Introduction to Carboxylic Acids

James Richard Fromm

Carboxylic acids are named systematically from their corresponding alkanes by changing the ending -ane to -oic acid.  Methane thus gives rise to methanoic acid, HCOOH, which is more widely known by its trivial name formic acid, while ethane gives rise to ethanoic acid, which is much more commonly called acetic acid.  Carboxylic acids have an easily ionizable acidic hydrogen.

Many carboxylic acids are known by both systematic and trivial names.  Those of some of the simplest carboxylic acids are given in the Table below.  Trivial names followed by an asterisk in the Table below are considered obsolete and their use is discouraged.

Table: Names of the Simplest Carboxylic Acids
Alkane Compound Systematic Name Trivial (Common) Name
Methane (CH4) HCOOH Methanoic Acid Formic Acid
Ethane (C2H6) CH3COOH Ethanoic Acid Acetic Acid
Propane (C3H8) C2H5COOH Propanoic Acid Propionic Acid*
Butane (C4H10) C3H7COOH Butanoic Acid Butyric Acid*
Pentane (C5H12) C4H9COOH Pentanoic Acid Valeric Acid*
Hexane (C6H14) C5H11COOH Hexanoic Acid Caproic Acid*
Heptane (C7H16) C6H13COOH Heptanoic Acid Enanthic Acid*

The -COOH group is characteristic of carboxylic acids and is called a carboxyl group.  Below is the structural formula of ethanoic acid.  These diagrams identify the location of the double bond.

Ethanoic2.gif (552 bytes) ethanoic.gif (1034 bytes)
Ethanoic Acid

Carboxylic acids containing long unbranched carbon chains are called fatty acids.  Fatty acids are major components of the fats or lipids found in living organisms.  Although over 70 different fatty acids are known, most have an even number of between 12 and 24 carbon atoms.  Of these, the most common and most important have either 16 or 18 carbon atoms and may contain a few double bonds.   Fatty acids are usually known by their trivial names, which are given in the Table below.

Table: Systematic and Trivial Names of Simple Fatty Acids
Systematic Name Trivial (Common) Name Carbon Atoms
n-dodecanoic acid Lauric Acid 12
n-tetradecanoic acid Myristic Acid 14
n-hexadecanoic acid Palmitic Acid 16
n-hexadecan-9-enoic acid Palmitoleic Acid 16
n-octadecanoic acid Stearic Acid 18
n-octadecan-9-enoic acid Oleic Acid 18
n-octadecan-9,12-dienoic acid Linoleic Acid 18
n-octadecan-9,12,15-trienoic acid Linolenic Acid 18
n-eicosanoic acid Arachidic Acid 20
n-eicosan-5,8,11,14-tetraenoic acid Arachidonic Acid 20
n-tetracosanoic acid Lignoceric Acid 22

The organic acids with double acid groups or containing other substituents are usually known by their trivial names, which are given in the Table below.

Table: Names of Other Selected Organic Acids
Compound Systematic Name Trivial (Common) Name
HOOCCOOH 2-Ethanedioic Acid Oxalic Acid
HOOCC2COOH 3-Propanedioic Acid Malonic Acid
HOOC(CH2)2COOH 4-Butanedioic Acid Succinic Acid
HOOC(CH2)3COOH 5-Pentanedioic Acid Glutamic Acid
HOOC(CH2)4COOH 6-Hexanedioic Acid Adipic Acid
HOOCCH=CHCOOH cis-butenedioic Acid Maleic Acid
HOOCCH=CHCOOH trans-butenedioic Acid Fumaric Acid
HOOCCH(OH)CH(OH)COOH 2,3-Dihydroxybutanedioic Acid Tartaric Acid
HOC(CH2COOH)2COOH 3-Hydroxy-3-carboxylpentanedioic Acid Citric Acid
HOCH2CH2COOH 1-Hydroxypropanoic Acid Lactic Acid
C6H5CH=CHCOOH 3-Benzyl-2-propanoic Acid Cinnamic Acid
C6H5COOH 1-Carboxyl-1,3,5-cyclohexatriene Benzoic Acid
H3CCOCOOH 2-Ketopropanoic Acid Pyruvic Acid

It is neither necessary nor practical to remember all of these trivial names, although practicing organic chemists remember many of them.  Trivial names, these and others, are very widely used in both organic and biological chemistry.

The melting and boiling points are higher for carboxylic acids than the melting and boiling points of alkanes due to the polarity of the carboxyl group.  As with alcohols short carboxylic acids dissolve in water, but long carboxylic acids do not dissolve well.   When etanoic acid is dissolved in water the hydrogen atom of the carboxyl group becomes the cation H+ and H3CCOO- becomes an anion.  Carboxylic acids (organic acids) are weaker than inorganic acids due to the the low degree of dissociation.

H3CCOOH + NaOH arrow2.gif (113 bytes) H3CCOONa + H2O

When ethanoic acid (H3CCOOH) reacts with caustic soda (NaOH) sodium acetate (H3CCOONa) and water are produced. sodium acetate is a salt of the ethanoic acid and can be used as a soap.  Often salts of the carboxylic acids with long chains are used as soaps.

Methanoic acid (Formic acid):  It smells very strong and is corrosive.   The melting point is 8 degrees with a boiling point of 100 degrees.  The name formic acid is comes from the Latin, formica which means ant.  This acid was discovered by the distillation of ants.  A bite of an ant can be painful due to the presence of this acid.  It is the most simple and strongest carboxylic acid.

Ethanoic acid (Acetic acid):  This acid also smells strong and is corrosive, but less so than methanoic acid.  As shown above ethanoic acid is commonly called acetic acid.  Vinegar (Fr. Vin, wine + Egar, sour) means sour wine.  A 5% solution of ethanoic acid (acetic acid) and water is known as vinegar.  The melting point of ethanoic acid is 16 degrees with a boiling point of 118 degrees. Ethanoic acid originates when ethanol (grain alcohol) is oxidized (reacts with oxygen).

CH3CH2OH + O2 arrow2.gif (113 bytes) CH3COOH + H2O
Ethanol + Oxygen arrow2.gif (113 bytes) Ethanoic Acid + Water

Ethanoic acid often originates in wine by means of the above reaction causing the wine to become sour.  Acetic acid in the form of vinega is used as a preserving agent and in the textile industry.

Mulitple cation carboxylic acids, hydroxycarboxylic acids, aromatic carboxylic acids, amino acids:  There are carboxylic acids such as oxalic acid (COOH)2 which have more than one carboxyl group.  There are also hydroxycarboxylic acids which not only have a carboxyl group but also possess a hydroxy group.  Tartaric acid or dihydroxylbutanidioic acid has two carboxyl groups and two hydroxyyl groups.   Citric acid and lactic acid are also hydroxycarboxylic acids.  There are also acids produced from benzene known as aromatic acids.  The simplest is benzoic acid.   One of the hydrogen atoms from benzene is replaced by a carboxyl group.   Benzoic acid has the formula C6H5COOH.  Replacing the next hydrogen atom with a hydroxyl group produces salicylic acid.  An ester (to be discussed later)  of ethanoic acid and the hydroxyl group of salicylic acid produces acetylsalicylic acid or aspirin.  Certain amino acids possess  carboxyl groups, hydroxyl groups and benzene rings in addition to the ever present  amino group (NH2 group).  The amino acids are important genetic components.  Proteins are made up of approximately twenty amino acids. Glutamic acid, sodium glutamate and potassium glutamate (both salts of the glutamic acid) are used in the food industry.

citric2.gif (1077 bytes) citric.gif (1498 bytes)
Citric Acid
Aspirin2.gif (1191 bytes) aspirin.gif (1455 bytes)
Acetylsalicylic Acid (Aspirin)
Glutamic.gif (1027 bytes)

Glutamic Acid

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Copyright 1997 James R. Fromm - Revised May, 1998