Question

Both, alkenes and alkynes are unsaturated hydrocarbons. State the one most significant difference between them.

Answer 1

Answer. Although both of the compounds are unsaturated, they decolorize pink colour of acidified dilute solution of potassium permanganate. Alkenes (ethene) produce ethyl glycol (1,2-ethenediol) by addition of two hydroxyl groups at the double bond. Such as:

  3CH2=CH2   +   2KMn04   +   4H2O ——>   3H2C—CH2 + 2MnO2 + 2KOH
                                                      OH OH
 While alkynes (ethene) adds four hydroxyl groups at the triple bonds. Such as:

                                                          OH OH  
HC=CH   +     2KMnO4   +   2H2O —————>     H-C—C-H     +   2MnO2   +   2KOH
\                                      \
                                                          OH OH

This intermediate product eliminates water molecules to from glyoxal, which further oxidize to form oxalic acid.

  OH OH       2H2O        O O        2[O]              O O
   │  │                      ║ ║        KMnO4            ║ ║
H-C—C-H                 H-C-C-H                     HO-C-C-OH
   │  │                                                   oxalic acid