Alkenes and Alkynes

Oxidation of Alkenes:

(a)Combustion : The combustion of alkenes is also exothermic with high calorific values and thus used for welding purposes in oxy-ethylene welding.

        CH₂=CH₂ + 3O₂ → 2CO₂ + 2H₂O; ΔH = -ve

        C_nH_2n + (3n/2)O₂ → nCO₂ + nH₂O; ΔH = -ve

 

(b)Oxidation by Baeyers' reagent or hydroxylation : A test for unsaturation

Alkenes on passing through dilute alkaline, 1% cold KMnO₄ (i.e. Baeyers reagent) decolorize the pink colour of KMnO₄ and forms dihydroxy compounds (e.g. glycols)

           

 

(c)Oxidation by alkaline KMnO₄ : Oxidation of alkenes by hot alkaline KMnO₄gives two acid salts showing fission of C=C bond

                   RCH=CHR'  RCOOK + R'COOK

                               ^{alk. KMnO}₄

 

(d)Oxidation by acidic KMnO₄ or K₂Cr₂O₇ :

(i) Oxidation of alkenes by acidic KMnO₄ or K₂Cr₂O₇ gives carboxylic acids. If HCOOH is acid, it is further oxidized to CO₂ & H₂O.

            

 

         CH₃-CH = CH₂  CH₃COOH + HCOOH

 

         RCH = CHR'   RCOOH + R'COOH

         

(ii)Same products are obtained if oxidation is made by per iodic acid or lead tetra acetate.

(iii)The nature of acid formed decides the position of unsaturation in molecule.

Note : Alkenes on oxidation by osmium tetraxide gives an intermediate product which on refluxing with NaHSO₃ (alc.) gives glycols.