Q.Explain the mechanism of the SN1 reaction, highlighting the stereochemistry behind it.

Step 1. SN1 proceeds in two distinct steps. The FIRST step, slow and rate-determining, is heterolytic ionisation of the C-X bond: the halogen departs with both bonding electrons, leaving behind a carbocation.

Step 2. This carbocation is sp2 hybridised and PLANAR -- three sigma bonds in one plane, with an empty p-orbital perpendicular to that plane carrying two equal, symmetric lobes (one above the plane, one below).

Step 3. The SECOND step, fast, is nucleophilic attack on this planar cation. Because the empty p-orbital has two equivalent lobes, the nucleophile can attack from EITHER face with essentially equal probability -- there is no preference for one side over the other, unlike SN2's single, defined backside approach.

Step 4. If the original halide was chiral (with the leaving halogen sitting on the stereocentre), attack from one face regenerates the original spatial arrangement (retention) while attack from the other face gives the inverted arrangement -- and since both happen with roughly equal likelihood, the product is an equal (50:50) mixture of both enantiomers. …

Walk through the two SN1 steps in order, emphasising the PLANAR, two-equal-lobed geometry of the intermediate carbocation, since that geometry is exactly what fo …

• Describing SN1 as giving 'inversion' like SN2 -- SN1's defining stereochemical outcome is RACEMISATION (a mixture of both inverted and retained product, in equal amounts), not a si …