(c) H C . + C . → H C–C1 Though in (c), CH – Cl, the one of the products is formed but free radicals are consumed and the chain is terminated. The above mechanism helps us to understand the reason for the formation of ethane as a byproduct during chlorination of methane.
. 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. è - c è - c CH (g) (g) CO (g) 2H O( ); Ä H 890kJmol ( . ) C H (g) /2O (g) 4CO (g) 5H O( ) Ä H .84kJmol ( .
) → − → = − The general combustion equation for any alkane is : n 2n+ 3n C H O nCO (n )H O → ( . ) 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. CH (g) + O (g) incomplete combustion C(s)+2H O( ) ( .
) . 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. (i) 2CH + O Cu/523K/100atm 2CH OH Methanol ( . ) (ii) CH + O Mo O ∆ HCHO + H O Methanal ( .
) (iii) 2CH CH + 3O (CH COO)Mn ∆ 2CH COOH Ethanoic acid + 2H O ( . ) (iv) Ordinarily alkanes resist oxidation but alkanes having tertiary H atom can be oxidized to corresponding alcohols by potassium permanganate. (iCH ) CH KMnO Oxidation (CH ) COH -Methylpropane -Methylpropane- - ( . ) .
Isomerisation n -Alkanes on heating in the presence of anhydrous aluminium chloride