Structure and Naming of Carboxylic Acids
• Carboxylic acids contain a carbonyl attached to a hydroxyl
group; this is called a carboxyl group
• Parent name ends in -oic acid
• Find longest chain containing the carboxyl group carbon
• Number C’s starting at carboxyl group carbon
• Locate and number substituents and give full name
• The smallest carboxylic acids are usually named by their
common names
• The carboxyl group takes precedence over all other groups
O
O
O
O
O
OH
H
OH
OH
OH
HO
Formic Acid
(Methanoic Acid)
Acetic Acid
(Ethanoic Acid)
3-Oxobutanoic Acid
para-Hydroxybenzoic Acid
Physical Properties of Carboxylic Acids
• Carboxylic acids are very polar due to both the carbonyl
group and the hydroxyl group
• Carboxylic acids can H-bond with each other, and in fact
exist primarily of dimers (two molecules held together by
H-bonding)
• Because of the above properties, carboxylic acids have
high boiling points (higher than corresponding alcohols)
• Those with less than 5 carbons are soluble in water
- those with more than 5 C’s can be soluble when ionized
O
C
+

-
H
O
O
H +

O
H
O
O
O
O
Na
Acidity and Salts of Carboxylic Acids
• Carboxylic acids are weak acids (partially ionize in water)
• They are stronger acids than alcohols, or even phenols, due
to the high stability of their conjugate bases (resonance)
• Carboxylic acids are neutralized by bases to form salts
• Salts of carboxylic acids are useful because they are solids
at room temperature, and most are soluble in water
O
O
+
H 2O
+
H 3O +
O
OH
O
O
O
=
O
O
O
O
O
+
+ NaOH
OH
O
Na
H 2O
Preparation of Carboxylic Acids
• Recall from chapters 14 and 15 that carboxylic acids can
be prepared by oxidation of primary alcohols or aldehydes
• Primary alcohols form acids when treated with Jones’
reagent (CrO3/H3O+), as well most other oxidizing agents
(except PCC, which forms aldehydes)
• Aldehydes can be oxidized to carboxylic acids with most
oxidizing agents, such as Tollens’reagent (AgNO3/NH3)
- alcohols do not react with Tollens
CrO3
OH
O
H3O+
O
OH
O
Tollens
H
OH
Preparation of Esters from Carboxylic Acids
• A carboxylic acid can be reacted with an alcohol to form
an ester using an acid catalyst and heat (called Fischer
esterification)
- esters have an alkoxy group attached to the carbonyl
• There are other types of esterification that we won’t study
• Fischer esterification is a type of intermolecular dehyration
• Because this is a reversible reaction, an excess of either the
acid or the alcohol is normally used to shift the equilibrium
towards products (sometimes H2O is removed as it forms)
O
Acid Cat.
O
+
+
OH
HO
Heat
O
H2O
Mechanism of Fischer Esterification
•
•
•
•
First, the carbonyl oxygen is protonated
Next, the alcohol oxygen attacks the carbonyl carbon
A proton is then transferred from the alkoxy to a hydroxyl
Finally, the proton is removed from the carbonyl oxygen, and water
is expelled
O
H
H
O
O
+
OH
H
H
O
OH
H
+
O
OH
H
O
H
+
H
H
H
O
O
O
O
OH
O
H
H
H
O
H
+
O
H
O
O
H
O
+
O
O
H
+
O
H
H
Naming Esters
•
•
•
•
•
Parent name ends in -oate
First name the alkyl group attached to the oxygen
Follow by the carboxylic acid name converted to end in -oate
For IUPAC use the IUPAC name for both alkyl group and acid
Many small esters are also know by their common names
O
O
H
O
O
Methyl formate
(Methyl methanoate)
Ethyl Acetate
(Ethyl ethanoate)
O
O
O
O
Methyl butanoate
Methyl benzoate
Physical Properties of Esters
• Because esters can’t H-bond with themselves, they have lower
boiling points than carboxylic acids and alcohols
• However, they are fairly polar, and have higher boiling points
than ethers or hydrocarbons (similar to aldehydes and ketones)
• Esters with less than 5 carbons are somewhat soluble in water
- solubility is between that of ethers and aldehydes or ketones
• Esters are not as flammable as ethers or hydrocarbons, but are
similar to alcohols and ketones
• Most esters have a pleasant smell and are often used as food
additives to simulate or enhance natural flavors
Hydrolysis of Esters
• Esters can be hydrolyzed by heating with water and an acid
catalyst (the reverse of Fischer esterification)
- the hydrolysis is favored by adding an excess of water
• Esters can also be hydrolyzed by heating with aqueous base
(called saponification, this is how soap is made)
- saponification produces a salt of the carboxylic acid
O
H3O+
O
O
O
+
Heat
NaOH
HO
OH
O
+
O
Heat
O
Na
HO
Mechanism of Base Hydrolysis of an Ester
• First, the hydroxyl group attacks the carbonyl carbon
• Next, the alkoxide ion is eliminated as the carbonyl reforms
• Finally, the alkoxide removes the proton from the acid and the
resulting carboxylate ion forms a salt with the metal ion
O
O
+
Na
OH
OH
+
O
O
Na
O
O
OH
H
+
O
O
O
O
O
H
O
+
+
O
O
Na
HO