Hi everybody, just wanted to give out a quick tip when looking at SN1 reactions. Many students believe that SN1 reactions give totally racemic products, but this is not actually the case.
We all know that the SN1 reaction proceeds through a carbocation intermediate, which cannot hold the stereochemistry that the carbon atom previously possessed. (In other words, if you start with a chiral carbon and go through a cation, you will have a trigonal, achiral cation)
However, this is not the whole story. Many times, the leaving group can come back to the cation on the same side. This partially blocks one face of the cation. The net result is that your carbocation CANNOT be equally attacked from both sides. What you get out is both R and S at that carbon, but not in equal ratios. This means that the SN1 reaction will lose some chirality, but will not usually become totally racemic.
Good luck on those exams and happy reacting.