Electrophilic addition reactions of alkenes

Mechanism of the electrophilic addition of HBr to 2-methyl-propene. The reaction occurs in two steps and involves a carbo-cation intermediate. 

0 Bromide ion donates an electron pair to the positively charged carbon atom, forming a C-Br bond and yielding the neutral addition product. 

Electrophilic addition of HX to alkenes is successful not only with HBr but with HC1 and HI as well. Note that HI is usually generated in the reaction mixture by treating potassium iodide with phosphoric acid. 

This is a good time to mention that organic reaction equations are sometimes written in different ways to emphasize different points. In describing a laboratory process, for example, the reaction of 2-methylpropene with HCI just shown might be written in the format A + B C to emphasize that both reactants arc equally important for the purposes of the discussion. The solvent and notes about other reaction conditions, such as temperature, are written either above or below the reaction arrow. 

Alternatively, we might write the same reaction in a format to emphasize that 2-methylpropene is the reactant whose chemistry is of greater interest. The second reactant, HCI, is placed above the reaction arrow together with notes about solvent and reaction conditions. 

Thomson Click Organic Process to view an animation of this alkene addition reaction. 

O A hydrogen atom on the electrophile HBr is attacked by tt electrons from the nucleophilic double bond, forming a new C-H bond. This leaves the other carbon atom with a + charge and a vacant p orbital. Simultaneously, two electrons from the H-Br bond move onto bromine, giving bromide anion. 

Before beginning a detailed discussion of alkene reactions, let s review briefly some conclusions from the previous chapter. We said in Section 5.S 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). Tor example, reaction of 2-methylpropene with HBr yields 2-bromo-2-methyipropane. A careful study of this ajid 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. 

1 he reaction begins with an attack on the electrophile, HBr, by the electrons of the nucleophilic tt bond. Two electrons from the tt bond form a new 7 bond between tlie entering hydrogen and an alkene carbon, as shown by the curved arrotv at the top of figure 6.7. I he carbocation intermediate that results is itself an electrophile, whicli can accej t an electron pair from nucleophilic Br ion to form a C-Br bond and yield a neutral addition product. 

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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. 

Alkyl substituents accelerate electrophilic addition reactions of alkenes and retard nucleophilic additions to carbonyl compounds. The bonding orbital of the alkyl groups interacts with the n bonding orbital, i.e., the HOMO of alkenes and raises the energy (Scheme 22). The reactivity increases toward electron acceptors. The orbital interacts with jt (LUMO) of carbonyl compounds and raises the energy (Scheme 23). The reactivity decreases toward electron donors. 

Alkenes are electron-rich species. The double bond acts as a nucleophile, and attacks the electrophile. Therefore, the most important reaction of alkenes is electrophilic addition to the double bond (see Section 5.3.1). An outline of the electrophilic addition reactions of alkenes is presented here. 

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. 

Such structure reactions have existed in organic chemistry since long. Wilson gives an example of classification of electrophilic addition reactions of alkenes and alkynes. Patterns in organometalhc chemistry with applications in organic synthesis have been discussed by Schwartz and Labinger. 

Alkene chlorinations and brominations are very general reactions, and mechanistic study of these reactions provides additional insight into the electrophilic addition reactions of alkenes. Most of the studies have involved brominations, but chlorinations have also been examined. Much less detail is known about fluorination and iodination. The order of reactivity is F2 > CI2 > Br2 > I2. The differences between chlorination and bromination indicate the trends for all the halogens, but these differences are much more pronounced for fluorination and iodination. Fluorination is strongly exothermic and difficult to control, whereas for iodine the reaction is easily reversible. 

Electrophilic addition reactions of alkenes lead to the synthesis of alkyl halides, vicinal dihalides, halohydrins, alcohols, ethers, and alkanes. 

Section 5.19 Stereochemistry of Electrophilic Addition Reactions of Alkenes... 

The first step in the mercuric-ion-catalyzed hydration of an alkyne is formation of a cyclic mercurinium ion. (Two of the electrons in mercury s filled 5d atomic orbital are shown.) This should remind you of the cyclic bromonium and mercurinium ions formed as intermediates in electrophilic addition reactions of alkenes (Sections 4.7 and 4.8). In the second step of the reaction, water attacks the most substituted carbon of the cyclic intermediate (Section 4.8). Oxygen loses a proton to form a mercuric enol, which immediately rearranges to a mercuric ketone. Loss of the mercuric ion forms an enol, which rearranges to a ketone. Notice that the overall addition of water follows both the general rule for electrophilic addition reactions and Markovnikov s rule The electrophile (H in the case of Markovnikov s rule) adds to the sp carbon bonded to the greater number of hydrogens. 

Ozone and the alkene undergo a concerted cycloaddition reaction—the oxygen atoms add to the two sp carbons in a single step. The addition of ozone to the alkene should remind you of the electrophilic addition reactions of alkenes discussed in Chapter 4. An electrophile adds to one of the sp carbons, and a nucleophile adds to the other. The electrophile is the oxygen at one end of the ozone molecule, and the nucleophile is the oxygen at the other end. The product of ozone addition to an alkene is a... 

Because carbocations are involved in the electrophilic addition reactions of alkenes, it is important to understand the bonding in these chemical intermediates. Describe the bonding in carbocations in orbital terms. 

The reaction of bromine with (E)-stilbene (47) to give meso-stilbene dibromide (48) as the major product (Eq. 10.21) is another example of an electrophilic addition reaction of alkenes. The addition of bromine to many alkenes is a stereospecific reaction that proceeds by anti addition to the double bond. However, the addition of bromine to 47 is not stereospecific because small amounts of dl-stilbene dibromide (49) are also formed in this reaction. The formation of wreso-stilbene dibromide presumably occurs via the nucleophilic attack of bromide on the intermediate cyclic bromonium ion, 50. The possible interconversion of 50 and the acyclic carbocation 51 (Eq. 10.22) is one possible way to account for the presence of dl-stilbene dibromide in the product. 

Vladimir Markovnikov was the first to recognize that the major product obtained when a hydrogen halide adds to an alkene is a result of the addition of the to the sp carbon bonded to the most hydrogens. Consequently, this is often referred to as Markovnikov s rule. However, Markovnikov s rule is valid only for addition reactions in which the electrophile is H. A better rule and the one we will use in this book, is one that applies to all electrophilic addition reactions of alkenes ... 

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