chemical reactions of alkenes

We have previously examined briefly two addition reactions of ethene, the first member of the homologous series of alkenes. These were addition of hydrogen, catalyzed by surfaces of finely divided metals such as nickel, and the addition of bromine. These reactions also occur with the higher homologs. For example, the colorless, volatile liquid 4-methyl-2-hexene reacts as follows:

Note that in the names of the three compounds shown, the number 1 carbon atom in one case is at the opposite end of the chain to that for the other two compounds. This is necessary to make the names conform to systematic usage (Sections 3.1 and 4.1).

The ease with which addition reactions to alkenes occur is the result of the repulsions between the two pairs of electrons that make up the double bond. Cleavage of one half of a carbon-carbon double bond requires 63 kcal, while cleavage of a carbon-carbon single bond requires 83 kcal (Table 2.1). Furthermore, because the repulsions push the electrons to average positions further from the bond axis than the electron positions of a single bond, the alkenes will be more readily attacked by electrophiles, that is, reagents that act to acquire electrons. On the other hand, nucleophiles (" nucleus-loving " reagents) are rather poor at reacting with carbon-carbon double bonds, unless one or more groups with a high degree of electronwithdrawing power are attached to one of the carbon atoms.

Of the two reagents so far considered, Hz and Br, , the latter, like all the halogens, is electrophilic, as we shall see when the mechanism of the reaction is considered in the next section. We have already noted that the addition of hydrogen to alkenes occurs on activated surfaces, and the availability of the electrons in the double bond is here reflected in part in the ease of adsorption of the alkene on the metallic surface.

A. ELECTROPHILIC ADDITION TO ALKENES. THE STEPWISE POLAR MECHANISM

Reagents such as the halogens (CI, , Br, , and, to a lesser extent, I,), hydrogen halides (HCI, HBr, and HI), hypohalous acids (HOC1 and HOBr), water, and sulfuric acid commonly add to the double bonds of alkenes to give

saturated compounds. These reactions have much in common in their mechanisms and have been much studied from this point of view. They are also of considerable synthetic and analytical utility. The addition of water to alkenes (hydration) is particularly important for the preparation of a number of commercially important alcohols. Thus ethyl alcohol and t-butyl alcohol are made on a very large scale by hydrating the corresponding alkenes (ethene and 2- methylpropene), using sulfuric or phosphoric acids as catalysts.

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