CHEMCA
EXAM MASTER REVIEW SHEET
Organic Chemistry: Alcohols
1. Classification & Physical Properties
Alcohols ($R-OH$) are classified based on the number of hydroxyl groups and the nature of the carbon atom attached.
Alcohols have significantly higher B.P. than hydrocarbons/ethers of comparable mass due to Intermolecular Hydrogen Bonding.
B.P. Order: $1^\circ > 2^\circ > 3^\circ$ (for isomeric alcohols)
Reason: Branching decreases surface area and Van der Waals forces.
Lower alcohols are highly soluble in water due to H-bonding with water molecules.
Solubility $\propto \frac{1}{\text{Size of Alkyl Group}}$
2. Methods of Preparation
From Carbonyl Compounds
• Reduction: Aldehydes $\rightarrow 1^\circ$; Ketones $\rightarrow 2^\circ$.
Reagents: $LiAlH_4$, $NaBH_4$, or $H_2/Pd$.
• Grignard Reagent:
1. $HCHO + RMgX \rightarrow 1^\circ$ Alcohol.
2. $R'CHO + RMgX \rightarrow 2^\circ$ Alcohol.
3. $R'COR'' + RMgX \rightarrow 3^\circ$ Alcohol.
From Alkenes
• Acid Hydration: Markovnikov (Rearrangement possible).
• Hydroboration-Oxidation:
Anti-Markovnikov; SYN addition.
• Oxymercuration-Demercuration:
Markovnikov (No rearrangement).
3. Acidity & O-H Cleavage
Alcohols are weak Brönsted acids. Reactivity of $O-H$ cleavage: $1^\circ > 2^\circ > 3^\circ$.
Electron releasing alkyl groups ($+I$) increase electron density on Oxygen, making $O-H$ bond harder to break.
Acidic Strength: $H_2O > 1^\circ > 2^\circ > 3^\circ$
$ROH + R'COOH \xrightarrow{H^2SO^4} R'COOR + H_2O$
Note: The Oxygen in water comes from the Acid, not the Alcohol!
4. Differentiation by Reaction
| Alcohol Type | Oxidation ($K_2Cr_2O_7/H^+$) | Dehydrogenation ($Cu, 573K$) |
|---|---|---|
| $1^\circ$ (Primary) | Aldehyde $\rightarrow$ Carboxylic Acid | Aldehyde |
| $2^\circ$ (Secondary) | Ketone (No further oxidation easily) | Ketone |
| $3^\circ$ (Tertiary) | Resistant (Cleavage gives mix of acids) | Alkene (Dehydration!) |
5. Distinguishing Tests
Reagent: Anhydrous $ZnCl_2 + \text{Conc. } HCl$
- • $3^\circ$ Alcohol: Immediate Turbidity.
- • $2^\circ$ Alcohol: Turbidity after 5-10 mins.
- • $1^\circ$ Alcohol: No turbidity at room temperature.
Sequence: $P/I_2 \rightarrow AgNO_2 \rightarrow HNO_2 \rightarrow NaOH$
- • $1^\circ$ Alcohol: Blood Red color.
- • $2^\circ$ Alcohol: Blue color.
- • $3^\circ$ Alcohol: Colorless.
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