Orgo II chp. 18 - Reactions at the alpha carbon of carbonyls

a-hydrogen acidity

Weakly acidic (pka = 19-20)

Not likely to dissociate

What is the ion that results with a hydrogen dissociating

Enolate ion

Enolate resonance structures

• DB on O and neg. charge on a-C

• DB on a-C and neg. charge on O

Conjugate base of enolate ion

Hydrogen can be added to O or a-C

Rations of keto and enol form

Enol form is extremely minor and DB on O for keto is highly favored

When will enol be favored

when conjugation is possible

ex. Pentane - 2,4 - dione is favored in enol form

Different types of a- halogenation

• Base-promoted

• Acid-promoted

• Hell-Volhard-Zelinski rxn

• Harpp Halogenation

Effect on B carbonyl on a-C hydrogen

Increases acidity

EWG makes H more likely to dissociate

Weaker bases can react with these enolates

B carbonyl effect on enolate resonance

Increases resonance through both carbonyls

Useful B-dicarbonyls

• Ethyl Acetoacetate

• Diethyl malonate