Download Chapter 19 Carboxylic Acids

Carboxylic Acids
Structure and Bonding
Electron Delocalization
R
C
•• •
O•
•• O ••
R
+
C
•• •–
O•
••
•• O ••
H
H
Electron Delocalization
R
C
•• •
O•
•• O ••
R
+
C
•• •–
O•
••
R
•• O ••
H
+ O ••
H
stabilizes
C
•• •–
O•
••
carbonyl group
H
Formic acid is planar
O
H
C
120 pm
H
O
134 pm
Carboxylic Acid Nomenclature
NOMENCLATURE
systematic IUPAC names replace "-e" ending
of alkane with "oic acid"

Systematic Name
O
HCOH
methanoic acid
O
CH3COH
O
CH3(CH2)16COH
ethanoic acid
octadecanoic acid
NOMENCLATURES

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
NOMENCLATURE
Systematic Name Common Name
O
CH3CHCOH
OH
2-hydroxypropanoic
acid
O
CH3(CH2)7
(CH2)7COH
C
H
lactic acid
C
H
(Z)-9-octadecenoic
acid
oleic acid
Physical Properties
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

form hydrogen bonds to water
H
O
H
O
H3CC
H
O
H
O
H
Greater acidity of carboxylic acids is attributed
stabilization of carboxylate ion by
inductive effect of carbonyl group
O
–
RC O
d+
resonance stabilization of carboxylate ion
••
O ••
RC
•• –
O ••
••
•• –
•O•
• •
RC
O ••
••
Dicarboxylic Acids
Dicarboxylic Acids
O
HOC
O
COH
1.2
Malonic acid
2.8
Heptanedioic acid
4.3
O
HOC(CH2)5COH

Oxalic acid
O
HOCCH2COH
O
pKa
O
one carboxyl group acts as an electronwithdrawing group toward the other; effect
decreases with increasing separation
Reactions of Carboxylic Acids
Acidity
Reduction
Esterification
Reaction
with Thionyl Chloride
Reactions of Carboxylic Acids
New reactions in this chapter

a-Halogenation

Decarboxylation

But first we revisit acid-catalyzed esterification
to examine its mechanism.
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
Ka = 1.8 x 10-5
pKa = 4.7
Ka = 10-16
pKa = 16
Substituent Effects on Acidity
O
X

CH2COH
X
Ka
pKa
H
1.8 x 10-5
4.7
F
2.5 x 10-3
2.6
Cl
1.4 x 10-3
2.9
electronegative substituents increase acidity
Salts of Carboxylic Acids
Carboxylic acids are neutralized by strong bases
O
RCOH +
stronger
acid


O
HO–
RCO– +
H2O
weaker
acid
equilibrium lies far to the right; K is ~ 1011
as long as the molecular weight of the
acid is not too high, sodium and potassium
carboxylate salts are soluble in water
Micelles

unbranched carboxylic acids with 12-18 carbons
give carboxylate salts that form micelles in water
O
ONa
sodium stearate
(sodium octadecanoate)
O
–
CH3(CH2)16CO Na+
Micelles
O
ONa
nonpolar




polar
sodium stearate has a polar end (the carboxylate
end) and a nonpolar "tail"
the polar end is "water-loving" or hydrophilic
the nonpolar tail is "water-hating" or hydrophobic
in water, many stearate ions cluster together to form
spherical aggregates; carboxylate ions on the
outside and nonpolar tails on the inside
A micelle
a -Halogenation of Carboxylic Acids
O
R2CCOH + X2
H
O
R2CCOH
+ HX
X

analogous to a-halogenation of aldehydes and
ketones

key question: Is enol content of carboxylic
acids high enough to permit reaction to occur
at reasonable rate? (Answer is NO)
Reactions
O
CH3CH2CH2COH
O
Br2
P
CH3CH2CHCOH
Br
(77%)

a-Halogen can be replaced by
nucleophilic substitution
Decarboxylation of Carboxylic Acids
Simple carboxylic acids do not decarboxylate
readily.
O
RH + CO2
RCOH
But malonic acid does.
O
O
HOCCH2COH
150°C
O
CH3COH +
CO2
Carboxylic Acid Derivatives
Nucleophilic Acyl Substitution
Acyl Halides
O
RC
X

name the acyl group and add the word
chloride, fluoride, bromide, or iodide as appropriate

acyl chlorides are, by far, the most frequently
encountered of the acyl halides
Acyl Halides
O
acetyl chloride
CH3CCl
O
H2C
CHCH2CCl
3-butenoyl chloride
O
F
CBr
p-fluorobenzoyl bromide
Acid Anhydrides
O O
RCOCR'
when
both acyl groups are the same, name the
acid and add the word anhydride
when
the groups are different, list the names of
the corresponding acids in alphabetical order and
add the word anhydride
Acid Anhydrides
O O
CH3COCCH3
acetic anhydride
O O
C6H5COCC6H5
benzoic anhydride
O O
C6H5COC(CH2)5CH3
benzoic heptanoic anhydride
Some anhydrides are industrial chemicals
O O
O
O
CH3COCCH3
O
Acetic
anhydride
Phthalic
anhydride
O
O
O
Maleic
anhydride
Esters
O
RCOR'
name as alkyl alkanoates
 cite the alkyl group attached to oxygen first (R')
 name the acyl group second; substitute the suffix
-ate for the -ic ending of the corresponding acid

Esters
O
CH3COCH2CH3
ethyl acetate
O
CH3CH2COCH3
methyl propanoate
O
COCH2CH2Cl
2-chloroethyl benzoate
Esters are very common natural products
O
CH3COCH2CH2CH(CH3)2
3-methylbutyl acetate
also
called "isopentyl acetate" and "isoamyl
acetate"
contributes to characteristic odor of bananas
Lactones
Lactones
Formed
are cyclic esters
by intramolecular esterification in a
compound that contains a hydroxyl group and
a carboxylic acid function
Examples
O
HOCH2CH2CH2COH
4-hydroxybutanoic acid


O
+
O
H2O
4-butanolide
IUPAC nomenclature: replace the -oic
acid ending of the carboxylic acid by -olide
identify the oxygenated carbon by number
Common names
b
a
O
b
g
O
g-butyrolactone



a
g
O
d
O
d-valerolactone
Ring size is designated by Greek letter
corresponding to oxygenated carbon
A g lactone has a five-membered ring
A d lactone has a six-membered ring
Amides having an NH2 group
O
RCNH2


identify the corresponding carboxylic acid
replace the -ic acid or -oic acid ending by amide.
Amides having an NH2 group
O
CH3CNH2
acetamide
O
(CH3)2CHCH2CNH2
3-methylbutanamide
O
CNH2
benzamide
Amides having substituents on N
O
RCNHR'
O
and
RCNR'2
name the amide as before
 precede the name of the amide with the name of
the appropriate group or groups
 precede the names of the groups by the letter N(standing for nitrogen and used as a locant)

Amides having substituents on N
O
N-methylacetamide
CH3CNHCH3
O
CN(CH2CH3)2
N,N-diethylbenzamide
O
CH3CH2CH2CNCH(CH3)2
CH3
N-isopropyl-N-methylbutanamide
Nitriles
RC
N
add the suffix -nitrile to the name of the parent
hydrocarbon chain (including the triply bonded
carbon of CN)
 or: replace the -ic acid or -oic acid name of the
corresponding carboxylic acid by -onitrile
or: name as an alkyl cyanide (functional class
name)

Nitriles
CH3C
C6H5C
N
N
CH3CHCH3
C
N
ethanenitrile
or: acetonitrile
or: methyl cyanide
benzonitrile
2-methylpropanenitrile
or: isopropyl cyanide