CBSE Class 12 Chemistry Notes: Carboxylic Acids – Important Questions for JEE

Prob 5. Two organic compounds A and B have vapour densities 15 and 30 respectively. (A) reduces Fehling solution, but does not react with Na₂CO₃. (B) does not reduce Fehling solution but gives effervescences with Na₂CO₃. B when treated with a concentrated solution of base followed by prolonged heating gives a compound C(C₃H₆O). Identify A, B and C.

Sol.

Prob 6.An organic acid A, C₃H₄O₃ is catalytically reduced in presence of ammonia to give B, C₃H₇NO₂. B reacts with acetyl chloride, hydrochloric acid and alcohols. It can also react with nitrous acid to give another compound C, C₃H₆O₃, along with the evolution of nitrogen. What are A, B and C. Give reasons?

Sol.      Compound A is acid having one –COOH group only, the remaining part C₂H₃O can be

on catalytic reduction keto group is converted into secondary alcohol which with ammonia will give amino acid, i.e.,

with nitrous acid, B, react to give

Prob 7.    Compound A (C₆H₁₂O₂) on reduction yields two compounds B and C. The compound B on oxidation gave D, which on treatment with aqueous alkali and subsequent heating furnished E. The latter on catalytic hydrogenation gave C. The compound D was oxidised further to give F which was found to be a monobasic acid (mw = 60). Deduce the structures of A to E. 

Sol.

From the molecular weight data F comes out to be CH₃COOH. If we work back B is CH₃CH₂­OH and D is CH₃CHO.

From here we can deduce that reduction of A with LiAlH₄ gives two alcohols. So A must be an ester. Hence (A) comes out to be CH₃CH₂CH₂COOC₂H₅.

 Prob 8.    An organic compound (A) on treatment with acetic acid in the presence of sulfuric acid produces an ester (B). (A) on mild oxidation gives (C). (C) with concentrated KOH followed by acidification with dil. HCl regenerates (A) and produces (D). (D) with phosphorous pentachloride followed by reaction with ammonia gives (E). (E) on dehydration produces hydrocyanic acid. Identify compounds A, B, C, D and E.

Sol.

(A) reacting with acetic acid is giving an ester (B). So (A) must be an alcohol. (A) alcohol on mild oxidation gives  (C). Hence (C) is an aldehyde. (C) with conc. KOH

followed by acidification is yielding (A) alcohol and  (D). So this reaction is cannizzaro reaction. So A must be CH₃OH.

Prob 9.    An aromatic compound A on treatment with CHCl₃ and KOH gives B & C, both of which, in turn give the same compound D when distilled with Zn dust.Oxidation of D yields E of formula C₇H₆O₂. The sodium salt of E on heating with soda lime gives F which can also be obtained by distilling A with Zn dust. Identify A, B, C, D, E and F.

 Sol.         Molecular formula of (E) is C₇H₆O₂ and reaction of its sodium salt with soda lime (decarboxylation) to form (F) indicates that (E) and (F) should be C₆H₅COOH and C₆H₆ respectively. Since (F) is also obtained from (A) by reaction with Zn dust, it indicates that (A) should be phenol. Nature of (A) as phenol is confirmed by the fact that it explains all the given reactions.

 

Prob 10.           Five isomeric para – disubstituted aromatic compounds A to E with molecular formula C₈H₈O₂ were given for identification. Based on the following observations, give structures of the compounds?

i)  both A and B form a silver mirror with Tollen’s reagents; B gives a positive test with FeCl₃ solution also.

 (ii)  C gives positive iodoform test.

 (iii)  D is readily extracted in NaHCO₃ solution.

(iv)  E on acid hydrolysis gives 1, 4 dihydroxy benzene. 

Sol. 

(A) and (B) gives tollen’s test. Hence must contain aldehyde group. But (B) shows phenolic test also. Hence (A) & (B) are

(C) shows Iodoform test show it must contain  group. Therefore (C) is

(D) shows presence of carboxylic group. Hence (D) and (E) are

Prob 11.  An organic compound A, C₈H₄O₃ in dry benzene in the presence of anhydrous AlCl₃ gives compound B. The compound B on treatment with PCl₅, followed by reaction with H₂/Pd (BaSO₄) gives compound C, which on reaction with hydrazine gives cyclised compound D (C₁₄H₁₀N₂). Identify A, B, C and D.

 

Sol.  Nature of reagents in the conversion of A to B indicates that the reaction must be  Friedal – crafts reaction.

Prob 12.  An organic compound A, C₆H₁₀O on reaction with CH₃MgBr followed by acid treatment gives compound B. The compound B on ozonolysis gives compound C, which in presence of a base gives 1- acetyl cyclopentene. The compound B on reaction with HBr gives compound D. Write the structures of A, B, C and D.

 

Sol.  As B undergoes ozonolysis to form C, if must have a double bond. C in the presence of base gives a, b unsaturated ketone, so it is aldol condensation.

 

Prob 13.  A phenolic compound (A), C₇H₆O₂ on mild oxidation gives a highly volatile oil (B).
A forms (C) on reaction with dimethyl sulphate in alkali. Oxidation of (C) with hot KMnO₄ gives (D), the silver salt of which reacts with bromine water followed by heating gives (E) containing about 72% bromine. Give structures A to E. 

Sol.

But it must be ortho because on oxidation, it gives a highly volatile oil indicating that there is intramolecular hydrogen bonding.

Prob 14.  An aromatic compound (A) gave a mixture of two isomeric products B and C on reaction with NH₂OH. C rearranged to D (C₈H₉NO) on heating with H₂SO₄. D on hydrolysis produced E and F. A was oxidised with per benzoic acid to G. Hydrolysis of G gave H and E. An anhydride of E and its sodium salt on condensation with PHCHO produced cinnamic acid. H on reaction with phthalic anhydride in H₂SO₄ gave phenolphthalein. Suggest structures for A to H.

Sol.



Formation of cinnamic acid proves that the anhydride is acetic anhydride. Hence E is acetic acid.

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