Alkene addition reactions forming carbocation intermediates

In this chapter, we will look at a wide variety of alkene reactions. You will see that some of the reactions form carbocation intermediates like the one formed when HBr reacts with an alkene, some form other kinds of intermediates, and some don t form an intermediate at all. At first, the reactions covered in this chapter might appear to be quite different, but you will see that they all occur by similar mechanisms. So as you study each reaction, notice the feature that all alkene reactions have in common The relatively loosely held tt electrons of the carbon-carbon double bond are attracted to an electrophile. Thus, each reaction starts with the addition of an electrophile to one of the sp carbons of the alkene and concludes with the addition of a nucleophile to the other sp carbon. The end result is that the tt bond breaks and the sp carbons form new cr bonds with the electrophile and the nucleophile. 

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... 

A second difference between alkene addition and aromatic substitution occurs after the carbocation intermediate has formed. Instead of adding Br- to give an addition product, the carbocation intermediate loses H+ from the bromine-bearing carbon to give a substitution product. Note that this loss of H+ is similar to what occurs in the second step of an El reaction (Section 11.10). The net effect of reaction of Br2 with benzene is the substitution of H+ by Br+ by the overall mechanism shown in Figure 16.2. 

There are, however, serious problems that must be overcome in the application of this reaction to synthesis. The product is a new carbocation that can react further. Repetitive addition to alkene molecules leads to polymerization. Indeed, this is the mechanism of acid-catalyzed polymerization of alkenes. There is also the possibility of rearrangement. A key requirement for adapting the reaction of carbocations with alkenes to the synthesis of small molecules is control of the reactivity of the newly formed carbocation intermediate. Synthetically useful carbocation-alkene reactions require a suitable termination step. We have already encountered one successful strategy in the reaction of alkenyl and allylic silanes and stannanes with electrophilic carbon (see Chapter 9). In those reactions, the silyl or stannyl substituent is eliminated and a stable alkene is formed. The increased reactivity of the silyl- and stannyl-substituted alkenes is also favorable to the synthetic utility of carbocation-alkene reactions because the reactants are more nucleophilic than the product alkenes. 

We may seem to have contradicted ourselves because Equation 10-1 shows a carbocation to be formed in bromine addition, but Equation 10-5 suggests a bromonium ion. Actually, the formulation of intermediates in alkene addition reactions as open ions or as cyclic ions is a controversial matter, even after many years of study. Unfortunately, it is not possible to determine the structure of the intermediate ions by any direct physical method because, under the conditions of the reaction, the ions are so reactive that they form products more rapidly than they can be observed. However, it is possible to generate stable bromonium ions, as well as the corresponding chloronium and iodonium ions. The technique is to use low temperatures in the absence of any strong nucleophiles and to start with a 1,2-dihaloalkane and antimony penta-fluoride in liquid sulfur dioxide ... 

Markovnikov s rule can be restated by saying that, in the addition of HX to an alkene, the more stable carbocation intermediate is formed. I his result is explained by the Hammond postulate, which says that the transition state of an exergonic reaction step structurally resembles the reactant, whereas the transition state of an endergonic reaction step structurally resembles the product. Since an alkene protonation step is endergonic, the stability of the more highly substituted carbocation is reflected in the stability of the transition state leading to its formal ion. 

Why does alkene hydroboration take place with non-Markovnikov regiochemistry, yielding the less highly substituted alcohol Hydroboration differs from many other alkene addition reactions in that it occurs in a single step without a carbocation intermediate. We can view the reaction as taking place through a four-center, cyclic transition state, as shown in Figure 7.6 p. 244). Since both C-H and C-B bonds form at the same time and from the same face of the alkene, syn stereochemistry is observed. 

The mechanism of the substitution reaction depends on the structure of the alcohol. Secondary and tertiary alcohols undergo SnI reactions. The carbocation intermediate formed in the SnI reaction has two possible fates It can combine with a nucleophile and form a substitution product, or it can lose a proton and form an elimination product. However, only the substitution product is actually obtained, because any alkene formed in an elimination reaction will undergo a subsequent addition reaction with HX to form more of the substitution product. 

This should remind you of the first step in an electrophilic addition reaction of an alkene A nucleophilic alkene reacts with an electrophile, thereby forming a carbocation intermediate (Section 3.6). In the second step of an electrophilic addition reaction, the carbocation reacts with a nucleophile (Z ) to form an addition product. 

Section 10.1 discussed strong acids such as HCl or HBr reacting with alkenes via an acid-base reaction to form carbocation intermediates that lead to an addition product (see 5). It was noted at that time that water and alcohols are weak acids and do not react directly with alkenes. Is it possible to design an experiment that allows weak acids to react The answer is yes. 

When an alkene undergoes an electrophilic addition reaction with HBr, the first step is a relatively slow addition of a proton (an electrophile) to the alkene (a nucleophile). A carbocation intermediate (an electrophile) is formed, which then reacts rapidly with a bromide ion (a nucleophile) to form an alkyl halide. Notice that each step involves the reaction of an electrophile with a nucleophile. The overall reaction is the addition of an electrophile to one of the sp carbons of the alkene and the addition of a nucleophile to the other sp carbon (Section 5.6). 

When two substituents add to the same side of a double bond, the addition is called syn addition. When two substituents add to opposite sides of a double bond, the addition is called anti addition. Both syn and anti additions occur in alkene addition reactions that form a carbocation intermediate. Equal amounts of the four stereoisomers are obtained so the reaction is not stereoselective and, because the four stereoisomers formed by the cis alkene are identical to the four stereoisomers formed by the trans alkene, the reaction is also not stereospecific. 

In Section 6.6, we saw that an alcohol can be prepared from the acid-catalyzed addition of water to an alkene. The reaction forms a carbocation intermediate, which will rearrange if the rearrangement leads to a more stable carbocation. 

The reaction begins with an attack on the electrophile, HBr, by the electrons of the nucleophilic tt bond. Two electrons from the 7t bond form a new u bond between the entering hydrogen and an alkene carbon, as shown by the curved arrow at the top of Figure 6.7. The carbocation intermediate that results is itself an electrophile, which can accept an electron pair from nucleophilic Br ion to form a C Brbond and yield a neutral addition product. 

The mechanism of alkyne additions is similar but not identical to that of alkene additions. When an electrophile such as HBr adds to an alkene (Sections 6.7 and 6.8), the reaction takes place in two steps and involves an alkyl carbocation intermediate. If HBr were to add by the same mechanism to an alkyne, an analogous vinylic carbocation would be formed as the intermediate. 

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