What is the difference of SN1 and an SN2 mechanism?

SN1= only 1 species involved in the rate determining step = carbocation intermediate = racemic mixture = not optically active SN2= 2 species involved in the rate determining step =transition state = single optical isomer = optically active

SN1 Mechanism:---- would draw during the session...key points:-Step 1 is the RATE DETERMINING step (slow step). The bond in step one has to be polarized. The electrons move onto the electronegative atom as it is better at accommodating the negative charge.-You form a carbocation which is PLANAR (think trigional planar). So the nucleophile can attack ABOVE AND BELOW the plane.-You form two optical isomers so you have a racemic mixture. Each isomer roataes plane polarized light in equal but OPPOSITE directions so the effects CANCEL. Thus the product is NOT optically active .
SN2 Mechanism:---- would draw during the session...key points:-It is only a one step process (the rate determining step). The nucleophile attacks the opposite side to the leaving group.-You form a TRANSITION STATE. (Note- a transition state is DIFFERENT to an intermediate because it cannot be isolated)-The nucleophile approaches along a defined pathway so you only form ONE optical isomer. Thus the product is OPTICALLY ACTIVE.