Steric effects

Steric effects are nonbonding interactions that influence the shape (conformation) and reactivity of ions and molecules.

Steric repulsive forces between overlapping electron clouds result in structured groupings of molecules stabilized by the way that opposites attract and like charges repel.

Steric hindrance is responsible for the observed shape of rotaxanes and the low rates of racemization of 2,2'-disubstituted biphenyl and binaphthyl derivatives.

Relative rates of chemical reactions provide useful insights into the effects of the steric bulk of substituents.

It is defined as the solid angle formed with the metal at the vertex and the hydrogen atoms at the perimeter of the cone (see figure).

In organic chemistry, steric effects are nearly universal and affect the rates and activation energies of most chemical reactions to varying degrees.

In biochemistry, steric effects are often exploited in naturally occurring molecules such as enzymes, where the catalytic site may be buried within a large protein structure.

The parent cyclobutadiene (R = H) readily dimerizes but the R = tert -butyl derivative is robust. [ 1 ]
Regioselective dimethoxytritylation of the primary 5'- hydroxyl group of thymidine in the presence of a free secondary 3'-hydroxy group as a result of steric hindrance due to the dimethoxytrityl group and the ribose ring (Py = pyridine ). [ 2 ]
The A-value for a methyl group is 1.74 as derived from the chemical equilibrium above. It costs 1.74 kcal/mol for the methyl group to adopt to the axial position compared to the equatorial position.
Ligand cone angle.
The steric effect of tri-(tert- butyl ) amine makes electrophilic reactions, like forming the tetra alkyl ammonium cation, difficult. It is difficult for electrophiles to get close enough to allow attack by the lone pair of the nitrogen (nitrogen is shown in blue)