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Ozonolysis, Electrophilic Addition Reactions of Alkenes, Markownikoff’s Orientation

A chemist will perform an ozonolysis test on a compound to determine whether it has double bonds.

Ozonolysis

A chemist will perform an ozonolysis test on a compound to determine whether it has double bonds. Compounds react with oxygen to form ozonides, and ozonides react with hydrogen or acids to form aldehydes, ketones, or carboxylic acids. The double bonds inside aldehydes and ketones determine their structure.

The ozonolysis of natural products is extensively used in order to determine the structure of their terpenes. It is used not only for researching the structure of aromatic compounds but also for synthesizing rare aldehydes and ketones. Also known as pelargonic aldehyde and azelaic semialdehyde, oleic acid releases n-nonanal (NNA) during ozonolysis.
  • Alcohols, aldehydes, ketone, and carboxylic acids can be formed through the oxidation of alkenes by ozone.
  • Ozonolysis of alkynes leads to acid anhydrides and diketones. Aqueous solutions of acid anhydride can hydrolyze to form two carboxylic acids.
  • Elastomers are subject to a process known as ozonolysis, which results in cracking. Elastomers are damaged by ozone because double bonds are broken.
  • The azo compounds that result from ozonolysis are nitrosamines.

Reaction

Ozone is a highly reactive allotrope of oxygen. Alkenes and alkynes are oxidatively cleaved by oxygen upon contact with ozone. As shown below, carbonyl is formed by substituting double bonds for carbonyl triple bonds.


Mechanism

Oxidative cleavage is a critical step in the ozonolysis mechanism. In addition to breaking carbon pi bonds, ozone also breaks carbon-carbon sigma bonds. It comes about when ozone attacks a given reactant to produce ozonides. The intermediate stage in the process of removing oxygen is zinc dust. Different reactions and procedures will result in different final products.


Ozonolysis of alkenes

The oxidation of an alkene results in the formation of alcohol, aldehyde, ketone, or carboxylic acid. For the general procedure, methanol is used. About 780 degrees Celsius are required to bubble ozone into the solution. Unreacted ozone gives the alkene a blue color, indicating it has been consumed. Potassium iodide solution may be used in chemical reactions to indicate the end of a reaction. This indicator is pumped through a mixture of reactants with an oxygen stream enriched with ozone. An iodide solution can have gas bubbles channeled out. After the alkene is consumed, ozone is not allowed to react with it, causing potassium iodide to form. Ozone reacts with potassium iodide to create iodine, which gives off the violet color of iodine.

In order to convert the ozonide to the necessary carbonyl derivative, a reaction mixture must first be exposed to ozone. It can be converted one of two ways:
  • Oxidative
  • Reductive

Ozonolysis of alkynes

Oxonolysis produces acids anhydrides and diketones from alkynes. An alkene does not need to be fragmented completely to undergo this reaction. Reducing agents are not used in the aqueous procedure. Two carboxylic acids are formed from the hydrolysis of the acid anhydride by water. Ozonolysis can also provide information about a triple bond in an unknown alkyne.


Electrophilic addition reactions of alkenes

There is at least one double bond in each molecule of an alkene, a characteristic of unsaturated hydrocarbons. Electrophiles attack double bonds in carbon-carbon atoms, resulting in addition products. Alkene addition reactions occur in the presence of electrophiles, and they are referred to as electrophilic addition reactions. Free radicals are often involved in these reactions. Alkenes can also undergo oxidation and ozonolysis reactions.

Electrophilic addition reactions of alkenes - Alkenes undergo a variety of electrophilic addition reactions. When hydrogen halides such as bromine or chloride are added to alkenes, they undergo electrophilic addition reactions. The general trend in hydrogen halide is: HI >HBr > HCl. The predictability of end products for alkenes with symetrical structures, such as ethene, is longer than that of those with unsymmetrical structures, like propene.

Example -

CH2 = CH2 + H – Br —> CH3 – CH2 – Br



Markownikoff’s orientation

A rule devised by Markownikoff to predict major products in such cases has been referred to as the Markownikoff rule. Molecular negative portions should be attached to carbon atoms that have the least amount of hydrogen. Based on this rule, 2-bromopropane is predicted to be present. Electrophilic substitution reactions can then be used to explain alkenes.

An electrophile, hydrogen bromide attacks double bonds to generate H+.



Since secondary carbocations are more stable than primary carbocations, they dominate the formation of ions.

Br– produces alkyl halos after attacking carbohydrate.

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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of pharmaguideline.com, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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