CHE 230

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.

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