1.

Discuss the important physical and chemical properties of alcohols. Give the tests of alcohols.

Answer»

Solution :Properties of alcohols
(A) Physicial properties
1. The lower members of alcohols are colourless, volatile liquids having typical alcoholic smell and burning taste.
2. Lower alcohols are completely soluble in water. However, the solubility decreases with increase in molecular mass.
3. The boilng points of alcohols are higher than those of alkanes of comparable molecular masses.
4. The common alcohols are lighter than water.
B. Chemical Properties.
1. Action with sodium. Alcohols react with sodium metal to form sodium alkoxide and hydrogen GAS is liberated. For example,
`underset("Methanol")(2CH_(3)OH) + 2Na rarr underset("Sodium methoxide")(2CH_(3)ONa) + H_(2)`
`underset("Ethanol")(2C_(2)H_(5)OH) + 2Na rarr underset("Sodium ethoxide")(2C_(2)H_(5)ONa) + H_(2)`
2.Combustion. Alcohols burns readily in air and produce carbon dioxide, water and heat. For example.
`underset("Methanol")(CH_(3)OH) + O_(2) rarr CO_(2) + 2H_(2)O + "Heat"`
`underset("Ethanol")(C_(2)H_(5)OH) + 3O_(2) rarr 2CO_(2) + 3H_(2)O + "Heat"`
3. Oxidation. Alcohols are easily oxidised to corresponding carboxylic acids by oxidising agents such asacidified potassium dichromate. For example,
`underset("Methanol")(CH_(3)OH) + 2[O] overset(K_(2)Cr_(2)O_(7), H_(2)SO_(4))rarr underset("Methanoic acid")(HCOOH) + H_(2)O`
`C_(2)H_(5)OH + 2[O] overset(K_(2)Cr_(2)O_(7), H_(2)SO_(4))rarr underset("Ethanoic acid")(CH_(3)COOH) + H_(2)O`
During this reaction, the organge COLOUR of `K_(2)Cr_(2)O_(7)` changes to green.
4. Reaction with carboxylic acids (esterification). Alcohols react with carboxylic acids in the presence of concentration sulphuric acid as catalyst to form ester which have sweet fruit smell. This reaction is called esterification. For example :
`underset("Methanol")(CH_(3)OH) + underset("Ethanoic acid")(CH_(3)COOH) underset("Heat")overset("Conc. " H_(2)SO_(4))hArr underset("Methyl ethanoate")(CH_(3)COOCH_(3)) + H_(2)O`
`underset("Ethanol")(CH_(3)CH_(2)OH) + underset("Ethanoic acid")(CH_(3)COOH) underset("Heat")overset("Conc. " H_(2)SO_(4))hArr underset("Ethyl ethanoate")(CH_(3)COOC_(2)H_(5)) + H_(2)O`
Tests for Alcohols. The presence of an alcoholic group or hydroxyl group in an organic compound can be tested by the following tests :
1. Sodium metal test. Add a small piece of sodium metal of the organic compound in a dry test tube. If the bubbles of hydrogen gas are produced, it shows the presence of alcoholic group (-OH) in the organic compound.
2. Ester formation test. Warm the organic compound with ACETIC acid and a few drops of conc. `H_(2)SO_(4)`. A sweet smell (due to the formation of ester) indicates the presencce of alcohlolic group.


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