Q.Compare SN1 and SN2 reaction mechanisms.

Step 1. Kinetics. SN2's rate depends on BOTH the alkyl halide and the nucleophile's concentration (second order overall, Rate = k2[RX][Nu-]); SN1's rate depends ONLY on the alkyl halide's concentration (first order overall, Rate = k[RX]), since the nucleophile plays no part in the slow step.

Step 2. Number of steps. SN2 is a single, concerted step -- nucleophile attack and halide departure happen together, with one transition state and no discrete intermediate. SN1 is two steps -- a slow ionisation to a carbocation, then a fast nucleophilic attack on that cation.

Step 3. Stereochemistry. SN2 proceeds by backside attack (the nucleophile approaches opposite the leaving group), inverting the configuration at the reacting carbon (Walden inversion) -- a single, defined stereochemical outcome. SN1 proceeds through a planar, sp2 carbocation that can be attacked from either face with roughly equal probability, so a chiral substrate gives a racemic (optically inactive) mixture. …

Compare the two mechanisms point by point across the same four criteria every time: rate law/order, number of steps, stereoch …

• Mixing up which substrate favours which mechanism -- it is specifically the STERIC bulk of a tertiary carbon that blocks SN2's backside attack (favouring SN1 instead), while that same bulk happens to stabilise the SN1 carbocation e …