Halogen, addition

Elemental fluorine is rarely used in addition to multiple bonds. This is due mainly 

In contrast to other halogens, the addition of fluorine is predominantly syn. This is compatible with the formation and rapid collapse of a 2-fluorocarbocation-fluoride tight ion pair [Eq. (6.31)]. Extra stabilization for the 2-fluorocarbocation, as in stilbene, decreases stereoselectivity this mechanism also accounts for the formation of 1,1,2-trifluoro compounds [Eq. (6.32)] and solvent incorporation  

A four-center molecular syn addition ([2 + 2]-cycloaddition) is also consistent with experimental observations.202 Molecular-orbital calculations and experimental observations by NMR spectroscopy in superacidic systems rule out the existence of the bridged fluoronium ion.204-206 

Xenon fluorides may add to alkenes in the presence of HF or CF3COOH as catalyst.207-209 The addition, though, takes place via predominant anti attack and is not stereoselective.210 The formation of trifluoro and trifluoromethyl derivatives via radical intermediates, and trifluoroacetate incorporation were also observed.211,212 

The rate law for the addition of chlorine across a carbon-carbon double bond is first-order in both alkene and chlorine independently on the polarity of solvent used.215,216 Strong acceleration of the reaction rate is brought about by increasing 

The addition of bromine to cycloalkenes can potentially form two stereoisomeric vicinal dibromides. However, only the trans isomer forms. The reaction is stereospecific. The stereochemistry of the products indicates that the halogen atoms bond from opposite faces of the double bond. 

The absence of rearrangement and the anti stereochemistry of the addition product must be accommodated in a proposed mechanism for the addition of a halogen to an alkene involving a carbocation intermediate. The first step of the reaction mechanism is the electrophilic addition of bromine to the Jt bond to give a three-membered ring called a tydic bromonium ion. 

The cyclic bromonium ion reacts stereospecifically with a nucleophilic bromide ion, as indicated by the formation of rra r-l,2-dibromocyclopentane from the addition of bromine to cyclopen tene. The bromonium ion intermediate has a bromine atom bonded to one face of the molecule. 

The Jt electrons of the alkene act as a nucleophile to displace bromide ion from bromine. The resulting cychc bromonium ion can be viewed as the addition of Br to the double bond. Bromine has two covalent bonds and a formal 1+ charge in this intermediate. Attack of the nucleophilic bromide ion occurs from the opposite face because the bromine atom that is already there blocks approach from the same face. 

(PhCH2NEt3)a (polymer-C6H4CH2NMej)ia2 (PhCH2NEt3)Mn04, CICOCOQ 

(n-Bu4N)HF2 XeF2, R3COCl (CF3S02)NF, HQ N-chlorosaccharin, HF-py review BrF 

3- dibromo-5,5-dimethyl-hydantoin, pyridinium poly(hydrogen fluoride) 

cat n-Bu4NF 3H20 NIS, (NH4)HF2-A1F3 NIS, ( -Bu4N)HF2 NIS, ( -BU4N)HjF3 

Modem Synthetic Reactions, 2nd ed, W. A. Benjamin, Inc. (1972), pp 422-429 Comprehensive Organic Synthesis, Eds. B. M. Trost and I. Fleming, Pergamon, Oxford (1991), 

(n-Bu4N)HS04 KBr, H202, cat NILVO3, H20, HCCI3 

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The reaction of chlorine and bromine with cycloalkenes illustrates an important stereo chemical feature of halogen addition Anti addition is observed the two bromine atoms of Br2 or the two chlorines of CI2 add to opposite faces of the double bond... 

These observations must be taken into account when considering the mechanism of halogen addition They force the conclusion that a simple one step bond switching process of the following type cannot be correct A process of this type requires syn addi tion It IS not consistent with the anti addition that we actually see... 

MECHANISM OF HALOGEN ADDITION TO ALKENES HALONIUM IONS... 

Mechanism of Halogen Addition to Alkenes Halonium Ions... 

Both parts of the Lapworth mechanism enol formation and enol halogenation are new to us Let s examine them m reverse order We can understand enol halogenation by analogy to halogen addition to alkenes An enol is a very reactive kind of alkene Its carbon-carbon double bond bears an electron releasing hydroxyl group which makes it electron rich and activates it toward attack by electrophiles... 

Table 7. Chlorinated and Mixed Halogen Additive Flame Retardants...

When cyclohexane is nitrated to produce nitrocyclohexane [1122-60-7] the foUowiag techniques minimise undesired C—C breakage ia the ring low temperature nitrations with NO2 (31), carefiil control of the reactor temperatures, and use of halogen additives. Oxygen iacreases the level of C—C bond breakage. 

DMPPO—polystyrene blends, because of the inherent flame resistance of the DMPPO component (oxygen index ca 29.5), can be made flame retardant without the use of halogenated additives that tend to lower impact strength and melt stabiUty in other polymers. Approximately one-half of total Noryl sales volume is in flame-retarded grades, ie, VO or VI in a 1.6-mm section (UL-94). 

The composition of an oil and the progress of its hydrogenation is expressed in terms of its iodine value (IV). Edible oils are mixtures of unsaturated compounds with molecular weights in the vicinity of 300. The IV is a measure of this unsaturation. It is found by a standardized procedure. A solution of ICl in a mixture of acetic acid and carbon tetrachloride is mixed in with the oil and allowed to reac t to completion, usually for less than I h. Halogen addition takes place at the double bond, after which the amount of unreacted iodine is determined by analysis. The reaction is... 

Neutral HX addition X = P, S, Se, Si Allylic bromination Carbon-halogen addition... 

Allenes react with other typical electrophiles such as the halogens and mercuric ion. In systems where bridged-ion intermediates would be expected, nucleophilic capture generally occurs at the allylic position. This pattern is revealed, for example, in the products of solvent capture in halogen additions and by the structures of mercuration products. ... 

Degree of unsaturation. Unsaturation accounts for the existence of carbon-carbon double bonds in resins. It is generally indicated by the bromine or iodine number. Both methods are based on the halogen addition to the double carbon-carbon bonds. Because the different reactivity of bromine and iodine, both numbers cannot be compared. The bromine or iodine number does not necessarily correlate with the reactivity of the resin, for instance in the ageing process. However, within a given resin series of the same structure, relative comparisons can be made. 

In other work Rozen added molecular fluorine to a steroidal ene-one dissolved in ethanol at low temperatures to produce a vicinal difluonde in a cleaner, better yield reaction than previously obtainable [55] Although the reaction was not general, the stereoselectivity was very high, and contrary to addition of other halogens, addition was r>ii, characteristic of an electrophilic addition pathway... 

Additions of elemental halogens to unsaturated compounds are among the most common preparations of halogenated fluoroorganics. The transformations are usually fairly clean and proceed in good yields. Besides the numerous examples of halogen addition tofluoroalkenes and fluoroalkyl-substituted alkenes, additions to perfliioropropyl vinyl ether [2] and fluormated styrenes [7, 4] have been reported. Both ionic and free-radical processes occur (equations 1 and 2)... 

Halohydrin fonnation, as depicted in Figure 6.13, is mechanistically related to halogen addition to alkenes. A halonium ion intennediate is fonned, which is attacked by water in aqueous solution. 

The bromonium ion postulate, made more than 75 years ago to explain the stereochemistry of halogen addition to alkenes, is a remarkable example of deductive logic in chemistry. Arguing from experimental results, chemists were able to make a hypothesis about the intimate mechanistic details of alkene electrophilic reactions. Subsequently, strong evidence supporting the mechanism came from the work of George Olah, who prepared and studied stable... 

HC1, HBr, and HI add to alkenes by a two-step electrophilic addition mechanism. Initial reaction of the nucleophilic double bond with H+ gives a carbo-cation intermediate, which then reacts with halide ion. Bromine and chlorine add to alkenes via three-membered-ring bromonium ion or chloronium ion intermediates to give addition products having anti stereochemistry. If water is present during the halogen addition reaction, a halohydrin is formed. 

Negative Ions. Negative ions in flames have been studied less widely than positive ions because they are present in much smaller concentrations typically, n+//i 100. The primary source of negative ions has been suggested (20) to be Reaction 3, comparable to Reaction 4 (see the section on flames with halogen additives). 

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