Addition of Halogens to Alkenes

The reactivity of alkenes toward mercuration varies over a considerable range and is governed by a combination of steric and electronic factors.Terminal double bonds are more reactive than internal ones. Disubstituted terminal olefins, however, are more reactive than monosubstituted cases, as would be expected for electrophilic attack. The differences in relative reactivities are large enough that selectivity can be achieved in certain dienes. Relative reactivity data for some pentene derivatives are given in Table 4.2. 

The addition of chlorine and bromine to alkenes is a very general reaction. Considerable insight has been gained into the mechanism of halogen addition 

CHAPTER 4 ELECTROPHILIC ADDITIONS TO CARBON-CARBON MULTIPLE BONDS 

Substantial amounts of syn addition have been observed for c/5-l-phenylpropene (27-80% syn addition), rraM5-l-phenylpropene (17-29% syn addition), and c/5-stilbene (up to 90% syn addition in polar solvents). A common feature of the compounds that give extensive syn addition is the presence of a 

Although chlorination of aliphatic olefins gives largely anti addition, syn addition is often dominant for phenyl-substituted olefins.These results again 

The bridging bromine prevents rotation about the remaining bond, and back-side nucleophilic opening of the ring by bromide ion would lead to the observed anti addition. Direct evidence for the existence of bromonium ions has been obtained from NMR measurements. A bromonium ion salt (with BrJ as the counterion) has been isolated from the reaction of bromine with the very hindered alkene adamantylideneadamantane.  

The diminished stereospecificity is similar to that noted for hydrogen halide addition to phenyl-substituted alkenes. 

A mechanism in which groups become attached to the same face of the double bond would be termed 

Bromine and chlorine both react via cyclic halonium cations, which we term bromonium and chloronium cations respectively. Fluorine and iodine are hardly ever used for halogenations iodine is a rather unreactive halogenating agent, whereas at the other extreme, fluorine is too vigorous to give controllable reactions. 

The stereochemical consequences of the electrophilic addition of, say, bromine to certain alkenes can be predicted as follows  

(Z)-but-2-ene will react to give 2,3-dibromo-butane as a pair of enantiomers, R,R and S,S, a result of the anti addition. A racemic product will thus be formed, because there is equal probability of 

Similarly, cyclohexene will form 1,2-dibromo-cyclohexane as a racemic product, again R,R and S,S. Note that the three-membered ring of the bromonium ion must be planar and can only be defused to the cyclohexane ring (see Section 3.5.2). 

The bridging by bromine prevents rotation about the remaining bond and back-side nucleophilic opening of the bromonium ion by bromide ion leads to the observed anti 

Interactive to use a web-based palette to predict products of the addition of halogens to alkenes. 

Bromine and chlorine add rapidly to alkenes to yield 1,2-dihalides, a process called fialogenation. For example, approximately 6 million tons per year of 1,2-dichloroethane (ethylene dichloridc) are synthesized industrially by addition 

Based on what we ve seen thus far, a possible mechanism for the reacuon of bromine with alkenes might involve electrophilic addition of Br to the alkene, giving a carbocation that could undergo further reaction with Br to yield the dibromo addition prt)duct. 

Although this mechanism seems plausible, it s not fully consistent with known facts. In particular, it doesn t explain the stereociieniistiy of the addition reaction. That is, the mechanism doesn t tell which product steieoisomer is formed. 

When the halogenation reaction is carried out on a cycloalkene, such as cyclopentene, only the tmns stereoisomer of the dihalide addition product is formed rather than the mixture of cis and trans isomers that might have been expected if a planar carbocation intermediate were involved. We say that the reaction occurs with anti stereochemistry, meaning that the two bromine atoms come from opposite faces of the double bond—one from the top face and one from the bottom face. 

In contrast to the free-radical substitution observed when halogens react with alkanes, halogens normally react with alkenes by electrophilic addition. 

The products of these reactions are called vicinal dihalides. Two substituents, in this case the halogens, are vicinal if they are attached to adjacent carbons. The word is derived from the Latin vicinalis, which means neighboring. The halogen is either chlorine (CI2) or bromine (Br2), and addition takes place rapidly at room temperature and below in a variety of solvents, including acetic acid, carbon tetrachloride, chloroform, and dichloromethane. 

Rearrangements do not normally occur, which can mean either of two things. Either carbocations are not intermediates, or if they are, they are captured by a nucleophile faster than they rearrange. We shall see in Section 6.16 that the hrst of these is believed to be the case. 

Fluorine addition to alkenes is a violent reaction, difficult to control, and accompanied by substitution of hydrogens by fluorine (Section 4.15). Vicinal diiodides, on the other hand, tend to lose I2 and revert to alkenes, making them an infrequently encountered class of compounds. 

Fluorine is too reactive and difficult to control for most laboratory applications, and iodine does not react with most alkenes. 

An explanation for the observed anti stereochemistry of addition was suggested in 1937 by George Kimball and Irving Roberts, who proposed that the true reaction intermediate is not a carbocation but is instead a bromonium 

Electrophilic Additions to Carbon-Carbon Multiple Bonds 

In biological pathways, dehydrations rarely occur with isolated alcohols but instead normally take place on substrates in which the -OH is positioned two carbons away from a carbonyl group. In the biosynthesis of fats, for instance, /3-hydroxybutyry) ACP is converted by dehydration to tram-crotonyl ACP, where ACP is an abbreviation for acyl carrier protein. We ll see the reason for this requirement in Section 11.10. 

One problem with elimination reactions is that mixtures of products are often formed. For example, treatment of 2-bromo-2-methylbutane with KOH in ethanol yields a mixture of two alkene products. What are their likely structures  

How many alkene products, including E,Z isomers, might be obtained by dehydration of 3-methy)-3-hexanol with aqueous sulfuric acid  

Zeolites can facilitate the shape-selective bromination of olefins. When a mixture of cyclohexene and oct-2-ene are reacted with bromine the two alkenes are brominated to a similar extent. However, when the zeolite catalyst silicalite-1 is present the reaction becomes selective [143]. This selectivity depends upon the order in which the reactants are introduced to the catalyst. If the alkene mixture is stirred with the zeolite prior to the addition of bromine then the straight chain octene enters the zeolite pores. The more bulky cyclohexene remains in solution and is halogenated in preference to the octene when the bromine is introduced. If the bromine is pre-absorbed on the zeolite before the alkene mixture is added then the selectivity of the process is reversed [144]. 

A halogen molecule (Br2, CI2, or I2) is electrophilic a nucleophile can react with a halogen, displacing a halide ion  

In this example, the nucleophile attacks the electrophilic nucleus of one bromine atom, and the other bromine serves as the leaving group, departing as bromide ion. Many reactions fit this general pattern for example  

In the last reaction, the pi electrons of an alkene attack the bromine molecule, expelling bromide ion. A bromonium ion results, containing a three-membered ring with a positive charge on the bromine atom. This bromonium ion is similar in structure to the mercurinium ion discussed in Section 8-5. Similar reactions with other halogens form other halonium ions. The structures of a chloronium ion, a bromonium ion, and an iodonium ion are shown next. 

Unlike a normal carbocation, all the atoms in a halonium ion have filled octets. The three-membered ring has considerable ring strain, however, which, combined with a positive charge on an electronegative halogen atom, makes the halonium ion strongly electrophilic. Attack by a nucleophile, such as a halide ion, opens the halonium ion to give a stable product. 

When a solution of bromine (red-brown) is added to cyclohexene, the bromine color quickly disappears because bromine adds across the double bond. When bromine is added to cyclohexane (at right), the color persists. 

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Many of the features of the generally accepted mechanism for the addition of halogens to alkenes can be introduced by referring to the reaction of ethylene with bromine... 

Interactions between the C=C double bond and halogens deserve particular attention. The importance of these interactions arises from the fact they are the first step of the reactions of addition of halogens to alkenes (and, in general, of the electrophilic addition to the carbon-carbon double bond68), which is a very useful functionalization reaction of the alkene double bond. 

When mechanisms involving radicals can be discounted, the main and general reaction pathway of addition of halogens to alkenes is shown in Scheme 10, where X is mainly Cl and Br69-70. 

Mechanism 8-6 Hydroboration of an Alkene 345 8-8 Addition of Halogens to Alkenes 349... 

Mechanism 8-7 Addition of Halogens to Alkenes 350 8-9 Formation of Halohydrins 352... 

Halogen addition is another example of a stereospecific reaction, in which different stereoisomers of the starting material give different stereoisomers of the product. Figure 8-4 shows additional examples of the anti addition of halogens to alkenes. 

Examples of the anti addition of halogens to alkenes. The stereospecific anti addition gives predictable stereoisomers of the products. 

CHAPTER 8 Ionic Addition of HX to an Alkene 332 Free-Radical Addition of HBr to Alkenes 334 Acid-Catalyzed Hydration of an Alkene 338 Oxymercuration of an Alkene 340 Hydroboration of an Alkene 345 Addition of Halogens to Alkenes 350 Formation of Halohydrins 352 Epoxidation of Alkenes 360 Acid-Catalyzed Opening of Epoxides 362 Olefin Metathesis 376... 

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