Carboxylic acids are a family of organic compounds characterized by the presence of the carboxyl group. The term "carboxyl" is a contraction of "carbonyl" and "hydroxyl".
 

    
Carboxylic acids are weak acids. In general, the dissociation of a carboxylic acid can be represented as:
   RCO₂H(aq) <====> RCO₂^1-(aq) + H¹⁺(aq)

In other words, carboxylic acids do not completely dissociate (typically less than 5%) into the carboxylate ion and the hydronium ion. For example, in a 1.0 molar solution of acetic acid, the acetic acid is only about 0.5% dissociated.

HC₂H₃O₂(aq) <======> H^l+(aq) + C₂H₃O₂^1-(aq)
A strong acid, such as hydrochloric acid, completely dissociates (100%) in aqueous solution: HCl(aq) -----> Cl^1-(aq) + H¹⁺(aq)

Note: The hydronium ion can be indicated as H¹⁺(aq) or H₃O¹⁺.

Like the carbonyl of aldehydes and ketones, the carboxyl group is polar.

The carboxyl group can hydrogen bond with water and with other carboxyl groups. Because of hydrogen bonding, carboxylic acids form dimers. Most carboxylic acids exist as dimers. Boiling points of carboxylic acids are higher than the alcohols with the same number of carbon atoms.

Carboxylic acids follow the rule of thumb solubility guideline. In other words, carboxylic acid molecules that do not exceed the ratio of 4 carbon atoms for every 1 oxygen atom are soluble in water.

Carboxylic acids are characterized by sharp, sour tastes and unpleasant odors.

Long chain carboxylic acids are referred to as fatty acids. Normally this includes carboxylic acids with carbon chains of 10 or more atoms.

Carboxylic acid derivatives include esters, acid chlorides, acid anhydrides, and amides.

IUPAC Nomenclature of Carboxylic Acids (Systematic Nomenclature)
Determine the parent compound, the longest continuous carbon chain bearing the carboxyl group.
    1.  Replace -e with -oic acid. If there are two carboxyl groups, the suffix -dioic is used.
    2.  Number the chain so that the carboxyl carbon is carbon number 1.
    3.  Name and number substituents in the usual way.

In the common system of nomenclature, names are derived from Latin or Greek words.   Substituted carboxylic acids are named as derivatives of the parent compound with Greek letters used to indicate the position of the substituent.

Aromatic carboxylic acids are usually named, in either system, as derivatives of benzoic acid.
Generally, the -oic acid or -ic acid suffix is attached to the appropriate prefix.
    Benzoic acid 

IUPAC and Common Names of Several Carboxylic Acids

Structure	Systematic Name	Common Name	Source
HCO2H	Methanoic	Formic	Found in ant venom.
CH3CO2H	Ethanoic	Acetic	Found in vinegar (4 to 5%).
CH3CH2CO2H	Propanoic	Propionic	Swiss cheese
CH3(CH2)2CO2H	Butanoic	Butyric	Rancid butter
CH3(CH2)3CO2H	Pentanoic	Valeric
CH3(CH2)4CO2H	Hexanoic	Caproic
CH3(CH2)6CO2H	Octanoic	Caprylic
CH3(CH2)8CO2H	Decanoic	Capric
CH3(CH2)10CO2H	Dodecanoic	Lauric	Saturated fatty acid
CH3(CH2)12CO2H	Tetradecanoic	Myristic	Saturated fatty acid
CH3(CH2)14CO2H	Hexadecanoic	Palmitic	Saturated fatty acid
CH3(CH2)16CO2H	Octadecanoic	Stearic	Saturated fatty acid
CH3-(CH2)7-CH=CH-(CH2)7-CO2H		Oleic acid	Unsaturated fatty acid

Dicarboxylic Acid Common Names

Structure	Common Name	Mnemonic
HO2CCO2H	Oxalic	Oh
HO2CCH2CO2H	Malonic	My
HO2C(CH2)2CO2H	Succinic	Such
HO2C(CH2)3CO2H	Glutaric	Good
HO2C(CH2)4CO2H	Adipic	Apple
HO2C(CH2)5CO2H	Pimelic	Pie
HO2C(CH2)6CO2H	Suberic	Sweet
HO2C(CH2)7CO2H	Azelaic	As
HO2C(CH2)8CO2H	Sebacic	Sugar

Aromatic Dicarboxylic Acids

Phthalic

Isophthalic

Terephthalic

Common Carboxylic Acids  [Note: The following information was taken from http://www.bartleby.com]

Citric acid is a colorless translucent crystalline acid, C₆H₈O₇, principally derived by fermentation of carbohydrates or from lemon, lime, and pineapple juices and used in preparing citrates and in flavorings and metal polishes.

Lactic acid is a syrupy, water-soluble liquid, C₃H₆O₃, produced in muscles as a result of anaerobic glucose metabolism, and present in sour milk, molasses, various fruits, and wines. A synthetic form of the compound is used in foods and beverages as a flavoring and preservative, in dyeing and textile printing, and in pharmaceuticals.

Adipic acid is a white crystalline dicarboxylic acid, C₆H₁₁O₄, that is derived from oxidation of various fats, slightly soluble in water and soluble in alcohol and acetone, and used especially in the manufacture of nylon and polyurethane foams.

Reactions and Preparation of Carboxylic Acids
1. Preparation by oxidation of primary alcohols and aldehydes.

                       [O]                  [O]
    R-CH₂-OH -----> RCHO ----> RCO₂H
    1^o alcohol           aldehyde     acid

It is difficult to stop the oxidation of a primary alcohol to the aldehyde.
 

2. Acid-Base Reactions / Neutralization

The reaction between a strong base and a carboxylic acid is called a neutralization.
The products of such a reaction are the carboxylate metal salt and water.

Acetic acid + Sodium hydroxide ---> Sodium acetate + Water

3.   Esterification
    Carboxylic acids react with alcohols to produce esters.

    RCO₂H + ROH ----> RCO₂R + H₂O

Esters
Esters are carboxylic acid derivatives that have the general formula of RCO₂R.
Esters generally have pleasant odors and are found in nature.

Ester Nomenclature
1.  Identify and name the alkyl group attached to the oxygen of the carboxyl group
2.  Use the name of the carboxylic acid, with the -ic ending replaced with -ate.
        Methyl acetate CH₃CO₂CH₃ or CH₃COOCH₃
        Ethyl acetate CH₃CO₂CH₂CH₃ or CH₃COOCH₂CH₃
        Ethyl butanoate CH₃CH₂CH₂CO₂CH₂CH₃
        Methyl benzoate PhCO₂CH₃
        Dimethyl malonate CH₃OOCCH₂COOCH₃

Reactions Involving Esters
1.  Preparation of Esters
    Acid + Alcohol <---> Ester + Water
    RCO₂H + R'OH ---> RCO₂R'

2.  Hydrolysis
Esters are hydrolyzed, either by aqueous acid or aqueous base.
Under acidic conditions, the carboxylic acid and alcohol are produced.
    RCO₂R' + H₃O¹⁺ ----> RCO₂H + R'OH
Under basic conditions, the salt of the acid and the alcohol are produced.
    RCO₂R' + NaOH ----> RCO₂^1-Na¹⁺ + R'OH

Base hydrolysis is called saponification.
A soap is a sodium or potassium salt of a fatty acid. A fatty acid is a long chain carboxylic acid containing 10-22 carbon atoms.  The role of soap in the removal of soil and grease is best understood by considering the functional groups in soap molecules.  The long, continuous hydrocarbon side chains in a soap molecule resemble an alkane, and they dissolve other nonpolar compounds such as oils and greases.  This part of the molecule is referred to as hydrophobic, which means "water-hating." This part of the molecule is repelled by water.  The carboxylate end of the molecule is called hydrophilic, which means "water-loving."  The hydrophobic end can also be viewed as being lipophilic that is oil-soluble.  The hydrophilic end can be viewed as being lipophobic that is oil-insoluble.  When soap is dissolved in water, the carboxylate end actually dissolves.  The hydrocarbon part is repelled by the water molecule so that a thin film of soap is formed on the surface of the water with the hydrocarbon chains protruding outward. This greatly lowers the surface tension of the water. When soap solution comes in contact with oil or grease, the hydrocarbon part dissolves in the oil or grease, but the polar carboxylate group remains dissolved in water. When particles of oil or grease are surrounded by soap molecules, the resulting "units" formed are called micelles.  Micelles repel one another because they are surrounded by the negatively charged carboxylate ions. Mechanical action (scrubbing or washing machine tumbling) causes oil and grease to be surrounded by soap molecules and broken into small droplets so that relatively small micelles are formed. These small micelles are then washed away. Careful examination of this solution shows that it is an emulsion containing suspended micelles.

Acid Chlorides
Acid chlorides are named by dropping the -ic of the acid name and replacing it with -oyl chloride.
    CH₃COCl is acetyl chloride and PhCOCl is benzoyl chloride.

Acid chlorides are produced by reacting an acid with thionyl chloride.
    RCO₂H + SOCl₂ ---> RCOCl

Acid chlorides react violently with water to produce the parent acid and HCl.
    RCOCl + Water -----> Acid + HCl

Acid Anhydrides
Acid anhydrides are named by replacing acid with anhydride.

Anhydrides are prepared by reacting the carboxylic acid with an acid chloride.

Phosphoesters and Thioesters
Alcohols react with phosphoric acid to produce a phosphate ester, or phosphoester.

Suggested Problems
15.15, 15.16, 15.24, 15.29, 15.31, 15.37, 15.39