carboxylic acids, esters, and other acid derivatives chapter 16
TRANSCRIPT
Carboxylic acids, esters, and other acid derivatives
Chapter 16
Structure of carboxylic acids and their derivatives
• The functional group present in a carboxylic acid is a combination of a carbonyl group and a hydroxyl group; however, the resulting carboxyl group ( -COOH) possesses properties that are unlike those present in aldehydes/ketones and alcohols.
carbonyl group hydroxyl group carboxyl group
C O H
O
O HC
O
Structure of carboxylic acids and their derivatives
• Carboxylic acids have the following general formula:
• Some simple carboxylic acids:
• Since carbon can have only four bonds, there are no cyclic carboxylic acids (i.e. the carboxyl group cannot form part of a carbon ring)
CR O H
O
formic acidIUPAC: methanoic acid
acetic acidIUPAC: ethanoic acid IUPAC: benzoic acid
O H
O
CCH 3 O H
O
CH O H
O
C
Structure of carboxylic acids and their derivatives
• The following molecules have a similar structure to carboxylic acids, and will be encountered in this chapter and the next.
carboxylic acid acid chloride acid anhydride amideester
Ch-16 Ch-16 Ch-16 Ch-16 Ch-17
OR'C
O
R C N H 2
O
RR
O
OC
O
RC Cl
O
RC O H
O
R
IUPAC nomenclature for carboxylic acids
• For monocarboxylic acids (one –COOH group):– Select the longest, continuous carbon chain that involves the
carboxyl group. This is the parent chain and the –COOH carbon is designated as C-1.
– Name the parent chain by dropping the “e” from the corresponding alkane name and changing to “oic acid”
– Indicate the identity and location of substituents on the parent chain at the front of the carboxylic acid’s name
Butanoic acid
2-Methylpropanoic acid3,3-Dibromobutanoic acid
3,5-Dichlorobenzoic acid
C H 3
C H 3
C H
O
C
OH
Cl
Cl
O H
O
Br
Br
C H 3 O H
O
CC H 2CC H 3 C H 2 O H
O
CC H 2
Benzoic acid
IUPAC nomenclature for carboxylic acids
• Dicarboxylic acids:– For these compounds, both ends of a chain will end with a
–COOH group. The parent chain is the one that involves both –COOH groups.
– The parent chain is named as an alkane and the term “dioic acid” is added afterwards to indicate the diacid structure.
(Succinic acid) Bromosuccinic acid)
Butanedioic acid Bromobutanedioic acid
Br O
O H
O
OH
O
O H
O
OH
Common names for carboxylic acids
Common names for dicarboxylic acids
Common names for carboxylic acids
• For common-name carboxylic acids and diacids, substituents are often numbered using a Greek system:
• So the following molecule could be called -Methylpropionic acid (or, using the IUPAC system, 2-Methylpropanoic acid)
12345carbon number
Greek letter
C O H
O
C H 2C H 2C H 2C H 3
CH
CH 3
C O H
O
CH 3
Polyfunctional carboxylic acids
• Carboxylic acids that contain other functional groups besides the –COOH group are called polyfunctional carboxylic acids. Some examples are shown below:
an unsaturated acid a hydroxy acid a keto acid
C
O
C C C O H
O
CC C
O H
C O H
O
C
O
CCC O H
Polyfunctional carboxylic acids
• Unsaturated acids possess a unit of unsaturation (double/triple C-C bond) in the structure. Three of the most common unsaturated acids are shown:
Unsaturated acids
acrylic acidIUPAC: Propenoic acid
maleic acidIUPAC: cis-Butenedioic acid
fumaric acidIUPAC: trans-Butenedioic acid
HOOC
COOHHOOC COOH
C
H
C
H
C
H
C
H
C
O
C HC H 2 O H
a metabolic acid
Polyfunctional carboxylic acids
• Four of the simplest hydroxy acids:
Hydroxy acids
malic acidhydroxysuccinic acid
IUPAC: Hydroxybutanedioic acid
lactic acid-hydroxypropionic acid
IUPAC: 2-Hydroxypropanoic acid
glycolic acidhydroxyacetic acid
IUPAC: 2-Hydroxyethanoic acid
tartartic acid,-dihydroxysuccinic acid
IUPAC: 2,3-Dihydroxybutanedioic acid
C HC H 3
O H
C O H
O
C H 2
O H
C O H
O
C H
O H
COH
O
C H
O H
C O H
O
COH
O
C H 2 C H
O H
C O H
O
citric acid
C
O H
O
CCOH
O O H
C O H
O
Polyfunctional carboxylic acids
• For keto acids, a carbonyl group is present as part of a carbon chain that involves a carboxyl group:
For IUPAC naming of this structure, the O-atom of the carbonyl group is treated as an “oxo” substituent and the molecule is called 2-Oxopropanoic acid
Keto acids
pyruvic acid
C
O
C H 3 C O H
O
C
O
CH3 C O H
O
Oxo-group
Metabolic carboxylic acids
• Polyfunctional acids are intermediates in metabolic reactions that occur as food is processed to obtain energy.
• Eight key intermediates in these processes are derived from only three carboxylic acids (propanoic, butanoic, and pentanoic acids, see below)
Physical properties of carboxylic acids
• Carboxylic acids are the most polar functional group we have seen so far. The presence of the carbonyl group next to the OH causes the O-H bond to be even more polar.
alcohols carboxylic acids
O HR O HC
O
R
Physical properties of carboxylic acids
• Because of the very polar –COOH group, carboxylic acids exhibit strong intermolecular attractions.
• As expected, carboxylic acids of a given number of carbon atoms have higher boiling points than alcohols.
• Carboxylic acids also tend to dimerize, producing molecules that are twice as heavy which have enhanced London forces (and thus still higher boiling points).
two H-bonds between two molecules
"dimer"
O
R
O
H
H
O
R
O
Physical properties of carboxylic acids
• In terms of water-solubility, because of H-bonding, carboxylic acids dissolve well in water (up to 4-carbon chains).
• Beyond 4 carbons, water-solubility drops off rapidly.
Preparation of carboxylic acids
• We saw in Ch-15 that carboxylic acids can be prepared from aldehydes (which can be prepared from primary alcohols):
• Aromatic carboxylic acids can be made by oxidizing alkyl-substituted aromatic molecules:
1o alcohol
[O] [O]
aldehyde carboxylic acid
HO
O
CRH
O
CRHOR
K2Cr2O7
H2SO4
2CO2 3H2O
O H
O
CC H 3C H 2C H 2
+ +
Acidity of carboxylic acids
• When carboxylic acids are placed in water, they undergo de-protonation as discussed in Ch-10:
H2O - H3O+
carboxylate ion hydroniumcarboxylic acid
O
O
CRHO
O
CR + +
Remember from Ch-10:HA + H2O A- + H3O+
Acidity of carboxylic acids
oxalic acidIUPAC: Ethanedioic acid
oxalate ionIUPAC: Ethanedioate ion
2H2O -
H2O -
-
H3O+
2H3O+
acetate ionIUPAC: Ethanoate ion
acetic acidIUPAC: Ethanoic acid
O
O
C
O
C OOH
O
C
O
C O H
O
O
CCH 3O H
O
CCH 3
+
+
+
+
Acidity of carboxylic acids
Carboxylic acid salts
• When carboxylic acids are reacted with strong bases, they are converted to salts as follows:
Na+
Na+
base
base sodium acetateIUPAC: Sodium ethanoate
salt
water
water
NaOH
NaOH
-
-
H2O
H2O
acetic acidIUPAC: Ethanoic acid
carboxylic acid
CH 3
O
C O H CH 3
O
C O
O
O
CRHO
O
CR
+
+
+
+
Carboxylic acid salts
• Salts of carboxylic acids are much more water-soluble than the acids themselves. Also, they can be converted back to the acid form by reacting them with a strong acid:
strong acid
Na+
sodium acetateIUPAC: Sodium ethanoate
-
salt
HCl NaCl
acetic acidIUPAC: Ethanoic acid
O
O
CCH 3 CH 3
O
C O H ++
Uses of carboxylic acid salts
• Because of their enhanced solubility in water compared to the acid form, many drugs and medicines that possess acid groups are marketed as carboxylic acid salts (sodium or potassium salts).
Benzoic acid
sorbic acid
sodium sorbateNa+
K+
-
-
Potassium benzoate
OC H 3 C H C H C H C H C
O
O
O HCC HC HC HC HC H 3
O
O H O
O
frogurt
Structure of esters
• Esters are carboxylic acid derivatives having an alkoxy group instead of a hydroxyl group.
estercarboxylic acid
R'OC
O
RO HC
O
R
Preparation of esters
• Esters are prepared by condensation reactions involving carboxylic acids and alcohols. Such reactions are called esterification reactions:
• This an ester consists of an acid portion and an alcohol portion:
H2O
alcoholcarboxylic acid ester
O
O
CROHHO
O
CR R'R' ++
acidportion
alcoholportion
R'OC
O
R
Reaction is encouraged by thepresence of excess alcohol
(Le Chatelier’s Principle)
Preparation of esters
• Cyclic esters (lactones) are created from hydroxy acids (bear both a hydroxyl group and a carboxyl group) in an intramolecular esterification reaction:
1234 H2O
-hydroxypropionic acid -lactone
2
3 4
1
O
O
O H
O H
O
CC H 2C H 2C H 2 +
-lactone-lactone
O
O
O
O
Preparation of esters• Indicate the structures of the carboxylic acid and alcohol that
are needed to make each of the following esters:
OCH 3
C
O
CH 3
O
C
O
C
O
O
O
C
O
C
O
O
ester carboxylic acid + alcohol
OH
CH 3C
O H
O
CH 3
+
H+
H+
H+
H+
H+
OHC
O H
O
+
OHC
O H
O
+
CH 3
O H OH
CH 3C
OH
C
O H
OO
+ +
H2O +
H2O +
H2O +
H2O +
H2O +
Nomenclature for esters• Thinking of an ester in terms of an “alcohol portion” and a “carboxylic acid
portion” is important for naming esters using the IUPAC system:1. The name for the alcohol portion comes first: name the alkyl part of
the alcohol (e.g., for the ester shown below, the first part of the ester’s name is methyl (alcohol part comes from methanol). Present the alkyl name separate from the remainder of the ester name.
2. The carboxylic acid portion is named as if it were deprotonated, changing the “-ic acid” part of that name to “-ate”
derives frompropanoic acid
derives frommethanol
O C H 3C
O
C H 2C H 3
Methyl propanoate
O CH3C
O
CH2CH3
This part would be called “propanoate”
Nomenclature for esters
• Some other examples:
Ethyl butanoate
Ethyl 2-methylbutanoateEthyl -methylbutyrate
2-Butyl butanoateSec-Butyl butanoate
O
O
O
O
C H 2
O
C
C H 3
C H 2
C H 3 C H 2
O
O
O
2-Methylpropyl butanoateIsobutyl butanoate
Selected common esters• Flavor/fragrance agents
Selected common esters
• Pheromones:
• Medications:
alarm pheromone for honeybee sexual attractant for canines
C H 3O
O
C H 3 C
C H 3
C H 3C HC H 2C H 2O
O
CC H 3
benzocaine Aspirin
C H 3
O
CO
O H
O
CH 3NH 2 C H 2O
O
C
Selected common esters
• Synthesis of Aspirin
• Synthesis of oil of wintergreen:
acetic acidsalicylic acid
H+H2O
heat
Aspirin
O H
O
C H 3
O
CO
O H
O
O H C H 3
O
C
OH
+ +
salicylic acid
H+H2O
heatMethanol
Aspirin
C H 3
O
O
O H
O H
O
O H
C H 3
OH
+ +
Isomerism in carboxylic acids and esters
• Recall that constitutional isomers are molecules that share the same formula but differ in their atom-to-atom connectivities.
• Three kinds of constitutional isomers (in the order we encountered them):– positional isomers– skeletal isomers– functional group isomers
• Carboxylic acids and esters that have a given number of carbon atoms form another example of functional group isomers:
C4H8O2
functionalgroup
isomers
Butanoic acid Methyl propanoate
OC H 3 C
O
C H 2 C H 3C
O
C H 2 C H 3C H 2OH
Isomerism in carboxylic acids and esters
• For both carboxylic acids and esters, skeletal isomers are possible:
Butanoic acid 2-Methylpropanoic acid
skeletalisomers
C H
C H 3
C H 3C
O
OHC H 2
C H 3C H 2
C
O
OH
Methyl butanoate Methyl 2-methylpropanoate
skeletalisomers
C H
C H 3
O
C H 3 C H 3C
O
O
C H 3 C H 2
C H 3C H 2
C
O
(carboxylic acids)
(esters)
Isomerism in carboxylic acids and esters
• Positional isomers are possible for esters, but not carboxylic acids.
Methyl butanoate Ethyl propanoate
positionalisomers
C H 2
C H 3 O C H 2
C H 3C
O
O
C H 3 C H 2
C H 3C H 2
C
O
Physical properties of esters
• Because they don’t possess OH groups, esters cannot form H-bonds with other ester molecules. As a result, esters have lower boiling points than carboxylic acids and alcohols that have approximately the same molar mass.
• Water molecules can H-bond to esters, at the oxygen atoms. This makes low molecule weight esters water-soluble.
Chemical reactions of esters
• Ester hydrolysis: the hydrolysis of an ester is accomplished by reacting water with the ester in the presence of an acid catalyst (this is the reverse reaction of esterification).
• An example:
carboxylic acidalcoholester
H+
H2OO HR
O
COHR
O
CO R'R' +
H+
H2O
Methyl propanoate Methanol Propanoic acid
O
C H 3C H 2COHO HC H 3C H 3
O
C H 3CH 2CO +
Chemical reactions of esters
• Ester saponification: another hydrolysis reaction, but this time, under basic conditions. Rather than a carboxylic acid, the acid salt is produced here.
• Example:
-Na+
carboxylic acid saltalcoholester
NaOH
H2OO
O
CO HRR
O
CO R'R' +
2-Propyl propanoate 2-Propanol Sodium propanoate
-Na+NaOH
H2OC H 3C H 2O H
C H 3
C H
C H 3
C H 3
C H 3
C H C H 3C H 2 O
O
C
O
CO +