In the SN1 reaction, leaving group leaves (taking the bonding electrons) in the first step of the reaction. The intermediate formed in this step is a planar carbocation. More substituted carbocations are more stable, so this reaction is faster for more substituted alkyl groups. The stereochemistry around the carbon is racemized in the course of the reaction, since attack of the nucleophile from either side of the planar carbocation is equally likely.