For electrophilic addition

Neither bromine nor ethylene is a polar molecule but both are polarizable and an induced dipole/mduced dipole force causes them to be mutually attracted to each other This induced dipole/mduced dipole attraction sets the stage for Br2 to act as an electrophile Electrons flow from the tt system of ethylene to Br2 causing the weak bromine-bromine bond to break By analogy to the customary mechanisms for electrophilic addition we might represent this as the formation of a carbocation m a bimolecular elementary step... 

Thus the mechanism for electrophilic addition of Bi2 to ethylene as presented m Figure 6 12 IS characterized by the direct formation of a cyclic bromonium ion as its... 

FIGURE 9 5 (a) Curved arrow notation and (b) transition state for electrophilic addition of a hydrogen halide HXto an alkyne... 

Recall from Chapter 6 the general mechanism for electrophilic addition to alkenes... 

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. 

How do we know that the carbocation mechanism for electrophilic addition reactions of alkenes is correct The answer is that we don t know it s correct at least we don t know with complete certainty. Although an incorrect reaction mechanism can be disproved by demonstrating that it doesn t account for observed data, a correct reaction mechanism can never be entirely proved. The best we can do is to show that a proposed mechanism is consistent with all known facts. If enough facts are accounted for, the mechanism is probably correct. 

Evidence in support of a carbocation mechanism for electrophilic additions comes from the observation that structural rearrangements often take place during reaction. Rearrangements occur by shift of either a hydride ion, H (a hydride shift), or an alkyl anion, R-, from a carbon atom to the adjacent positively charged carbon. The result is isomerization of a less stable carbocation to a more stable one. 

At least three mechanisms have been proposed for electrophilic addition (these mechanisms are shown for attack by HX, but analogous mechanisms can be written for other electrophiles). 

Scheme 4.3. Sulfur and Selenium Reagents for Electrophilic Addition Reactions...

In this chapter, both intermolecular and intramolecular electrophilic [1] and nucleophilic additions [2, 3] to allenes will be discussed. For electrophilic addition, the regio- and stereoselectivity depend on the steric and electronic effects of the substituents on the allenes and the nature of the electrophiles. However, nucleophilic addition usually occurs at the central carbon atom with very limited exceptions. 

One general, and as yet unsolved problem, is understanding and predicting the regiochemistry of arylnitrenium ion addition to DNA bases. This reaction is complicated because phenylnitrenium ion has four potential sites of addition (Fig. 13.21) the nitrenium center as well as the ortho and para ring carbons. Likewise, DNA has numerous sites for electrophilic addition. In guanine bases alone, there is a possibility for addition to N7, 06, N2, or C8. 

They react with alkyl halides to give internal alkynes (see Section 5.5.2) via nucleophilic substitution reactions. This type of reaction also is known as alkylation. Any terminal alkyne can be converted to acetylide and alkynide, and then alkylated by the reaction with alkyl halide to produce an internal alkyne. In these reactions, the triple bonds are available for electrophilic additions to a number of other functional groups. 

Elliott and Shepherd (70) have investigated the effect of Os(II) coordination on dienes and report that the metal acts as a protecting group for electrophilic addition of bromine. Thus, the action of bromine on [Os(NH3)5 T 2-l,2-(l,3-butadiene) ]2+ is thought to result in the 3,4-dibromo-1-butene analog. 

There do not appear to be any direct approaches for electrophilic addition of amines across C—C double or triple bonds. However, aminomercuration-demercuration affords a very useful process to effect this transformation (equation 151). This reaction has been thoroughly reviewed recently.194... 

The extensive data accumulated by Nakamura and Otsuka, although interpreted by them as being due to the intervention of metal carbene and metallocyclobutane intermediates, can also be rationalized by an alternative mechanism in which coordination of the chiral Co(II) catalyst with the alkene activates the alkene for electrophilic addition to the diazo compound (Scheme 5.4). Subsequent ring closure can be envisioned to occur via a diazonium ion intermediate, without involving at any stage a metal carbene intermediate. 

Large inverse DKIE have been observed400 in the reaction402 of 370 and 371 with Br2 in AcOH and MeOH at 25 °C in the presence of LiBr, and explained by a pronounced steric DKIE401-403 on the partitioning of a reversibly formed bromonium ion 372. There is less compression of the erido C-L groups in the rate-limiting TS for electrophilic addition to 371 (D20 species) than to 370 (H20 species). 

Stereoselectivity found for electrophilic additions to norbornene and l-methoxy-2-cyclohexene is believed to originate from secondary orbital interactions rather than from orbital distorsion at the reaction centre144. Rather different results have been obtained from an ab initio MO study of the norbornene hydroboration145. 

Electrophilic addition is an important reaction for alkenes. When you see an alkene on the MCAT, check for electrophilic addition. An electrophile j.s an eiec-LrOn-loving species, so il will have at leas I a partially positive charge, even if it is only from a momentary dipole. The double bond of an alkene is an electron-rich environment and will attract electrophiles,... 

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