SN2 - Second-order Nucleophilic Substitution
Substrate Effects in Reactions
The rate of a reaction is frequently influenced to a significant degree by the spatial shape of the substrate. In reactions, the reaction rate depends considerably on the reaction center's degree of substitution. The influences on the reaction rate that arise from the substituent's spatial structure are called steric effects. Additionally, the reaction rate may be controlled by the substituent's ability to stabilize (delocalize) electrical charge (electronic effect).
|Alkyl halide||Structure||Class||Relative reaction rate|
|tert-butyl bromide||Tertiary||negligible small|
The cause of the reaction rate's large decrease in the halogen exchanges with different alkyl bromides is the increasing steric shielding of the central carbon. In an reaction, the nucleophile has to approach the central carbon from the side opposite to the leaving group (here: bromide). This approach is more sterically hindered the more alkyl substituents the central carbon carries. In the three-dimensional, interactive molecular models below, the steric effects of alkyl substituents are illustrated, in particular, when the spacefill presentation is activated.
Due to extremely low steric shielding by hydrogen atoms, methyl halides are particularly reactive in reactions. Primary substrates react considerably more rapidly than secondary and tertiary substrates do. In tert-butyl bromide, the reaction center is almost completely blocked by the three methyl substituents. Therefore, the reaction with tert-butyl bromide can only proceeds extremely slowly.
The connection between the reaction rate of reactions and the degree of substitution of the substrate's reaction center is depicted by the following illustration:
In connection with secondary and tertiary alkyl compounds, a competition between the reaction, on the one hand, and the reaction or elimination, on the other hand, is to be expected. Particularly in the case of tertiary alkyl compounds, the reaction is usually completely superseded by the reaction or elimination. In reactions and eliminations, the reaction rate is influenced to a much smaller degree by steric effects. The reaction rate is rather controlled by electronic effects. In other words, the more the reaction center's positive charge, which appears in the transition state, is stabilized by substituents, the higher the reaction rate is, as well. Therefore, in contrast to the reaction, the reaction rate of an reaction and an elimination usually increases with the reaction center's degree of substitution.