Hydroboration, Oxidation and Cleavage of Alkenes and Alkynes
A double or triple bond is a handle for putting oxygen where you want it. Different reagents attack the same bond with different regiochemistry and different depth of oxidation, up to cutting the molecule in two.
What a learner can do afterwards
- Predicts the anti-Markovnikov, syn product of hydroboration and oxidation and explains both features from the addition step
- Distinguishes the products of cold dilute and hot concentrated oxidation of the same alkene
- Uses ozonolysis products to work backwards to the position of the original double bond
- Predicts whether an alkyne gives an aldehyde, a ketone or an acid under stated conditions
1 · Read
A carbon double bond is a handle for attaching oxygen. Two ways add water across it, putting OH on opposite carbons. Direct acid hydration follows Markovnikov: the OH lands on the more substituted carbon, so propene gives 2-propanol. Hydroboration followed by oxidation does the opposite: the OH lands on the less substituted carbon, so propene gives 1-propanol.
Both the reversed position and the geometry come from one step. Boron and hydrogen add across the same face, with boron going to the less crowded carbon. Oxidation swaps boron for OH, keeping that geometry, so H and OH finish on the same face. Chemists call this an anti-Markovnikov, syn addition.
Osmium tetroxide adds two OH across the same face to give a syn diol; cold dilute potassium permanganate does the same. Hot concentrated potassium permanganate cuts the double bond entirely: cyclohexene gives hexanedioic acid. A peracid instead gives an epoxide, so the reagent decides, not temperature alone.
Ozonolysis with a reductive workup is the cleanest cut: each double bond carbon becomes a carbonyl carbon. To work backwards, split each product at its C=O and rejoin the halves with a double bond: ethanal plus propanone rejoin to 2-methyl-2-butene. Alkynes follow the same logic: acid hydration with mercury(II) sulfate of a terminal alkyne gives the Markovnikov ketone, hydroboration with a bulky borane followed by oxidation gives the aldehyde instead, and harsh cleavage gives acids.
Boron sends OH to the less substituted carbon on the same face, osmium or cold dilute permanganate stops at the diol, and hot concentrated permanganate cuts the bond into pieces you can rejoin.
The rewritten readings were written for this dataset.
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