Electrophiles in polar reactions

In Chapter 30, you came across the idea of umpolung, the inversion of the usual reactivity pattern of a molecule. You may have already noticed that radicals often have an umpolung reactivity pattern. Alkyl halides are electrophiles in polar reactions yet they generate nucleophilic radicals that react with electrophilic alkenes. 

Alkyl halides behave as electrophiles in polar reactions. 

The electrophile in oxymercuration reactions, HgX or Hg " , is a soft acid and strongly polarizing. It polarizes the n electrons of an alkene to the extent that a three-center, two-... 

Before beginning a detailed discussion of alkene reactions, let s review briefly some conclusions from the previous chapter. We said in Section 5.5 that alkenes behave as nucleophiles (Lewis bases) in polar reactions. The carbon-carbon double bond is electron-rich and can donate a pair of electrons to an electrophile (Lewis acid), for example, reaction of 2-methylpropene with HBr yields 2-bromo-2-methylpropane. A careful study of this and similar reactions by Christopher Ingold and others in the 1930s led to the generally accepted mechanism shown in Figure 6.7 for electrophilic addition reactions. 

Carbozincations constitute undoubtedly one of the most fascinating fields of organozinc chemistry. Whereas alkenes and alkynes are not traditionally considered as electrophiles in polar organometallic chemistry, carbometallation reactions offer many synthetically... 

Both carbocations and carbanions are unstable reactive intermediates. A carbocation contains a carbon atom surrounded by only six electrons, and a carbanion has a negative charge on carbon, which is not a very electronegative atom. Carbocations (electrophiles) and carbanions (nucleophiles) can be intermediates in polar reactions— reactions in which a nucleophile reacts with an electrophile. 

There is, of course, also some polarity in the bond between a saturated carbon atom and a leaving group, say, a bromine atom, but this is a much smaller effect leading only to very small charge separation represented as 5-i-. In alkyl iodides, one of the best electrophiles in Sn2 reactions, there is in fact almost no dipole at all—the electronegativity of C is 2.55 and that of I is 2.66. Electrostatic attraction is unimportant in Sn2 reactions. 

In polar reactions, nucleophiles react with electrophiles. Furthermore, most polar reactions are carried out under either acidic or basic conditions. 

Where do mercuration reactions fit into this picture A mercurinium ion has both similarities and differences, as compared with the intermediates that have been described for other electrophilic additions. The electrophile in oxymercuration reactions, +HgX or Hg +, is a soft Lewis acid and polarizes the TT-electrons of an alkene to the extent that a three-center two-electron bond is formed between mercury and the two carbons of the double bond. However, there is also back bonding from Hg +(i orbitals to the alkene tt orbital. There is weaker bridging in the mercurinium ion than in the three-center four-electron bonding of the bromonium ion. 

In polar reactions, nucleophiles react with electrophiles. 

Electrophilic additions occur frequently in the biosynthesis of steroids and terpenoids. The electrophile in such reactions is a positively charged or a positively polarized carbon atom, which often adds to an unsaturated (electron-rich) partner, usually an alkene, and leads to the formation of a saturated product. In most cases, biochemical pathways have evolved in such a way that electrophilic addition reactions... 

---