Physical Properties
of Alcohols and Phenols
Physical state
Alcohols : Lower alcohols are colourless liquids at normal temperature. The higher alcohols are colourless, odourless waxy solids.
Phenols : Phenols, like alcohols, are either colourless liquids or solids. But they usually turn reddish brown due to atmospheric oxidation.
Boiling points
Alcohols : The boiling point of alcohols is higher than that of corresponding aliphatic hydrocarbons, haloalkanes, aldehydes and ketones because alcohols form intermolecular hydrogen bonding.
The boiling point of isomeric alcohols decreases with branching due to decrease in surface area.
CH3CH2CH2CH2OH > (CH3)2CHCH2OH > (CH3)3COH
The boiling point of alcohols increases with molecular mass.
Phenols : The boiling point of phenols is higher than that of corresponding aromatic hydrocarbons, haloarenes, aldehydes and ketones because phenols, like alcohols, form intermolecular hydrogen bonding.
Arrange 'butan-1-ol, isobutyl alcohol, tert-butyl alcohol' in order of increasing boiling points.
Solubility
Alcohols : Alcohols are soluble in water because they form intermolecular hydrogen bonding with water molecules. The solubility decreases with increase in mass because the hydrocarbon part becomes larger and resists the formation of hydrogen bonds with water molecules.
The solubility of isomeric alcohols increases with branching because the surface area of the hydrocarbon part decreases with branching.
Phenols : Phenols also form hydrogen bonds with water and hence are soluble in water. However, the solubility of phenols is much lower than that of alcohols due to the presence of the larger hydrocarbon part (benzene ring).
Arrange 'C6H5OH, C6H5CH2OH, C2H5OH' in increasing order of solubility in water.
Questions
Question : Explain, why propanol has higher boiling point than that of hydrocarbon, butane?
Answer : The boiling point of propanol is much higher than that of butane because the molecules of propanol are held together by intermolecular hydrogen bond.
Question : Alcohols are comparatively more soluble in water than hydrocarbons of comparable molecular masses. Explain this fact.
Answer : Alcohols have tendency to form H-bonds with water and break the already existing H-bonds between water molecules. Hence, they are soluble in water.
Hydrocarbons, on the other hand, cannot form H-bonds; hence, they are insoluble in water.