Organic reaction mechanisms (A2)
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What is the difference between SN1 and SN2 and in what case scenarios do they each occur?
Corrie writes ....
SN1 and SN2 reactions usually turn up at A-level in the context of the substitution reactions of halogenoalkanes, such as:
R-Cl + HO- -> ROH + Cl-
As you probably know, the 'S' stands for substitution, and the 'N' stands for nucleophilic - the OH- ion above is acting as a nucleophile.
The '1' or '2' indicates the order of the reaction, obtained from kinetic studies, and tells you about the mechanism of the reaction. The mechanism is the series of steps that take you from reactants to products. One of the steps will be the slowest (the rate determining step) and will determine the order of the reaction.
The overall reaction above could happen by two different mechanisms:
A. In a first step the polar C-Cl bond in the halogenoalkane (R-Cl) breaks, forming a carbocation, R+ and a Cl- ion. In a second step the R+ ion combines with an HO- ion to form the alcohol, R-OH.
1. R-Cl -> R+ + Cl-
2. R+ + HO- -> R-OH
The first step will be relatively slow compared to the second step which, not unexpectedly, will be fast. So the rate of this reaction will depend only on the slow first step (the rate determining step), giving a 1st order reaction, SN1.
B. The HO- approaches the electron deficient C-atom on the halogenoalkane and begins to bond to it. At the same time the C-Cl weakens as the Cl leaves. The reaction is completed in a single step involving both the halogenoalkane and the HO- ion, with a transition state halfway through the process looking something like this:
1. R-Cl + HO- -> HO - - -R- - - Cl -> R-OH + Cl-
Here the slowest step is the only step and leads to a second order reaction, where the rate = k[R-Cl][HO-]
The SN1 mechanism is favoured with tertiary halogenoalkanes, whereas primary halogenoalkanes favour the SN2 mechanism.
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updated: 14 November 2007
