steric hindrance

Meaning: Atoms and groups take up space, and electron clouds repel when forced close together. Crowding raises energy in two ways. In a reaction, bulky groups block the approach of a reactant and raise the energy of the transition state, so the rate drops. In a molecule, bulky groups forced near each other cause strain, as in gauche butane, eclipsed conformations, and axial substituents on cyclohexane (1,3-diaxial interactions). The larger group prefers the equatorial position.

On the MCAT: Expect SN2 rate comparisons. Methyl and primary halides react fastest, and tertiary halides react very slowly by SN2 because the nucleophile cannot reach the back side of the carbon. Branching next to the reacting carbon also slows SN2. A bulky base such as tert-butoxide tends to remove an accessible proton rather than attack carbon, so it favors E2 elimination over substitution. You may also be asked to choose the more stable chair or to explain why a reagent attacks one site and not another.

Common confusion: Hindrance does not slow every mechanism. SN1 is fastest at tertiary carbons, because the carbocation is stabilized and is planar, so the nucleophile can approach from either face. Also, a bulky base can still be strong: size lowers nucleophilicity more than it lowers basicity.