Electrophilic addition reversibility

Is the electrophilic addition of hydrogen chloride to 2-methyl-propene the reverse of the El or the E2 elimination reaction of fe/t-butyl chloride ... 

Just the reverse trend characterizes fc2. This reaction is aided by electron donation to the carbonyl carbon, as would be expected if the mechanism consists of electrophilic addition of the proton to the OH group. The reaction constant is p = -1.74. 

All the above solvolysis data are rationalized by the process depicted in Scheme 1. What is important for our consideration here is that a solvolytically produced bromonium ion of a "normal" olefin has been shown to react in both MeOH and HOAc by preferential attack of Br- on Br" ". This simple set of experiments might be taken as indicating bromonium ion reversal during electrophilic addition of Br2 to olefins is more prevalent than was originally believed. 

This is comparabie to the eiectrophiiic addition of HBr to butadiene (see Section 8.2), though the addition is in the reverse sense overaii, in that Br adds before H in the radicai reaction, whereas H adds before Br in the ionic mechanism. As with the electrophilic addition, we shall nsnally obtain a mixtnre of the two prodncts. 

Large inverse DKIE have been observed400 in the reaction402 of 370 and 371 with Br2 in AcOH and MeOH at 25 °C in the presence of LiBr, and explained by a pronounced steric DKIE401-403 on the partitioning of a reversibly formed bromonium ion 372. There is less compression of the erido C-L groups in the rate-limiting TS for electrophilic addition to 371 (D20 species) than to 370 (H20 species). 

In this reaction, carbamate esters were obtained after long reaction times (70 h), in 0.06-11% yield, depending on the olefin/amine molar ratio (l-10mol/mol) and temperature (308-353 K). Since 1-ethoxyethyl carbamate was formed as the sole regio-isomer product, but there was no formation of the 2-ethoxyethyl ester, the reaction was believed to proceed by an electrophilic addition to the vinyl ether of carbamic acid, formed by reversible reaction of C02 with amine (see Equation 6.1). 

Alkyl halides having a proton attached to a neighbouring (1-carbon atom can undergo an elimination reaction to produce an alkene and a hydrogen halide (Following fig.)- This reaction is the reverse of the electrophilic addition of a hydrogen halide to an alkene. There are two mechanisms by which this elimination can occur. These are E2 mechanism and the El mechanism. 

Alternative mechanistic routes for electrophilic additions to acetylene derivatives are also possible. According to the principle of microscopic reversibility, the same spectrum of different mechanisms is expected for addition to the triple bond as for elimination from a double bond. 

A silver-silyl intermediate was detected by Si nuclear magnetic resonance (NMR) at 5 = 97 ppm and inverse saturation behavior was detected with mono-substituted alkenes. Based on their studies, a silver-mediated reversible releasing of silylene followed by an irreversible electrophilic addition of the silver-silyl intermediate to the alkene was suggested (Fig. 32). 

The full paper on the reactions of iodine(l) thiocyanate and 5a-androst-2-ene and other alkenes has appeared." An investigation into the stereochemistry of the addition of BrCl to cholesterol revealed that the ratio of a /8 attack was 4.5 1, which was higher than that for other electrophilic additions. It was suggested that the reaction involved rapid reversible formation of the bromonium ions followed by attack by chloride ion and that the opening of the 5/8,6/3-bromonium ion was slow relative to that of the 5a,6a-bromonium ion owing to steric interactions with the la-, 3a-, 7a-, and 9a-axial hydrogen atoms. 

Electrophilic additions to alkenes (the reverse of the reactions in this chapter) ch20... 

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