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