Question

State true or false: acetylene on ozonolysis gives glyoxal.

• A. True
• B. False

Answer 1

The correct option is A True

Explanation for the correct option

Ozonolysis

• Ozonolysis is the reaction in which an alkene reacts with ozone in an inert solvent (ether, ${\mathrm{CCl}}_4$ etc.), to form an explosive and unstable ozonide.
• Ethylene reacts with ozone to produce acetylene ozonide which is further hydrolyzed in the presence of zinc to form glyoxal.
• The reaction is given as: $\underset{\mathrm{acetyline}}{\mathrm{CH}\equiv \mathrm{CH}}+\underset{\mathrm{ozone}}{{\mathrm{O}}_3}\stackrel{}{\to }\mathrm{ozonoide}\stackrel{\mathrm{Zn}-{\mathrm{H}}_2\mathrm{O}}{\to }\underset{\mathrm{glyoxal}}{\mathrm{OHC}-\mathrm{CHO}}$
• Mechanism of the reaction:
  • Step 1: Initial electrophilic addition of ozone to the carbon-carbon triple bond.

• This forms the molozonide intermediate.
• It is an unstable intermediate and continues further with the reaction
• It undergoes fragmentation to form a molecule with a carbonyl group and a carbonyl oxide group.
• Step 2: The carbonyl and the carbonyl oxide rearrange themselves and form a stable ozonide intermediate.

• The ozonide intermediate undergoes a reductive workup using zinc in water.
• A reducing agent is added that decomposes the ozonide into glyoxal.

$\mathrm{ozonoide}\stackrel{\mathrm{Zn}-{\mathrm{H}}_2\mathrm{O}}{\to }\underset{\mathrm{glyoxal}}{\mathrm{OHC}-\mathrm{CHO}}$

Hence the given statement is true.