Download 18_01_06rw

Chapter 18
Carboxylic Acids
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
18.1
Carboxylic Acid Nomenclature
Table 18.1
Systematic IUPAC names replace "-e"
ending of alkane with "oic acid"
Systematic Name
O
HCOH
methanoic acid
O
CH3COH
ethanoic acid
O
CH3(CH2)16COH
octadecanoic acid
Table 18.1
Common names are based on natural
origin rather than structure.
Systematic Name Common Name
O
HCOH
methanoic acid
formic acid
ethanoic acid
acetic acid
octadecanoic acid
stearic acid
O
CH3COH
O
CH3(CH2)16COH
Table 18.1
Systematic Name Common Name
O
CH3CHCOH 2-hydroxypropanoic acid
lactic acid
O
OH
CH3(CH2)7
(CH2)7COH
C
H
C
H
(Z)-9-octadecenoic acid
oleic acid
or (Z)-octadec-9-enoic acid
18.2
Structure and Bonding
Formic Acid is Planar
O
H
C
120 pm
O
H
134 pm
Electron Delocalization
Stabilizes carbonyl group
R
C
•• •
O•
•• O ••
R
+
C
•• •–
O•
••
•• O ••
H
R
C
•• •–
O•
••
+ O ••
H
H
18.3
Physical Properties
Boiling Points
O
bp (1 atm): 31°C
80°C
OH
O
OH
99°C 141°C
Intermolecular forces, especially hydrogen
bonding, are stronger in carboxylic acids than
in other compounds of similar shape and
molecular weight.
Hydrogen-bonded Dimers
O
H
O
CCH3
H3CC
O
H
O
Acetic acid exists as a hydrogen-bonded
dimer in the gas phase. The hydroxyl group
of each molecule is hydrogen-bonded to the
carbonyl oxygen of the other.
Hydrogen-bonded Dimers
Acetic acid exists as a hydrogen-bonded
dimer in the gas phase. The hydroxyl group
of each molecule is hydrogen-bonded to the
carbonyl oxygen of the other.
Solubility in Water
Carboxylic acids are similar to alcohols in respect
to their solubility in water.
They form hydrogen bonds to water.
H
O
H
O
H3CC
H
O
H
O
H
18.4
Acidity of Carboxylic Acids
Most carboxylic acids have a pKa close to 5.
Carboxylic Acids are Weak Acids
But carboxylic acids are far more acidic than alcohols.
O
CH3COH
CH3CH2OH
pKa = 4.7
pKa = 16
Free Energies of Ionization
CH3CH2O– + H+
G°= 64 kJ/mol
G°= 91 kJ/mol
O
CH3CO– + H+
G°= 27 kJ/mol
O
CH3CH2OH
CH3COH
Greater Acidity of Carboxylic Acids is Attributed
Stabilization of Carboxylate Ion by
Inductive effect of carbonyl group
O
–
RC O
+
Resonance stabilization of carboxylate ion
••
O ••
RC
•• –
O ••
••
•• –
•O•
• •
RC
O ••
••
Figure 18.3(b): Electrostatic Potential Maps of
Acetic Acid and Acetate Ion
Acetic acid
Acetate ion
18.5
Substituents and Acid Strength
Substituent Effects on Acidity
standard of comparison is acetic acid (X = H)
O
X
CH2COH
pKa = 4.7
Substituent Effects on Acidity
O
X
Alkyl groups have
negligible effect
CH2COH
Electronegative groups
increase acidity
X
pKa
X
pKa
H
4.7
H
4.7
CH3
4.9
F
2.6
CH3(CH2)5
4.9
Cl
2.9
Substituent Effects on Acidity
O
X
CH2COH
Electronegative substituents withdraw
electrons from carboxyl group; increase K for
loss of H+.
Effect of electronegative substituent decreases
as number of bonds between it and
carboxyl group increases.
pKa
CH3CH2CHCO2H
2.8
Cl
CH3CHCH2CO2H
4.1
Cl
ClCH2CH2CH2CO2H
4.5
18.6
Ionization of
Substituted Benzoic Acids
Hybridization Effect
O
pKa
COH
4.2
O
H2C
HC
CH
COH
O
4.3
C
COH
1.8
sp2-hybridized carbon is more electronwithdrawing than sp3, and sp is more
electron-withdrawing than sp2.