140
ACIDS AND BASES
H 3 C
C
O
R
O
H 3 C
C
O
R
O
delocalization of oxygen
lone pair into π system
H 3 C
C
O
R
O
H 3 C
C
O
R
O
H
H 3 C
C
O
R
O
H
H 3 C
C
O
R
O
H
lone pairs in sp
2 orbitals
lone pairs in sp
3 orbitals
resonance
stabilization
no resonance
stabilization
pK a about −6
R = H, carboxylic acid
R = alkyl/aryl, ester
This is a consequence of delocalization, with resonance stabilization being possible when the carbonyl
oxygen is protonated, but not possible should the OR
oxygen become protonated. This additional resonance
stabilization is not pertinent to aldehydes and ketones,
which are thus less basic than the carboxylic acid
derivatives. However, these oxygen derivatives are
still very weak bases, and are only protonated in the
presence of strong acids.
In the case of the sulfur analogues thioesters and
thioacids, this delocalization is much less favourable.
In the oxygen series, delocalization involves overlap
between the oxygen sp
3 orbital and the π system
of the carbonyl, which is composed of 2p orbitals.
Delocalization in the sulfur series would require
overlap between a sulfur 3p orbital and a carbon 2p
orbital, which is much less likely because of the size
difference between these orbitals.
H 3 C
C
S
R
O
H 3 C
C
S
R
O
resonance of this type is less favourable in the sulfur
esters and acids due to the larger S atom, and less
orbital overlap
Amidines are stronger bases than amines. The pK a
for acetamidine is 12.4.
N
C
Me
N
H
N
C
Me
NH 2
H
H
N
C
Me
NH 2
H
H
pK a 12.4
sp
2 orbital
sp
3 orbital
H
H
acetamidine
(ethanamidine)
Amidines are essentially amides where the carbonyl
oxygen has been replaced with nitrogen, i.e. they
are nitrogen analogues of amides. It is the nitrogen
replacing the oxygen that becomes protonated. This
is easily rationalized, even though the hydridization
here is sp
2 , which in theory should be less basic than
the sp
3 -hybridized nitrogen. Protonation of the imine
nitrogen allows resonance stabilization in the cation,
which could not happen if the amide nitrogen were
protonated. In addition, the two resonance structures
both have charge on nitrogen, and in fact are identical.
We have a similar situation in the carboxylate anion.
Amidines, therefore, are quite strong bases, with the
potential for electron delocalization being a greater
consideration than the hybridization state of the
orbital housing the lone pair.
ACIDS AND BASES
H 3 C
C
O
R
O
H 3 C
C
O
R
O
delocalization of oxygen
lone pair into π system
H 3 C
C
O
R
O
H 3 C
C
O
R
O
H
H 3 C
C
O
R
O
H
H 3 C
C
O
R
O
H
lone pairs in sp
2 orbitals
lone pairs in sp
3 orbitals
resonance
stabilization
no resonance
stabilization
pK a about −6
R = H, carboxylic acid
R = alkyl/aryl, ester
This is a consequence of delocalization, with resonance stabilization being possible when the carbonyl
oxygen is protonated, but not possible should the OR
oxygen become protonated. This additional resonance
stabilization is not pertinent to aldehydes and ketones,
which are thus less basic than the carboxylic acid
derivatives. However, these oxygen derivatives are
still very weak bases, and are only protonated in the
presence of strong acids.
In the case of the sulfur analogues thioesters and
thioacids, this delocalization is much less favourable.
In the oxygen series, delocalization involves overlap
between the oxygen sp
3 orbital and the π system
of the carbonyl, which is composed of 2p orbitals.
Delocalization in the sulfur series would require
overlap between a sulfur 3p orbital and a carbon 2p
orbital, which is much less likely because of the size
difference between these orbitals.
H 3 C
C
S
R
O
H 3 C
C
S
R
O
resonance of this type is less favourable in the sulfur
esters and acids due to the larger S atom, and less
orbital overlap
Amidines are stronger bases than amines. The pK a
for acetamidine is 12.4.
N
C
Me
N
H
N
C
Me
NH 2
H
H
N
C
Me
NH 2
H
H
pK a 12.4
sp
2 orbital
sp
3 orbital
H
H
acetamidine
(ethanamidine)
Amidines are essentially amides where the carbonyl
oxygen has been replaced with nitrogen, i.e. they
are nitrogen analogues of amides. It is the nitrogen
replacing the oxygen that becomes protonated. This
is easily rationalized, even though the hydridization
here is sp
2 , which in theory should be less basic than
the sp
3 -hybridized nitrogen. Protonation of the imine
nitrogen allows resonance stabilization in the cation,
which could not happen if the amide nitrogen were
protonated. In addition, the two resonance structures
both have charge on nitrogen, and in fact are identical.
We have a similar situation in the carboxylate anion.
Amidines, therefore, are quite strong bases, with the
potential for electron delocalization being a greater
consideration than the hybridization state of the
orbital housing the lone pair.