SN

watch a molecule turn inside out like an umbrella.

tap the three groups to build a crowded or open carbon, then pull the nucleophile in yourself and watch which mechanism you built

A nucleophile attacking a carbon has to approach from directly opposite the group that is leaving -- the backside -- because that is the only angle where the old bond can break exactly as the new one forms. If nothing blocks that angle, the attack happens in one step and the molecule inverts completely, like an umbrella catching a gust: this is SN2. Pack three bulky groups around the same carbon and the backside is physically walled off, so the leaving group has to leave first, on its own, forming a flat carbocation with an open face on both sides -- so the nucleophile attacks from either face with equal odds, and you get a fifty-fifty mix of both mirror-image outcomes. This is SN1. Which one happens is decided entirely by what is already standing at the back door.

Concepts: chemistry, atomic-structure.

An interactive you play with one thumb, on Wundr.