  • Heat (high heat favors elimination reactions)

Unimolecular = SN1 and E1

Bimolecular = SN2 and E2