Organic Reaction Mechanisms.
2. Haloalkane reaction with electron rich species.
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The role of the electron rich species in these reactions is to provide electrons to the carbon substrate, the halogen leaves as the halide ion. If the electron rich species reacts directly at the carbon bearing the halogen, then a substitution occurs (S), the incoming atom or group replacing the halogen on the carbon. If the electron rich species reacts with a hydrogen on the carbon adjacent to the carbon carrying the halogen (the beta-carbon) then the electron flow is from the beta-C-H bond into a pi bond which provides electrons to the carbon bearing the halogen. In this case the attacked hydrogen (as H+) and the halogen (X-) have both been lost and a carbon-carbon double bond formed in an elimination (E) reaction.
These two reactions can occur in either a one step or a two step process.
The one step processes involves the incoming electron rich species reacting with the haloalkane as the halogen simultaneously leaves. There is no intermediate formed, and both reacting species are involved in the one step: the reaction is bimolecular.
If the electron rich species is not a strong base, and if the carbon bearing the halogen is reasonably accessible to the attacking group, then a nucleophilic substitution can take place. (The electron rich species being known as a nucleophile.) The reaction mechanism is termed SN2, substitution, nucleophilic, bimolecular.
If the electron rich species acts as a base, and particularly if it is bulky and has difficulty approaching the carbon bearing the halogen, then attack can take place at the more readily available beta-hydrogens. In this reaction the beta hydroge and the halogen are lost simultaneously and a carbon,carbon double bond forms between the carbons losing the atoms. The resulting alkene has been produced by the elimination of the equivalent of a small molecule, HX. The reaction mechanism is termed E2, elimination, bimolecular.
The unimolecular substituion and elimination reactions occur with the same first step, producing a carbocation intermediate, differing in the second step as to the fate of the carbocation.
The loss of the halide ion to form the carbocation is the rate determining step in each case and hence the unimolecular kinetics.
If the carbocation is generated in the absence of any base, then the major product will be the addition of a nucleophile at the carbocation centre. Rearrangement of the first formed carbocation may occur before this happens.
In the presence of a base, even a weak one, the carbocation will lose a beta proton and a carbon,carbon double bond will form between the cation centre and that beta carbon. Again, rearrangement of the first formed carbocation may take place before the proton is lost.
Date created: 2005 06 24.