This is a Nucleophilic Substitution reaction where there are 2 components in the rate determining step. It occurs simultaneously as bonds are made and broken at the same time. During the backside attack, a nucleophile attacks the electrophile from the back and causes an inverse in configuration. This attack causes a leaving group to leave. A transition state is formed while this reaction occurs, which is when the leaving group and nucleophile are attached at the same time.
What is SN1?
This is a Nucleophilic Substitution reaction which is unimolecular. First, there is a loss of a leaving group (typically, the leaving group should be very stable). This loss forms a carbocation intermediate. A carbocation has a carbon atom with