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Carboxylic acids are organic compounds containing carboxyl (–COOH) groups. The carboxyl group derives its name from carbonyl (C=O) and hydroxyl (-OH) groups, which it is made up of. Here it may be noted that the carboxyl group shows different properties from the carbonyl and hydroxyl group.
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Keyterms: Carboxyl Group, Carbonyl Group, Hydoxyl group, Organic Compounds, Hydrocarbons, Hydrogen, Hydrocarbon
What are Carboxylic Acids?
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Carboxylic acids are considered to be the derivatives of hydrocarbons, which are obtained by the displacement of one or more hydrogen atoms by carboxylic radicals. Carboxylic acids are also considered as oxidation products of hydrocarbons.
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| Dimethyl Sulfoxide | Gattermann Reaction | Malonic Acid |
| 2 4 dinitrophenylhydrazine | Acetamide | Acetonitrile |
| Aldehydes, Ketones and Carboxylic | Clemmensen reduction | Uses of Carboxylic Acids |
Nomenclature and Isomerism
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The carboxyl group is generally written as –COOH. The carboxylic acid may be aliphatic (R–COOH) or aromatic (Ar–COOH) depending upon whether –COOH is attached to an alkyl group (or a hydrogen atom) or an aryl group.
The acidic character of carboxylic acid is due to the presence of replaceable hydrogen atoms in the carboxyl group. Since, one carboxyl acid contains one replaceable hydrogen, an acid containing one carboxyl group is monobasic and known as monocarboxylic acid.Similarly,acids containing two or three carboxyl groups are designated as dicarboxylic (dibasic) and tricarboxylic( tribasic) acids respectively.
Classification of Carboxylic Acids
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On the basis of the number of carboxyl (–COOH) groups they are classified into:
Monocarboxylic Acids: Organic acids having one carboxyl group is known as monocarboxylic acids. Aliphatic monocarboxylic acids are known as fatty acids because some of their higher members like palmitic acid (C15H31COOH) and stearic acid (C17H35COOH) were first obtained by hydrolysis of fats.
Their general formula is CnH2nO2 or CnH2n-1COOH. They are monobasic in nature. Example: Formic acid (HCOOH), acetic acid (CH3COOH), etc.
Dicarboxylic Acids: Organic acids having two carboxyl groups are known as dicarboxylic acids.
Tricarboxylic Acids: Organic acids having three carboxyl groups are known as tricarboxylic acids.
Physical Properties of Carboxylic Acids
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Physical State: The first three acids (aliphatic) are colourless, pungent smelling and corrosive liquids. Carboxylic acids from C4 to C9 are oily liquids with an odour of rancid butter. Higher acids (C > 10) are odourless waxy solids with densities much less than water.
Melting and Boiling Point: The boiling point of a carboxylic acid is quite higher even to that of an alkane and alcohol of comparable molecular mass.
Solubility: Due to the possibility of the formation of hydrogen bonds with water molecules; carboxylic acids are soluble in water.
Structure of Carboxylic Acid
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Modern spectroscopic and diffraction studies suggest that the acid group (–COOH) is planar. That means the carbon atom of the (–COOH) group and the two oxygen atoms are sp2 hybridized and the alkyl carbon atoms are sp3 hybridized.
(i) The –R group has one sp3 orbital unpaired, so it overlaps axially with the sp2 orbital of carbon forming σ- bond.
(ii) The carbon atom is sp2 hybridized. The remaining pz orbital is perpendicular to the plane of three sp2 orbitals. One sp2 orbital overlaps with the sp3 orbital of –R group. The remaining two overlap with the sp2 orbital of two oxygen atoms forming two σ- bonds.
(iii) One oxygen is sp2 hybridized. The remaining pz orbital lies perpendicular to the plane. One sp2 orbital overlaps with the sp2 orbital of carbon forming σ- bond. Other two sp2 orbitals have lone pairs of electrons.
(iv) Second oxygen is also coplanar of which one sp2 orbital overlaps with the s orbital of hydrogen forming σ- bond.
In the complete orbital picture, all the three pz orbitals lying perpendicular to the plane can overlap laterally forming π-bonds. So, these are delocalized π orbital.
General Methods of Preparation of Carboxylic Acids
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- Oxidation Method: The oxidation of 10 alcohol or aldehyde produces carboxylic acid with the same number of carbon atoms. The oxidation is carried out with acidified K2Cr2O7 or KMnO4.
- By the oxidation of alkyl benzene: Aromatic carboxylic acid can be prepared by the oxidation of alkyl benzene with alkaline potassium permanganate, acidic potassium dichromate or even with dilute HNO3.
- Hydrolysis Method: Acid or alkaline hydrolysis of ester gives carboxylic acid. Example: When ethyl acetate is reacted with mineral acid (HCL or H2SO4), acetic acid is formed.
- Heating gem-dicarboxylic acid: The dicarboxylic acids in which both –COOH groups are linked to the same carbon atom give monocarboxylic acids on heating. This method cannot be used for formic acid (HCOOH)
- From Grignard reagent: The carbonation of Grignard reagent or organolithium gives an adduct, the hydrolysis of which produces acid. For the purpose of carbonation, dry ice (solid CO2) is used or CO2 is bubbled through ethereal solution of Grignard reagent or organolithium compound. The carboxylic acid obtained depends upon the organometallic compound taken. The acid formed contains one carbon atom more than the starting organometallic compound.
- Carboxylation of alkenes: Carboxylation means addition of carbon monoxide. Alkenes are heated with CO and steam under pressure and temperature in the presence of phosphoric acid. Higher fatty acids are obtained by this process. The method is known as Koch reaction.
- Reaction of Sodium alkoxide and carbon monoxide: Sodium alkoxide is heated under pressure with CO and the product is acidified to give the acid.
- By catalytic oxidation of long chain hydrocarbons: Air is passed through a long chain hydrocarbon at 120o C in the presence of manganese stearate as catalyst, when carboxylic acid is formed.
- Reaction of acid amides with nitrous acid: The acid amides on treatment with HNO2 give parent carboxylic acids.
Common and IUPAC Names of Carboxylic Acids
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| S.No. | Formula | Common Name | IUPAC Name |
|---|---|---|---|
| 1. | HCOOH | Formic acid | Methanoic acid |
| 2. | CH3COOH | Acetic acid | Ethanoic acid |
| 3. | CH3CH2COOH | Propionic acid | Propanoic acid |
| 4. | CH3(CH2)2COOH | Butyric acid | Butanoic acid |
| 5. | CH3(CH2)3COOH | Valeric acid | Pentanoic acid |
| 6. | CH3(CH2)4COOH | Caproic acid | Hexanoic acid |
| 7. | C4H8O2 | Isobutyric acid | 2-methylpropanoic acid |
| 8. | C4H7BrO2 | α – bromobutyric acid | 2-bromobutanoic acid |
| 9. | CH3(CH2)8COOH | Capric acid | Decanoic acid |
| 10. | CH3(CH2)10COOH | Lauric acid | Dodecanoic acid |
| 11. | CH3(CH2)12COOH | Myristic acid | Tetradecanoic acid |
| 12. | CH3(CH2)14COOH | Palmitic acid | Hexadecenoic acid |
| 13. | CH3(CH2)16COOH | Stearic acid | Octadecanoic acid |
Things To Remember
- Carboxylic acids are considered to be the derivatives of hydrocarbons.
- The acidic character of carboxylic acid is due to the presence of replaceable hydrogen atoms in the carboxyl group
- On the basis of the number of carboxyl (–COOH) groups they are classified into monocarboxylic acid, dicarboxylic acid and tricarboxylic acid.
- Modern spectroscopic and diffraction studies suggest that the acid group (–COOH) is planar.
- Carboxylic acids are soluble in water.
- The boiling point of a carboxylic acid is quite higher even to that of an alkane and alcohol of comparable molecular mass.
- Depending upon the type of group attached to COOH (alkyl or aryl), these are classified into aliphatic and aromatic compounds.
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Sample Questions
Ques. Why are carboxylic acids called fatty acids? (2 marks)
Ans. Carboxylic acids are called fatty acids because higher carboxylic acids are obtained from the hydrolysis of fats and oils.
Ques. Illustrate the following name reactions Hell-Volhard-Zelinsky reaction. (CBSE 2010) (2 marks)
Ans. Carboxylic acid reacts with chlorine or bromine in presence of small quantities of red phosphorus to give exclusively α-chloro or α-bromo acids.
Ques. Acetic acid is weaker than formic acid. Why? (CBSE 2011) (3 marks)
Ans. After the release of H+, formic acid gives formate ion and acetic acid gives acetate ion. Due to the +I effect of –CH3 group in acetate ion, the negative charge on the oxygen acetate is further increased, hence stability of acetate ion will decrease. No such +I effect in formate ion.Therefore, formic acid will comparatively dissociate more into HCOO— and H+ than acetic acid.
Ques. The boiling points of acids are higher than the corresponding alcohols. Give reasons. (CBSE 2014) (2 marks)
Ans. The boiling points of acids are higher due to the presence of intermolecular hydrogen bonding between the two molecules of acids. Due to the formation of dimer, the effective molecular mass becomes just double than the normal mass which increases the boiling point. Moreover, the hydrogen bonding in acid is stronger than alcohols because of the more polar –OH group of acid than alcohols which further increases the boiling point of acids.
Ques. Why is chloroacetic acid a stronger acid than acetic acid? (CBSE 2004) (3 marks)
Ans. The acidic nature of carboxylic acids is due to its ability to release H+.
In case of phenoxide ion, structures (V – VII) carry a negative charge on the less electronegative carbon atom.Therefore, their contribution towards the resonance stabilization of phenoxide ion is very small.
In structures I and II, (carboxylate ion), the negative charge is delocalized over two oxygen atoms while in structures III and IV, the negative charge on the oxygen atom remains localized only the electrons of the benzene ring are delocalized. Since delocalization of benzene electrons contributes little towards the stability of phenoxide ion therefore, carboxylate ion is much more resonance stabilized than phenoxide ion. Thus, the release of a proton from carboxylic acids is much easier than from phenols. In other words, carboxylic acids are stronger acids than phenols.
Ques. What is Decarboxylation? (2 marks)
Ans. Carboxylic acids lose carbon dioxide to form hydrocarbons when their sodium salts are heated with soda lime. The reaction is known as decarboxylation.abilizes the carboxylate ion would facilitate the release of H+ and thus increase the acid strength. The electron withdrawing group increases the acid strength. Due to the electron withdrawing effect of chlorine atoms, the negative charge of carboxylate ions formed gets dispersed. As such, this ion gets stabilized and is therefore more easily formed.
But acetate ions get destabilized due to electron releasing effects of the methyl group and are not formed easily. Hence we can say that; chloroacetic acid is more acidic than acetic acid.
Ques. The treatment of an ester with LiAIH4 followed by acid hydrolysis produces what? (2 marks)
Ans. The treatment of an ester with LiAIH4 followed by acid hydrolysis produces two alcohols.
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