one forms a carbocation and the other an anion.
For determining stability of leaving groups, an easy method is to look at the pka of conjugate acid of
the leaving group. The lower the pka or stronger the acid is, the better the leaving group. Ex. HI is a
better acid than HF, likewise I- is a better leaving group than F-
. The only shortcoming of this method is
that it does not take into account kinetics, so pka trends don’t perfectly match the LG trends.
Sometimes poor LGs can be converted into better LGs (see other reactions section).
E2 & Sn2 (wip)
Size/kinetics, polarizability and electronegativity play a role in Nu: strength. The LG is important to a
lesser degree in E2/Sn2 than E1/Sn1. You’ll notice a that size and polarizability are in contest This is
Prediction of Elimination and Substitution Products and their
Sn2: Always undergoes 100% inversion at the chiral centre only. This may induce other chiral centres
to flip configuration, but their physical connectivity is not changed.
Sn1: Forms racemic mixtures which sometimes favor the inverted structure due to ion pairing.
Additionally, Sn1 can undergo hydride shifts, methyl shifts and resonance shifts due to formation of a
carbocation intermediate, moving the location of the + charge. This results in a mixture of products,
depending on how probable the shift is.
E2: Follows 3 rules Antiperiplanar/Anti-coplanar rule, Zaitsev's rule and E/Z Isomerism rule. The first
rule trumps the second two.
Antiperiplanar/Anti-coplanar Rule - The hydrogen to be deprotonated must be antiperiplanar (180
degrees apart) from the leaving group. In cyclic compounds only axial groups can be removed.
Zaitsev's Rule - If multiple products are possible, the the more substituted alkene is favored unless
*the base is large *the alkyl halide is an alkyl fluoride *the alkyl halide contains one or more double
bonds. In both cases the minor products occur.
E/Z Isomerism rule - If multiple products are possible, the product with Z or trans isomerism is
favoured, though minor cis products occur.
E1: This mechanism forms racemic mixtures which tend to favour Zaitsev products and trans products.
Alkene Addition Reactions
Reaction mechanisms are given under the table. Reaction mechanisms and intermediates are not
given for mechanisms you aren’t required to know, and are marked N/A. The reaction conditions
given are to favour products. Know the relationship between reversible reactions, and how reaction
conditions change to favor each side.