What is a Carboxylic Acid?
Carboxylic acids make up one of the most important classes of organic compounds. The general formula for a carboxylic acid is R-C(O)OH, where R is an alkyl or aryl group. Many carboxylic acids widely occur in nature while a major part of them are manufactured synthetically. The presence of double bonded oxygen atom in carboxylic acids greatly affects the properties of these compounds and the properties of carboxylic acids are greatly governed by their structure and the presence of the carbonyl group in it. Let us first understand what a Carboxyl Group is.
What is a Carboxyl Group?
The Carboxyl Group is a functional organic compound that comprises of the double-bonded carbon atom to an oxygen and a single bond to a hydroxyl group. The organic compounds consisting of carboxyl group are termed as a carboxylic acid. They include acetic acid and amino acid.
Carboxyl groups are present on the side of a molecule. It ionizes, discharging the H from the hydroxyl aggregate as a free proton (H+), with the rest of the O conveying a -ve charge. This charge turns about forward and backward between the 2 oxygen molecules, which makes this ionized state moderately steady.
Properties of Carboxyl Group:
Carboxyl group comprises of electronegative oxygen double bond to a carbon atom. As a result, there will be an increase in the polarity of a bond. A compound comprising a carboxyl group will possess a high melting point, hydrophilic centers, and boiling point.
The reason behind high boiling point and melting point can be dissipated because of the formation of hydrogen bond in the solid and liquid state. One of the typical examples is fatty acids.
Chemical Properties of Carboxylic Acids:
On heating mineral acids such as \(H_2SO_4\) or \(P_2O_5\) with carboxylic acid we get the corresponding anhydride.
When alcohols or phenols react with carboxylic acids then esters are formed in the presence of conc. H2SO4 or HCl as a catalyst.
Reaction with PCl5, SOCl2:
The hydroxyl group of carboxylic acid is easily replaced by chlorine atom when it is treated by \(PCl_5\), \(SOCl_2\) as the products which are produced are gaseous in nature and can escape from the reaction mixture making the purification easier.
Reaction with ammonia:
Ammonium salt is formed when carboxylic acids react with ammonia and on further heating it gives amides.
Carboxylic acids lose carbon dioxide to form hydrocarbons when their sodium salts are heated with soda lime and this reaction is called decarboxylation.
Carboxylic acids having α-hydrogen are halogenated at α-position when treated with bromine or chlorine in presence of small amount of red phosphorus to give α-halo-carboxylic acids. This reaction is known as Hell-Volhard –Zelinsky reaction.
Carboxyl group acts as a deactivating and meta-directing group when aromatic carboxylic acid undergoes electrophilic substitution reaction. They do not undergo Friedel-Crafts reaction as carbonyl group is a deactivating group and aluminum chloride acts as a catalyst and gets bonded to a carboxyl group.
Physical Properties of Carboxylic Acids:
- Aliphatic carboxylic acids having nine or fewer carbon atoms. They are colorless at room temperature and have an unpleasant smell.
- The higher acids are wax-like solids and are odorless because of their low volatility.
- Carboxylic acids have a higher boiling point when compared to ketones, aldehydes or alcohols of comparable masses. This high boiling point is because of extensive association of their molecules via intermolecular hydrogen bonding. Even in the vapor phase, the hydrogen bonds remain intact and most of the carboxylic acids exist as dimers in the vapor phase or in aprotic solvents.
- The simple aliphatic carboxylic acids get dissolved in water as they can form hydrogen bonds with water.
- As the number of carbon atoms in carboxylic acids increases their solubility decreases. This happens due to the increase in the hydrophobic interaction of hydrocarbon part of the acid.
- Higher carboxylic acids do not dissolve in water. Less polar organic solvents like benzene, alcohols, and chloroform dissolve carboxylic acids.
Chemical reactions involved (Acidity):
On reaction with metals and alkalis: With electropositive metals just like alcohols, carboxylic acids also liberate hydrogen and form salts with alkalis just like phenols. Unlike phenols, they react with the weaker bases in order to produce carbon dioxide. The strength of acids is measured by pK
The strength of acids is measured by pKa, smaller the value of pKa, stronger is the acid. The pKa value of strong acids is less than 1, the acids which have pKa values between 1 and 5 are moderate acids and weak acids have pKa values between 5 to 15. Although carboxylic acids are weak acids than many mineral acids, they are stronger than many simple phenols and alcohols. Carboxylic acids are more stable than the phenoxide ions that’s why carboxylic acids have higher acidity than phenols.
Nomenclature of carboxylic acids:
A carboxylic acid is an organic compound containing carboxyl group (COOH) attached to an alkyl or aryl group. The general formula of a carboxylic acid is Ar/R−COOH where Ar/R represents the aryl or alkyl group attached. In carboxylic acids, three of the four bonds of a carboxyl carbon atom are to two oxygen atoms. Earlier, most of the compounds with same structural formula were known by different names depending on the regions where they were synthesized. This naming system was very trivial since it raised a lot of confusion. Finally, a common naming system enlisting standard rules was set up by IUPAC (The International Union for Pure and Applied Chemistry) for the naming of compounds. This method of naming is IUPAC naming or IUPAC nomenclature. Nomenclature of carboxylic acids revolves around two standard ways of naming:
Nomenclature of carboxylic acids revolves around two standard ways of naming:
- Common nomenclature
- IUPAC nomenclature
Carboxylic acids are mainly known to us by their common names only as they are amongst the earliest organic compounds that were isolated. Common nomenclature of carboxylic acids involves replacement of the prefix “e” of the corresponding alkane with “ic acid”. For example, HCOOH isolated from red ants is commonly known as formic acid.
IUPAC nomenclature of carboxylic acids:
- According to IUPAC nomenclature of carboxylic acids, the suffix “e” in the name of the corresponding alkane is replaced with “oic acid”.
- When the aliphatic chain contains only one carboxyl group, the carboxylic carbon is always numbered one. For example, CH3COOH is named as ethanoic acid.
- When the aliphatic chain contains more than one carboxyl group, the total number of carbon atoms is counted and the number of carboxyl groups is represented by Greek numeral prefixes “di-”, “tri-“etc.
- A carboxylic acid is named by adding these prefixes to the parent alkyl chain. Arabic numerals are used for indicating the positions of the carboxyl group.
COOH-(CH2)4-COOH is named as hexanedioic acid,
HOOC–CH2-CH(COOH)-CH2-COOH is named as Propane-1, 2, 3-tricarboxylic acid.
Uses of carboxylic acid:
- Nylon-6,6 is manufactured using hexanedioic acid.
- For making perfumes, an ester of benzoic acid is used.
- Sodium benzoate is used as food preservatives.
- Lastly, for the manufacture of soaps and detergents fatty acids are used.
- Carboxylic acids also find application in rubber, textile and leather industries.