Q.Alcohols are compounds with general formula R-OH.

Hydrogen bonding between the -OH group and water explains alcohol solubility; ethanol is manufactured industrially by fermentation of molasses; and phenol is reduced to benzene by distillation over zinc dust.

a) Why alcohols are soluble in water

The polar –OH group of an alcohol can form hydrogen bonds with water molecules (the alcohol's O–H can donate a hydrogen bond, and the O lone pairs can accept one). This hydrogen bonding with the solvent compensates for the energy needed to break up the water's own hydrogen-bonded structure, allowing the alcohol to dissolve. (Solubility decreases as the non-polar hydrocarbon part of the alcohol gets larger, since it disrupts the water's hydrogen-bond network without contributing any compensating bonding of its own.)

b) (i) Manufacture of ethanol — fermentation

Molasses (containing sucrose) is diluted with water; the enzyme invertase, from yeast, hydrolyses sucrose into glucose and fructose:

C12H22O11 + H2O →(invertase) C6H12O6 (glucose) + C6H12O6 (fructose)

The enzyme zymase, also from yeast, then ferments these simple sugars into ethanol, releasing carbon dioxide:

C6H12O6 →(zymase) 2C2H5OH + 2CO2

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