JEE Chemistry Alcohols Phenols and Ethers Notes Set 08

Classification and Preparation of Alcohols

1. Alcohols contain the –OH group on a saturated carbon. They are primary, secondary or tertiary depending on whether the carbon holding –OH is attached to one, two or three other carbon atoms.
2. Hydration of alkenes: acid-catalysed hydration follows Markovnikov's rule (propene gives propan-2-ol). Hydroboration–oxidation (B₂H₆ then H₂O₂/OH⁻) gives the anti-Markovnikov alcohol (propene gives propan-1-ol).
3. From carbonyl compounds: aldehydes are reduced to primary alcohols and ketones to secondary alcohols by NaBH₄, LiAlH₄ or H₂/Pd. Carboxylic acids and esters give primary alcohols with LiAlH₄.
4. From Grignard reagents: HCHO gives a primary alcohol, other aldehydes give secondary alcohols and ketones give tertiary alcohols after hydrolysis. Esters give tertiary alcohols with excess Grignard reagent.
5. Other methods: hydrolysis of alkyl halides with aqueous KOH, hydrolysis of esters (saponification), and treatment of primary aliphatic amines with HNO₂: RNH₂ + HNO₂ → ROH + N₂ + H₂O.
6. Ethanol is made industrially by fermentation of sugar (zymase) and by hydration of ethene. Methanol is made from water gas: CO + 2H₂ → CH₃OH (ZnO–Cr₂O₃, high pressure).

Physical Properties and Acidity

1. Alcohols have much higher boiling points than hydrocarbons, ethers and haloalkanes of similar mass because they form intermolecular hydrogen bonds. Boiling point rises with chain length and falls with branching.
2. Lower alcohols are miscible with water through hydrogen bonding, and solubility falls as the alkyl part gets larger.
3. Alcohols are weak acids (pKa about 16–18). Their acidity falls in the order methanol > primary > secondary > tertiary because alkyl groups push electrons and destabilise the alkoxide ion. Alcohols are weaker acids than water and phenols.
4. They react with active metals to give alkoxides and hydrogen: 2ROH + 2Na → 2RONa + H₂.

Chemical Reactions of Alcohols

1. Reaction with hydrogen halides, PCl₅, PCl₃ or SOCl₂ replaces –OH by halogen. The Lucas reagent (conc. HCl and anhydrous ZnCl₂) distinguishes the three classes: tertiary alcohols give turbidity at once, secondary in about 5 minutes and primary only on heating.
2. Dehydration: ethanol with conc. H₂SO₄ at 443 K gives ethene, and at 413 K gives diethyl ether. The ease of dehydration is tertiary > secondary > primary.
3. Oxidation: primary alcohols give aldehydes (PCC) or acids (KMnO₄, K₂Cr₂O₇); secondary alcohols give ketones; tertiary alcohols resist oxidation, and under drastic conditions break into smaller fragments. The orange-to-green colour change of chromic acid is a test for primary and secondary alcohols.
4. Dehydrogenation: passing the vapours of an alcohol over copper at 573 K gives an aldehyde (primary), a ketone (secondary) or an alkene (tertiary).
5. Esterification with carboxylic acids (H₂SO₄ catalyst) or acid chlorides and anhydrides gives esters. Methanol and salicylic acid give methyl salicylate (oil of wintergreen).
6. Victor Meyer test: primary alcohols give red colour, secondary give blue and tertiary give none. Ethanol and other CH₃CH(OH)– alcohols give a yellow iodoform precipitate with I₂/NaOH; methanol does not.
7. Ethylene glycol is cleaved by periodic acid to two molecules of HCHO. Glycerol reacts with excess HI to give 2-iodopropane, and its nitrate (nitroglycerine) is an explosive. Heating glycerol with KHSO₄ gives acrolein.