Halide To alkene

N,N,N, N -tetramethyl-l,8,-naph-thalenediamiDe M.P. 51 C. A remarkably strong mono-acidic base (pKg 12-3) which is almost completely non-nucleophilic and valuable for promoting organic elimination reactions (e.g. of alkyl halides to alkenes) without substitution. 

This concludes discussion of our second functional group transformation mvolv mg alcohols the first was the conversion of alcohols to alkyl halides (Chapter 4) and the second the conversion of alcohols to alkenes In the remaining sections of the chap ter the conversion of alkyl halides to alkenes by dehydrohalogenation is described... 

It IS worth remembering that a theory can never be proven correct It can only be proven incor rect incomplete or inadequate Thus theories are always being tested and refined As important as anything else in the scientific method is the testable hypothesis Once a theory is proposed experiments are designed to test its validity If the results are con sistent with the theory our belief in its soundness is strengthened If the results conflict with it the theory IS flawed and must be modified Section 6 7 describes some observations that support the theory that car bocations are intermediates in the addition of hydro gen halides to alkenes... 

Our belief that carbocations are intermediates m the addition of hydrogen halides to alkenes is strengthened by the fact that rearrangements sometimes occur For example the reaction of hydrogen chloride with 3 methyl 1 butene is expected to produce 2 chloro 3 methylbutane Instead a mixture of 2 chloro 3 methylbutane and 2 chloro 2 methylbutane results... 

We have seen this situation before m the reaction of alcohols with hydrogen halides (8ection 4 11) m the acid catalyzed dehydration of alcohols (8ection 5 12) and m the conversion of alkyl halides to alkenes by the El mechanism (8ection 5 17) As m these other reactions an electronic effect specifically the stabilization of the carbocation intermediate by alkyl substituents is the decisive factor The more stable the carbo cation the faster it is formed... 

The addition of hydrogen halides to alkenes has been studied from a mechanistic point of view over a period of many years. One of the first aspects of the mechanism to be established was its regioselectivity, that is, the direction of addition. A reaction is described as regioselective if an unsymmetrical alkene gives a predominance of one of the two possible addition products the term regiospecific is used if one product is formed... 

Addition of hydrogen halides to alkenes is not stereospecific. In contrast, addition of Br2 proceeds exclusively with anti stereochemistry. 

The free radical additions of sulfonyl halides to alkenes, catalyzed by light or typical chemical radical initiators (In), were first investigated in the 1950s69. The products which are / -halo sulfones (22) were obtained via a chain reaction in which RSO j acts as the chain carrier, namely61-62,70,71... 

Free-radical addition of acyl halides to alkenes... 

A more complete discussion of the mechanism of addition of hydrogen halides to alkenes is given in Chapter 6 of Part A. In particular, the question of whether or not discrete carbocations are involved is considered there. Even when a carbocation is not involved, the regioselectivity of electrophilic addition is the result of attack of the electrophile at the more electron-rich carbon of the double bond. Alkyl substituents increase the electron density of the terminal carbon by hyperconjugation (see Part A, Section 1.1.8). 

The stereochemistry of addition of hydrogen halides to alkenes depends on the structure of the alkene and also on the reaction conditions. Addition of hydrogen bromide to cyclohexene and to E- and Z-2-butene is anti.6 The addition of hydrogen chloride to 1 -methylcyclopentene is entirely anti when carried out at 25° C in nitromethane.7... 

Table 4.1. Stereochemistry of Addition of Hydrogen Halides to Alkenes...

The addition of hydrogen halide to alkene is another classical electrophilic addition of alkene. Although normally such reactions are carried out under anhydrous conditions, occasionally aqueous conditions have been used.25 However, some difference in regioselectivity (Markovnikov and anti-Markovnikov addition) was observed. The addition product formed in an organic solvent with dry HBr gives exclusively the 1-Br derivative whereas with aq. HBr, 2-Br derivative is formed. The difference in the products formed by the two methods is believed to be due primarily to the difference in the solvents and not to the presence of any peroxide in the olefin.26... 

Nitroso compounds are formed during the addition of nitrous oxide," " dinitrogen trioxide, and nitrosyl halides to alkenes, and in some cases, from incomplete oxidation of amines with peroxyacids like peroxyacetic acid. Quenching of carbanions with nitrosyl halides is also a route to nitroso compounds. A full discussion on this subject is beyond the scope of this work and so the readers are directed to the work of Boyer. ... 

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