Introduction to Carbonyl Compounds
What are Carbonyl Compounds?
Carbonyl compounds are organic substances characterized by the presence of a carbonyl group (C=O). This functional group is highly reactive and serves as the main structural feature of many important organic molecules.
Key Characteristics:
- The carbonyl carbon is sp² hybridized
- The C=O bond is polar due to oxygen’s higher electronegativity
- Carbonyl compounds undergo nucleophilic addition reactions
- They can be classified based on groups attached to carbonyl carbon
Classification of Carbonyl Compounds
| Type | General Formula | Examples |
|---|---|---|
| Aldehydes | RCHO | Formaldehyde (HCHO), Acetaldehyde (CH₃CHO) |
| Ketones | RCOR’ | Acetone (CH₃COCH₃), Butanone (CH₃COC₂H₅) |
| Carboxylic Acids | RCOOH | Acetic acid (CH₃COOH), Benzoic acid (C₆H₅COOH) |
| Esters | RCOOR’ | Ethyl acetate (CH₃COOC₂H₅) |
| Amides | RCONH₂ | Acetamide (CH₃CONH₂) |
| Acyl Chlorides | RCOCl | Acetyl chloride (CH₃COCl) |
Acidity of Carbonyl Compounds
Relative Acidities
Carboxylic acids are the most acidic among simple organic compounds due to resonance stabilization of their conjugate base (carboxylate ion).
Acidity Order:
Carboxylic acids > Phenols > Alcohols
pKa values: Carboxylic acids (~5), Phenols (~10), Alcohols (~16)
Effect of Substituents on Acidity
Electron-Withdrawing Groups INCREASE Acidity:
- -Cl, -F, -Br, -I (Halogens)
- -NO₂ (Nitro group)
- -CN (Cyano group)
Electron-Donating Groups DECREASE Acidity:
- -CH₃, -C₂H₅ (Alkyl groups)
- -OH (Hydroxyl group)
- -NH₂ (Amino group)
| Compound | Formula | pKa |
|---|---|---|
| Trichloroacetic acid | CCl₃COOH | 0.9 |
| Chloroacetic acid | CH₂ClCOOH | 2.9 |
| Acetic acid | CH₃COOH | 4.7 |
| Phenol | C₆H₅OH | 10.0 |
| Ethanol | CH₃CH₂OH | ~16 |
Preparation Methods
Preparation of Benzoic Acid
Benzoic acid can be prepared by oxidation of alkyl benzenes using hot alkaline KMnO₄ followed by acidification.
Key Points:
- Alkyl side chain is completely oxidized to -COOH group
- Benzene ring remains intact during oxidation
- Reaction requires heating under reflux conditions
Preparation of Acyl Chlorides
Acyl chlorides can be prepared from carboxylic acids using various chlorinating agents:
Note:
SOCl₂ is preferred because the byproducts (SO₂ and HCl) are gases that easily escape, leaving pure acyl chloride.
Multiple Choice Questions
Correct Answer: a) KMnO₄ and dilute acid
Toluene (methylbenzene) is oxidized to benzoic acid using hot alkaline KMnO₄ followed by acidification with dilute acid.
Correct Answer: b) Acetyl chloride
PCl₅ reacts with carboxylic acids to form acyl chlorides: CH₃COOH + PCl₅ → CH₃COCl + POCl₃ + HCl
Short Answer Questions
Explanation: Thionyl chloride (SOCl₂) converts carboxylic acids to acyl chlorides. The gaseous byproducts make purification easy.
Answer: Methanoic acid (HCOOH) has a hydrogen atom attached directly to the carbonyl carbon, making it structurally similar to aldehydes. This allows it to be oxidized to CO₂ and H₂O. Other carboxylic acids have alkyl groups that resist oxidation under normal conditions.
Long Answer Questions
Complete Process:
- Toluene is heated with alkaline KMnO₄ under reflux
- The methyl group is oxidized to carboxyl group
- The mixture is acidified with dilute acid
Chemical Equations:
Reason for Reagent Choice:
- KMnO₄ is a strong oxidizing agent
- It selectively oxidizes side chains without affecting benzene ring
- Alkaline medium prevents further oxidation