Steric effects

Steric effects are influences on molecular structure, stability, or reactivity caused by spatial crowding between atoms or groups. They arise when atoms’ occupied regions constrain how molecules can arrange or react.

Steric hindrance: Reduced access or reactivity at a molecular site because nearby atoms or groups occupy space. This is the direct kinetic effect of crowding near a reaction site.
Van der Waals radius: A measure of an atom’s effective size based on its nonbonded contacts with other atoms. Atomic radii help estimate when nonbonded groups are crowded together.
Steric number: The total number of atoms bonded to a central atom plus its lone pairs in a simple electron-domain model. Unlike steric effects, steric number is a counting rule used to predict geometry.
Inductive effect: A shift in electron density transmitted through sigma bonds because of electronegativity differences. It changes reactivity through bond polarization rather than physical crowding.
A-value: A measure of the energetic preference for an equatorial rather than axial substituent on a cyclohexane ring. A-values quantify a substituent’s steric penalty in the axial cyclohexane position.
Torsional strain: An increase in molecular energy caused by repulsion between bonds on adjacent atoms. Crowding can force bonds into eclipsed conformations, adding torsional strain.
Noncovalent interaction: An interaction between molecules or parts of molecules that does not involve forming a covalent bond. Steric repulsion is one contributor to the broader family of noncovalent interactions.
Asymmetric catalysis: Catalysis that preferentially produces one stereoisomer from a reaction. Catalysts use differently crowded environments to favor one product configuration.
Resonance effect: A change in electron distribution caused by conjugation among adjacent orbitals. Resonance alters electron distribution, while steric effects arise from spatial constraints.
1,3-diaxial interaction: A repulsive interaction between an axial substituent on cyclohexane and axial groups on carbons three atoms away. This is the main source of axial strain measured by substituent A-values.

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