Short Answer Questions

Carbonyl Compounds and Carboxylic Acids | Chemistry

Short Answer Questions

1. How is propanoic acid is prepared starting from

(a) an alcohol (b) an alkylhalide (c) an alkene

(a) an alcohol:

Oxidation of propanol gives propanoic acid

(b) an alkylhalide

Ethyl bromide treated with KCN gives ethyl cyanide which on further hydrolysis produce propanoic acid

(c) an alkene

Oxidation of 3-hexene with acidified KMnO₄ gives propanoic acid

2. A Compound (A) with molecular formula C2H3N on acid hydrolysis gives(B) which reacts with thionylchloride to give compound(C). Benzene reacts with compound (C) in presence of anhydrous AlCl3 to give compound(D). Compound (D) on reduction with Zn/Hg and Conc.HCl gives (E). Identify (A), (B), (C), (D) and (E). Write the equations.

A Compound (A) with molecular formula C₂H₃N is CH₃CN on acid hydrolysis gives acetic acid (B).

Acetic acid (B) which reacts with thionylchloride gives acetyl chloride (C)

CH₃COOH + SOCl₂ → CH₃COCl + HCl + SO₂

Benzene reacts with acetyl chloride (C) in presence of anhydrous AlCl₃ gives acetophenone (D)

Acetylchloride on reduction gives Acetaldehyde (E)

3. Identify X and Y.

CH3 COCH2CH2 COOC2H5 -----CH3 MgBr→ X ----H3O+→Y

4. Identify A, B and C

5. A hydrocarbon A (molecular formula C₈H₁₀) on ozonolysis gives B(C₄H₆O₂) only. Compound C (C₃H₅Br) on treatment with magnesium in dry ether gives (D) which on treatment with CO₂ followed by acidification gives(B). Identify A, B and C.

Answer:

6. Identify A, B, C and D

7. An alkene (A) on ozonolysis gives propanone and aldehyde (B). When (B) is oxidised (C) is obtained. (C) is treated with Br2/P gives (D) which on hydrolysis gives (E). When propanone is treated with HCN followed by hydrolysis gives (E). Identify A, B, C, D and E.

Answer:

2, 4-dimethylpent-2-ene on ozonolysis gives propanone and isobutyraldehyde.

Oxidation of isobutyraldehyde (B) gives isobutyric acid.

isobutyric acid with P / Br2 gives α – bromo substituted compound D

Hydrolysis of D gives 2- hydroxyl – 2 – methylpropanoic acid

A → 2,4 – dimethyl pent-2-ene

B → Isobutyraldehyde

C → Isobutyric acid

D → 2-bromo-2-methylpropanoic acid

E → 2-hydroxy-2-methylpropanoic acid

8. How will you convert benzaldehyde into the following compounds?

(i) benzophenone (ii) benzoic acid (iii) β-hydroxyphenylaceticacid.

9. What is the action of HCN on

(i) propanone (ii) 2,4-dichlorobenzaldehyde. iii) ethanol

10. A carbonyl compound A having molecular formula C5H10O forms crystalline precipitate with sodium bisulphate and gives positive iodoform test. A does not reduce Fehling solution. Identify A.

• A does not reduce Fehling solution.

Therefore A is ketone.

• A gives positive iodoform test. A contains -COCH₃ group.

• A has molecular formula C₅H₁₀O and one - COCH₃ group. Hence A is 2-pentanone.

CH₃—CO—CH₂—CH₂—CH₃

              2-pentanone

11. Write the structure of the major product of the aldol condensation of benzaldehyde with acetone.

CH₃COCH₃ --^OH- → ^-CH₂COCH₃

 acetone

12. How are the following conversions effected

(a) propanal into butanone

(b) Hex-3-yne into hexan-3-one.

(c) phenylmethanal into benzoic acid

(d) phenylmethanal into benzoin

13. Complete the following reaction.

14. Identify A, B and C

15. Oxidation of ketones involves carbon – carbon bond cleavage. Name the product (s) is / are formed on oxidising 2,5 – dimethyhexan – 3- one using strong oxidising agent.

Ketones are not easily oxidized under drastic condition (or) with powerful oxidizing agent like con.HNO₃, H⁺/KMnO₄, H⁺/ K₂Cr₂O₇, cleavage of carbon - carbon bond takes place to give mixture of carboxylic acids having less number of carbon atom than the parent ketone.

The oxidation of unsymmetrical ketones is governed by popoff's rule. It states that during the oxidation of an unsymmetrical ketone, a (C -CO) bond is cleaved in such a way that the keto group stays with the smaller alkyl group.

16. How will you prepare

i. Acetic anhydride from acetic acid

ii. Ethylacetate from methylacetate

iii. Acetamide from methylcyanide

iv. Lactic acid from ethanal

v. Acetophenone from acetylchloride

vi. Ethane from sodium acetate

vii. Benzoic acid from toluene

viii. Malachitegreen from benzaldehyde

ix. Cinnamic acid from benzaldehyde

x. Acetaldehyde from ethyne

Answer:

i) Acetic anhydride from acetic acid:

Acetic acid on heating in the presence of a strong dehydrating agent such as P₂O₅ forms acid anhydride.

ii) Ethyl acetate from methyl acetate:

Methyl acetate is treated with ethanol to form ethyl acetate.

iii) Acetamide from methyl cyanide:

Partial hydrolysis of alkyl cyanides with cold con.HCl gives amides.

iv) Lactic acid from ethanal:

Ethanol is treated with HCN to form acetaldehyde cyanohydrins and then hydrolysed with H₃O⁺ gives lactic acid.

v) acetophenone from acetyl chloride:

Acetyl chloride is treated with benzene in presence of AlCl₃ and H₂O gives acetophenone.

 vi) Ethane from sodium acetate:

The aqueous solutions sodium or potassium salts of carboxylic acid on hydrolysis gives alkanes at anode. This reaction is called Kolbes electrolysis.

vii) Benzoic acid from toluene:

The oxidation of alkyl benzene with chromic acid or acidic or alkaline KMnO₄ to form benzoic acid.

viii) Malachite green from benzaldehyde:

Benzaldehyde condenses with N, N - dimethyl aniline in the presence of strong acids forms malachite green.

ix) Cinnamic acid from benzaldehyde: (Perkin's reaction)

Benzaldehyde is treated with acetic anhydride, condensation takes place and cinnamic acid is obtained.

x) Acetaldehyde from ethyne:

The hydration of alkynes in presence of 40% dilute H₂SO₄ and 1% HgSO₄ to give corresponding aldehyde.

EVALUATE YOURSELF:

1. Write the IUPAC name for the following compound

2. Write all possible structural isomers and position isomers for the ketone represented by the molecular formula C₅H₁₀O.

3. What happens when the following alkenes are subjected to reductive ozonolysis?

1) propene 2) 1 - Butene 3) Isobutylene

4. What happens when n-propyl benzene is oxidised using H⁺ / KMnO₄?

n-propyl benzene is oxidised using H⁺ / KMnO₄ gives benzoic acid

5. How will you prepare benzoic acid using Grignard reagent?

Phenyl magnesium bromide reacts with carbon dioxide (dry ice) followed by acidification gives benzoic acid

6. Why is acid anhydride are preferred to acyl chloride for carrying out acylation reactions?

i) Acid anhydride is safer to use than acyl chloride. It is less corrosive and not so readily hydrolysed (Its reaction with water is slower)

ii) Acid anhydride does not produce dangerous (corrosive and poisonous) fumes of HCl 

CARBONYL COMPOUNDS