Steric effects

Steric hindrance in organic chemistry referred to the influence of the spatial extent of a molecule to the course of the reaction. The term was first coined in 1894 by Victor Meyer for the observation that some reactions only very slowly or not run when in the vicinity of the reacting atoms are large and bulky groups.

A known example of the influence the steric hindrance is the reaction of ketones in a Grignard reaction. If di-tert -butyl ketone used in the reaction, the reaction is due to the very bulky tert -butyl groups as strongly slowed down so that a maximum of one methyl group can be introduced, even more remains no longer respond. This is justified by the fact that the transition state of the reaction at a very high energy level is because the bulky residues very close to each other back and there is an electrostatic repulsion of the electron shells. Thermodynamically thus a higher activation energy for the elapse of a response is required, which is not or only barely reaches under the usual reaction conditions. This is associated with a kinetic inhibition: If the reaction takes place, it is slowed down by steric hindrance.

Another observable effect is the hindrance of rotation about a single bond in a molecule. Thus, the molecule 1,1 '-bi -2-naphthol ( BINOL ) is due to the hydrogen atoms between the two rings (not shown ) is always present in a twisted conformation. It is worth noting that is prevented by the right - to the left-handed conformation due to the steric hindrance of the change, which is why these molecules are chiral, even though they have no atomic chiral center. This phenomenon is called axial chirality.

Use

The inhibitory effects of steric hindrance be used specifically in the context of multi-step syntheses. Exist in a molecule a plurality of functional groups which may be involved in a particular reaction, as these groups can be shielded by steric hindrance and thus protected. Known protecting groups are, for example, tert-butyl ester and the di -tert-butyl -methyl -silyl group.

The hindrance of the rotation about a bond is especially used in stereoselective reactions.