|Lecture Notes: 15 June||
IUPAC Nomenclature: Alkyl halides are named just as alkanes, alkenes and alkynes with the halide considered as a substituent.
Haloalkanes are also known as alkyl halides as common names. In the common naming system the alkyl group is named with the halogen named with an -ide ending as in inorganic compounds. Thus chloroethane becomes ethyl chloride, chloroethene becomes vinyl chloride, 2-chloro-2methylpropane becomes t-butyl chloride, etc. A number of halogenated solvents are also known by common names, including: methylene chloride (CH2Cl2), chloroform (CHCl3), methyl chloroform (CH3CCl3).
Nucleophilic substitution reactions are of wide occurrence and importance. First some terms:
We'll first look at substitution reactions with alkyl halides as a model reaction system. If we react a variety of alcohols with HX, two slightly different types of reactions may occur.
For example, if we react a chiral tertiary alcohol under neutral or acidic conditions with, say HBr, we get a racemic mixture of the haloalkane:
For this reaction we find that the product is achiral, and the rate of the reaction depends only on the concentration of the alcohol, [HBr] does not affect the rate (its first order).
On the other hand, in Walden's classic reaction of 1896, a chiral secondary alcohol, Malic acid, gave a chiral product:
Note that only the first of these reactions causes an inversion. The rate of the first reaction also depends not only on the concentration of the alcohol, it also depends on the concentration of PCl5 (its second order). So what's going on with these reactions? Let's look at the malic acid reaction first.
In considering a possible mechanism for this reaction we might note the following:
With these thoughts in mind let's consider a mechanism:
This is called an SN2 reaction (Substitution Nucleophilic 2nd order). Looked at mechanism with protonation. Reaction coordinate diagram.
Last modified 15 June 2004