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Carboxylic Acid

 Organic compounds are mainly composed of carbon atoms. Hydrocarbons are simplest organic compounds which are composed of carbon and hydrogen atoms. They are non-polar molecules with single or multiple covalent bonds in the molecule. The group of atom which imparts different physical and chemical properties of the molecule is called as a functional group. For example; if a halogen group is bonded to the parent carbon chain of the alkane molecule by the replacement of hydrogen atom, it forms haloalkane. Haloalkanes are polar molecules due to the presence of the halogen group on the carbon atom.

Carboxylic Acid
Other examples of functional groups are carbonyl group (>C=O), a hydroxyl group (-OH), an amino group (-NH2), the carboxy group (-COOH), Nitro group (-NO2). The presence of the carboxy group on the carbon chain of the molecule forms carboxylic acid. The general formula of carboxylic acids is RCOOH. The presence of the carboxy group makes the molecule acidic compare to other molecules. This functional group is a combination of two functional groups; >C=O and –OH group but –COOH group show quite different physical and chemical properties compared to both of these functional groups.

Let’s discuss the preparation methods of carboxylic acid from other organic compounds like aldehyde, alcohols etc. The chemical reactions of carboxylic acids are almost same as carbonyl compounds and alcohols.

 

Carboxylic Acid Group

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The carbonyl compounds in which the carbonyl group (>C=O) is bonded with one hydroxyl group (-OH) and another valency get attached with either hydrogen or aryl/ alkyl group are called as carboxylic acid group. The generic formula for carboxylic acid compounds is RCOOH. Since these are acidic compounds, therefore easily lose H+ ion and covert in carboxylate ion (RCOO-).

Carboxylic Acid Group

Many carboxylic acids are batter known by their common or trivial names which are based on their origin. For example, methanoic acid first isolated from formica (red ant) and called as formic acid.
Similarly acetic acid was isolated from acetum, ’vinegar’ and Butyric acid (Latin butyrum, "butter") is
found in rancid butter. Caproic, caprylic, and capric acids (Latin caper, "goat") are obtained from goat fat. Different carboxylic acids with their IUPAC and common name are as follows.

S.No
Common name
Formula
IUPAC name
1 Formic acid HCOOH Methanoic acid
2
Acetic acid
CH3COOH
Ethanoic acid
3
Proprionic acid
CH3CH2COOH
Propanoic acid
4
Butyric acid
CH3(CH2)2COOH
Butanoic acid
5
Caproic acid
CH3(CH2)4COOH
Hexanoic acid
6
Caprylic acid
CH3(CH2)6COOH
Octanoic acid
7
Capric acid
CH3(CH2)8COOH
Decanoic acid
8
Lauric acid
CH3(CH2)10COOH
Dodecanoic acid
9
Myristic acid
CH3(CH2)12COOH
Tetradecanoic acid
10
Palmitic acid
CH3(CH2)14COOH
Hexadecanoic acid
11
Stearic acid
CH3(CH2)16COOH
Octadecanoic acid

Carboxylic Acid Structure

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The carboxyl group is made up of a carbonyl group joined with a hydroxyl group.

>C=O + -OH -C(=O)OH

The carbon atom in carboxyl group is bonded with two oxygen atoms; one by a double bond and another by single bond. The remaining valency can be satisfied either by hydrogen or by alkyl group in aliphatic carboxylic acids and by aryl group in aromatic carboxylic acids.
Carboxylic Acid Structure
  1. The carboxylic acid group is a planer structure which suggests that the carbonyl carbon atom and two oxygen atoms are sp2 hybridized.
  2. The two sp2 hybridized orbital of carboxyl carbon atom get overlap with one sp2 hybridized orbital of each oxygen atom to form sigma bond with them, while the third sp2 hybridized orbital of carbon get overlap with a s-orbital of hydrogen atom or a sp3 hybridized orbital of alkyl group in aliphatic carboxylic acid.
  3. In aromatic carboxylic acids, the third sp2 orbital of carboxyl carbon atom gets overlap with sp2 hybridized orbital of aromatic ring.
  4. Now carboxyl carbon atom and both oxygen atoms left with one unhybridised p-orbital which lie perpendicular to sigma bonds.
  5. These unhybridised p-orbitals overlap to form pi bond which is partly delocalized between carbon and oxygen atom of one side and C-O of the hydroxy group on the other side.

Hence carboxylic acids can be represented by to resonance structures.

Carboxylic Acids Resonance Structures

Because of this resonance in carboxylic acid group, the >C = O bond length becomes 123 pm which is lesser compare to alcohols (143 pm) and more than ketones (120 pm). The Carbon-oxygen bond length on hydroxyl group side is around 136 pm which is more than >C = O due to lack of double bond.

Carboxylic Acid Derivatives

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The functional group of carboxylic acids is the carboxyl group; -C ( = O ) OH. If the hydroxyl group of carboxyl group gets replace by other functional groups like halogen(-X), -OCOR’ , -OR’ and-NH2, they called as acid halide, anhydrides, esters and amide respectively. These compounds are collectively called as acid derivatives or functional derivatives of carboxylic acids as hydrolysis of all these compounds again form parent carboxylic acid.

Carboxylic Acid Derivatives

Carboxylic acid derivatives have wide applications like esters are found in fats and cell membranes, amides involve in the manufacturing of synthetic fibres and bio molecules like proteins. Acyl chloride and acid anhydride mainly involve in preparation methods for other carboxyl derivatives. Some acid derivatives with their property are as follows.

Acid Derivatives with Their Property

Aldehydeto Carboxylic Acid

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Aldehydes can be easily oxidized to carboxylic acids which have same number of carbon atoms as in Aldehydes. Since the oxidation of aldehyde involve the cleavage of Carbon-hydrogen bond which is compare to carbon-carbon bond in ketones.

Aldehyde to Carboxylic Acid
Hence oxidation of aldehyde can occur with strong as well as weak oxidizing agents. Strong oxidizing agents like chromic acid, potassium chromate, potassium permanganate, potassium dichromate and concentrate nitric acid can oxidize Aldehydes to carboxylic acid containing same number of carbon atoms. Oxidation of aldehyde can be done under acidic and alkaline medium. In acidic conditions, Aldehydes get oxidise to form carboxylic acid while in alkaline medium carboxylate salt is formed.

RCHO +H2O RCOOH +2H+ +2e-
RCHO +3OH-
RCOO- +2H2O +2e-

Oxidation of aldehyde completed in three steps.
  1. In acidic medium, the carbonyl group of aldehyde form hydrate.
  2. This hydrated product; an alcohol reacts with the chromium species and form a chromate ester.
  3. During E2 elimination reaction, a base abstracts a proton from the chromate ester and carboxylic group with a Cr species forms as product.

Oxidation of Aldehyde
Oxidation of Aldehydes with different oxidizing agents is as follows.

1. Oxidation with potassium dichromate salt in acidic medium


A small amount of acidified potassium dichromate solution acts as good oxidizing agent for aldehyde to convert it in carboxylic acid. The orange color of dichromate solution turns to green after oxidation due to the formation of chromium sulphate in solution. During oxidation, dichromate ion (VI) gets reduced to Cr3+ ion and oxidized aldehyde to carboxylic acid.

Cr2O72- +14H+ +6e- 2Cr3+ +7H2O
RCHO+H2O
RCOOH +2H+ +2e-

The overall oxidation of aldehyde in the presence of dichromate salt is as follows;

3RCHO + Cr2O72- +8H+ 3RCOOH + 2Cr3+ +4H2O

2. Oxidation with Tollen’s reagent


Tollen’s reagent is an ammonical solution of silver nitrate contains diamminesilver(I) ion[Ag(NH3)2]+. It is prepared by addition of ammonia hydroxide (NH4OH) solution in silver nitrate solution till gray precipitate of Ag2O gets re-dissolved. By heating this solution with aldehyde, Ag2O is reduced to metallic silver and deposited on the inner wall of test tube and aldehyde oxidized to carboxylic acids.

2AgNO3 +2NH4OH Ag2O +2NH4NO3 +H2O
Ag2O +4NH4OH
2[Ag(NH3)2]+OH- +3H2O
RCHO + 2[Ag(NH3)2]+OH- +3OH-
RCOO- +2Ag +4NH3 +2H2O

Both fehling's solution and Benedict's solution are weak oxidizing agent but can oxidize aldehyde to carboxylic acid. Both oxidizing agents contain complexed copper (II) ions in an alkaline solution. The only difference is that Fehling's solution contains copper (II) ions with tartrate ions in sodium hydroxide solution and Benedict's solution contains copper (II) ions with citrate ions in sodium carbonate solution. In both solutions the presence salt ions prevents the precipitation of copper (II) hydroxide. With these oxidizing agents, aldehyde oxidized to carboxylic acid and copper (II) ion reduced to copper (I) ion which precipitated as red color ppt.

RCHO +2Cu2+ +5OH- RCOO- +Cu2O+ 3H2O

Reductionof Carboxylic Acid

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Carboxylic acids and their salts undergo many reactions which involve either the elimination or the decomposition of the carboxyl group. Reduction of carboxylic acid is one of them. Carboxylic acids can reduced to either alkane or alcohols depends upon the reducing agent involve in reaction.

1. Reduction of carboxylic acid to alcohols.


When the carbonyl group of carboxyl acid is reduced to >CH2 group, it gets converted to carboxylic acid. Lithium aluminium hydride (LiAlH4) and sodium borohydride (NaBH4) are good reducing for this conversion. For example, propanoic acid is reduced to propanol and benzoic acid to benzyl alcohol.

Reduction of Carboxylic Acid to Alchols

Reduction of carboxylic acid to alcohols follows nucleophilic acyl substitution followed by nucleophilic
addition and completed in four steps;
  1. The hydride reagent reacts with carbonyl carbon atom of carboxyl group to form an intermediate; metal alkoxide complex.
  2. Decomposition of tetrahedral intermediate and displace the hydroxyl part of acid as a leaving group and produces a ketone as an intermediate.
  3. The nucleophilic H from the hydride reagent reacts with polar carbonyl group of the aldehyde and form an intermediate metal alkoxide complex.
  4. Protonation of the alkoxide oxygen forms the primary alcohol product.

Hence overall reaction mechanism is

4RCOOH+3LiAlH4 $\overset{Ether}{\rightarrow}$ 4H2+4RCH2OM+ metal oxides $\overset{H_2O}{\rightarrow}$ 4RCH2OH+ Metal hydroxides

Reduction of Carboxylic Acid to Alkanes

2. Reduction of carboxylic acid to alkanes


Carboxylic acid reduced to alkanes in the presence of reducing agent red phosphorus at 473 K temperature.

RCOOH + 6HI RCH3+2H2O +3I2

Carboxylic Acid Reactions

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Carboxylic acid consider as a compound composed of two main parts; an alkyl or aryl group and a carboxyl group. Therefore the reactions of carboxylic acids can be due to carboxyl group or because of alkyl group. Overall all reactions of carboxylic compounds can be classified in three types.
  1. Reactions involve the cleavage of –O-H bond in hydroxyl group.
  2. Reactions involve the cleavage of C-O bond in carboxyl molecule.
  3. Reaction involving whole carboxyl group
  4. Reactions involve the alkyl group of carboxyl compounds.

1. Reactions involve the cleavage of -OH bond in hydroxyl group


(a) Acidity of Carboxylic Acids:
Carboxylic acids can be easily ionize and exist in a dynamic equilibrium between the carboxylate ion and hydronium ion. Hence they show acidic nature.

R COOH + H2O $\rightleftharpoons$ RCOO- + H3O+

The equilibrium constant K for given equation can be expressed as,

K = $\frac{[RCOO^-][ H_3O^+]}{[RCOOH][H_2O]}$

Since water present in large excess; the same equation is written as,

K x [H2O] = $\frac{[RCOO^-][ H_3O^+]}{[RCOOH]}$

Ka = $\frac{[RCOO^-][ H_3O^+]}{[RCOOH]}$

Ka is known as acid dissociation constant which is a measure of acid strength of an acid. The high
value of Ka corresponds to more acidity and low value relates to less acidic nature. Ka can also be expressed in terms of pKa.

pKa= -logKa

Various carboxylic acids with their pKa values are as follows.

Carboxylic Acid with pka Values
There are two reasons of acidic nature of carboxyl compounds

1. Carboxyl group can show resonance structures, in which the oxygen atom of hydroxyl group contain positive charge, which is not a stable condition, hence can lose bonded hydrogen atom in the form of proton and convert in carboxylate ion. That is the reason the equilibrium between carboxyl group and carboxylate ion lies towards right side.
Carboxyl Group Resonance Structures
2. The carboxylate anion is also stabilized by resonance like carboxylic acid. The stabilization of the anion is much greater than that of the neutral carboxyl group. In the carboxylate anion the C–O bonds are of equal length (between a double and a single bond) and the two contributing structures have equal weight in the hybrid.

Carboxylate Anion Stabilized by Resonance

The acidic nature of carboxylic acid increases in the presence of electron withdrawing group like -NO2, -CN , which makes the hydroxyl oxygen atom , electron deficient and easily proton from molecule. However the presence of electron donating group like alkyl groups decreases the acidity of carboxylic group.
The decreasing order of acidic strength of various halocarboxylic acids due to the presence of substituents is as follows.
  • As the electronegativity of halogen decreases, acidic nature decreases;
FCH2COOH > ClCH2COOH > BrCH2COOH > ICH2COOH > CH3COOH
  • As the number of halogen atom decreases, acidic nature of carboxylic acid decreases;
Cl3CCOOH > Cl2CHCOOH > ClCH2COOH > CH3COOH
  • As the distance of halogen with carboxylic group decreases, acidic nature decreases;
CH3CH2CH(Cl)COOH > CH3CHCl CH2 COOH > ClCH2CH2CH2COOH > CH3CH2CH2 COOH

Some reactions with show the acidic nature of carboxylic compounds are as follow.

1. Reaction with metal-Carboxylic acids react with active metals like; K, Ca, Mg to form their respective salts and release hydrogen gas.

2RCOOH + 2Na 2RCOONa +H2

2. Similarly carboxylic acids react with alkalis like sodium hydroxide to form salts and water and show simple acid-base neutralization reaction.

RCOOH +NaOH RCOONa + H2O

3. Carboxylic acids are weaker than mineral acids like sulphuric acid or nitric acid and able to react with weaker bases like carbonates and bicarbonates to evolve CO2 with water.

RCOOH + NaHCO3RCOONa + CO2 +H2O

The carbon dioxide gas evolved with brisk effervescence, thus reaction can be used to detect the presence of carboxyl group in a compound and differentiate between carboxylic acids and phenols as they cannot give effervescence with aqueous solution of NaHCO3.

2. Reactions involve the cleavage of C-O bond in carboxyl molecule

These reactions involve the replacement of hydroxyl group by some other groups like; -Cl, -OR, -NH2 to form acyl chloride, ester and acid amide.

(a) Ester Formation: Carboxylic acids react with alcohols to form esters which have a general structural formula of RCOOR. Reaction involves the replacement of hydroxyl group by –OR
group of alcohol or phenol and also known as Fischer-speier esterification. In Fisher esertification an
alcohol and an acid are reacted in an acidic medium such as concentrated sulphuric acid or dry HCl gas. For example, ethanoic acid reacts with ethanol to form ethylthanoate. The reaction exists in equilibrium, hence product ahs to remove fat from reaction system to complete the reaction.

Ethylethanoate


The reaction proceeds through a carbocation mechanism and completed in following steps.

1. Protonation of carboxyl carbon of the carboxylic acid and Nucleophilic attack of alcohol molecule to the carbocation.
Protonation Reaction
2. The oxonium ion loses a proton to generate an alkoxyalcohol as an intermediate which picked up a proton from solution by a hydroxyl group and form a tetrahedral intermediate.
Formation of Oxonium Ion

3. The tetrahedral intermediate undergoes proton transfer to give another tetrahedral intermediate which loss water molecule to form a protonated ester. This protonated ester finally loses a proton to gives the ester.
Ester Formation
(b) Amide Formation: Carboxylic compounds react with ammonia to form ammonium salts which upon heating lose water molecule to form amide.

RCOOH + NH3 RCOO-NH4+ RCONH2 + H2O

(c) Acid Halide Formation: Carboxylic acids can react with a number of halide derivatives; phosphorous trichloride (PCl3), phosphorous pentachloride (PCl5), thionyl chloride (SOCl2), and phosphorous tribromide (PBr3) to form acyl halides.

Formation of Acid Halide
(d) Acid Anhydride Formation: The dehydration of carboxylic group in the presence of a strong dehydrating agent likes phosphorus pentaoxide or concentrated sulphuric acid forms acid
anhydrides. For example, two molecules of acetic acid forms acetic anhydride in the presence of concentrated sulphuric acid.

CH3COOH + HOOCCH3 CH3COOCOCH3 + H2O

3. Reactions of whole carboxyl group


(a) Decarboxylation Reaction:
Sodalime that is a mix of sodium hydroxide with calcium oxide is a best reagent used for decarboxylation and forms alkanes. Simple copper salts such as copper chromate, copper hydroxide or copper carbonate are also good in decarboxylation of aliphatic and aromatic acids.
Decarboxylation Reaction

(b) Hunsdiecker Reaction: Silver salt of an aromatic carboxylic acid on treatment with bromine water in refluxing CCl4 gives bromoalkane or bromobenzene.
Hunsdiecker Reaction

(c) Kolbe Electrolysis: The electrochemical oxidation of sodium or potassium salts of carboxylic acids gives alkanes having twice the number of carbon atoms present in the alkyl group of the acid. This process is known as Kolbe’s electrolysis. For example; the electrolysis of potassium ethanoate forms ethane with carbon dioxide gas and hydrogen gas as side product.

2CH3COOK 2CH3COO+ 2K+
2H2O
2H+ + 2OH-
At cathode; 2H+ + 2e-
H2
At anode; 2CH3COO
2CH3COO·
2CH3COO·
2CH3· + 2CO2
2CH3·
CH3CH3

4. Reactions Involve the Alkyl Group of Carboxyl Compounds

The alkyl group of carboxylic acids can involve in halogenation to form haloacids. Halogenation takes place with chlorine or bromine in the presence of small quantities of red phosphorus which
form α-halo or β-haloacids. The reaction is called as Hell-Volhard-Zelinsky reaction.

RCH2-COOH + X2 $\xrightarrow[H_2O]{Red P}$ R-CH(X)-COOH

Bromination of carboxylic acids is more selective in nature and exclusively occurs at alpha position to form α-bromocarboxylic acids. However chlorination of carboxylic acid first occurs at alpha position, when all alpha position gets occupy, it occurs further along the chain.
More topics in Carboxylic Acid
Carboxylic acid functional group Carboxylic Acid Derivatives
Carboxylic Acid Reactions Naming Carboxylic Acids
Test for Carboxylic Acid
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