Electronegativity electrophilic addition reactions

It is important to be able to look at a molecular structure and deduce the possible reactions it can undergo. Take an alkene, for example. It has a 7t bond that makes it electron-rich and able to attack electrophiles such as water, halogens and hydrogen halides in electrophilic addition reactions. Haloalkanes, on the other hand, contain polar carbon-halogen bonds because the halogen is more electronegative than carbon. This makes them susceptible to attack by nucleophiles, such as hydroxide, cyanide and alkoxide ions, in nucleophilic substitution reactions. 

Since the rate-determining stage in an electrophilic addition reaction often involves the attack of the electrophile upon the unsaturated system, factors which affect the electronegativity of the atom being attacked will influence the rate of the reaction. In the acid-catalysed hydration of olefins, which in dilute solutions follows the simple kinetic form... 

The general observation that replacement of a hydrogen or carbon attached to the carbon-carbon double bond by halogen results in retardation of electrophilic addition reactions is justified by arguing that there is decreased electron density in the double bond. That is, halogen inductively withdraws electrons better than hydrogen or carbon (electronegativity, Chapter 1). 

The most common reaction of aldehydes and ketones is the nucleophilic addition reaction, in which a nucleophile, Nu , adds to the electrophilic carbon of the carbonyl group. Since the nucleophile uses an electron pair to form a new bond to carbon, two electrons from the carbon-oxygen double bond must move toward the electronegative oxygen atom to give an alkoxide anion. The carbonyl carbon rehybridizes from sp2 to sp3 during the reaction, and the alkoxide ion product therefore has tetrahedral geometry. 

As we saw in A Preview of Carbonyl Compounds, the most general reaction of aldehydes and ketones is the nucleophilic addition reaction. A nucleophile, Nu-, approaches along the C=0 bond from an angle of about 75° to the plane of the carbonyl group and adds to the electrophilic C=0 carbon atom. At the same time, rehybridization of the carbonyl carbon from sp2 to sp3 occurs, an electron pair from the C=0 bond moves toward the electronegative oxygen atom, and a tetrahedral alkoxide ion intermediate is produced (Figure 19.1). 

Both undergo addition reactions. They differ in that the C of C=0 is more electrophilic than a C of C=C, because O is more electronegative than C. Consequently, the C of C=0 reacts with nucleophiles. The C=C is nucleophilic and adds mainly electrophiles. 

Product orientation alone is insufficient for elucidation of the mechanisms of these addition reactions as the same products would be obtained by either electrophilic or nucleophilic addition. It is well knowTi that the greater electronegativity of the sp carbon in alkynes leads to lower reactivity with electrophiles and greater reactivity with nucleophiles compared with the sp carbon of alkenes. A number of nucleophilic additions to alkynes such as addition of alcohols and weak acids are known ° . 

An addition reaction is formally the reverse of an elimination reaction. We will see both electrophilic and nucleophilic additions in this section. There are few new generic classes of reactants to consider, and there are three new routes, but these are just the reverse of the elimination routes just covered. The new generic classes are discussed in much more detail in Chapter 5 and 6, but need to be introduced here. A carbon-carbon double bond can range from electrophilic to nucleophilic depending on what is attached to it (Fig. 4.33). Another way to make a double bond electrophilic is to replace one of its carbon atoms with an electronegative heteroatom like oxygen, C=0, a carbonyl. 

Similar addition reactions have been reported for perfluorinated Dewar benzenes, although few electrophilic reactions are known since the high electronegativity of fluorine makes the double bond much less nucleophilic. Thus, bromine adds to the double bonds of hexafluoro Dewar benzene in a stepwise manner (59)13). 

Two bonds are broken in this reaction—the weak 7C bond of the alkene and the HX bond— and two new (T bonds are formed—one to H and one to X. Because X is more electronegative than H, the H-X bond is polarized, with a partial positive charge on H. Because the electrophihc (H) end of HX is attracted to the electron-rich double bond, these reactions are called electrophilic additions. 

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