Electrophilic addition reactions mechanisms

The most common method of epoxidation is the reaction of olefins with per-acids. For over twenty years, perbenzoic acid and monoperphthalic acid have been the most frequently used reagents. Recently, m-chloroperbenzoic acid has proved to be an equally efficient reagent which is commercially available (Aldrich Chemicals). The general electrophilic addition mechanism of the peracid-olefin reaction is currently believed to involve either an intra-molecularly bonded spiro species (1) or a 1,3-dipolar adduct of a carbonyl oxide, cf. (2). The electrophilic addition reaction is sensitive to steric effects. 

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. 

What evidence is there to support the carbocation mechanism proposed for the electrophilic addition reaction of alkenes One of the best pieces of evidence was discovered during the 1930s by F. C. Whitmore of the Pennsylvania State University, who found that structural rearrangements often occur during the reaction of HX with an alkene. For example, reaction of HC1 with 3-methyl-1-butene yields a substantial amount of 2-chloro-2-methylbutane in addition to the "expected" product, 2-chloro-3-methylbutane. 

With regard to the composition of the electrical effect, examination of the p values reported in Table XVII shows that in six of the sets which gave significant correlation, the localized effect is predominant (in these sets, either Pr < 50 or / is not significant). Thus it would appear that in so far as substituent effects are concerned, there are two major classes of electrophilic addition to the carbon-carbon double bond predominance of the localized effect or predominance of the delocalized effect. This behavior may well be accounted for in terms of the reaction mechanism. The rate-determining step in the electrophilic addition reaction is believed to be the formation of an intermediate which may be either bridged or a free carbonium ion. 

The reactions of halogens and hydrogen halides with alkenes are electrophilic addition reactions. This means that the initial attack on the organic molecule is by an electron-deficient species that accepts a lone pair of electrons to form a covalent bond. This species is called an electrophile. In the case of the reaction with hydrogen bromide, the mechanism for the reaction is as shown. 

The most significant change in these reactions is the formation of the carbon-nncleophile bond so, in both types of mechanism, the reaction is termed a nucleophilic addition. It should be noted that the polarization in the carbonyl group leads to nucleophilic addition, whereas the lack of polarization in the C=C donble bond of an alkene leads to electrophilic addition reactions (see Chapter 8). Carbonyl groups in carboxylic acid derivatives undergo a similar type of reactivity to nucleophiles, but the... 

In general, the mechanisms of nucleophilic additions to double bonds have not been as much studied or systemized as those of electrophilic addition. Reactions 7.51 and 7.52 are examples of the very useful Michael condensation, in which a carbanion adds to an a,/ -unsaturated carbonyl or nitrile compound. The usefulness of these reactions arises from the fact that the number of ways of building longer carbon chains from smaller ones is limited. 

Write the steps in the mechanism of an electrophilic addition reaction. 

With unsymmetrical alkenes, the least substituted alcohol is obtained (Following fig.) and so the organoborane reaction is complementary to the electrophilic addition reaction with aqueous acid. Steric factors appear to play a role in controlling this preference with the boron atom preferring to approach the least sterically hindered site. Electronic factors also play a role as described in the mechanism below ... 

Detailed investigation of C-14 KIE in simple electrophillic addition reactions has been presented by Kanska and Fry610-612 regarding the mechanism of addition of 2,4-dini-trobenzensulphenyl chloride to substituted styrenes labelled with C-14. Additions of 2,4-dinitrobenzenesulphenyl chlorides to / -substituted a- and -labelled styrenes have been carried out in acetic acid at 30.1 °C (equation 267) ... 

The simplest version of the mechanism for this addition reaction occurs in two steps. First, the electrophile adds to the double bond, producing a carbocation intermediate. In the second step the nucleophile adds to the carbocation. This step is identical to the second step of the SN1 reaction. Because the initial species that reacts with the double bond is an electrophile, the reaction is called an electrophilic addition reaction. 

Most of the reactions discussed in this chapter involve the attack of an electrophile on an aromatic compound. Although the initial step of the mechanism resembles that of the electrophilic addition reactions of carbon-carbon double bonds discussed in Chapter 11, the final product here results from substitution of the electrophile for a hydrogen on the aromatic ring rather than addition. Therefore, these reactions are called electrophilic aromatic substitutions. 

The word mechanism will often be used loosely here. In contrast to the S l reaction of alkyl halides or the electrophilic addition reactions of alkenes, the details of some of the mechanisms presented in Chapter 12 are known with less certainty. For example, although the identity of a particular intermediate might be confirmed by experiment, other details of the mechanism are suggested by the structure or stereochemistry of the final product. 

Electrophilic addition reaction (Section 10.9) An addition reaction in which the first step of the mechanism involves addition of the electrophilic end of a reagent to a 7i bond. 

Mechanism of the o] mercuration of an alkene to yield an alcohol. TTiis electrophilic addition reaction involves a mercurinium ion intermediate, and its mechanism is similar to that of halohydrin formation. The product of the reaction is the more highly substituted alcohol, corresponding to Markovnikov regiochemistry. 

We ve already seen several methods for preparing alkyl halides, including the reactions of HX and X2 with alkenes in electrophilic addition reactions (Sections 6.8 and 7.2). The hydrogen halides HCl, HBr, and HI react with alkenes by a polar mechanism to give the product of Markovnikov addition. Bromine and chlorine yield trans 3,2 dihalogenated addition products. 

Most reactions of carbonyl groups occur by one of four general mechanisms nucleophilic addition, nucleophilic acyl substitution, alpha substitution, am carbonyl condensation. These mechanisms have many variations, just a alkene electrophilic addition reactions and 8 2 reactions do, but the varia tions are much easier to learn when the fundamental features of the mechanisms are understood. Let s see what the four mechanisms are and what kinds of chemistry carbonyl groups undergo. 

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