Generally, sigma bonds to carbon are stronger than is the pi bond between carbons and, as a result, addition reactions are generally exothermic.
Catalytic reduction of an alkene results in the addition of a hydrogen to each of the sp2 carbons and removal of the pi bond. The mechanism results in the addition of the hydrogens cis to each other but because the reaction is reversible, the product need not have this stereochemistry
Addition of a proton to an akene results in the formation of a carbocation that can then react with a nucleophile resulting in net addition. In the case of HBr and HCl, this addition converts the akene into an alky halide. When the starting alkene is unsymmetrical, the proton adds to the less substituted carbon so that the more substituted and therefore more stable carbocation results. This regiochemistry of addition is know as Markovnikov's Rule.
Alcohols can be prepared from alkenes by acid catalyzed hydration. The reaction involves protonation of one of the two sp2 carbons to form a carbocation. The stability of the carbocation dictates the regiochemistry of protonation. The general rule is that the proton is added to that sp2 carbon that already has the most hydrogen atoms---Markovnikov's Rule. There are exceptions to this rule. For example, an oxygen substituent on a carbocation is greatly stabilizing. Hydration is exothermic in most case but the reaction is reversible and can be "pulled" to the alkene by removal of the water formed. Because a carbocation intermidiate is involved, rearrangement of secondary to tertiary carbocations can be observed. The extent of rearrangement depends on may factors, For example, the increasing the concentration of water will increase the rate of reaction of the initially formed carbocation without affecting the rate of rearrangement.
Bromine and chlorine add to alkenes to fomr vicinal dibromides. The reaction proceeds through and intermidiate halonium ion with the halogen bridging between the original sp2 carbons of the alkene. Attack of halide ion on the halonium ion proceeds in much the same way as SN2 reactions, resulting in inversion of stereochemistry at the carbon undergoing substitution. The result is anti addition of the halogen to the alkene.
When halogenation is carried out in the presence of water, halohydrins are formed, the result of attack by water on the intermediate halonium ions. These halohydrins can be converted to epoxides by treatment with base.
With nucleophiles present other than halide ions, the intermediate bromonium ion (or chloronium ion) can form products with a halogen on one carbon and the other nucleophile on the other. When the degree of substitution of the sp2 carbons of the starting alkene is different, the generally the other nucleophile adds to the more substituted carbons of the halonium ion. You may ignore the reactions presented in Table 12-2.
This sequence produces the same outcome as acid catalyzed hydration. It has the advantage that rearrangement does not occur as carbocation intermediates are not involved.
This sequence produces the same outcome as acid catalyzed hydration with two important differences:
- Anti Markovnikov additions is observed, the result of simultaneous addition of B and H to the alkene with the larger B adding to the less substitued carbon of the alkene;
- The reaction adds the H and the OH groups cis to each other.
Carbenes are highly reactive, unstable carbon species with only two substituents and only six-valence electrons. They react with alkenes to form compounds with three-membered rings (cyclopropanes).
Epoxides are formed upon treatment of alkenes with carboxylic acid peracids. The reaction is concerted with all bond changes taking place in a single step.
Reaction of OsO4 with alkenes results in the formation of osmate esters by a concerted reaction in which two of the oxygen atoms of OsO4 add simultaneously to the two carbons of the alkene. The osmate ester is cleaved to a diol with oxidation of the Os to the +8 state by a hydroperoxide, ROOH. The addition of the two oxygens is syn.
Treatment of alkenes with O3 followed by reduction results in cleave of both the sigma and pi bond with each of the original sp2 carbons of the alkene being carbonyl group carbons in the product(s). Cyclic alkenes result in a single dicarbonyl group product whereas acyclic alkenes produce two molecules of carbonyl product. These can either be the same or different depending on the starting alkene. With symmetrical alkenes, a single product is observed.
In the presence of radicals, HBr adds to alkenes with Anti-Markovnikov regiochemistry. The reactions begins with the formation of RO radicals that abstract a hydrogen atom from H—Br, production ROH and and bromine radical. The bromine radical adds to the less substituted carbon of the alkene, thus generating the more substituted and therefore for stable carbon radical. This radical abstracts a hydrogen from H—Br, generating the organic product and another bromine radical that continues the process in a chain reaction.
The addition of a radical (e.g. a bromine radical) to an alkene forms a carbon radical while the pi bond is lost. This radical can react with another molecule of alkene with the formation of a new C—C bond and a new carbon radical. This process results in the formation of a C—C sigma bond at the expense of the pi bond and is thus roughly 20 kcal/mole exothermic.