CHEMCA
EXAM MASTER REVIEW SHEET
Organic Chemistry: Carboxylic Acids
1. Structure & Exceptional Acidity
Carboxylic acids ($R-COOH$) are more acidic than phenols and alcohols because the carboxylate ion is stabilized by two equivalent resonance structures.
Acidity depends on the stability of the conjugate base ($R-COO^-$).
Carboxylic Acid > Phenol > Alcohol
Electron Withdrawing Groups ($-I, -M$) increase acidity; Electron Releasing Groups ($+I, +M$) decrease acidity.
The carboxyl carbon is $sp^2$ hybridized. Bond angles are approximately $120^\circ$. Planar structure facilitates resonance.
2. Methods of Preparation
From Grignard Reagent
$RMgX + CO_2 (\text{Dry Ice}) \xrightarrow{\text{Dry Ether}} R-COOMgX \xrightarrow{H_3O^+} R-COOH$
• Excellent for increasing carbon chain length by one unit.
From Nitriles & Amides
$R-CN \xrightarrow{H^+/H_2O} R-CONH_2 \xrightarrow{H^+/H_2O, \Delta} R-COOH$
• Full hydrolysis of nitriles yields carboxylic acids.
Oxidation of Alkyl Benzenes
Alkyl Benzene $\xrightarrow{KMnO_4/KOH, \Delta} \text{Benzoic Acid}$.
• Requires at least one benzylic hydrogen. Tertiary alkyl groups do not react.
From Alcohols & Aldehydes
$R-CH_2OH \xrightarrow{KMnO_4 \text{ or } K_2Cr_2O_7} R-COOH$
• Primary alcohols and aldehydes are easily oxidized to acids.
3. Key Chemical Reactions
| Reaction Type | Reagents | Product / Note |
|---|---|---|
| Reduction | $LiAlH_4 / \text{Ether}$ or $B_2H_6$ | Primary Alcohol ($R-CH_2OH$). |
| Soda Lime Decarboxylation | $NaOH + CaO$, $\Delta$ | Alkane ($R-H$) with one less carbon. |
| PCl5 / SOCl2 | $PCl_5, PCl_3$ or $SOCl_2$ | Acid Chloride ($R-COCl$). |
| Esterification | $ROH, \text{conc. } H_2SO_4$ | Ester ($R-COOR'$) - Fruity Smell. |
4. Hell-Volhard-Zelinsky (HVZ) Reaction
Carboxylic acids having an $\alpha$-hydrogen are halogenated at the $\alpha$-position on treatment with chlorine or bromine in the presence of a small amount of red phosphorus.
Essential for introducing functional groups at the $\alpha$-position.
5. Distinguishing Tests
NaHCO3 Test
Carboxylic acids react with sodium bicarbonate to release $CO_2$.
Observation: Brisk Effervescence.
*Differentiates acids from most phenols (except Picric acid).*
Ester Test
Heat acid with ethanol and a few drops of conc. $H_2SO_4$.
Observation: Fruity / Sweet Smell.
No comments:
Post a Comment