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  Alkenes Electrophilic Additions Homework Help - K-12 Grade Level, College Level Chemistry

Introduction to Addition Reactions of Alkenes

The addition reaction is the most familiar chemical transformation of a carbon-carbon double bond. A large number of reagents, both organic and inorganic, have been found to add to this functional group and in this unit we shall review several of these reactions. A majority of these reactions are exothermic, because of the fact that the C-C pi-bond is comparatively weak (ca. 63 kcal/mole) relative to the sigma-bonds formed to the atoms or groups of the reagent. Keep in mind; the bond energies of a molecule are the energies that are required to break (homolytically) all the covalent bonds in the molecule. Subsequently, the reaction will be exothermic if the bond energies of the product molecules are larger than the bond energies of the reactants. The following calculations for the addition of H-Br are typical. Note: by convention exothermic reactions have a negative heat of reaction.

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1. Addition of Strong Brønsted Acids

As demonstrated by the preceding general equation, strong Brønsted acids like HCl, HI & HBr, H2SO4, fastly add to the C=C functional group of alkenes to give products wherein new covalent bonds are formed to hydrogen and to the conjugate base of the acid. Employing the above equation as a guide, write the addition products supposed on reacting each of these reagents with cyclohexene.

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Weak Brønsted acids like water acetic acid (pKa = 4.75) and (pKa = 15.7) do not generally add to alkenes. Though, the addition of a strong acid serves to catalyze the addition of water and in this way alcohols can be prepared from alkenes. For an instance, sulphuric acid is completely ionized to the hydronium ion, H3O(+) if it is dissolved in water and this strongly acidic (pKa = -1.74) species effects hydration of ethene and other alkenes. 

CH2=CH2   +   H3O(+)   -->   HCH2-CH2OH   +   H(+)

The significance of choosing a suitable solvent for these addition reactions should now be clear. Water and alcohols should not be used if the addition of HBr, HCl, or HI is desired. These strong acids will ionize in this type of solvents to give ROH2(+) and the nucleophilic oxygen of the solvent will compete with the halide anions in the last step, giving ether and alcohol products. By employing inert solvents like benzene, hexane, and methylene chloride, these competing solvent additions are avoided. Since these additions continue by way of ionic or polar intermediates, the rate of reaction is larger in polar solvents, like acetonitrile  and nitromethane, than in non-polar solvents, like cyclohexane and carbon tetrachloride.

Regioselectivity and the Markovnikov Rule

Only one result is possible from the addition of these strong acids to symmetrical alkenes like ethene and cyclohexene. Though, if the double bond carbon atoms are not structurally equal, like in molecules of 1-butene, 2-methyl-2-butene and 1-methylcyclohexene, the reagent possibly may add in two different ways. This is displayed for 2-methyl-2-butene in the following equation.

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When addition reactions to this type of unsymmetrical alkenes are carried out, we come to know that one of the two possible constitutionally isomeric products is formed preferentially. Selectivity of this type is termed as regioselectivity. In the above illustrations, 2-chloro-2-methylbutane is almost the exclusive product. likewise, 1-butene forms 2-bromobutane as the predominant product on treatment with HBr.

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3. Electrophilic Reagents

The proton is not the only electrophilic species that starts addition reactions to the double bond. Lewis acids such as the halogens, boron hydrides and specific transition metal ions are capable to bond to the alkene pi-electrons and by nucleophiles the resulting complexes rearrange or are attacked to give addition products. The electrophilic character of halogens is well known. Even though fluorine is uncontrollably chlorine, reactive, bromine and to a lesser degree iodine reacts selectively with the double bond of alkenes. The addition of bromine and chlorine to alkenes, as displayed in the following general equation, proceeds by a primary electrophilic attack on the pi-electrons of the double bond. Iodine includes reversibly to double bonds, but the equilibrium does not usually favor the addition product, so it is not a helpful preparative technique. Dihalo-compounds wherein the halogens are juxtaposed in the manner shown are called vicinal, from the Latin vicinalis, which meaning neighboring.

R2C=CR2   +   X2   -->  R2CX-CR2X

Another halogen consist of reagents that add to double bonds comprise HOX, hypohalous acids and sulfenyl chlorides, RSCl. These reagents are referred as unsymmetrical; therefore their addition to unsymmetrical double bonds may in principle take place in two ways. In fact, these addition reactions are regioselective, by one of the two possible constitutionally isomeric products being favored. These reagents' electrophilic moiety is the halogen.

(CH3)2C=CH2   +   HOBr   -->  (CH3)2COH-CH2Br
(CH3)2C=CH2   +   C6H5SCl   -->  (CH3)2CCl-CH2SC6H5

2. Addition Reactions Initiated by Electrophilic Halogen

The halogens bromine and chlorine add quickly to a wide range of alkenes without inducing the kinds of structural rearrangements noted for strong acids .The stereoselectivity of these additions is strongly anti, as displayed in the following examples.

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