Alkanes

Introduction

- saturated hydrocarbons because molecules of alkane contain only C-C and C-H

- straight chain, branched chain, rings

- non-polar, hence insoluble in water

- as the molecular mass increase, m.p./b.p. increases. (because vdwf increases)

- chemically inert: do not react with common O.A., R.A., acid and alkali

Combustion

- complete combustion: CxHy + ( x + y/4 ) O2 -----> x CO2 + y/2 H2O

- incomplete combustion: formation of carbon monoxide

Halogenation

- e.g.: CH4 + Cl2 ----(UV)----> CH3Cl + HCl

Free radical chain reaction

- example of homolytic fission

- initiated by light / other radicals

- formation of free radical

Chlorination of methane

- Evidences and deduction

  (1) rate of reaction is greater in light

  (2) light must have wavelength<400nm, equivalent to energy of 300kJmol-1

  (3) in suitable light, reaction occurs rapidly at room temperature and for each photon of light absorbed,

       many thousands of molecules of chloromethane are formed

- Mechanism of chlorination

  (1) initiation: free radicals are formed. Cl-Cl ---(UV)---> Cl* + *Cl

        (99/9(b)(i))

  (2) Propagation: free radicals are generated.

  (3) Termination: free radicals are destroyed.

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