The functional group of carboxylic acids consists of a C═O with —OH bonded to the same carbon.
Carboxyl group is usually written —COOH.
Aliphatic acids have an alkyl group bonded to —COOH.
Aromatic acids have an aryl group.
Fatty acids are long-chain aliphatic acids.
20-2 Nomenclature of Carboxylic Acids
Many aliphatic acids have historical names.
Positions of substituents on the chain are labeled with Greek letters starting at the carbon attached to the carboxylic carbon
Remove the final -e from alkane name, add the ending -oic acid.
The carbon of the carboxyl group is #1.
Remove the final -e from alkene name, add the ending -oic acid.
Stereochemistry is specified
Aromatic acids are named as derivatives of benzoic acid.
Ortho-, meta- and para- prefixes are used to specify the location of a second substituent.
Numbers are used to specify locations when more than 2 substituents are present.
A dicarboxylic acid (also called a diacid) is a compound with two carboxyl groups. The common names of simple dicarboxylic acids are used more frequently than their systematic names.
A common mnemonic for these names is “Oh my, such good apple pie,” standing for oxalic, malonic, succinic, glutaric, adipic, and pimelic acids.
Substituted dicarboxylic acids are given common names using Greek letters, as with the simple carboxylic acids. Greek letters are assigned beginning with the carbon atom next to the carboxyl group that is closer to the substituents
Benzenoid compounds with two carboxyl groups are named phthalic acids.
Phthalic acid itself is the ortho isomer. The meta isomer is called isophthalic acid, and the para isomer is called terephthalic acid.
Aliphatic dicarboxylic acids are named simply by adding the suffix -dioic acid to the name of the parent alkane. For straight-chain dicarboxylic acids, the parent alkane name is determined by using the longest continuous chain that contains both carboxyl groups. The chain is numbered beginning with the carboxyl carbon atom that is closer to the substituents, and these numbers are used to give the positions of the substituents.
20-3 Structure and Physical Properties of Carboxylic Acids
The sp2 hybrid carbonyl carbon atom is planar, with nearly trigonal bond angles.
The O—H bond also lies in this plane, eclipsed with the C═O bond.
The sp3 oxygen has a C—O—H angle of 106°.
Higher boiling points than similar alcohols, due to the formation of a hydrogen-bonded dimer.
Aliphatic acids with more than 8 carbons are solids at room temperature.
Double bonds (especially cis) lower the melting point. The following acids all have 18 carbons:
Stearic acid (saturated): 72°C
Oleic acid (one cis double bond): 16°C
Linoleic acid (two cis double bonds): -5°C
Water solubility decreases with the length of the carbon chain. With up to 4 carbons, acid is miscible in water. Very soluble in alcohols.
Also soluble in relatively nonpolar solvents like chloroform because the hydrogen bonds of the dimer are not disrupted by the nonpolar solvent
20-4 Acidity of Carboxylic Acids
A carboxylic acid may dissociate in water to give a proton and a carboxylate ion.
The equilibrium constant Ka for this reaction is called the acid-dissociation constant.
The acid will be mostly dissociated if the pH of the solution is higher than the pKa of the acid.
Values of are about pKa 5 1Ka = 10-5) for simple carboxylic acids. For example, acetic acid has a pKa of 4.7 (Ka = 1.8 x 10-5 ).
Energy Diagram of Carboxylic Acids and Alcohols
Stability of carboxylate ions. Carboxylic acids are more acidic than alcohols because carboxylate ions are more stable than alkoxide ions. A carboxylate ion has its negative charge delocalized over two oxygen atoms, compared with only one oxygen atom bearing the negative charge in an alkoxide ion.
Acetate Ion Structure