This is a summary of the principal characteristics of the S_N1, S_N2, E1 and E2 reactions. In the R-X‰s, X can be a halogen (Cl, Br, I) or something such as tosylate (TsO; p-CH₃C₆H₄SO₃).

S_N2 = substitution nucleophilic bimolecular

   a. CH₃-I + OH^- ----> CH₃-OH + I^-
 
   b. occurs with inversion of stereochemistry in a single step

   c. second order kinetics, rate = k [R-X][nucleophile]

   d. reaction coordinate vs E profile has single hump; the
   transition   state or activated complex is 5-coordinate with an
   sp²-hybridized central C; rxn is called "bimolecular" because
   2 species  (the R-X and nucleophile are involved in rate
   determining step

   e. R-X reactivity shows effect of steric hindrance to approach of
   nucleophile; most reactive R in R-X = 1^o > 2^o > neopentyl > 3^o; rnx
   effectively does not occur with neopentyl and 3^o R-X‰s

   f. rate increases with good nucleophiles; nucleophilicity
   increases as go down in PT; eg, Cl^- < Br^- < I^- and
   RO^- < RS^-

   g. rate of reaction largest in polar aproptic solvents;
   protic solvents can H-bond to nucleophile which effectively
   reduces the nucleophilicity (availability of e-pair)

S_N1 = substitution nucleophilic unimolecular

   a. (CH₃)₃C-I + H₂O ----> (CH₃)₃C-OH + H-I
   (in this reaction, H₂O is both the nucleophile and the
   solvent; called  a solvolysis reaction)

   b. occurs with partial racemization (usually more inversion than
   retention of configuration)

   c. first order kinetics, rate = k [R-X]; rate does not
   depend on concn of nucleophile

   d. reaction coordinate vs E profile has two humps (transition
   states) and two steps; step # 1 (rate-controlling step) is
   ionization of R-X to form a carbocation; step # 2 is fast and
   involves rxn of the carbocation with the nucleophile

   e. R-X reactivity parallels that of carbocation stability; 3^o
   most reactive, 1^o and methyl least

   f. rate of reaction largest in polar protic solvents (high
   dielectric constant) capable of ion stabilization

E2 = elimination bimolecular

   a. R-X + base ----> alkene + "H-X"
 
   b. 2nd order kinetics; base and R-X are involved in the
   single reaction step

   c. need a strong base, such as RO^-

   d. optimal conformation of R-X for loss of H and X is
   antiperiplanar; in cyclohexanes, H and X preferentially
   involved are both axial

   e. Zaitsev‰s rule applies; major organic product will be most
   stable (highly substituted) alkene

E1 = elimination unimolecular