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Test for Carboxylic Acid

The organic acid group contains carboxyl group $–COOH$ bonded to either an alkyl group $(RCOOH)$ or an aryl group $(ArCOOH)$. To understand how the test for this functional group works we need to get the facts about the various forms of acids that this group can result in.

In aliphatic series, we have formic acid or $HCOOH$, acetic acid or $CH_3COOH$ and $Ar-COOH$ as the benzoic acid. The longest chain having the functional group carboxyl is considered as the parent structure and is suitably named by removing the ‘-e’ suffix of the corresponding alkane with a suffix of ‘oic’ acid.

The name of salt of a carboxylic acid consists of the cation followed by the name of the acid with the ending ‘-ic’ acid changing to ‘-ate’. There are many tests which help in identifying these functional groups and their respective acids, like Tollens’s Fehling’s and Benedict’s tests etc.

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Test for Carboxylic Acid Functional Group

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The acids do not show the typical reaction of carbonyl groups because of the following resonance, however they have the tendency to donate protons and act as acids. This property of donating protons is helpful in the identification of a -COOH group.

Litmus test:
The carboxyl group because of their acidic nature, it turns blue litmus into red.

The blue litmus solution of around one drop is added to an aqueous solution of acid of around 1 mL. The appearance of a red colour indicates the presence of a carboxylic acid group. Blue litmus paper may be used as well in place of a blue litmus solution.

Sodium bicarbonate test:
Carboxylic acid group reacts with a solution of sodium bicarbonate with effervescence which shows the evolution of $CO_{2}$.

$RCOOH + NaHCO_{3} \rightarrow RCOONa + H_{2}O + CO_{2}$

To a saturated solution of sodium bicarbonate in water of around 1 mL we need to add the given compound (a pinch of solid sample or 2 -3 drops of the liquid, or even aqueous or alcoholic solution of the compound. The quick effervescence indicates the presence of carboxylic acid. The effervescence of CO2 can be tested with lime water which turn milky when this gas is passed through it.

$CO_{2} + Ca (OH)_{2} \rightarrow CaCO_{3} + H_{2}O$

The carbon di oxide in aqueous form acts as dilute acid turns the mild alkali to react and produce salt and water. The cloudy appearance of $CaCO_{3}$ is basically the presence of salt, white in colour which remains suspended. 

Ferric Chloride Test for Carboxylic Acid

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In order to complete this test, the ethanoic acid is first reacted with ammonium hydroxide and the resultant ammonium acetate is then further reacted with ferric chloride.

$CH_{3}COOH + NH_{4}OH \rightarrow CH_{3}COONH_{4} + H_{2}$O

The ammonium acetate so formed is put in warm bath where the ammonia vapor is lost to an extent. Once the vapor is lost it is then reacted with ferric chloride. The resultant ferric acetate formed is confirmed with the appearance of red hue colour.

$CH_{3}COONH_{4} + FeCl_{3} \rightarrow (CH_{3}COO)_{3} Fe + NH_{4}Cl$ 

Test for Carboxylic Acid and Alcohol

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The test for carboxylic acid and alcohol is better known as esterification. The esterification reactions are very slow as homogeneous and heterogeneous acids act catalytically in the esterification because the initiating step in the reaction mechanism is protonation of the carboxylic used. The presence of efficient homogeneous mineral acid H2SO4 helps in the process of protonation.

The heterogeneous catalyst containing sulphonic acid groups are active in esterification. The process of esterification is basically the reaction between alcohol and carboxylic acid in presence of sulphuric acid at about 170oC temperature. The formation of ester is important as most of the synthetic flavoured edibles are based on ester and their various derivatives. The alcohol reacts with carboxylic acid in presence of catalyst H2SO4 acid and produce ester along with water as only other product.

Carboxylic acid caries a proton or hydrogen ion from the catalyst sulphuric acid which finally attaches to one of the oxygen’s lone pairs. This oxygen is bonded with carbon with a double bond and this transfer of proton eventually gives the positive charge to oxygen.

The delocalisation of positive charge results in the shifting of electron pair resulting in canonical structures. 

Carboxylic Acid
The carbon atoms charge attracts the ethanol molecule’s lone pair which finally results in the formation of a water molecule. The remaining resultant is the ester that we have as the other product. Test ends in the formation of a sweet smelling compound ester.

$CH_{3}COOH + C_{2}H_{5}OH \rightarrow H_{2}SO_{4} / 170 C \rightarrow CH_{3}COOC_{2}H_{5} + H_{2} O$

Test for Carboxylic Acid Using Sodium Carbonate

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The test involving sodium carbonate with carboxylic acid results in the evolution of carbon di oxide gas which turns lime water milky.
Ethanoic acid when poured into sodium carbonate solution, there is an immediate effervescence of carbon di oxide which is then confirmed by passing it through lime water.

The evolution of carbon di oxide is confirmed once the lime water turns milky which also confirms the presence of carboxylic acid.

$2 CH_{3}COOH + Na_{2}CO_{3} \rightarrow 2 CH_{3}COONa + CO_{2} + H_{2}O$

$CO_{2} + Ca (OH)_{2} \rightarrow CaCO_{3} + H_{2}O$

Functional Group Test Chart

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Functional Group Test Chart
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