Q.SN1 and SN2 mechanism of substitution reaction ?

Step 1. Kinetics. SN2 shows second-order kinetics -- rate depends on the concentration of BOTH the substrate and the nucleophile (rate = k[R-X][Nu-]) -- because both species are involved in the single, rate-determining step. SN1 shows first-order kinetics -- rate depends ONLY on the substrate's concentration (rate = k[R-X]) -- because the nucleophile plays no part in the slow, rate-determining step.

Step 2. Number of steps and the transition state / intermediate. SN2 is a single-step mechanism: bond-breaking (C-X) and bond-forming (C-Nu) happen simultaneously, through a crowded, pentacoordinate transition state in which the nucleophile attacks from directly opposite the leaving group (backside attack). SN1 is a two-step mechanism: Step 1 (slow) is heterolysis of the C-X bond on its own, forming a flat, sp2-hybridised carbocation intermediate; Step 2 (fast) is attack of the nucleophile on that carbocation, from either face.

Step 3. Stereochemical outcome. At a chiral carbon, SN2 always inverts the configuration (Walden inversion), because the nucleophile can only approach from the one side opposite the leaving group. SN1 gives a nearly racemic product mixture (racemisation), because the planar carbocation intermediate is equally open to attack from either face. …

Working through the four standard axes of comparison -- kinetics, mechanism/steps, stereochemistry, and favouring f …

• Mixing up which order (1st vs 2nd) belongs to which mechanism.
• Saying SN2 gives 'racemisation' and SN1 gives 'inversion' -- this is backwards; SN2 inverts cleanly …