Electrophiles Addition reactions

Electrophile Addition Reactions. The addition of electrophilic (acidic) reagents HZ to propylene involves two steps. The first is the slow transfer of the hydrogen ion (proton) from one base to another, ie, from Z to the propylene double bond, to form a carbocation. The second is a rapid combination of the carbocation with the base, Z . The electrophile is not necessarily limited to a Lowry-Briiinsted acid, which has a proton to transfer, but can be any electron-deficient molecule (Lewis acid). 

Addition Reactions. 1,3-Butadiene reacts readily via 1,2- and 1,4-free radical or electrophilic addition reactions (31) to produce 1-butene or 2-butene substituted products, respectively. 

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. 

Markovnikov s rule is used to predict the regiochemistry of HX (electrophilic) addition reactions. The rule states that HX adds to an unsymmetrical alkene mainly in the direction that bonds H to the less substituted alkene carbon and X to the more substituted alkene carbon. 

The reaction is an example of a polar reaction type known as an electrophilic addition reaction and can be understood using the general ideas discussed in the previous section. Let s begin by looking at the two reactants. 

Carbon-carbon double bonds are present in most organic and biological molecules, so a good understanding of their behavior is needed. In this chapter, we ll look at some consequences of alkene stereoisomerism and then focus on the broadest and most general class of alkene reactions, the electrophilic addition reaction. 

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. 

Let s summarize our knowledge of electrophilic addition reactions up to this point. We know that ... 

How does the Hammond postulate apply to electrophilic addition reactions The formation of a catbocation by protonation of an alkene is an endergonic step. Thus, the transition state for alkene protonation structurally resembles the... 

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. 

Aikene chemistry is dominated by electrophilic addition reactions. When HX reacts with an unsymmetrically substituted aikene, Markovnikov s rule predicts that the H will add to the carbon having fewer alky) substituents and the X group will add to the carbon having more alkyl substituents. Electrophilic additions to alkenes take place through carbocation intermediates formed by reaction of the nucleophilic aikene tt bond with electrophilic H+. Carbocation stability follows the order... 

The following carbocation is an intermediate in the electrophilic addition reaction of HCl with two different alkenes. Identify both, and tell which C-H bonds in the carbocation are aligned for hyperconjugation with the vacant p orbital on the positively charged carbon. 

When an unsymmetrically substituted vinyl monomer such as propylene or styrene is polymerized, the radical addition steps can take place at either end of the double bond to yield either a primary radical intermediate (RCH2-) or a secondary radical (R2CH-). Just as in electrophilic addition reactions, however, we find that only the more highly substituted, secondary radical is formed. 

The chemistry of alkynes is dominated by electrophilic addition reactions, similar to those of alkenes. Alkynes react with HBr and HC1 to yield vinylic halides and with Br2 and Cl2 to yield 1,2-dihalides (vicinal dihalides). Alkynes can be hydrated by reaction with aqueous sulfuric acid in the presence of mercury(ll) catalyst. The reaction leads to an intermediate enol that immediately isomerizes to yield a ketone tautomer. Since the addition reaction occurs with Markovnikov regiochemistry, a methyl ketone is produced from a terminal alkyne. Alternatively, hydroboration/oxidation of a terminal alkyne yields an aldehyde. 

Practically everything we ve said in previous chapters has been stated without any proof. We said in Section 6.8, for instance, that Markovnikov s rule is followed in alkene electrophilic addition reactions and that treatment of 1-butene with HC1 yields 2-chJorobutane rather than 1-chlorobutane. Similarly, we said in Section 11.7 that Zaitsev s rule is followed in elimination reactions and that treatment of 2-chlorobutane with NaOH yields 2-butene rather than 1-butene. But how do we know that these statements are correct The answer to these and many thousands of similar questions is that the structures of the reaction products have been determined experimentally. 

One of the most striking differences between conjugated dienes and typical alkenes is in their electrophilic addition reactions. To review briefly, the addition of an electrophile to a carbon-carbon double bond is a general reaction of alkenes (Section 6.7). Markovnikov regiochemistry is found because the more stable carbo-cation is formed as an intermediate. Thus, addition of HC1 to 2-methylpropene yields 2-chloro-2-methylpropane rather than l-chloro-2-methylpropane, and addition of 2 mol equiv of HC1 to the nonconjugated diene 1,4-pentadiene yields 2,4-dichloropentane. 

Interactive to use a web-based palette to predict products from electrophilic addition reactions to conjugated dienes. 

Conjugated dienes also undergo electrophilic addition reactions readily, but mixtures of products are invariably obtained. Addition of HBr to 1,3-butadiene, for instance, yields a mixture of two products (not counting cis-trans isomers). 3-Bromo-l-butene is the typical Markovnikov product of 1,2-addition to a double bond, but l-bromo-2-butene appears unusual. The double bond in this product has moved to a position between carbons 2 and 3, and HBr has added to carbons 1 and 4, a result described as 1,4-addition. 

When the allylic cation reacts with Br to complete the electrophilic addition, reaction can occur either at Cl or at C3 because both carbons share the positive charge (Figure 14.4). Thus, a mixture of 1,2- and 1,4-addition products results. (Recall that a similar product mixture was seen for NBS bromination of alkenes in Section 10.4, a reaction that proceeds through an allylic radical.)... 

Predicting the Product of sn Electrophilic Addition Reaction of a Conjugated Diene... 

Electrophilic addition reaction (Section 6.7) The addition of an electrophile to a carbon-carbon double bond to yield a saturated product. 

Markovnikov s rule (Section 6.8) A guide for determining the regiochemistry (orientation) of electrophilic addition reactions. In the addition of HX to an alkene, the hydrogen atom bonds to the alkene carbon thal has fewer alkyl substituents. 

Diels-Alder reaction, 493 El reaction, 391-392 ElcB reaction, 393 E2 reaction, 386 Edman degradation, 1032 electrophilic addition reaction, 147-148. 188-189 electrophilic aromatic substitution, 548-549 enamine formation, 713 enol formation, 843-844 ester hydrolysis, 809-811 ester reduction, 812 FAD reactions. 1134-1135 fat catabolism, 1133-1136 fat hydrolysis, 1130-1132 Fischer esterification reaction, 796 Friedel-Crafts acylation reaction, 557-558... 

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