Solvents alkene bromination mechanisms

Scheme 2. Mechanisms for alkene bromination in aprotic and protic solvents. 

The reaction of an alkene with diatomic bromine is quite facile, often occurring at room temperature in a few minutes. Based on previous discussions, breaking the n-bond and forming a new C-Br bond is expected to generate a carbocation intermediate. It does not In fact, detailed experiments into the mechanism of this reaction show that there is indeed an intermediate cation, but it is not a carbocation in the solvent CCI4. Any mechanism must also address the stereochemical issue that the bromine atoms in 38 are trans. These issues are discussed in the following section. 

The first possibility envisages essentially the same mechanism as for the second-order process, but with Bt2 replacing solvent in the rate-determining conversion to an ion pair. The second mechanism pictures Bt2 attack on a reversibly formed ion-pair intermediate. The third mechanism postulates collide of a ternary complex tiiat is structurally similar to the initial charge-transfer complex but has 2 1 bromine alkene stoichiometry. There are very striking similarities between the second-order and third-order processes in terms of magnitude of p values and product distribution. In feet, there is a quantitative correlation between the rates of the two processes over a broad series of alkenes, which can be expressed as... 

When NBS is used to brominate non-alkenyl substrates such as alkanes, another mechanism, involving abstraction of the hydrogen of the substrate by the succinimidyl radical " 14 can operate. " This mechanism is facilitated by solvents (such as CH2CI2, CHCI3, or MeCN) in which NBS is more soluble, and by the presence of small amounts of an alkene that lacks an allylic hydrogen (e.g., ethene). 

Chapter 18 by C. Chiappe focuses on the mechanism of bromination of alkenes, exploring the role of solvent on the formation of cyclic bromonium ion versus P-bromocarbemium ion, as key intermediates. In Chapter 19, H. P. A. Mercier et al. discuss the utility of a novel class of noble-gas onium salts as oxidants for generation and isolation of various trihalomethyl cation salts. 

The second piece of evidence against the mechanism of Figure 10-7 is that bromine addition reactions carried out in the presence of more than one nucleophilic reagent usually give mixtures of products. Thus the addition of bromine to an alkene in methanol solution containing lithium chloride leads not only to the expected dibromoalkane, but also to products resulting from attack by chloride ions and by the solvent ... 

Iodine, less reactive than bromine, is best added to alkenes by use of IC1 and IBr (see Section 1.8.3.3). Iodine itself adds rapidly but reversibly to alkenes forming diiodides by mechanisms that can be either ionic or radical. The position of the equilibrium depends upon the structure of the alkene, the solvent and the temperature. Simple vicinal diiodides survive distillation in the dark, but are unstable toward iodine or radicals. In the presence of functions containing free OH groups, such as alumina, HI generated from I2 adds to alkenes irreversibly with the result that the HI adduct, rather than the I2 adduct, is the exclusive product.86 A comparison of the reaction of 1,5-cyclooctadiene with chlorine, bromine and iodine in CH2CI2 reveals that chlorine gas at-50 C gives a 93 7 mixture of trans- and cis-5,6-dichlorocyclooc-... 

It was reported by Rozhkov and Chaplina130 that under mild conditions perfluorinated r-alkyl bromides (r-RfBr) in nonpolar solvents can be added across the n bond of terminal alkenes, alkynes and butadiene. Slow addition to alkenes at 20 °C is accelerated in proton-donating solvents and is catalyzed by readily oxidizable nucleophiles. Bromination of the it bond and formation of reduction products of t-RfBr, according to Rozhkov and Chaplina, suggest a radical-chain mechanism initiated by electron transfer to the t-RfBr molecule. Based on their results they proposed a scheme invoking nucleophilic catalysis for the addition of r-RfBr across the n bond. The first step of the reaction consists of electron transfer from the nucleophilic anion of the catalyst (Bu4N+Br , Na+N02, K+SCN , Na+N3 ) to r-RfBr with formation of an anion-radical (f-RfBr) Dissociation of this anion radical produces a perfluorocarbanion and Br, and the latter adds to the n bond thereby initiating a radical-chain process (equation 91). 

The reaction of an alkene with bromine in an alcohol as solvent produces an ether as the product. Show a mechanism for the following reaction and explain the stereochemistry of the product ... 

Chlorine and bromine commonly add to alkenes by the halonium ion mechanism. Iodination is used less frequently because diiodide products decompose easily. Any solvents used must be inert to the halogens methylene chloride (CH2CI2), chloroform (CHCI3), and carbon tetrachloride (CCI4) are the most frequent choices. 

A major factor in determining the magnitude of relative rates is the solvent used. Thus, high alkene alkyne reactivity ratios are found in organic solvents of low dielectric constant (acetic acid), but the rates of reaction are comparable when water is the solvent. This is true not only for the hydration but also for bromination. Polar solvents, water in particular, are evidently capable of minimizing the energy difference between the two transition states. Whether this is accomplished by very strong solvation or by some other mechanism is not entirely clear. Addition reactions of alkenes and alkynes with trifluoroacetic acid also take place at comparable rates. ... 

For many experimental conditions, the mechanism in Scheme 10.3 is a simplification. This mechanism typically occurs only at low concentrations of bromine or in water and alcohol solvents. In solvents of lower polarity, even acetic acid, the reaction is second order in bromine. The second bromine assists the first step by polarizing the bromine that is adding to the alkene, creating Bra" instead of Br" as a leaving group (Eq. 10.20). Similarly, iodinations of alkenes are commonly second order in I2, and sometimes even third order. Kinetic expressions such as that given in Eq. 10.21 are often observed. The kinetic dependence on Bra" indicates a reaction between the alkene and Br2, as well as Br, which is equivalent to Bra". 

Normally, the nonnucleophilic solvent carbon tetrachloride (CCI4) is used for the bromination or chlorination of alkenes. But the reaction will also work in pro-tic solvents such as water and simple alcohols. In these reactions, new products appear that incorporate molecules of the solvent (Rg. 10.14). How do the OH or OR groups get into the product molecule To see the answer, write out the mechanism and look for an opportunity to make the new products. The first step... 

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