Steric hindrance

Steric hindrance is destabilization or slowed reaction caused when atoms or groups occupy overlapping regions of space. It can alter molecular shape, reactivity, and selectivity.

Van der Waals radius: A measure of an atom’s effective size based on its nonbonded contacts with other atoms. Atomic size determines when nonbonded groups approach closely enough to create steric repulsion.
Nucleophilic substitution: A reaction in which a nucleophile replaces a leaving group attached to an electrophilic atom. Crowding near the electrophilic carbon can strongly impede concerted substitution.
Molecular geometry: The three-dimensional arrangement of atoms in a molecule. Steric effects depend on the spatial arrangement of atoms, not merely their connectivity.
Electronic effect: A change in molecular behavior caused by the distribution or movement of electrons. Chemical trends attributed to hindrance may instead result from altered electron distribution.
Conformational analysis: The study of molecular shapes produced by rotation around single bonds and their relative energies. Rotations can relieve or intensify crowding, changing which molecular conformations are favored.
Esterification: A reaction that forms an ester, commonly by combining a carboxylic acid with an alcohol. Bulky reactants can slow ester formation by obstructing approach to the carbonyl group.
Noncovalent interaction: An interaction between molecules or parts of molecules that does not involve forming a covalent bond. Steric repulsion is one contribution to nonbonded interactions between nearby groups.
Aromaticity: The unusual stability of cyclic, conjugated molecules with delocalized electrons. Aromatic stabilization can outweigh destabilization from crowded substituents.
Transition state: A high-energy arrangement of atoms at the boundary between reactants and products along a reaction pathway. Crowding can raise a transition state's energy and slow the reaction.
Chiral ligand: A ligand with a nonsuperimposable mirror-image structure that coordinates to a metal center. Its spatial bulk helps control which face of a substrate reacts in asymmetric catalysis.

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