Salt effect
A salt effect (also called a kinetic salt effect) is observed when soluble electrolytes ( salts ) are added to a reaction mixture and can influence the reaction rate.
Positive salt effect
This salt effect is often a purely electrostatic effect, comparable to the stabilizing effect of solvating molecules of a polar solvent on the (ionic) transition state of a reaction - mostly an S N 1 reaction. This reaction-accelerating salt effect is particularly important in S N 1 reactions in solvents with a rather low polarity. Reaction accelerating acts z. B. the addition of a salt that contains a non-nucleophilic anion (e.g. perchlorate ).
Negative salt effect (self-ion effect)
There are also cases in which the ions of the added salt delay the progress of a substitution reaction. If the following reaction takes place as an S N 1 reaction, the addition of similar anions (X - ) can slow down the formation of R – Nu, since the regression of the starting material R – X is accelerated:
| Nucleophilic substitution : the addition of "own ions" (X - ) slows down the reaction | ||||
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| Haloalkane + nucleophile –––––> substitution product + halide | ||||
This influence of "own ions" (in the example X - ) is also called the common-ion effect .
See also
Individual evidence
- ^ A b Ivan Ernest: Binding, Structure and Reaction Mechanisms in Organic Chemistry , Springer-Verlag, 1972, p. 111, ISBN 3-211-81060-9 .
- ^ Ulrich Lüning: Organic reactions , 2nd edition, Elsevier GmbH, Munich, 2007, p. 42, ISBN 978-3-8274-1834-0 .