Substitution and Elimination Reaction Mechanisms
Leaving Group
Leaving group is the group on the substrate that leaves. In the case of an alkyl halide, this
is the halide ion that leaves the carbon atom when the nucleophile attacks. The tendency
of the nucleophile to leave is
Relative Reactivity of Leaving GroupsI-> Br-> Cl->> F-Fluoride ions are very poor leaving groups because they bond very strongly and are very
rarely used in alkyl halide substitution reactions. Reactivity of a leaving group is related
to its basicity with stronger bases being poorer leaving groups.
Solvent
The solvent can play an important role inSN2 reactions, particularly inSN2 involving
secondary alkyl halide substrates, where it can be the determining factor in mechanism.
Solvent can also have a great effect on reaction rate ofSN2 reactions.
TheSN2 mechanism is preferred when the solvent is an aprotic, polar^4 solvent. That is, a
solvent that is polar, but without a polar hydrogen. Polar, protic solvents would include
water, alcohols, and generally, solvents with polar NH or OH bonds. Good aprotic, polar
solvents are HMPA^5 , CH 3 CN, DMSO^6 , and DMF^7.
A polar solvent is preferred because it better allows the dissociation of the halide from the
alkyl group. A protic solvent with a polar hydrogen, however, forms a ’cage’ of hydrogen-
bonded solvent around the nucleophile, hindering its approach to the substrate.
Relative Reactivity of SolventsHMPA > CH 3 CN > DMF > DMSO >> H 2 O56.1.3 SN1 Reactions
TheSN1 mechanism is very different from theSN2 mechanism. In some of its preferences,
its exactly the opposite and, in some cases, the results of the reaction can be significantly
different.
Like theSN2 mechanism, structure plays an important role in theSN1 mechanism. The
role of structure in theSN1 mechanism, however, is quite different and because of this, the
reactivity of structures is more or less reversed.
Reactivity Due to Structure of SN1CH 3 X < RCH 2 X << R 2 CHX < R 3 CX4 Chapter 6 on page 23
5 https://en.wikipedia.org/wiki/Hexamethylphosphoramide
6 https://en.wikipedia.org/wiki/Dimethyl%20sulfoxide
7 https://en.wikipedia.org/wiki/Dimethylformamide