steric hindrance

Try to thread a needle while wearing thick mittens — the bulk of the gloves gets in the way, no matter how steady your hand. Atoms feel the same kind of crowding. Steric hindrance is the slowing or blocking of a reaction caused by groups physically getting in each other's way, taking up space and preventing reactants from reaching where they need to go. It is about size and shape, not about charge or electronics.

In substitution, steric hindrance is the single biggest factor deciding whether an SN2 reaction can happen. The SN2 nucleophile must reach the back of the reacting carbon. If that carbon carries only small hydrogens (a methyl or primary substrate), the back door is wide open and SN2 is fast. Add bulky alkyl groups and they crowd the approach: a secondary carbon is partly blocked and reacts slowly, and a tertiary carbon — ringed by three groups — is so shielded that SN2 essentially cannot occur. Hence the SN2 reactivity order methyl > primary > secondary >> tertiary. The same crowding also slows the SN2 transition state, where the carbon is briefly bonded to five things.

Tellingly, steric hindrance affects SN1 hardly at all in the same way — in fact, bulky tertiary substrates favor SN1 (their carbocations are the most stable, and ionizing relieves crowding). So the two pathways respond oppositely to a crowded substrate: piling on bulk shuts down SN2 but opens the door to SN1. Steric hindrance also drives a bulky base toward elimination rather than substitution, since reaching a peripheral hydrogen is easier than reaching a buried carbon. It is one of the four levers (substrate, nucleophile, leaving group, solvent) that steer the outcome.