In the study of chemistry, Chapter 12 of the Class 12 NCERT textbook holds significant importance. This chapter, titled “Aldehydes, Ketones, and Carboxylic Acids,” covers essential concepts related to organic chemistry and is crucial for understanding the chemistry of carbonyl compounds. For students preparing for their board exams or competitive exams, mastering this chapter is vital. In this article, we will explore important questions from Chapter 12, providing solutions and explanations to help students grasp key concepts effectively.
Overview of Chapter 12: Aldehydes, Ketones, and Carboxylic Acids
Chapter 12 of the Class 12 NCERT Chemistry textbook delves into the following topics:
Aldehydes and Ketones:
- Preparation methods
- Properties
- Reactions including nucleophilic addition
- Aldol condensation
- Cannizzaro reaction
Carboxylic Acids:
- Preparation methods
- Properties
- Reactions including esterification
- Decarboxylation
- Comparative study of carboxylic acids with other organic compounds
Understanding these concepts thoroughly is crucial as they form the foundation for many reactions and mechanisms in organic chemistry.
Important Questions and Solutions
1. Define Aldehydes and Ketones. How are they classified?
Solution:
Aldehydes and ketones are carbonyl compounds characterized by the presence of a carbonyl group (C=O) in their structure.
Aldehydes have the general formula R-CHO, where R is an alkyl or aryl group, and the carbonyl group is attached to a hydrogen atom. They are classified into:
- Aliphatic Aldehydes: e.g., Methanal (HCHO), Ethanal (CH3CHO)
- Aromatic Aldehydes: e.g., Benzaldehyde (C6H5CHO)
Ketones have the general formula R-CO-R', where R and R' are alkyl or aryl groups, and the carbonyl group is flanked by two hydrocarbon groups. They are classified into:
- Aliphatic Ketones: e.g., Propan-2-one (CH3COCH3)
- Aromatic Ketones: e.g., Acetophenone (C6H5COCH3)
2. Describe the methods of preparation of aldehydes and ketones.
Solution:
Preparation of Aldehydes:
Oxidation of Primary Alcohols:
- Primary alcohols are oxidized to aldehydes using oxidizing agents like PCC (Pyridinium Chlorochromate).
- Example: CH3CH2OH + PCC → CH3CHO (Ethanal)
Hydrolysis of Nitriles:
- Nitriles can be hydrolyzed to yield aldehydes.
- Example: CH3CN + 2H2O → CH3CHO + NH3
Reduction of Carboxylic Acids:
- Carboxylic acids can be reduced to aldehydes using lithium aluminum hydride (LiAlH4) or borane (BH3).
- Example: CH3COOH + LiAlH4 → CH3CHO + LiAlO2
Preparation of Ketones:
Oxidation of Secondary Alcohols:
- Secondary alcohols are oxidized to ketones using oxidizing agents like potassium permanganate (KMnO4).
- Example: CH3CH(OH)CH3 + [O] → CH3COCH3 (Propan-2-one)
Friedel-Crafts Acylation:
- Ketones can be prepared by acylation of aromatic hydrocarbons.
- Example: C6H6 + RCOCl + AlCl3 → C6H5COR + HCl
Dehydrogenation of Secondary Alcohols:
- Secondary alcohols can be dehydrogenated using catalysts like copper at high temperatures.
- Example: CH3CH(OH)CH3 → CH3COCH3 + H2
3. Explain the reaction of aldehydes with sodium bisulfite.
Solution:
The reaction of aldehydes with sodium bisulfite (NaHSO3) results in the formation of bisulfite addition compounds, which are often used for the detection and purification of aldehydes.
- Reaction:
- R-CHO + NaHSO3 → R-CH(OH)SO3Na
In this reaction, sodium bisulfite reacts with the aldehyde to form a soluble bisulfite addition compound. This reaction is useful for identifying and separating aldehydes from mixtures. The resulting compound is a white crystalline solid that can be easily purified and characterized.
4. Discuss the aldol condensation reaction. Provide an example.
Solution:
Aldol Condensation is a reaction where an aldehyde or ketone with at least one alpha-hydrogen reacts with another molecule of the same or different aldehyde/ketone under basic or acidic conditions to form a β-hydroxyaldehyde (aldol) or β-hydroxyketone.
General Reaction:
- In Basic Medium:
- 2 R-CHO → R-CH(OH)-CH(R)-O
- In Acidic Medium:
- The aldol undergoes dehydration to form α,β-unsaturated carbonyl compounds.
Example:
Aldol Reaction:
- CH3CHO + NaOH → CH3CH(OH)CH=O (3-Hydroxybutanal)
Dehydration:
- CH3CH(OH)CH=O → CH3CH=CH-COH (Crotonaldehyde)
The aldol condensation is significant in organic synthesis for constructing carbon-carbon bonds and producing complex molecules.
5. What is the Cannizzaro Reaction? Provide an example.
Solution:
The Cannizzaro Reaction is a disproportionation reaction where non-enolizable aldehydes (those without alpha-hydrogens) are oxidized and reduced simultaneously in the presence of a strong base.
General Reaction:
- 2 R-CHO + 2 NaOH → R-COONa + R-CH2ONa + 2 H2O
In this reaction, one molecule of the aldehyde is oxidized to a carboxylate salt, while another is reduced to an alcohol.
Example:
- Reaction of Formaldehyde:
- 2 HCHO + 2 NaOH → HCOONa + CH3ONa + 2 H2O
Here, formaldehyde (HCHO) reacts with sodium hydroxide (NaOH) to form sodium formate (HCOONa) and methanol (CH3ONa).
6. Describe the methods of preparation of carboxylic acids.
Solution:
Preparation of Carboxylic Acids:
Oxidation of Primary Alcohols:
- Primary alcohols can be oxidized to carboxylic acids using strong oxidizing agents like potassium permanganate (KMnO4) or chromium trioxide (CrO3).
- Example: CH3CH2OH + [O] → CH3CH2COOH (Propanoic acid)
Oxidation of Aldehydes:
- Aldehydes can be oxidized to carboxylic acids using agents like Tollen’s reagent or Benedict’s solution.
- Example: CH3CHO + [O] → CH3COOH (Acetic acid)
Hydrolysis of Nitriles:
- Nitriles can be hydrolyzed to carboxylic acids.
- Example: CH3CN + 2H2O → CH3COOH + NH3
Carbonation of Grignard Reagents:
- Grignard reagents react with carbon dioxide (CO2) to form carboxylic acids.
- Example: RMgX + CO2 → RCOOH
7. Discuss the properties and reactions of carboxylic acids.
Solution:
Properties of Carboxylic Acids:
Physical Properties:
- Carboxylic acids have higher boiling points compared to other organic compounds due to strong hydrogen bonding.
- They are usually soluble in water due to their ability to form hydrogen bonds.
Chemical Reactions:
Esterification:
- Carboxylic acids react with alcohols in the presence of an acid catalyst to form esters and water.
- Example: CH3COOH + CH3OH → CH3COOCH3 + H2O
Decarboxylation:
- Carboxylic acids can lose carbon dioxide (CO2) when heated with soda lime.
- Example: RCOOH → RH + CO2
Reduction:
- Carboxylic acids can be reduced to primary alcohols using reducing agents like lithium aluminum hydride (LiAlH4).
- Example: CH3COOH + 4[H] → CH3CH2OH + H2O
Reaction with Bases:
- Carboxylic acids react with bases to form salts and water.
- Example: CH3COOH + NaOH → CH3COONa + H2O
8. Compare the reactivity of aldehydes and ketones towards nucleophilic addition reactions.
Solution:
Aldehydes vs. Ketones:
Aldehydes are generally more reactive than ketones towards nucleophilic addition reactions. This increased reactivity is due to the following reasons:
- Aldehydes have one alkyl group and one hydrogen atom attached to the carbonyl group, which leads to less steric hindrance compared to ketones.
- The carbonyl carbon in aldehydes is more electrophilic due to the presence of only one alkyl group which is less electron-donating compared to two alkyl groups in ketones.
Ketones have two alkyl groups attached to the carbonyl carbon, which results in increased steric hindrance and decreased electrophilicity of the carbonyl carbon, making them less reactive towards nucleophiles.
9. Explain the mechanism of nucleophilic addition reaction of aldehydes with sodium borohydride.
Solution:
Mechanism:
Nucleophilic Attack:
Sodium borohydride (NaBH4) is a mild reducing agent that provides hydride ions (H-) for the reduction of aldehydes.
The hydride ion attacks the electrophilic carbonyl carbon of the aldehyde, forming a tetrahedral intermediate.
Reaction:
- R-CHO + NaBH4 → R-CH(OH) + NaBH3
Formation of Alcohol:
The tetrahedral intermediate collapses, and the hydride transfer reduces the carbonyl group to a primary alcohol.
Intermediate:
- R-CH(OH) → R-CH2OH
Final Product:
The reaction yields the corresponding alcohol and sodium borate as a byproduct.
Product:
- R-CH2OH
10. What is the role of Tollens' reagent in the oxidation of aldehydes?
Solution:
Tollens' Reagent:
Composition:
- Tollens' reagent is a solution of silver nitrate (AgNO3) in ammonia (NH3) and is used to test for the presence of aldehydes.
Reaction with Aldehydes:
Aldehydes are oxidized to carboxylic acids by Tollens' reagent, and in the process, silver ions (Ag+) are reduced to metallic silver (Ag), which forms a silver mirror on the inner surface of the test tube.
Reaction:
- R-CHO + 2[Ag(NH3)2]+ + 3OH- → R-COOH + 2Ag + 2NH3 + 2H2O
Positive Test:
- The formation of a silver mirror is a positive test for aldehydes, indicating their presence.
Conclusion
Chapter 12 of the Class 12 NCERT Chemistry textbook covers fundamental concepts related to aldehydes, ketones, and carboxylic acids. Understanding these concepts and practicing important questions is crucial for mastering organic chemistry and performing well in exams. This comprehensive guide has provided solutions and explanations to key questions, offering a solid foundation for students to build upon. By thoroughly studying these topics and solving related problems, students can enhance their understanding and achieve academic success in their chemistry studies.
