Section 19.1 Acidity of -Hydrogensa 78919.1 Acidity of -Hydrogens
Hydrogen and carbon have similar electronegativities, which means that the electrons
binding them together are shared almost equally by the two atoms. Consequently, a
hydrogen bonded to a carbon is usually not acidic. This is particularly true for hydro-
gens bonded to hybridized carbons, because these carbons are the most similar to
hydrogen in electronegativity (Section 6.9). The high of ethane is evidence of the
low acidity of hydrogens bonded to hybridized carbons.
A hydrogen bonded to an hybridized carbon adjacent to a carbonyl carbon is
much more acidic than hydrogens bonded to other hybridized carbons. For example,
the for dissociation of an from an aldehyde or a ketone ranges from 16
to 20, and the for dissociation of an from an ester is about 25
(Table 19.1). Notice that, although an is more acidic than most other
carbon-bound hydrogens, it is less acidic than a hydrogen of water A
compound that contains a relatively acidic hydrogen bonded to an hybridized carbon
is called a carbon acid.
ORCH 2 CORORCH 2 CRORCH 2 CHpKa ∼ 16 − 20 pKa ∼ 25sp^31 pKa=15.7 2.a-hydrogenpKa a-hydrogenpKa a-hydrogensp^3sp^3CH 3 CH 3
pKa = 50sp^3pKasp^3A
Table 19.1 The pKa Values of Some Carbon AcidsOCH 2 CN(CH 3 ) 2HOCH 2 COCH 2 CH 3HOCH 2 CCH 3HCH 2 CNHOCH 2 CHHCH 3 CHNO 230252025178.6HN CCHC NHOOCH 3 CCHCOCH 2 CH 3HOOCCHCCH 3HOOCH 3 CCHCCH 3HOOCH 3 CCHCHHO 2 NCHNO 211.810.79.48.95.93.6HpKa pKa