Chapter 8: Alkenes: Reactions and Synthesis
Catalytic hydrogenation adds H₂ with syn stereochemistry
This chapter adds five transformations to the alkene toolkit - hydrogenation, hydroboration-oxidation, epoxidation, dihydroxylation, and ozonolysis - and then uses them in synthesis. Every one of them answers the same two questions: which carbon receives the new group (regiochemistry), and which face receives it (stereochemistry). Watch for those two decisions in each section. Catalytic hydrogenation converts an alkene to an alkane by adding one molecule of H₂ across the double bond. The reaction does not proceed at an appreciable rate without a catalyst; a finely divided transition metal…
Hydroboration-oxidation gives the anti-Markovnikov alcohol
Hydroboration–oxidation converts an alkene to an alcohol with regiochemistry opposite to that of acid-catalyzed hydration. In the first stage, borane adds across the double bond in a single concerted step. Boron becomes bonded to the less hindered, less substituted carbon for two reasons: boron with its attached groups is bulky, and in the four-center transition state the developing positive charge sits on the more substituted carbon, where alkyl groups stabilize it — the same stability logic that governs Markovnikov addition of HBr. Hydrogen adds to that more substituted carbon on the same…
A peroxyacid transfers one oxygen to give an epoxide
Epoxidation adds a single oxygen atom across the double bond to give an epoxide, a three-membered cyclic ether. The reagent is a peroxyacid, most commonly meta-chloroperoxybenzoic acid (mCPBA), which contains a weak, electrophilic oxygen–oxygen bond. The oxygen is transferred to the alkene in one concerted step, and the corresponding carboxylic acid is released as the byproduct. Because both new carbon–oxygen bonds form at the same time and on the same face of the double bond, the reaction is a syn addition and is stereospecific: the substituent relationships present in the alkene are…
Osmium tetroxide gives a syn 1,2-diol
Dihydroxylation adds a hydroxyl group to each carbon of the double bond, giving a vicinal (1,2) diol. The usual reagent is osmium tetroxide, frequently used in catalytic amount together with a stoichiometric oxidant that regenerates it. Osmium tetroxide adds to one face of the alkene to form a five-membered cyclic osmate ester, so both oxygen atoms are delivered to the same face. Hydrolysis then releases the diol without changing that arrangement, so the two hydroxyl groups are syn (cis) to each other. Cold, dilute, basic potassium permanganate gives the same syn diol and is a less expensive,…
Ozonolysis cleaves the double bond to two carbonyls
Oxidative cleavage breaks the carbon–carbon double bond entirely, replacing it with two carbonyl groups. Treatment of an alkene with ozone gives an unstable ozonide, which is then reduced, commonly with zinc and aqueous acid or with dimethyl sulfide. Each carbon that was part of the double bond becomes the carbon of a carbonyl group. A double-bond carbon that bears a hydrogen gives an aldehyde, and a fully substituted double-bond carbon gives a ketone. For example, 2-methylbut-2-ene is cleaved to acetone and acetaldehyde. Ozonolysis serves two purposes. Synthetically, it is a route to…
Choosing a reaction from the required product
The alkene reactions become most useful when they are selected deliberately to construct a target molecule. Retrosynthetic analysis reverses the usual direction: beginning from the desired product, one identifies the bond to be formed and the alkene reaction that would form it with the correct regiochemistry and stereochemistry. The complementary pairs established across this chapter and the previous one (Alkenes: Structure and Reactivity) make the choice explicit. Acid-catalyzed hydration gives the Markovnikov alcohol, and hydroboration–oxidation gives the anti-Markovnikov alcohol.…
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