Organic Chemistry - Part IX

Organic acids
Organic compounds having functional group –COOH behave like acids and hence they are known as organic acids. The functional group –COOH is called the carboxyl group and thus the organic acids are also called as carboxylic acids. The bonds in a carboxyl group are such that the C has a double bond to O and a single bond to which the hydroxyl group OH is attached. The simplest carboxylic acid is methanoic acid HCOOH. This is commonly known as formic acid. Since it has one carbon atom,  the prefix “meth” is used for it from the IUPAC nomenclature. The next acid with two carbon atoms is called ethanoic acid CH₃COOH. Its common name is acetic acid. The IUPAC name has prefix “eth” attached to the ethanoic acid because it has two carbon atoms.

The carboxyl acids with functional group –COOH attached to alkane molecule forms a homologous series. Two hydrogen atoms of one of the carbon atoms in an alkane are replaced simultaneously by an oxygen atom and a hydroxyl group, to make a carboxylic acid. The oxygen atom is double bonded to the carbon atom. Thus general formula for carboxylic acid is R-COOH where R is an alkyl group, like methyl CH₃, ethyl C₂H₅, propyl C₃H₇, etc. Only it has to be remembered that in case of the simplest of the acids namely formic acid H-COOH, R is a hydrogen atom.

Naming of the organic acids
Organic acids are derived from alkanes, the IUPAC names of  organic acids are also derived from the IUPAC names of alkanes.  The carboxylic group is given a suffix “oic” which is then attached to the IUPAC name of the alkane. A word acid is added in the end. The suffix “oic” shows that there is a presence of a carboxylic group.  

Examples below will clarify the way the IUPAC nomenclature is used.

Example 1 :  Let us see how formic acid HCOOH is named by IUPAC method.  

The compound contains one carbon atom, so its parent hydrocarbon is methane. The carboxyl group –COOH indicates that it is an acid. Hence the suffix “oic” is attached to methane  by replacing the last “e” of methane. Hence the IUPAC name of formic acid HCOOH is methan-oic acid or methanoic acid. Its common name is formic acid.

Example 2 :  Let us see how acetic acid CH₃COOH is named by IUPAC method.  

The compound has 2 carbon atoms. So its parent hydrocarbon is an ethane. The presence of -COOH shows that the compound is an organic acid. Hence attach the suffix “oic” and thus the compound CH₃COOH is named as ethanoic acid by IUPAC. Its common name is acetic acid.

Example 3 :  Let us see how C₂H₅COOH is named by IUPAC method.   

The compound has 3 carbon atoms. So its parent hydrocarbon is a propane. The presence of -COOH shows that the compound is an organic acid. Hence attach the suffix “oic” and thus the compound C₃H₅COOH is named as propanoic acid by IUPAC. Propanoic acid can also be written as CH₃CH₂COOH, as this molecular formula clearly indicates its structure.

Example 4 :  Let us see how C₃H₇COOH is named by IUPAC method.  

The compound has 4 carbon atoms in a straight chain. So its parent hydrocarbon is a butane. The presence of -COOH shows that the compound is an organic acid. Hence attach the suffix “oic” and thus the compound C₄H₇COOH is named as butanoic acid by IUPAC. Butanoic acid can also be written as CH₃CH₂CH₂COOH, as this molecular formula clearly indicates its structure.

Preparation of organic acids
Organic acids are prepared by oxidation of alcohols. These reactions we have seen in the earlier sections. Incomplete oxidation of alcohols leads to organic acids.

Incomplete oxidation is achieved by providing nascent oxygen for the reaction. When air is made to pass over heated copper, oxygen from air can break into atomic or nascent oxygen. This is made to react with an alcohol to give organic acid and water.  Copper is a catalyst.  

When methanol is used as a starting material, methanoic acid is formed. If ethanol is used as a starting material, ethanoic acid is formed. Ethanoic acid is also called acetic acid or vinegar. Vinegar has many uses. For industrial production of vinegar, ethanol is acted upon by bacteria which gives rise to incomplete oxidation of ethanol.

Nascent oxygen can be obtained from a reaction of potassium dichromate and sulphuric acid. The result is an organic acid and water.  

Properties of organic acids

Physical Properties
1. Physical State : The first three organic acids are colourless liquids at ordinary room temperatures. Organic acids having 4 to 9 carbon atoms are colourless oily liquids at ordinary room temperatures. Carbonic acids with higher number of carbon atoms are colourless wax like substances at ordinary room temperatures.
2. Odour : The first three members have very pungent smell. Next acids with 4 to 9 carbon atoms, smell pungent like goat’s butter.  Still higher molecular weighted organic acids are odourless.
3. Solubility : The first four carbonic acids are soluble in water. As the molecular weight increases, the solubility in water decreases. Organic acids with 10 or more carbon atoms are insoluble in water. All organic acids are soluble in organic solvents like benzene, ethanol, ether, etc.
4. Acidic nature : Organic acids are weak acids as they can be only partially ionized in water to give H^- ions. The acidic nature decreases as we go higher in the homologous series. The lower members readily show the litmus test turning a blue litmus paper to red.
5. Fatty acids : Higher members of organic acids that have one –COOH group are also called mono carboxylic fatty acids. Most of these acids are obtained from hydrolysis of fats and are hence known as fatty acids. They have many industrial uses especially in soap and detergent industry.  

Chemical Properties
1. Action on litmus : Methanoic and ethanoic acids turn blue litmus to red quite easily. This indicates the acidic nature of the compounds. Higher members of the organic acid do not show this test as readily.
2. Reaction with sodium bicarbonate :  Organic acid reacts with sodium bicarbonate to release water and carbon dioxide.  If ethanol is reacted with sodium bicarbonate, sodium ethanoate, an ester is formed along with water and carbon dioxide. The reaction is shown below:

The sodium bicarbonate test is a test for the presence of a carboxyl group –COOH in a compound because the reaction causes effervescence and the release of carbon dioxide.
3. Reaction with alcohols :  We have seen earlier how esters are formed when alcohols are made to react with organic acids in the presence of concentrated sulphuric acid. Equation below shows the reaction for ethanoic acid reacting with ethanol.

Ethanol and ethanoic acid is mixed and warmed and few drops of concentrated sulphuric acid is added, A sweet smell will emanate from the reaction. The sweet smell is the smell of ester ethyl ethanoate.

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