Welcome to Learn Chemistry by Dr. Inam Jazbi! In this post, we’ll study one of the most important oxidation reactions of alkynes — the Ozonolysis (Ozonide Reaction). This reaction is very useful in organic analysis for locating the position of the triple bond and identifying unknown alkynes through their oxidation products. Let’s understand how this reaction works, step by step, with examples and chemical equations.
Ozonolysis/Ozoniation (Ozonide Reaction)
Ozonolysis is an organic redox chemical reaction involving
oxidative cleaving of the unsaturated bonds of alkenes, alkynes and azo compounds (compounds with the diazenyl functional group; −N=N−) using ozone (O3; a reactive allotrope of oxygen). The process allows for carbon-carbon double or triple bonds to be replaced by double bonds with oxygen.
Definition of Ozonolysis or Ozoniation
The addition of ozone or trioxygen (O3) to an alkene or alkyne to give an adduct called ozonide is known as ozoniation. The hydrolysis of ozonide (product of ozoniation) is called ozonolysis. In other words, the process of preparing the ozonide by the addition of ozone involving the cleavage of an alkene or alkyne followed by its subsequent decomposition by hydrolysis in which the pi bonds between carbon-carbon atoms are broken down in the presence of reducing agent such as zinc or oxidizing agent such as hydrogen peroxide is called ozonolysis. Ozonolysis is a chemical reaction in which the pi bonds between carbon-carbon atoms are broken down by the addition of ozone (O3).
Products of Ozoniation of Alkynes
The hydrolysis of ozonide in presence of reducing agents such as Zinc dust or dimethyl sulfide is known as reductive ozonolysis. In this type, aldehydes and ketones are formed as products.
The hydrolysis of ozonide in presence of oxidizing agent such as H2O2 is known as oxidative ozonolysis. In this type, already-formed aldehydes and ketones are converted to carboxylic acids.
Terminal alkynes undergo oxidative cleavage to yield one carboxylic acid and CO2. (If the alkyne is terminal i.e. contains R≡CH, then the products are RCO2H and CO2 since unstable carbonic acid, HOC(=O)OH, is formed).
Internal alkynes undergo oxidative cleavage to yield two carboxylic acids. For a symmetrical, internal alkyne this would mean two equivalents of the same carboxylic acid. For an asymmetrical, internal alkyne this would mean two different carboxylic acids. Note that each of the CC bonds in the C≡C becomes a CO bond
General Reaction
ozone (O3) adds across the triple bond of an alkyne to form an unstable intermediate called ozonide which on decomposition with hydrogen peroxide to form glyoxal.
Alkyne reacts with ozone to form ozonide. The ozonide may react with water in presence of zinc dust to give ketone (1,2-dicarbonyl compound) which are finally oxidized to acids by H2O2 produced in the reaction.
