Chapter 8: Aldehydes, Ketones and Carboxylic Acids
Key Topics: Carbonyl Group Structure, Named Preparation Reactions, Nucleophilic Addition Mechanism, Aldol & Cannizzaro Reactions, Distinction Tests, and Acidity of Carboxylic Acids.
1. Carbonyl Structure & Reactivity
Both aldehydes and ketones contain the carbonyl functional group ($\text{>C=O}$). The carbonyl carbon is $sp^2$ hybridized, making it planar and electrophilic.
- Aldehydes: Have at least one hydrogen atom bonded to the carbonyl carbon ($\text{R-CHO}$).
- Ketones: Have two alkyl/aryl groups bonded to the carbonyl carbon ($\text{R-CO-R'}$).
- Nucleophilic Addition Order: Aldehydes are generally more reactive than ketones toward nucleophilic addition due to steric hindrance and electronic factors.
2. Key Named Preparation Reactions
Rosenmund Reduction (Aldehydes from Acyl Chlorides):
$$\text{R-COCl} + \text{H}_2 \xrightarrow{\text{Pd-BaSO}_4} \text{R-CHO} + \text{HCl}$$
Stephen Reaction (Aldehydes from Nitriles):
$$\text{R-CN} + \text{SnCl}_2 + \text{HCl} \rightarrow \text{R-CH=NH}\cdot\text{HCl} \xrightarrow{\text{H}_3\text{O}^+} \text{R-CHO} + \text{NH}_4\text{Cl}$$
Etard Reaction (Benzaldehyde from Toluene):
$$\text{C}_6\text{H}_5\text{CH}_3 + 2\text{CrO}_2\text{Cl}_2 \xrightarrow{\text{CS}_2} \text{Complex} \xrightarrow{\text{H}_3\text{O}^+} \text{C}_6\text{H}_5\text{CHO}$$
Gattermann-Koch Reaction:
$$\text{C}_6\text{H}_6 + \text{CO} + \text{HCl} \xrightarrow{\text{Anhyd. AlCl}_3 / \text{CuCl}} \text{C}_6\text{H}_5\text{CHO} + \text{HCl}$$
3. Distinction Tests for Aldehydes & Ketones
| Test | Aldehydes ($\text{R-CHO}$) | Ketones ($\text{R-CO-R'}$) |
|---|---|---|
| Tollen's Test (Ammoniacal $\text{AgNO}_3$) | Forms a shiny silver mirror ($\text{Ag}^0$). | No reaction. |
| Fehling's Test ($\text{Cu}^{2+}$ complex) | Reduces solution to reddish-brown precipitate ($\text{Cu}_2\text{O}$). | No reaction (except $\alpha$-hydroxy ketones). |
| Iodoform Test ($\text{I}_2 + \text{NaOH}$) | Positive for $\text{CH}_3\text{CHO}$ (Yellow precipitate of $\text{CHI}_3$). | Positive for methyl ketones ($\text{R-COCH}_3$). |
4. Important Condensation & Oxidation Reactions
Aldol Condensation (Requires $\alpha$-Hydrogen):
$$2\text{CH}_3\text{CHO} \xrightarrow{\text{Dil. NaOH}} \text{CH}_3\text{-CH(OH)-CH}_2\text{-CHO} \xrightarrow[\Delta]{-\text{H}_2\text{O}} \text{CH}_3\text{-CH=CH-CHO}$$
Cannizzaro Reaction (No $\alpha$-Hydrogen - Disproportionation):
$$2\text{HCHO} + \text{Conc. KOH} \rightarrow \text{CH}_3\text{OH} + \text{HCOOK}$$
$$2\text{C}_6\text{H}_5\text{CHO} + \text{Conc. NaOH} \rightarrow \text{C}_6\text{H}_5\text{CH}_2\text{OH} + \text{C}_6\text{H}_5\text{COONa}$$
Clemmensen Reduction (Acidic Medium):
$$\text{>C=O} + 4[\text{H}] \xrightarrow{\text{Zn-Hg / conc. HCl}} \text{>CH}_2 + \text{H}_2\text{O}$$
Wolff-Kishner Reduction (Basic Medium):
$$\text{>C=O} + \text{NH}_2\text{NH}_2 \xrightarrow{-\text{H}_2\text{O}} \text{>C=N-NH}_2 \xrightarrow[\Delta, \text{ Ethylene glycol}]{\text{KOH}} \text{>CH}_2 + \text{N}_2$$
5. Carboxylic Acids & Reactions
Acidity & Dissociation Constant ($K_a$):
$$\text{R-COOH} + \text{H}_2\text{O} \rightleftharpoons \text{R-COO}^- + \text{H}_3\text{O}^+$$
$$pK_a = -\log_{10} K_a$$
$$\text{Smaller } pK_a \text{ value } \implies \text{Stronger Acid}$$
Hell-Volhard-Zelinsky (HVZ) Reaction (Halogenation at $\alpha$-position):
$$\text{R-CH}_2\text{-COOH} \xrightarrow{(i) \text{ X}_2 / \text{Red Phosphorus}}{(ii) \text{ H}_2\text{O}} \text{R-CH(X)-COOH} + \text{HX} \quad (\text{X = Cl, Br})$$
Decarboxylation Reaction:
$$\text{R-COONa} + \text{NaOH} \xrightarrow[\Delta]{\text{CaO}} \text{R-H} + \text{Na}_2\text{CO}_3$$