University of Kentucky
Identify the electrophilic atom
An electrophilic C atom is one that is attached to a leaving group. Typical leaving groups are:
- most often, I, Br, or Cl;
- a sulfonate such as tosylate (OTs), mesylate (OMs), or triflate (OTf);
- if the conditions are acidic, OH, OR (R = Me, Et, etc.), or O2CR such as OAc;
- occasionally, a positively charged group that can break off to make a small, neutral molecule such as N2, SMe2, NMe3, or OMe2.
The OH group is sometimes converted into a better leaving group in a first step before the substitution or elimination is executed:
- treatment with TsCl, MsCl, or Tf2O and a base (often pyridine) gives OTs, OMs, or OTf;
- treatment with SOCl2 or PBr3 gives Cl or Br.