HBr addition

Electrophilic addition of HBr to 2-butene gives 2-butyl bromide. The product is chiral, and HBr addition may selectively form one enantiomer. 

Use Markovnikov s rule to predict the products of HBr addition to the alkenes shown above. 

You knowr the mechanism of HBr addition to alkenes, and you know the effects of various substituent groups on aromatic substitution. Use this knowledge to predict which of the following two alkenes reacts faster with HBr. Explain your answer by drawing resonance structures of the carbocation intermediates. 

Thus the observed orientation in both kinds of HBr addition (Markovnikov electrophilic and anti-Markovnikov free radical) is caused by formation of the secondary intermediate. In the electrophilic case, it forms because it is more stable than the primary in the free-radical case because it is sterically preferred. The stability order of the free-radical intermediates is also usually in the same direction 3°>2°>1° (p. 241), but this factor is apparently less important than the steric factor. Internal alkenes with no groups present to stabilize the radical usually give an approximately 1 1 mixture. 

This 1,4-dlfunctlonalised compound cannot easily be made from diketone (24) because of the chemoselectivity problem. The o-halo ketone (25) is a better starting material and this could be made by HBr addition to (26). 

As we saw in the previous section, Markovnikov s rule tells us to place the H on the less substituted carbon, and to place the X on the more substituted carbon. The rule is named after Vladimir Markovnikov, a Russian chemist, who first showed the regiochemical preference of HBr additions to alkenes. When Markovnikov recognized this pattern in the late 19th century, he stated the rule in terms of the placement of the proton (specifically, that the proton will end up on the less substituted carbon atom). Now that we understand the reason for the regiochemical preference (carbocation stability), we can state Markovnikov s rule in a way that more accurately reflects the underlying principle The regiochemistry will be determined by the preference for the reaction to proceed via the more stable carbocation intermediate. 

Even with HBr addition the reaction chains tend to be rather short— much shorter than those in halogen addition—and more than a trace of peroxide is thus needed to provide sufficient initiator radicals for preparative purposes up to 0 01 mol peroxide per mol of alkene. In practical terms, however, there may already be sufficient peroxide... 

Poly(4-vinylstyrene) No.1 IR-Spectroscopy RU [%] determined by ICl-Addition HBr-Addition... 

Table 11.1 Selectivity data for HBr addition to allenes in solution and in the gas phase [13, 37],...

Scheme 11.4 Products of HBr addition to buta-1,2-diene (lb) in the gas phase (top), preferred site of hydrogen atom trapping of 2-bromobutenyl radical 7b (center) and isomerization of methylallyl radical (Z)-8 (bottom) [13, 37, 38].

The first solid-state linear dimerization was observed with N-vinylpyrrolidi-none. It was first quantitatively converted to its Markovnikov HBr addition product (by application of HBr gas at -40 °C), which upon warming to room temperature lost HBr and formed ( )-l,r-(3-methyl-l-propene-l,3-diyl)bis-(2-pyrrolidinone), but the yield was less than 100% [58]. Interestingly, such head-to-tail dimerizations of alkenes lead to shrinking and that may create reactivity even if the crystal lattice does not allow for molecular migrations due... 

Recently, tryptophane has been converted to the methyl ester of lysergic acid in ten steps, involving a stereoselective HBr-addition to a C=C double bond 236). 

For addition of liquid bromine, a 5-mL piston burette was used delivering 0.4629 p.L s . In the calorimeter was placed 400 mL of a 0.4 M alkylmagnesium bromide in diethyl ether. The measuring procedure followed the same principles as used for HBr addition. 

Problem 8.41 Assign numbers from 1 for least to 5 for most to indicate the relative reactivity on HBr addition to the following compounds ... 

Problem 9.25 Although cyclopropanes are less reactive than alkenes, they undergo similar addition reactions, (fl) Account for this by geometry and orbital overlap, (b) How does HBr addition to 1,1-dimethylcyclopropane resemble Markovnikov addition <... 

Thus the observed orientation in both kinds of HBr addition (Markovnikov electrophilic and anti-Markovnikov free radical) is caused by formation of the secondary intermediate. 

The stereochemistry of HBr addition to alkenes has been thoroughly studied. The reaction of cis- or fnwu-2-butene with DBr in DO Ac gives a mixture consisting of 60% threo and 40% erythro products.116 While cyclohexene117 and alkene (l)118 undergo predominant anti addition, the alkenes (4 -(6) give... 

With other vinylic halides, the substitution pattern determines the direction of HBr addition (equations 79—83).61,65,128-136... 

Polyhalogenated alkenes also undergo HBr addition (equations 84-87).61,63,131 Even tetrahaloethylenes undergo HBr addition in a continuous flow reactor in the presence of a catalyst.132... 

Conjugated dienes generally produce mixtures of 1,2- and 1,4-addition compounds.101 1,3-Butadiene reacts rapidly with HBr even at low temperatures and gives mixtures in which either l-bromo-2-butene or 3-bromo-l-butene predominate, depending on the reaction conditions (equation 100.)72146,147 Further HBr addition gives mixtures of 1,3-dibromobutane and 2,3-dibromobutane.147... 

Dimethyl-1,3-butadiene cleanly affords l-bromo-2,3-dimethyl-2-butene,150 but further HBr addition produces a mixture of dibromides (equation 102).151... 

Totally different products are observed from aqueous82 and gas phase133 HBr addition to tetrasub-stituted allenes (equation 107). 

The stereochemistry of HBr addition to l-alkyn-3-ones can be controlled by proper choice of reagents (equation 114).164... 

However, HBr addition to tetrolic acid and phenylpropiolic acid in benzene is reported to afford the opposite regioisomers (equation 122),170 and the stereochemistry of HBr addition to PhCWCCChEt171 and NCC CCN172 is unclear. 

The reversal of orientation of HBr addition to alkenes in the presence of peroxides. A free-radical mechanism is responsible for the peroxide effect, (p. 336)... 

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