Carboxylic Acids- Aliphatic and Aromatic – Preparation and Properties

  • Post Author:

Carboxylic acids

Organic compounds containing carboxyl group (–COOH) as functional group are called carboxylic acids. Examples:

Classification of Carboxylic acids

On the basis of number of –COOH groups in their molecules carboxylic acids are classified as:

1. Monocarboxylic acids: The carboxylic acids containing one –COOH group in their molecule.

2. Dicarboxylic acids: The carboxylic acids containing two –COOH groups in their molecule.

3. Tricarboxylic acids: The carboxylic acids containing three –COOH groups in their molecule.

Isomerism in carboxylic acids

1. Chain isomerism: eg.

2. Functional isomerism:

Monocarboxylic acids show functional isomerism with ester. Eg.

General methods of preparation of monocarboxylic acids

1. By the oxidation of primary alcohols and aldehydes:

Primary alcohols are easily oxidized first to aldehyde and then to carboxylic acids. Eg.

2. By hydrolysis of alkyl cyanides [i.e. alkane nitriles]:

Complete hydrolysis of alkane nitriles give carboxylic acids. Eg.

3. By hydrolysis of 1,1,1-trihalides:

Carboxylic acids are obtained when 1,1,1-trihalides are hydrolysed in presence of strong alkalies like KOH. The unstable intermediate formed undergoes dehydration to give carboxylic acid. Eg.

4. From Grignard reagent:

When carbon dioxide gas is bubbled into the ethereal solution of Grignard reagent followed by subsequent hydrolysis with dilute acid then carboxylic acid is obtained.

5. From sodium alkoxides and carbonmonoxide:

When sodium alkoxide is heated with CO under pressure it gives sodium salt of carboxylic acid which upon subsequent acidification gives carboxylic acid. Eg.

6. From dicarboxylic acid:

A dicarboxylic acid having two –COOH groups on same carbon atom on heating undergoes decarboxylation giving a monocarboxylic acid. Eg.

Formic acid is prepared in the laboratory by the decarboxylation of oxalic acid with glycerol at 1100C.

7. Preparation of benzoic acid from the oxidation of alkyl benzene:

Benzoic acid can be obtained by oxidation of alkyl benzene with acidic KMnO4 and K2Cr2O7. During oxidation, side chain is oxidized to –COOH group irrespective of the length of the carbon chain. Eg.

Physical properties of carboxylic acids

1. Solubility: The first four carboxylic acids are soluble in water, the next two acids are slightly soluble. Acids having seven or more carbon atoms are insoluble in water. This is due to the fact that lower acids can form H-bond with water but with increased number of carbon atom the polarity of molecules decreases and it cannot form H-bond.

However, all carboxylic acids are soluble in less polar organic solvents such as ether, alcohol, etc.

2. Boiling point:

(Q) The boiling point of methanoic acid is higher than ethanol though they have same molecular mass, explain.

The boiling point of carboxylic acids is much higher than those of alcohols of comparable molecular mass. This is due to the fact that acids form stronger intermolecular H-bond than alcohol as the O-H bond in acids is more polarized due to presence of adjacent electron withdrawing C=O group. It is also due to the fact that carboxylic acid can form a cyclic dimer by forming hydrogen bond between –COOH group.

Acidity of carboxylic acids

Due to presence of polar O-H group, carboxylic acids ionize to give proton and hence behave as acids.

Both carboxylic acid as well as carboxylate anion are stabilized by resonance.

The resonating structures of carboxylic acid are:

The resonating structures of carboxylate anion are:

However, carboxylate anion is more stabilized by resonance as both the resonating structures of carboxylate anion are equivalent. Due to the more stabilized carboxylate ion the equilibrium lies very much in forward direction. Hence, carboxylic acids behave as acids.

Effect of substituents on acidic strength of carboxylic acids:

1. Effect of electron donating (releasing) substituent :

Q) Why is methanoic acid stronger than ethanoic acid?

Positive inductive effect (+I effect) of electron donating(releasing) groups like alkyl groups( CH3 – , CH3CH2 -, etc.) increases the electron density on O – H bond(group). It makes the release of H+ ion difficult. Therefore, the acidic nature decreases with the increase in + I effect. Hence, methanoic acid (formic acid) is stronger acid than ethanoic acid (acetic acid).

2. Effect of electron withdrawing substituent :

Q) Why is chloroacetic acid stronger than acetic acid?

Negative inductive effect (- I effect) of electron withdrawing groups like halogens, – NO2, -CN, etc. decreases the electron density on O – H bond (group). It makes the release of H+ ion easier. Therefore, the acidic nature increases with the increase in – I effect. Hence, chloroacetic acid is stronger acid than acetic acid.

Chemical properties of carboxylic acids

1. Acidic nature:

a. Reaction with metals:

Carboxylic acids react with active metals like Na, K, Ca, Zn, Mg, etc. forming their respective salt and hydrogen gas. Eg.

b. Reaction with alkalies:

Carboxylic acids can neutralize alkalies like NaOH or KOH to form salt and water. Eg.

c. Reaction with carbonates and bicarbonates:

Carboxylic acids decomposes metal carbonates and bicarbonates which produces effervesce due to liberation of CO2 gas. Eg.

4. Reaction with metal oxides:

Carboxylic acids react with basic metal oxides to form salt and water. Eg.

2. Reactions involving cleavage of –OH group:

a. Reaction with alcohols ( Formation of ester) :

Carboxylic acids react with alcohols in the presence of conc. H2SO4 or dry HCl to form esters. This reaction is called esterification reaction. Eg.

b. Reaction with ammonia(Formation of amide) :

Carboxylic acids react with ammonia to form ammonium salt which on heating give amides. Eg.

c. Reaction with PCl5, PCl3 or SOCl2 ( formation of acid chloride) :

Carboxylic acids react with phosphorus pentachloride(PCl5), phosphorus trichloride (PCl3) or thionyl chloride (SOCl2) to form acid chloride. Eg.

d. Formation of acid anhydrides (Dehydration):

Carboxylic acids on heating in the presence of dehydrating agent like P2O5 form acid anhydrides. Eg.

3. Reduction:

If carboxylic acids are reduced with Lithium aluminium hydride (LiAlH4), only the CO group of carboxylic acid is reduced to CH2 to yield alcohols. Eg.

4. Reactions involving alkyl group:

Halogenation : Hell-Volhard Zelinsky [HVZ] reaction:

Carboxylic acids (except formic acid) reacts with chlorine or bromine in presence of red phosphorous to give α-chloro or α-bromo acids. The reaction does not stop at monosubstituted product but continues till all α-hydrogen atoms are replaced.

Abnormal behaviour of formic acid

In formic acid molecule, the carboxylic acid group is attached to hydrogen atom (not to an alkyl group as in case of other higher members). As a result, it possesses dual functional groups i.e. carboxylic group as well as aldehyde group.

Hence, it behaves as an acid and an aldehyde.

1. Action with Tollen’s reagent: When formic acid is boiled with Tollen’s reagent, a silver mirror is deposited.

Acetic acid does not give this test.

2. Action with Fehling’s solution: When formic acid is warmed with Fehling’s solution, a brick red ppt. of cuprous oxide is obtained.

Acetic acid does not give this test.

Reactions of aromatic carboxylic acid (Benzoic acid)

1. Reactions due to carboxyl group:

Reaction due to benzene ring:

-COOH group present in benzoic acid is electron withdrawing group. Thus it deactivates benzene ring by decreasing electron density at ortho- and para- position. Electron density at meta position is comparatively high and hence electrophile attacks the benzene ring at meta position to give meta substituted product.

Q) Aqueous formic acid can not be dried by conc. H2SO4, P2O5 and solid KOH, why?

Aqueous formic acid can not be dried by conc. H2SO4, anhy.P2O5 and solid KOH because formic acid itself reacts with these compounds.

Aqueous formic acid can not be dried by conc. H2SO4, P2O5 and solid KOH, why?

EXERCISE

1. An organic compound ‘A’ C3H6O2 on reaction with ammonia followed by heating yield B. Compound B on reaction with Br2 and alc. NaOH gives compound C (C2H7N). Compound C forms a foul smelling compound D on reaction with chloroform and NaOH. Identify A, B, C, D and write the equations of reactions involved.

2. An organic compound ‘A’ on treatment with ethyl alcohol gives a carboxylic acid ‘B’ and a compound ‘C’. Hydrolysis of ‘C’ under acidic conditions gives ‘B’ and ‘D’. Oxidation of ‘D’ with KMnO4 also gives B. B upto heating with Ca(OH)2 gives ‘E’ (molecular formulae C3H6O). ‘E’ does not gives Tollen’s test and does not reduce Fehling solution , but forms a 2,4-dinitrophenyl hydrazone. Identify A,B,C,D and E with essential reacyions.

[Ans. A= (CH3CO)2O B= CH3COOH C= CH3COOC2H5 D=C2H5OH E=CH3COOCH3]

3. Compound A (C6H12O2) on reduction with LiAlH4 yielded two compounds B and C. The compound B on oxidation give D, which on treatment with aqueous alkali and subsequent heating furnished E. The latter, on catalytic hydrogenation gave C. The compound D was oxidized further to gave F, which was found to be a monobasic acid (mol.wt= 60). Deduce the structures of A,B,C,D and E.

[Ans. A=CH3CH2CH2COOCH2CH3 B=CH3CH2OH C= CH3CH2CH2CH2OH D=CH3CHO

E=CH3CH=CHCHO]

4. Compound (A) having molecular formula (C3H602) which upon ammonolysis gives (B). (B) when heated with Br2 in presence of KOH, gives (C). (C) when heated with nitrous acID gives compound (D) which gives positive iodoform test. (D) again on oxidation gives compound (E) which when reacted with ethyl alcohol gives pleasant smell. Find (A), (B), (C) (D) and (E).

5. An aliphatic compound (A) reacts with SOCl2 to give (B). The compound (B) is heated with ammonia to produce (C). The compound (C) is further heated with Br2/KOH to yield (D). The compound (D) gives (E) when treated with NaN02/HCI at low temperature. The compound (E) is primary alcohol which gives positive iodoform test. Identify (A), (B), (C), (D) and (E). Write reactions involved.

6. An unknown ester C5H1202 was hydrolysed with water and acid to give carboxylic acid (A) and alcohol (B). Treatment of (B) with PBr3 gave an alkyl bromide (C). When (C) was treated with KCN, a product (D) was formed which on acid hydrolysis gave the carboxylic acid (A). Give the structure and name of the original ester. Identify (A), (B), (C) and (D) and write equations for the reactions involved.

7. An organic compound ‘P’ reduces Tollen’s reagent and on oxidation with potassium permanganate forms a compound ‘Q’ having the same number of carbon atoms as ‘P’. ‘Q’ reacts with aq. Na2CO3 to give carbondioxide. ‘Q’ on reaction with ethanol in the presence of sulphuric acid forms an ester having molecular formula C4H8O2 ‘R’. identify P, Q and R and also write their IUPAC names.

8. You are given two test tubes, one containing methanoic acid and other ethanoic acid. Suggest a suitable chemical test to identify them. Give chemical reaction too.

9. Methanoic acid is different from its higher member and it gives reactions of both an acid and an aldehyde group.

a. Write one peculiar behavior of methanoic acid.

b. How does methanoic acid act upon (a) Methanol/H+ (b) conc.H2SO4 (c) acidified KMnO4.

10. There are carboxylic acid and ester as isomers of molecular formula C4H8O2.

a. Write the structural formula and IUPAC name of possible isomers of carboxylic acid and ester of given formula.

b. Which isomers produce CO2 from NaHCO3? Write the involved reaction.

c. Write an use of both types of isomers.

11. Acetic acid is mostly found in fruit juices.

a. Write a test reaction of acetic acid.

b. Write functional isomer of acetic acid.

c. How do you prepare acetic acid from- (i) sodium methoxide (ii) ethane nitrile (iii) chloroform

d. Acetic acid is treated with – (a)P2O5 (b) NaOH/CaO (c) Ca (d) PCl5

12. An alkene (A) with molecular formula (C7H14) on ozonolysis yields an aldehyde. The aldehyde is easily oxidized to an acid (B). When B is treated with bromine in presence of phosphorous it yields a compound (C) which on hydrolysis gives a hydroxyl acid (D). This acid can also be obtained from acetone by the reaction with hydrogen cyanide followed by hydrolysis. Identify A, B, C and D and write the chemical equations for the reactions involved.

13. Two isomeric compounds ‘A’ and ‘B’ have same molecular formula C3H5N. Predict the structure of A and B on the basis of following information:

a. A and B do not react with HNO2 or CH3COCl.

b. On refluxing with dil.HCl, A gives C, and B gives D, C and D being the monobasic acids.

c. Molecular mass of D is 74.

d. Write the reduction products of A and B.

14. Arrange the following compounds in order of their reactivity with reason:

(a) Acid amide (b) acid anhydride (c) ester (d) acid chloride.

Write short note on:

  1. Hell-Volhard-Zelinsky reaction
  2. Esterification reaction
  3. Carboxylation reaction
  4. Decarboxylation reaction
  5. Test of carboxylic acid
  6. Claisen condensation reaction

How will you distinguish:

1. Acid present in venom of bee and acid present in vinegar.

2. Ethanol and acetic acid.

[Ans. Ethanol gives iodoform test and acetic acid liberates CO2 from NaHCO3]

3. Phenol and benzoic acid.

4. Acetic acid and acetone

Account for the following:

  1. Chloroacetic acid is stronger acid than acetic acid.
  2. Formic acid is stronger acid than acetic acid.
  3. pKa of F-CH2-COOH is lower than that of Cl-CH2-COOH.
  4. Methanoic acid gives Tollen’s test.
  5. Carboxylic acids do not give the properties of aldehyde and ketone although both of them have carbonyl (>C=O) group.
  6. Boiling point of formic acid is higher than ethanol though both have same molecular mass.
  7. Dichloroacetic acid is stronger acid than monochloroacetic acid.
  8. Benzamide is less easily hydrolyzed than methyl methanoate.
  9. P2O5 is not used for the preparation of anhydrous formic acid.
  10. Methanoic acid is not halogenated.

What happens when:

  1. Methanoic acid is heated
  2. Tollen’s reagent is warmed with methanoic acid.
  3. Acetic acid is treated with sodium metal.
  4. Benzoic acid is treated with PCl5.
  5. Benzoic acid is treated with phenol in acidic medium.
  6. Benzoyl chloride is treated with H2 in presence of Pd and BaSO4.
  7. Ethanoic acid is heated with HI in presence of red phosphorous.
  8. Acetyl chloride reacts with phenol in presence of anhydrous AlCl3.
  9. Ethanoyl chloride and ammonia is heated with Br2 and aqueous KOH.
  10. Ethanoic anhydride is reduced.
  11. Ethanamide is reduced.

Convert:

  1. Benzene to benzoic acid.
  2. Aniline to benzoic acid.
  3. Methanoic acid to ethanoic acid and vice-versa.
  4. Benzonitrile to m-nitrobenzoic acid.
  5. Phenol to bemzoic acid and vice-versa.
  6. Ethanoyl chloride to methanol.
  7. Benzamide to toluene.
  8. Ethyl acetate to acetoacetic ester.