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Reactions of Alkenes. Some Reaction Types :. Substitution. Elimination. Addition. Addition Reactions. The vast majority of the reactions of alkenes are additions.
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Some Reaction Types: Substitution Elimination Addition
Addition Reactions • The vast majority of the reactions of alkenes are additions. • When the addition reaction is initiated by attack of the pi bond electrons of the double bond on an electrophile to form a carbocation, the reaction is known as an electrophilic addition.
Addition of HX to alkenes • In the addition of H-X to alkenes, H bonds to the carbon of the double bond with the greater number of hydrogens. This is called Markovnikov’s rule.
The reaction is regiospecific, and the product obtained is called the Markovnikov product. It is formed because the reaction proceeds to form the most stable carbo- cation intermediate.
When HBr is added in the presence of peroxides, the anti-Markovnikov product is formed. The reason for the change in regiochemistry is that the reaction proceeds via a free radical mechanism rather than a carbocation mechanism. The production of free radicals is initiated by the presence of the peroxide.
Mechanism: Note that reaction proceeds to form the most stable radical.
3. Addition of Water—Hydration of Alkenes General Reaction: This reaction follows Markovnikov’s rule.
The alkyl hydrogen sulfate can be converted to an alcohol by boiling in water. The reaction proceeds by an SN1 mechanism in which the OSO3H acts as a leaving group.
5. Oxymercuration-Demercuration The addition is anti, and Markovnikov orientation is observed.
6. Alkoxymercuration-Demercuration When mercuration takes place in an alcohol solvent, ethers are formed. The mechanism is the same as that for oxymercuration- demercuration, except that the mercuronium ion is attacked by the alcohol instead of water.
Mechanism for hydroboration step: The reaction is anti-Markovnikov. The addition is syn and the reaction is stereospecific.
8. Catalytic Hydrogenation: This reaction is formally a reduction, with H2 adding across the double bond to give an alkane. The process usually requires a catalyst containing Pt, Pd, or Ni. The addition is syn.
Mechanism: The reaction is stereospecific and anti.
11. Epoxidation of Alkenes. An epoxide (oxirane) is a three membered ring cyclic ether.
Mechanism. This is a concerted electrophilic addition. The addition is syn. One of the peracids that is often used is “meta-chloroperoxy- benzoic acid (MCPBA). If you see MCPBA as a reactant, this means that you are using a per acid.
12. Acid Catalyzed Epoxide Ring Opening. Epoxides react with water in the presence of a catalytic amount of acid to form diols. The orientation is of the -OH groups is anti.
13. Syn Hydroxylation of Alkenes. Hydroxylation is the addition of an OH group to each end of a double bond to form a glycol. The reaction can be accomplished with OsO4/H2O2, or KMnO4/OH-.
Example of oxidative cleavage with permanganate.
15. Ozonolysis. Ozone cleaves double bonds to give ketones and/or aldehydes.
17. Addition of Carbenes to Double Bonds. When CHBr3 or CHCl3 is treated with aqueous KOH, a carbene is formed. Carbenes react at double bonds to form cyclopropanes.
