Organic Chemistry

790 CHAPTER 19 Carbonyl Compounds III

Why is a hydrogen bonded to an hybridized carbon that is adjacent to a carbonyl

carbon so much more acidic than hydrogens bonded to other hybridized carbons?

An is more acidic because the base formed when the proton is removed

from the is more stable than the base formed when a proton is removed from

other hybridized carbons, and acid strength is determined by the stability of the

conjugate base that is formed when the acid gives up a proton (Section 1.17).

Why is the base more stable? When a proton is removed from ethane, the electrons

left behind reside solely on a carbon atom. Because carbon is not very electronegative,

a carbanion is relatively unstable and therefore difficult to form. As a result, the of

its conjugate acid is very high.

When a proton is removed from a carbon adjacent to a carbonyl carbon, two factors

combine to increase the stability of the base that is formed. First, the electrons left be-

hind when the proton is removed are delocalized, and electron delocalization increas-

es the stability of a compound (Section 7.6). More important, the electrons are

delocalized onto an oxygen, an atom that is better able to accommodate the electrons

because it is more electronegative than carbon.

PROBLEM 1

The of propene is 42, which is greater than the of the carbon acids listed in

Table 19.1, but less than the of an alkane. Explain.

Now we can understand why aldehydes and ketones are more acidic

than esters The electrons left behind when an is removed from

an ester are not as readily delocalized onto the carbonyl oxygen as are the electrons left

behind when an is removed from an aldehyde or a ketone. Because a lone

pair on the oxygen of the OR group of the ester can also be delocalized onto the carbonyl

oxygen, the two pairs of electrons compete for delocalization onto oxygen.

Nitroalkanes, nitriles, and N,N-disubstituted amides also have relatively acidic

(Table 19.1) because in each case the electrons left behind when the proton

is removed can be delocalized onto an atom that is more electronegative than carbon.

CH 3 CH 2 NO 2 CH 3 CH 2 CN CH 3 CN(CH 3 ) 2

O

nitroethane

pKa = 8.6

propanenitrile

pKa = 26

N,N-dimethylacetamide

pKa = 30

a-hydrogens

a-hydrogen

1 pKa= 252. a-hydrogen

1 pKa=16–20 2

pKa

pKa pKa

RCH C

O

R

H

RCH C

O

R RCH

−

C

O−

R

resonance contributors

electrons are better

accomodated on O

than on C

+ H+

delocalized

electrons

CH 3 CH 2 + H+

−

CH 3 CH 3

localized electrons

pKa

sp^3

a-carbon

a-hydrogen

sp^3

sp^3

RCH C

O

OR

−

−−

+

RCH C

O

OR

−

RCH C

O

OR

delocalization of

the lone pair on

oxygen

delocalization of

the negative charge

on the -carbon

resonance contributors