Alcohols — Part 2: Acidity, reactions, mechanisms and tests: visual lesson, ISC Class 12 Chemistry

Lesson

Reactions of alcohols

Part 2 — reactions breaking the O–H bond, reactions breaking the C–O bond, oxidation, and tests.

In this lesson you will

Explain why alcohols are weaker acids than water
Write reactions of alcohols with Na, HX, PCl₃, PCl₅, SOCl₂, acid chlorides and anhydrides; draw the esterification mechanism
Predict oxidation products and draw the dehydration mechanism of ethanol
Convert one alcohol into another and use the Lucas test

Concept

Acidic nature

Alcohols react with active metals (Na, K) to give alkoxides and H₂, showing weak acidity. They are weaker acids than water because the alkyl group (+I) pushes electron density towards O, destabilising the alkoxide ion; alkoxides are strong bases. Acidity: CH₃OH > 1° > 2° > 3°.

\ce{2CH3CH2CH2OH + 2Na -> 2CH3CH2CH2ONa + H2}

\ce{CH3CH2O- + H2O -> CH3CH2OH + OH-}

Table of pKa values of water and alcohols
Compound	pKa (approx.)
Water	15.7
Methanol	15.5
Ethanol	15.9
2-Methylpropan-2-ol	18

Concept

Replacement of –OH by halogen

The C–O bond breaks. Reactivity with HX: HI > HBr > HCl; 3° > 2° > 1° (via carbocations). PCl₃, PCl₅ and SOCl₂ (best, gaseous by-products) also convert ROH to RCl.

\ce{CH3CH2OH + HBr -> CH3CH2Br + H2O}

\ce{3CH3CH2OH + PCl3 -> 3CH3CH2Cl + H3PO3}

\ce{CH3CH2OH + PCl5 -> CH3CH2Cl + POCl3 + HCl}

\ce{CH3CH2OH + SOCl2 -> CH3CH2Cl + SO2 + HCl}

Table of halogenating reagents and their by-products
Reagent	By-products
HX	H₂O
PCl₃	H₃PO₃
PCl₅	POCl₃, HCl (white fumes)
SOCl₂	SO₂, HCl (gases)

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