Steric effects are nonbonding interactions that influence the shape and reactivity of ions and molecules. Steric effects complement electronic effects, which usually dictate shape and reactivity. Steric effects result from repulsive forces between overlapping electron clouds. Steric effects are widely exploited in applied and academic chemistry.
Steric hindrance
Steric hindrance is a consequence of steric effects. Steric hindrance is the slowing of chemical reactions due to steric bulk. It is usually manifested in intermolecular reactions, whereas discussion of steric effects often focus on intramolecular interactions. Steric hindrance is often exploited to control selectivity, such as slowing unwanted side-reactions. Steric hindrance between adjacent groups can also affect torsional bond angles. Steric hindrance is responsible for the observed shape of rotaxanes and the low rates of racemization of 2,2'-disubstituted biphenyl and binaphthyl derivatives.
Measures of steric properties
Because steric effects have profound impact on properties, the steric properties of substituents have been assessed by numerous methods.
s provide another measure of the bulk of substituents. A values are derived from equilibrium measurements of monosubstituted cyclohexanes. The extent that a substituent favors the equatorial position gives a measure of its bulk. 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.
is a measure of the steric properties of the monomers that comprise a polymer. is the temperature where the rate of polymerization and depolymerization are equal. Sterically hindered monomers give polymers with low 's, which are usually not useful.
Steric effects are critical to chemistry, biochemistry, and pharmacology. 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. In pharmacology, steric effects determine how and at what rate a drug will interact with its target bio-molecules. makes electrophilic reactions, like forming the tetraalkylammonium cation, difficult. It is difficult for electrophiles to get close enough to allow attack by the lone pair of the nitrogen
