Introduction
Carboxylic acids are the compounds containing the carboxyl functional group −C(=O)−OH.
The carboxyl group is made up of carbonyl (>C=O) and hydroxyl (–OH) group.
Classification
(1) Carboxylic acids are classified as monocarboxylic acids, dicarboxylic acids, tricarboxylic acids etc. depending on the number of –COOH groups present in the molecule.
CH₃COOH
HOOC−CH₂−COOH
HOOC−CH(COOH)−CH₂COOH
(2) Monocarboxylic acids of aliphatic series are commonly known as fatty acids such as palmitic acid (C₁₅H₃₁COOH) and stearic acid (C₁₇H₃₅COOH).
(3) The general formula for monocarboxylic acids is CₙH₂ₙ₊₁COOH or CₙH₂ₙO₂. Where n = number of carbon atoms.
(4) The carboxylic acids may be aliphatic or aromatic depending upon whether –COOH group is attached to aliphatic alkyl chain or aryl group respectively.
Methods of Preparation of Monocarboxylic Acid
(1) By oxidation of alcohols, aldehydes and ketones
Methanoic acid cannot be prepared by oxidation method.
(2) By Hydrolysis of nitriles, ester, anhydrides and acid chloride
(3) From Grignard Reagent
(4) From Alkene or Hydro-carboxy-addition (Koch reaction)
(5) Special methods
Physical Properties of Monocarboxylic Acids
First three members (upto 3 carbon atoms) are colourless, pungent smelling liquids. The next six members are oily liquids having unpleasant smell. The higher members are colourless and odourless waxy solids.
The lower members of the aliphatic carboxylic acid family (upto C₄) are highly soluble in water. The solubility decreases with the increase in the size of the alkyl group. All carboxylic acids are soluble in alcohol, ether and benzene etc.
The solubility of lower members of carboxylic acids is due to the formation of hydrogen bonds between the –COOH group and water molecules.
Acetic acid exists in the solution in dimer form due to intermolecular hydrogen bonding. The observed molecular mass of acetic acid is 120 instead of 60.
(i) The melting points of carboxylic acids donot vary smoothly from one member to another.
(ii) The melting point of the acids having even number of carbon atoms are higher than those containing an odd number immediately above and below them.
(iii) The acids with even number of carbon atoms have the –COOH group and the terminal –CH₃ group on the opposite side of the carbon chain.
(iv) In the case of odd numbers, the two groups lie on the same side of the chain.
When the terminal groups lie on the opposite sides the molecules fit into each other more closely. More effective packing of the molecule in the lattice. Therefore, results into higher melting point.
Boiling point of carboxylic acids increase regularly with increase of molecular mass. Boiling points of carboxylic acids are higher than those of alcohols of same molecular mass. This is due to intermolecular hydrogen bonding between two acid molecules.
Acidic Nature of Monocarboxylic Acids
(1) Cause of acidic nature
R−C(OH)=O
R−C(O⁻)=O⁺H
(i) A molecule of carboxylic acid can be represented as a resonance hybrid of the following structures.
(ii) Due to electron deficiency on oxygen atom of the hydroxyl group (Structure II), their is a displacement of electron pair of O–H bond toward the oxygen atom. This facilitate the release of hydrogen as proton (H⁺).
(iii) The resulting carboxylate ion also stabilized by resonance (As negative charge is dispersed on both the oxygen atom). This enhance the stability of carboxylate anion and make it weaker base or strong acid.
(2) Effect of substituent on acidic nature
(i) An electron withdrawing substituent (–I effect) stabilizes the anion by dispersing the negative charge and therefore increases the acidity.
(ii) An electron releasing substituent (+I effect) stabilizes negative charge on the anion resulting in the decrease of stability and thus decreased the acidity of acid.
Electron with drawing nature of halogen : F > Cl > Br > I
Thus, the acidic strength decreases in the order : FCH₂COOH > ClCH₂COOH > BrCH₂COOH > ICH₂COOH
Similarly : CCl₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH
(iii) Inductive effect is stronger at α-position than β-position similarly at β-position it is more stronger than at γ-position
Example: Cl−CH₂−CH₂−CH₂COOH > Cl−CH₂−CH₂COOH > Cl−CH₂COOH
(iv) Relative acid strength in different compounds RH > NH₃ > HC≡CH > ROH > H₂O > RCOOH
Greater the value of Kₐ or lesser the value of pKₐ stronger is the acid, i.e. pKₐ = –log Kₐ
Acidic nature (Kₐ) α 1/molecular weight
Kₐ Value HCOOH 17.5×10⁻⁵ > CH₃COOH 1.75×10⁻⁵ > C₂H₅COOH 1.3×10⁻⁵
The formic acid is strongest of all fatty acids.
Acetic acid is less weak acid than sulphuric acid due to less degree of ionisation.
Chemical Properties of Monocarboxylic Acids
(1) Reaction involving removal of proton from –OH group
All carboxylic acids turn blue litmus red.
Reaction of carboxylic acid with aqueous sodium carbonates solution produces bricks effervescence. However most phenols do not produce effervescence. Therefore, this reaction may be used to distinguish between carboxylic acids and phenols.
(2) Reaction involving replacement of –OH group
(a) The reaction is shifted to the right by using excess of alcohol or removal of water by distillation.
(b) The reactivity of alcohol towards esterification. tert-alcohol < sec-alcohol < pri-alcohol < methyl alcohol
(c) The acidic strength of carboxylic acid plays only a minor role. HCOOH < COOHCH₃ < RCH₂COOH < R₂CHCOOH < R₃CCOOH
When methanol is taken in place of ethanol, then reaction is called trans esterification.
(3) Reaction involving carbonyl (>C = O) group
Carboxylic acid are difficult to reduce either by catalytic hydrogenation or Na/C₂H₅OH
(4) Reaction involving attack of carboxylic group (–COOH)
When sodium formate is heated with sodalime H₂ is evolved. (Exception) HCOONa + NaOH →[CaO] H₂ + Na₂CO₃
In Hunsdiecker reaction, one carbon atom less alkyl halide is formed from acid salt.
In Schmidt reaction, one carbon less product is formed.
In the above reaction, the –COOH group is reduced to a CH₃ group.
(5) Reaction involving hydrogen of α-carbon Halogenation
Carboxylic acid having an α-hydrogen react with Cl₂ or Br₂ in the presence of a small amount of red phosphorus to give α-chloro acetic acid. The reaction is known as Hell Volhard-zelinsky reaction.
Individual Members of Monocarboxylic Acids
Formic Acid or Methanoic acid (HCOOH)
Formic acid is the first member of monocarboxylic acids series. It occurs in the sting of bees, wasps, red ants, stinging nettles, and fruits. In traces it is present in perspiration, urine, blood and in caterpillar's.
(i) Oxidation of methyl alcohol or formaldehyde
(ii) Hydrolysis of hydrocyanic acid : Formic acid is formed by the hydrolysis of HCN with acids or alkalies.
(iii) Laboratory preparation
Glycerol + Oxalic acid →[100-110°C] Glycerol monooxalate →[120°C] Glycerol monoformate →[Hydrolysis] HCOOH
The following procedure is applied for obtaining anhydrous formic acid.
(iv) Industrial preparation : Formic acid is prepared on industrial scale by heating sodium hydroxide with carbon monoxide at 210°C under a pressure of about 10 atmospheres.
Sodium formate thus formed is distilled with sodium hydrogen sulphate, when anhydrous formic acid distils over.
(i) It is a colourless pungent smelling liquid.
(ii) It melts at 8.4°C and boils at 100.5°C.
(iii) It is miscible with water, alcohol and ether. It forms azeotropic mixture with water.
(iv) It is strongly corrosive and cause blisters on skin.
(v) It exists in aqueous solution as a dimer involving hydrogen bonding.
(i) In the laboratory for preparation of carbon monoxide.
(ii) In the preservation of fruits.
(iii) In textile dyeing and finishing.
(iv) In leather tanning.
(v) As coagulating agent for rubber latex.
(vi) As an antiseptic and in the treatment of gout.
(vii) In the manufacture of plastics, water proofing compounds.
(viii) In electroplating to give proper deposit of metals.
(ix) In the preparation of nickel formate which is used as a catalyst in the hydrogenation of oils.
(x) As a reducing agent.
(xi) In the manufacture of oxalic acid.
Acetic Acid (Ethanoic Acid) (CH₃COOH)
Acetic acid is the oldest known fatty acid. It is the chief constituent of vinegar and hence its name (Latin acetum = vinegar)
(i) By oxidation of acetaldehyde (Laboratory-preparation)
(ii) By hydrolysis of methyl cyanide with acid
(iii) By Grignard reagent
(iv) By hydrolysis of acetyl chloride, acetic anhydride or acetamide and ester
(v) Manufacture of acetic acid
(a) From ethyl alcohol (Quick vinegar process) : Vinegar is 6-10% aqueous solution of acetic acid. It is obtained by fermentation of liquors containing 12 to 15% ethyl alcohol. Fermentation is done by Bacterium Mycoderma aceti in presence of air at 30-35°C. The process is termed acetous fermentation.
It is a slow process and takes about 8 to 10 days for completion.
In this process, the following precautions are necessary:
• The concentration of the ethyl alcohol should not be more than 15%, otherwise the bacteria becomes inactive.
• The supply of air should be regulated. With less air the oxidation takes place only upto acetaldehyde stage while with excess of air, the acid is oxidised to CO₂ and water.
• The flow of alcohol is so regulated that temperature does not exceed 35°C, which is the optimum temperature for bacterial growth.
Acetic acid can be obtained from vinegar with the help of lime. The calcium acetate crystallised from the solution is distilled with concentrated sulphuric acid when pure acetic acid distils over.
(b) From acetylene : Acetylene is first converted into acetaldehyde by passing through 40% sulphuric acid at 60°C in presence of 1% HgSO₄ (catalyst).
The acetaldehyde is oxidised to acetic acid by passing a mixture of acetaldehyde vapour and air over manganous acetate at 70°C.
Acetylene required for this purpose is obtained by action of water on calcium carbide.
The yield is very good and the strength of acid prepared is 97%. The method is also quite cheap.
(c) By the action of CO on methyl alcohol : Methyl alcohol and carbon monoxide react together under a pressure of 30 atmospheres and 200°C in presence of a catalyst cobalt octacarbonyl, Co₂(CO)₈ to form acetic acid.
(i) At ordinary temperature, acetic acid is a colourless, corrosive liquid with a sharp pungent odour of vinegar. It has a sour taste.
(ii) Below 16.5°C, it solidifies as an icy mass, hence it is named glacial acetic acid.
(iii) It boils at 118°C. The high boiling point of acetic acid in comparison to alkanes, alkyl halides or alcohols of nearly same molecular masses is due to more stronger hydrogen bonding between acid molecules. This also explains dimer formation of acetic acid in vapour state.
(iv) It is miscible with water, alcohol and ether in all proportions.
(v) It is good solvent for phosphorus, sulphur, iodine and many organic compounds.
(i) As a solvent and a laboratory reagent.
(ii) As vinegar for table purpose and for manufacturing pickles.
(iii) In coagulation of rubber latex.
(iv) For making various organic compounds such as acetone, acetic anhydride, acetyl chloride, acetamide and esters.
(v) For making various useful metallic acetates, such as:
(a) Basic copper acetate which is used for making green paints.
(b) Al, Fe and Cr acetates which are used as mordants in dyeing.
(c) Lead tetra-acetate which is a good oxidising agent.
(d) Basic lead acetate which is used in the manufacture of white lead.
(e) Aluminium acetate which is used in the manufacture of water-proof fabrics.
(f) Alkali acetates which are used as diuretics.
Table : 28.1 Comparison of Formic Acid and Acetic Acid
| Property | Formic acid | Acetic acid |
|---|---|---|
| 1. Acidic nature, (i) With electro-positive metals | Forms salts, Hydrogen is evolved. | Forms salts. Hydrogen is evolved. |
JEE Main Weightage
Typically 2-3 questions from Carboxylic Acids and Derivatives. Focus on preparation methods, acidic strength, named reactions (HVZ, Esterification, Decarboxylation), and distinction tests.