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Physical Properties of Alkanols

1. The lower members of alkanols are colourless, volatile liquids with a characteristic alcoholic smell and burning taste whereas higher alkanols are odourless and tasteless.

Higher alkanols having 12 or more carbon atoms are colourless waxy solids.

2. Solubility. The first three members are completely miscible with water. The solubility rapidly decreases with increase in molecular mass. The higher members are almost insoluble in water but are soluble in organic solvents like benzene, ether, etc.

The solubility of lower alkanols is due to the existence of hydrogen bonds between water and polar O– H group of alkanol molecules.

The solubility of alkilnols in water decreases with increase in molecular mass becaJJ.se with increase in molecular mass, the non-polar alkyl group becomes predominant and masks the effect of polar-OH group. In addition, among the isomeric alkanols the solubility increases with branching of chain.

3. Boiling Points. Boiling points of alcohols are much higher than those of alkanes, haloa1kanes or ethers of comparable molecular masses. This is because in alkanols intermolecular hydrogen bonding exists. Due  to which a large amount of energy is required to break these bonds.

Compound                              Molecular                    Boiling

Mass                          Point

CH3CH2OH (Ethanol)                   46                                351 K

CH3CH3 (Methoxymethane)          46                                297 K

CH3Cl (Chloromethane)                50.5                             249 K

CH3CH2CH3 (Propane)                  44                                 231 K

 

Among isomeric alkanols, the boiling point decreases with increase in branching in the alkyl group.

For isomeric alkanols the boiling points generally follow the order: 1° > 2° > 3°

4. Intoxicating Effect. Alcohols are known to have intoxicating effect. Methanol is poisonous and is not good for drinking purposes. It causes blindness. Ethanol on the other hand is used for drinking purposes. Sometimes it is rendered undrinkable by adding methanol