Carboxylic Acid

Carboxylic Acid Introduction A carboxylic acid contains a carboxy (carboxyl) group, COOH, which has one oxygen atom doubly bonded to the carbon atom and a hydroxyl (OH) group singly bonded to the carbon atom. The carbon atom of the carboxy group must also be attached to an organic group or hydrogen. Examples of common carboxylic acids with their structure are shown below.

Carboxylic acids are Bronsted acids — they are proton donors. Salts and anions of carboxylic acids are called carboxylates. Straight Chain carboxylic acid is called aliphatic acid. In the IUPAC system of nomenclature the carboxyl carbon is designated number 1, and other substituents are located and named accordingly. The characteristic IUPAC suffix for a carboxyl group is "oic acid"

In organic chemistry it is the strongest acid. Carboxylate ion which is formed after proton donation is resonance stabilized. Electron withdrawing group on ∀-carbon further stabilizes conjugate base which increases acidity of the carboxylic acid.

Carboxylic acid behaves as acid or as a substrate in nucleophilic substitution reaction. It is venerable to nucleophiles and when hydroxyl group is protonated the good leaving water is formed resulting in substitution reaction. Physical properties The boiling points increased with size in a regular manner, but the melting points did not. Un-branched acids made up of an even number of carbon atoms have melting points higher than the odd numbered homologues having one more or one less carbon. This reflects differences in intermolecular attractive forces in the crystalline state. The presence of a cis-double bond significantly lowers the melting point of a compound. The saturated carboxylic acids with more than eight carbons are generally solids. The double bond in unsaturated carboxylic acid breaks the lattice and lowers melting point.

Carboxylic acid forms dimmer which increases the boiling point. It is the presence of a intermolecular hydrogen bond which forms the dimmer as also increases the boiling point.

Carboxylic acid is miscible in water. The solubility decreases with increase in molecular mass or increase in number of carbon atoms. Carboxylic acids after ten carbons become insoluble in water. Solubility of carboxylic is due to hydrogen bonding. Carboxylic acid is also soluble in non-polar solvents. Decarboxylation Loss of carbon dioxide from carboxylic acid is called decarboxylation. The reaction is exothermic. Simple carboxylic acids rarely undergo decarboxylation as the energy of activation is high making the reaction difficult. However carboxylic acids with a carbonyl group at the 3- (or b-) position readily undergo thermal decarboxylation, e.g. derivatives of malonic acid.

There are two reasons for ∃-keto acid for decarboxylation. 1. The decarboxylation of carboxylate anion forms a resonance stabilized enolate anion.

This anion is much more stable than anion RCH2:- That would be produced by decarboxylation of an ordinary carboxylic acid anion. 2. When the acid itself decarboxylates, it can do so through a six member cyclic transition state.

This reaction produces an enol directly and avoids an anionic intermediate. The enol then tautomerizes to methyl ketone. Acetoacetice Ester Synthesis The process of synthesis of ketones using acetoacetic ester is called acetoacetic ester synthesis. The process involves decarboxylation and depends upon property of ∃-dicarbonyl compound to increase the acidity of ∀-hydrogen between the carbonyls. 1) Acetoacetic ester is treated with a base which removes ∀-hydrogen. 2) Removal of ∀-hydrogen creates enolate ion. 3) Enolate ion is allowed to react with alkyl halide forming alkylacetoacetic ester which is conducted absolute alcohol. 4) Alkylacetoacetic ester is hydrolyzed by dilute aqueous alkali and corresponding acid is produced. 5) The acid undergoes decarboxylation to form ketone.

Derivatives of carboxylic acid The derivatives of Carboxylic group are the compounds in which hydroxyl group is replaced by halogen, another carboxyl group, alkoxy group etc. The acyl group is common to derivatives of carboxyl group. Derivatives of carboxyl group are given below

Preparation of Derivatives of carboxylic acid The first step in preparation is to prepare acyl chloride (also called acid chloride.

Acyl chloride can be easily prepared by treating carboxylic acid with inorganic acid chloride such as SOCl2, PCl3, PCl5 etc. Acyl chloride are Bronsted acids capable of donating ∀-hydrogen. Acyl chloride is stronger acid than aldehyde.

Acyl chloride is the most reactive of all the derivatives of carboxylic acid. Acyl chloride is hydrolyzed to obtain all other derivatives of carboxylic acid. Acyl chloride is a very strong nucleophile.

Derivatives of carboxylic acid on hydrolysis give carboxylic acid.

Reactivity of carboxylic derivatives depend upon the substituting group or on formation of the compound whose leaving group is a stronger base.

Ester Esters are usually prepared by the reaction of alcohol or phenol with acid or acid derivatives. It is a nucleophilic substitution reaction. A strong acid catalyzes the reaction by protonating hydroxyl group on the carboxylic acid.

The reaction is known as esterification and it is best carried out by the reaction between acid chloride derivatives of carboxylic acid and alcohols.

In some reaction one alcohol is capable of displacing another alcohol from an ester. This alcoholysis (cleavage of an alcohol) of an ester is called transesterification.

An equilibrium is also achieved in the reaction and the result can be controlled by adding an excess of alcohol in the product or reactant.

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