8.3 Reactions Involving Enols

8.3 Reactions Involving Enols

  • There is a plausible mechanism for the following tautomerization.
    • A piece of paper is needed to record your answer.
  • The alpha carbon has a p bond on the enol.
  • The ability of a ketone to tautomerize is needed in order for the attack to occur.
    • Not every ketone will be in equilibrium with an enol.
  • A typical ketone will be in equilibrium with an enol if it has alpha protons.
    • In the previous section, we saw cases where the equilibrium can favor the enol, but in general, the equilibrium favors the ketone.
    • You will only have trace amounts of the enol in equilibrium with the ketone.
  • A small amount of enol is able to react as a nucleophile.
    • The equilibrium of the keto-enol is re-established by the production of more enol after the attack on the electrophile.
    • Slowly but surely, most of the ketone molecules convert into enols.
  • Let's look at how this happens.
    • A small amount of enol is generated by the tautomerizing of the ketone.
  • Most of the steps in this mechanism are only for protons.
    • The pattern of our mechanism is tautomerize, attack, and deprotonate.
    • "tautomerize" is a new name for a combination of two transfer steps.
    • When the enol attacks the electrophile is the only step where an attack takes place.
  • CH COOH can be used as a mild acid to facilitate tautomerization.
  • carboxylic acids are slower to react in this kind of reaction, so we don't have to worry about that.
  • The carboxylic acid needs to be converted into an acid halide.
    • The enol of an acid halide will attack a halogen very quickly.
  • The Hell-Volhard-Zelinsky reaction is a strategy for halogenating carboxylic acids.
  • In this section, we have seen two reactions that exploit the nature of enols.
  • We used some reagents.
    • The reagents were in acidic conditions.
  • H O is followed by br and pbr.
  • In the last step, water is used to convert the acid into a carboxylic acid.
  • We are starting with a ketone and subjecting it to acidic conditions.
  • Our product will have a br at one of the alpha positions.