The reaction of 1,2-diols (vicinal diols) with periodic acid ($HIO_4$) is known as Malaprade oxidation. It involves the oxidative cleavage of the carbon-carbon bond between the two carbons bearing the hydroxyl groups.

1. Reaction Principle: The reagent $HIO_4$ cleaves the C-C bond. Each carbon attached to an -OH group is oxidized. A primary alcohol group ($-CH_2OH$) is oxidized to an aldehyde (Formaldehyde, $HCHO$), and a secondary alcohol group ($-CHROH$) is oxidized to a higher aldehyde ($RCHO$).
2. Application to Ethylene Glycol: The reactant is $HO-CH_2-CH_2-OH$. Cleavage of the central C-C bond converts each $-CH_2OH$ group into Formaldehyde. Reaction: $CH_2OH-CH_2OH + HIO_4 \rightarrow 2HCHO + HIO_3 + H_2O$.