MP Board Class 12th Chemistry Important Questions Chapter 12 Aldehydes, Ketones and Carboxylic Acids

MP Board Class 12th Chemistry Important Questions Chapter 12 Aldehydes, Ketones and Carboxylic Acids

Aldehydes, Ketones and Carboxylic Acids Important Questions

Aldehydes, Ketones and Carboxylic Acids Short Answer Type Questions

Question 1.
Arrange the following compounds in increasing order of their boiling points :
CH₃CHO, CH₃CH₂OH, CH₃OCH₃, CH₃CH₂CH₃. (NCERT)
Answer:
CH₃CH₂CH₃ < CH₃O CH₃ < CH₃CHO < CH₃CH₂OH.
This order can be predicted on the basis of inter-molecular force operating between them, these are having comparable molecular mass. CH₃CH₂OH undergoes the strongest H – bonding. In CH₃OCH₃ and CH₃CHO dipole – dipole attraction is more in CH₃CHO, since CH₃CHO is more polar than CH₃OCH₃ therefore its boiling point is more than CH₃ – O – CH₃. Propane being non – polar therefore, weak van der Waals’ forces exist between them.

Question 2.

1. Why ketones are less reactive than aldehydes?
2. Benzaldehyde is less reactive than Acetaldehyde. Why?

Answer:
1. Ketones are less reactive than aldehydes because in ketones there are two alky group attached with carbonyl group, due to the positive inductive effect (+I) of both the alkyl group the positive charge on carbon atom decreases. Hence, the sensitivity of ketones to the nucleophilic reagents decreases. In aldehydes, they have only one alkyl group so they are more reactive than ketones.

2. – CHO group of benzaldehyde becomes stable due to resonance with benzene ring whereas resonance is not found in acetaldehyde. Benzaldehyde is aromatic and alde – hyde is aliphatic.

Question 3.
How is urotropine obtained from formaldehyde? Write its chemical name and structural formula.
Answer:
When formaldehyde is treated with ammonia, urotropine is formed. Its chemical name is hexamethylene tetra ammine or hexa ammine.

Question 4.
Write a short note on Tollen’s reagent.
or,
What is Tollen’s reagent? Write its reaction with acetaldehyde.
Answer:
Tollen’s Reagent:
Ammoniacal silver nitrate solution is known as Tollen’s reagent. When Tollen’s reagent is heated with aldehyde, aldehyde reduces Ag+ to Ag and forms a bright silver mirror on the wall.

Ketones do not give this test.

Question 5.
Why boiling point of carboxylic acid is higher than alcohols having same molecular mass?
Answer:
Carboxylic acid exist as dimer due to hydrogen bond. These bonds are more stronger in acids compared to alcohols, therefore boiling point of carboxylic acid is higher than alcohol.

Question 6.
Explain Fehling reaction with equation.
Answer:
Fehling Reaction:
Sodium, Potassium tartarate associated with alkaline CuSO₄ is known as Fehling solution. When aldehyde is heated with Fehling solution, then aldehyde is oxidized and red precipitate of cuprous oxide is obtained. This is known as Fehling test.

Ketones do not give this test.

Question 7.
Why is the boiling point of ketone little higher than its corresponding isomeric aldehyde?
Answer:
Ketones are comparatively more polar than their corresponding isomeric alde – hyde because the >C=O group in ketone is linked with two electron releasing alkyl group. Thus, the dipole attractive force of ketone is comparatively higher. This is the reason that the boiling point of ketone is comparatively higher than its corresponding isomeric aldehyde.

Question 8.
Among formaldehyde, acetaldehyde and acetone which is more reactive and why? Explain.
Answer:
Among HCHO, CH₃CHO and CH₃COCH₃, HCHO is more reactive. This can be explained on the basis of:
1. Electron releasing effect:
Alkyl groups are electron releasing in nature due to which magnitude of positive charge on carbonyl carbon decreases and hence it becomes less susceptible to nucleophilic attack.

2. Steric effect:
The bulkier groups in ketones hinders approach of the nucleophile to the carbonyl carbon. This is known as steric effect. Thus, HCHO with negligible electron releasing effect as well as steric effect is more reactive.

Question 9.
Compare acidic strength of acetic acid, formic acid and chloroacetic acid.
Answer:
Chlorine atom present in chloroacetic acid has strong negative inductive effect (-I). Due to this, electrons of O – H bond easily displaced towards oxygen and it releases H⁺ easily. CH₃ group present in CH₃COOH which produces (+I) effect causes decrease in acidic nature.

In formic acid there is no such group which produces (+1) or (-1) effect. Hence, formic acid is stronger than acetic acid and chloroacetic acid is stronger than acetic acid. In short chloroacetic acid is stronger than formic acid and formic acid is stronger than acetic acid.

Question 10.

1. What is Hell – Volhard – Zelinsky (HVZ) reaction?
2. What happens when formic acid is heated?

Answer:
1. Hell – Volhard – Zelinsky Reaction:
When carboxylic acid is treated with Cl₂ or Br₂ in presence of phosphorus, α – halogenated carboxylic acid is formed. This reaction is known as Hell – Volhard – Zelinsky reaction (HVZ).

2. When formic acid is heated to 160°C it dissociates into CO and H₂O.

Question 11.
Although phenoxide ion has more number of resonating structures than carboxylate ion, carboxylic acid is a stronger acid than phenol. Why? (NCERT)
Answer:

In carboxylate ion negative charge is delocalised over two oxygen atoms which are highly electronegative whereas in phenoxide ion negative charge is delocalised over only one oxygen atom. Carboxylate ion is more stable than phenoxide ion that is why carboxylic acid is more acidic than phenols.

Question 12.
Give chemical equation of the following :

1. Acetaldehyde from formaldehyde.
2. Formaldehyde from acetaldehyde.
3. Acetic acid from formic acid.

Answer:
1. Acetaldehyde from formaldehyde :

2. Formaldehyde from acetaldehyde :

3.

Question 13.
Write down the difference between formic acid and acetic acid on the basis of following points :

1. Effect of heat.
2. Reaction with acidified KMnO₄.
3. Distillation of Ca salt.
4. Reaction with ammoniacal silver nitrate solution.
5. Reaction with PCl₅.

Answer:
Differences between Formic Acid and Acetic Acid :

Question 14..
Explain Stephen’s reaction and Benzoin condensation with example.
Answer:
stephen’s reaction:
Alkyl cyanide on reduction with acidified stannous chloride (i.e., SnCl₂ + HCl) at room temperature forms aldimine hydrochloride, which on hydrolysis with boiling water gives aldehyde. This specific type of reduction of cyanide is known as Stephen’s reaction.
SnCl₂ + 2HCl → SnCl₄ + 2H

Benzoin condensation:
Two molecules of benzaldehyde in presence of alcoholic KCN or NaCN condenses to form benzoin.

Question 15.

1. Write a short note on Perkin’s reaction.
2. What happens when acetone is heated with H₂SO₄?

Answer:
1. Perkin’s reaction:
When aromatic aldehyde is heated in presence of sodium salt of aliphatic acid with anhydride of aliphatic acid, then α, β unsaturated acid is obtained.

2. In presence of H₂SO₄ three molecules of acetone get condensed and form mesitylene.

Aldehydes, Ketones and Carboxylic Acids Long Answer Type Questions

Question 1.
Write down the difference between compounds containing aldehydic group and ketonic group.
Answer:
Differences between Aldehydic group and Ketonic group :

Question 2.
Describe the following: (NCERT)

1. Acetylation
2. Cannizzaro reaction
3. Cross – aldol condensation
4. Decarboxylation.

Answer:
1. Acetylation:
The introduction of an acetyl functional group into an organic compound is known as acetylation. It is usually carried out in the presence of a base such as pyridine, dimethylaniline, etc. This process involves the substitution of an acetyl group for an active hydrogen atom. Ecetyl chloride and acetic anhydride are commonly used as acety – lating agents.
For example, acetylation of ethanol produces ethyl acetate.
CH₂CH₂OH + CH₃COCl → CH₃COOC₂H₅ + HCl

2. Cannizzaro reaction:
Aldehydes which do not contain α – hydrogen like HCHO, C₆H₅CHO react with cone. NaOH solution to form methyl alcohol and formic acid. This reaction is called Cannizzaro reaction.

3. Cross – aldol condensation:
When aldol condensation is carried out between different aldehydes or two different ketones or an aldehyde and a ketone, then the reaction is called a Cross – aldol condensation. If both the reactants contain α – hydrogens, four compounds are obtained as products.

4. Decarboxylation:
Decarboxylation refers to the reaction in which carboxylic acids lose carbon dioxide to form hydrocarbons when their sodium salts are heated with soda – lime.

Decarboxylation also takes place when aqueous solutions of alkali metal salts of carboxylic acids are electrolysed. This electrolytic process is known as Kolbe’s electrolysis.

Question 3.
Describe the laboratory method of preparation of acetone. Draw labelled diagram and write down chemical equations.
Answer:
In laboratory, acetone is prepared by dry distillation of anhydrous calcium acetate.

Method 30 – 40 gm of calcium acetate mixed with equal amount of sodium acetate is heated in a glass retort fitted with a condenser and receiver. Acetone is collected in the receiver. The acetone so obtained is not pure. To purify this, it is shaken with saturated solution of sodium bisulphite then crystals of acetone sodium bisulphite salt separate out. The crystal is washed and heated with sodium carbonate and then dried over anhydrous and CaCl₂ then distilled at 56°C to get pure acetone.

(CH₃)₂C = O + NaHSO₃ → (CH₃)₂C(OH)SO₃Na
2(CH₃)₂C(OH)SO₃Na + Na₂CO₃ → 2(CH₃)₂C = O + 2Na₂SO₃ + H₂O + CO₂.

Question 4.
Write down the following reaction giving example and equation :

1. Iodoform reaction
2. Tischenko reaction
3. Gattermann – Koch synthesis
4. Rosenmund’s reaction.

Answer:
1. Iodoform (Haloform) reaction:
Acetaldehyde or methyl ketone reacts with iodine in presence of alkali to form yellow coloured iodoform. This reaction is known as Iodoform test.
2NaOH + I₂ → NaI + NaOI + H₂O

2. Tischenko reaction:
Two molecules of benzaldehyde is coupled together in presence of aluminium ethoxide or isopropoxide then benzylbenzoate (ester) is formed.

3. Gattermann – Koch synthesis:
Mixture of CO and HCl bubbled through a solution of aromatic hydrocarbon in ether solution in the presence of anhydrous AlCl₃, then benzaldehyde is formed.

4. Rosenmund’s reaction:
Aldehydes are obtained by the reduction of acid chloride with hydrogen in boiling xylene in presence of a catalyst Pd suspended in BaSO₄.

This reaction is called Rosenmund reaction.

Question 5.
Give quick vinegar method of preparation of acetic acid. Give its reaction with phosphorus pentaoxide and phosphorus pentachloride and write its two uses.
Answer:
In this process, a dilute aqueous solution of ethyl alcohol is oxidized in presence of enzyme Mycoderma aceti.

In this process, a wooden vat is fitted with two wooden plates having holes. Between these plates is filled by beech wood savings, moistened with old vinegar solution which is the chief source of Mycoderma aceti. A 10% aqueous solution of ethyl alcohol is dropped slowly from the top of the vat and air is passed at a controlled rate through the holes near the bottom of the vat. Ethyl alcohol is oxidized to acetic acid. This process is called quick vinegar process because vinegar is formed very quickly.

(a) Reaction with P₂O₅:

(b) Reaction with PCl₅:
CH₃CHOOH + PCl₅ → CH₃COCl + POCl₃ + HCl

Uses:

1. As a reagent and solvent in the lab.
2. As vinegar in the preparation of pickel, chutney etc.
3. In the preparation of methyl acetate, ethyl acetate and other esters.

Question 6.
Write a brief note on :

1. Claisen condensation
2. Benzoin condensation.

Answer:
1. Claisen condensation:
When aromatic aldehyde reacts with aliphatic alde – hyde or ketone with α – hydrogen, in presence of weak base, α, β unsaturated aldehyde or ketone is formed. This type of condensation is called Claisen condensation.

2. Benzoin condensation:
Two molecules of benzaldehyde in presence of alcoholic KCN or NaCN condenses to form benzoin.

Question 7.
How will you convert ethanal into the following compounds : (NCERT)
(i) Butan – 1,3 – diol
(ii) But – 2 – enal
(iii) But – 2 – enoic acid.

Answer:

Question 8.
What happens when, (only equation):

1. On reacting acetone with Grignard reagent?
2. Reaction of acetone with chloroform in presence of KOH?
3. Benzaldehyde reacts with aniline?
4. On heating sodium salt of carboxylic acid with soda lime?
5. Benzene reacts with acetyl chloride in presence of anhydrous AlCl₃?

Answer:
1. Reaction of acetone with Grignard reagent.

2. Reaction of acetone with chloroform.

3. Reaction of benzaldehyde with aniline

4. Reaction of sodium salt of carboxylic acid with soda lime.

5. Reaction of benzene with CH₃COCl.

Question 9.
How will you obtain the following from acetic acid (Give only equations):

1. Acetamide
2. Ethyl acetate
3. Acetic Anhydride
4. Trichloro acetic acid.

Answer:
1. Acetamide:

2. Ethyl acetate:

3. Acetic anhydride:

4. Trichloro acetic acid:
CH₃ – COOH + 3Cl₂ → CCl₃ – COCH + 3HCl

MP Board Class 12th Chemistry Important Questions