Hydrocarbons · Lesson 8 of 14
Alkene Oxidation, Ozonolysis, and Polymers
“Distinguish reactions that add oxygen, reveal a double-bond position, or build long molecular chains.”
• Interpret permanganate decolourisation and vicinal diol formation. • Predict oxidative-cleavage and reductive-ozonolysis products. • Reconstruct an alkene from carbonyl fragments. • Relate monomer connectivity to polymer repeat units and applications.
Oxidation conditions decide whether the skeleton survives
Cold dilute alkaline aqueous potassium permanganate, traditionally called Baeyer’s reagent, converts an alkene into a vicinal diol. Two hydroxyl groups attach to neighbouring carbons while their connecting carbon bond survives. Disappearance of purple permanganate colour is a useful observation associated with reaction at an unsaturated site.
Propene similarly forms propane-1,2-diol. Stronger oxidative conditions, such as hot acidic permanganate or suitable acidic dichromate conditions, can instead cleave the double bond. A carbon carrying hydrogen can become part of a carboxylic acid; one carrying two carbon groups gives a ketone. A terminal carbon can be oxidised all the way to carbon dioxide. The reagent conditions and the groups on the original carbons both matter.
Problem
Compare but-2-ene under cold dilute permanganate and under vigorous acidic permanganate.
- 1.Under cold dilute conditions, put OH on both double-bond carbons and retain their connection. The product is butane-2,3-diol.
- 2.Under vigorous conditions, split that connection. Each original alkene carbon carries methyl and hydrogen, so each side forms ethanoic acid.
- 3.The initial four-carbon molecule therefore gives either an intact diol or two two-carbon acid molecules. “Oxidation” without conditions does not specify one product.
Strong oxidation of 2-methylpropene gives propanone from the carbon with two methyl groups and can give carbon dioxide from its terminal carbon. The aldehyde that would initially correspond to the terminal carbon does not necessarily survive these oxidising conditions.
Ozonolysis gives a map of the original double bond
Ozone adds at the double bond and leads to an ozonide. Zinc and water then provide a reductive work-up, giving aldehydes and/or ketones. To predict products, preserve the groups attached to each original alkene carbon and replace the inter-carbon double bond by a carbon–oxygen double bond on each fragment. A carbon retaining hydrogen gives an aldehyde; one retaining two carbon groups gives a ketone.
Problem
Find the reductive ozonolysis products of pent-2-ene.
- 1.Write and identify its two alkene carbons before cutting the connection.
- 2.One retains methyl and hydrogen, giving ethanal. The other retains ethyl and hydrogen, giving propanal.
- 3.The fragments contain two plus three carbons, recovering the original five. Counting fragment carbons is a useful check against omitting a carbonyl carbon.
Problem
An alkene gives ethanal and pentan-3-one on reductive ozonolysis. Find its structure and name.
- 1.Ethanal identifies one original alkene carbon carrying methyl and hydrogen. Pentan-3-one identifies the other carrying two ethyl groups.
- 2.Remove both carbonyl oxygens conceptually and join their carbons by a double bond: .
- 3.A five-carbon parent contains the double bond at position 2 and an ethyl group at position 3. The name is 3-ethylpent-2-ene.
Reductive ozonolysis can preserve aldehydes, while vigorous oxidation may oxidise the corresponding fragments further. The carbonyl carbon is one of the original double-bond carbons; it is not an extra atom added by ozone.
Joining monomers rather than cutting them
Polymerisation joins many small monomer molecules into a large polymer. In ethene addition polymerisation, pi bonding is consumed as new carbon–carbon sigma bonds connect units into a chain. The carbon backbone becomes saturated. Square brackets show a repeat unit and n indicates many repetitions, not an atom’s valence.
The methyl group of propene remains a side group in polypropene. The two double-bond carbons enter the main polymer backbone. Polythene is used for films, bags, squeeze bottles, toys, and pipes. Polypropene is used in milk crates, buckets, and moulded articles. Manufacturing conditions influence properties, so a polymer name does not by itself specify every product’s rigidity or strength.
Problem
Why is a three-carbon straight-chain repeat not the correct simple addition-polymer repeat for propene?
- 1.Propene has two connected double-bond carbons and a methyl group attached to one of them. Chain growth connects those two-carbon units.
- 2.The correct repeat is . The methyl carbon stays a branch rather than moving into a three-carbon straight backbone.
- 3.All original atoms remain accounted for, but the connectivity must also be preserved. A correct atom count alone does not validate a polymer drawing.
Durable plastic products are useful partly because their long carbon backbones persist. Excessive disposable use and poor waste handling consequently create environmental problems. The molecular explanation for a useful material should be connected to thoughtful use and disposal, rather than treating polymer formation as an equation without consequences.
Quiz
What does cold dilute permanganate commonly give from an alkene?
Which pair results from reductive ozonolysis of propene?
What does an alkene carbon carrying two carbon groups become on reductive ozonolysis?
What happens to ethene pi bonds during addition polymerisation?
Which feature occurs in a polypropene repeat unit?
Practice Problems
- Compare mild and vigorous oxidation of but-2-ene.
- Predict reductive ozonolysis products of 3,4-dimethylhept-3-ene and 2-ethylbut-1-ene.
- Reconstruct the alkene giving propanal and pentan-3-one on reductive ozonolysis.
- Explain the different fate of the terminal carbon of 2-methylpropene under reductive ozonolysis and strong oxidation.
- Draw polythene and polypropene repeat units, matching their atoms to monomers.
- Connect polymer durability to one useful application and one disposal concern.
Key Takeaways
• Mild permanganate oxidation can preserve the carbon skeleton while producing a diol. • Strong oxidation and reductive ozonolysis can leave different final fragments. • Ozonolysis can be used forwards to predict fragments and backwards to reconstruct a double bond. • Addition polymerisation preserves substituents while joining the double-bond carbons.