Question

Assertion: Boiling point of alkanes increases with increase in molecular mass. Reason: Van der Waal's forces increase with increase in molecular mass.

(i) Give reason for following a) Ethyne is more acidic than ethene b) Nitration of Toluene give ortho and para nitro toluene but not m-Nitro toluene (ii) Complete the following equation CH3CH(Cl)-CH2-CH3 with AlcKOH → (iii) Give a chemical test to distinguish the following pairs: (a) Ethene and Ethyne (b) Ethene and Ethane

Alkanes are generally inert towards acids, bases, oxidising and reducing agents. However, they undergo the following reactions under certain conditions. 1) Substitution reactions One or more hydrogen atoms of alkanes can be replaced by halogens, nitro group and sulphonic acid group. Halogenation takes place either at higher temperature (573-773 K) or in the presence of diffused sunlight or ultraviolet light. Lower alkanes do not undergo nitration and sulphonation reactions. These reactions in which hydrogen atoms of alkanes are substituted are known as substitution reactions. As an example, chlorination of methane. 2) Combustion Alkanes on heating in the presence of air or dioxygen are completely oxidized to carbon dioxide and water with the evolution of large amount of heat. Due to the evolution of large amount of heat during combustion, alkanes are used as fuels. During incomplete combustion of alkanes with insufficient amount of air or dioxygen, carbon black is formed which is used in the manufacture of ink, printer ink, black pigments and as filters. 3) Controlled oxidation Alkanes on heating with a regulated supply of dioxygen or air at high pressure and in the presence of suitable catalysts give a variety of oxidation products.

Assertion: Boiling point of alkanes increases with increase in molecular weight. Reason: Van der Waal's forces increase with increase in molecular weight