CHEMCA
EXAM MASTER REVIEW SHEET
Carbonyl Compounds: Aldehydes & Ketones
1. Carbonyl Group Structure
The carbonyl carbon is $sp^2$ hybridized and the group is highly polar due to electronegativity difference.
Aldehydes are generally more reactive than ketones towards Nucleophilic Addition ($N_A$) due to:
- • Steric Factor: Ketones have two bulky alkyl groups.
- • Electronic Factor: Two $+I$ groups in ketones reduce the positive charge on Carbon.
$HCHO > RCHO > R_2CO$
Aldehydes/Ketones have higher B.P. than hydrocarbons/ethers but lower than alcohols.
Solubility decreases with increase in alkyl chain length.
2. Specific Preparation Methods
Rosenmund Reduction
Acyl Chloride + $H_2 \xrightarrow{Pd/BaSO_4}$ Aldehyde.
• $BaSO_4$ "poisons" the catalyst to prevent reduction to alcohol.
Stephen Reaction
$R-CN + SnCl_2 + HCl \rightarrow R-CH=NH \cdot HCl \xrightarrow{H_2O} R-CHO$
Etard Reaction
Toluene $\xrightarrow{CrO_2Cl_2, CS_2}$ Complex $\xrightarrow{H_2O}$ Benzaldehyde.
Gattermann-Koch
Benzene + $CO + HCl \xrightarrow{Anhyd. AlCl_3/CuCl}$ Benzaldehyde.
3. Nucleophilic Addition ($N_A$)
| Nucleophile | Product | Key Observation |
|---|---|---|
| $HCN$ | Cyanohydrin | Base catalyzed reaction. |
| $NaHSO_3$ | Bisulphite adduct | Crystalline white precipitate. |
| $R-MgX$ | Alcohol | $HCHO \rightarrow 1^\circ$; $RCHO \rightarrow 2^\circ$; $R_2CO \rightarrow 3^\circ$. |
| $NH_2-Z$ | Imine Derivatives | Reaction with 2,4-DNP gives Yellow/Orange ppt. |
4. Aldol vs. Cannizzaro
Requires at least one $\alpha$-Hydrogen.
2 moles Carbonyl $\xrightarrow{dil. NaOH}$ $\beta$-hydroxy aldehyde/ketone.
Product: $\alpha, \beta$-unsaturated carbonyl (on heating).
For carbonyls without $\alpha$-Hydrogen.
Disproportionation: $HCHO \xrightarrow{conc. NaOH}$ $CH_3OH + HCOONa$.
Example: Formaldehyde, Benzaldehyde.
5. Reductions & Carbonyl Tests
Alkane Reduction
Clemmensen: $Zn-Hg / conc. HCl$
Wolff-Kishner: $NH_2NH_2 / KOH, \text{Ethylene glycol}$
Converts $>C=O$ group into $>CH_2$ group.
Identification Tests
- • Tollens' Test: Only Aldehydes give Silver Mirror.
- • Fehling's Test: Aliphatic Aldehydes give Red Ppt ($Cu_2O$).
- • Iodoform Test: Methyl Ketones give Yellow Ppt ($CHI_3$).
No comments:
Post a Comment