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# Carboxylic Acid

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 Sub Topics Typical organic acids include the simple carboxylic acids, which consist of an alkyl chain with a carboxyl group at one end. Long chain carboxylic acids are known as fatty acids, as they are a major component of fats and oils. The alkyl chain can vary in length up to 30 carbons and can contain alkene functional groups within its structure. Carboxylic acids can be part of a biological molecule such as citric acid which contains other functional groups. Carboxylic acids are named by reference to the parent alkane from which they are derived. Thus HCOOH has one carbon and is called methanoic acid and CH3COOH contains two carbons and is called ethanoic acid. The carboxylic acids of particular interest to life scientists contain long alkyl chains. Long chain carboxylic acids are also called fatty acids as they are one of the components of fats and oils.

## Naming

What is naming carboxylic acids?

When naming a carboxylic acid according to the IUPAC rules, the parent is the longest continuous carbon chain that includes the carboxyl group. Numbering begins at the carboxyl group and alkyl groups are identified by name, position and number of appearances.

Carboxylic acids are named by replacing -e in the name of the parent alkane with the suffix -oic acid. Fromic acid is the common name for methanoic acid; acetic acid is ethanoic acid and propionic acidis propanoic acid. The carboxylic acid group does not receive a number in the name because it is a terminal functional group, common names are included in parentheses. The carboxylic acid derivatives are the final category of functional groups. These include ester, amides and anhydrides.

## Properties

At room temperature low molecular weight carboxylic acids are liquids with distinctly sharp or unpleasant odors. High molecular weight long chain acids are wax like solids. Carboxylic acids are quite effective in forming dimers, two molecules held together by hydrogen bonds. Thus they have relatively high boiling points and those with lower molecular weights are soluble in water.

Carboxylic acids are both hydrogen bonding donors and hydrogen bonding acceptors. Thus carboxylic acids of low molecular weight are infinitely soluble in water because of extensive hydrogen bonding with water molecules.

As the molecular weight of a carboxylic acid increase, so does the size of the hydrocarbon portion of the molecule causing its hydrophobicity to increase. Eventually this hydrophobicity outweighs the hydrophilicity of the carboxyl group resulting in decreasing solubility.

## Structure

Typical organic acids include the simple carboxylic acids which consists of an alkyl chain with a carboxyl group at one end. Long chain carboxylic acids are known as fatty acids, as they are a major component of fats and oils. The alkyl chain can vary in length up to 30 carbonsa and can contain alkene functional groups within its structure. Carboxylic acids can be part of a biological molecule such as citric acid which contains other functional groups.

Carboxylic acids are named by reference to the parent alkane from which they are derived. Thus HCOOH has one carbon and is called methanoic acid and CH3COOH contains two carbons and is called ethanoic acid. The carboxylic acids of particular interest to life scientists contain long alkyl chains.

## Carboxylic Acid Derivatives

Carboxylic acid derivatives are compounds with functional groups that can be converted to carboxylic acids by a simple acidic or basic hydrolysis. The most important acid derivatives are esters, amides and nitriles. Acid halides and anhydrides are also included, although the activated forms of the parent acids rather than completely different compounds.

Many advances in organic chemistry involve making and using derivatives of carboxylic acids. The reactivity trends among the carboxylic acid derivatives can be recognized by taking account of the effect of the substituents on the stability of the carbonyl center and the ability of the various substituents to act as leaving groups from the tetrahedral intermediate.

## Functional Group

The functional group of a carboxylic acid is located at the end of the carbon chain. To name a carboxylic acid using IUPAC system, first name the carbon chain, then replace the -e at the end of the alkane name with -oic acid. Note that the functional group carbon counts as part of the principal chain. For example, the four carbon carboxylic acid that follows is called butanoic acid.

Carboxylic acids were among the first organic compounds to be discovered, so most carboxylic acids have trivial names that were given before the IUPAC system was devised. Many of these trivial names are still used in preference to the IUPAC names. For example, the carboxylic acids with one and two carbon atoms are almost always called formic acid and acetic acid respectively.

## Aldehyde to Carboxylic Acid

Aldehydes RCHO can be oxidized to carboxylic acids, RCO2H under aqueous conditions.

Typical reagents are based on aqueous Cr(VI), chromic acid H2CrO4, chromate salts, eg. Na2CrO4, dichromate salts eg, K2Cr2O7. The oxidation actually proceeds on the hydrate, RCH(OH)2, formed by the reaction of water with the aldehyde. R can be alkyl or aryl.

## Carboxylic Acid Reactions

Acyl halides

Carboxylic acid undergo nucleophilic substitution reaction with SOCl2 in the presence of base forms acyl chlorides.

Acyl chlorides named from corresponding acid remove -ic add -yl chloride. For example, CH3COCl as ethanoyl chloride and C6H5COCl as benzene carbonyl chloride.

Acid anhydrides

Acid anhydrides named from corresponding acid remove acid add anhydride and is more useful, because it is cheaper, less corrosive, less vulnerable to hydrolysis and less dangerous while reacting.

Esters

Esters are obtained by refluxing the parent carboxylic acid with the appropriate alcohol with an acid catalyst. The equilibrium can be driven to completion by using an excess of either the alcohol or carboxylic acid, or by removing the water as it forms.

## Test for Carboxylic Acid

The organic compounds containing the carboxyl group as the functional group is called carboxylic acids. The presence of carboxylic acid can be identified by the following tests.
• Litmus test
• Sodium bicarbonate test
• Ester test
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Litmus test

Dissolve a pinch of organic compound in water in a test tube and add 2-3 drops of blue litmus solution. Turning of blue red shows the presence of carboxylic group. Phenols also give this test.

Sodium bicarbonate test

Carboxylic acids react with sodium hydrogen carbonate to evolve CO2 gas. Dissolve a pinch of the organic compound in water in a test tube and add 5% aq.NaHCO3 solution to it with a dropper and observe. Effervescence due to the evolution of CO2 shows the presence of -COOH group.

RCOOH + NaHCO3 $\rightarrow$ RCOONa + H2O + CO2$\uparrow$

Ester test

To the given organic compound in a dry clean test tube add 1ml of ethyl alcohol and 2-3 drops of conc.H2SO4. Warm the test tube in a water bath for 10 min. Cool and pour the contents of the test tube into aq.Na2CO3. A pleasant fruity smell of ester indicates the presence of -COOH group.

RCOOH + CH3CH2OH $\overset{conc.H_{2}SO_{4}}{\rightarrow}$ RCOOCH2CH3 + H2O