91Ó°ÊÓ

The acidity of a-hydrogens bonded to carbons flanked by two carbonyl groups increases because the electrons left behind when the proton is removed can be delocalized onto two oxygens. $\mathbf{β}$-Diketones have lower ${\mathbf{pK}}_{\mathbf{a}}$values than $\mathbf{β}$-keto esters because electrons are more readily delocalized onto the carbonyl oxygen of a ketone than they are onto the carbonyl oxygen of an ester.

(a) A $\mathrm{β}$- keto nitrile has a CN group (nitrile) and $>\mathrm{C}=\mathrm{O}$group at the$\mathrm{Î\pm }$ position with respect to CN group. For example:

$\mathrm{β}$-Keto nitrile (a)

(b)A $\mathrm{β}$-diester has COOR (ester) group and also has another ester group at $\mathrm{β}$-position with respect to the first ester group. For example:

$\mathrm{β}$-Diester (b)

(c) A $\mathrm{β}$-keto aldehyde has CHO (aldehyde) group and keto group ($>\mathrm{C}=\mathrm{O}$) at position with respect to the aldehyde group. For example

$\mathrm{β}$-keto aldehyde (c)