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Halogenations alkenes, bromine

So, a modification of our earlier addition mechanism suffices to explain the reaction. In contrast to the addition to aikenes we saw in Chapter 9 (HX, carbocations, and boranes), in alkene halogenation (bromination and chlorination) a three-membered ring is produced as an intermediate. In some, very spedalized cases the bromonium ion can even be isolated, and it seems its intermediacy in most brominations is assured. [Pg.416]

Indeed, more intermediates are involved in alkenes bromination than were previously considered. The first intermediates formed in the early steps are the alkene-halogen molecular complexes, whose ionization gives the corresponding bromonium or p-bromocarbenium bromide (tribromide) ion pairs. (2) The reversibility of the ionization step has been widely discussed in the last years... [Pg.388]

Halohydrins are easily generated by treating alkenes with aqueous solutions of halogens. Bromine water and chlorine water add across double bonds with Markovnikov orientation (Section 8-11). The following reaction shows cyclopentene reacting with chlorine water to give the chlorohydrin. Treatment of the chlorohydrin with aqueous sodium hydroxide gives the epoxide. [Pg.647]

Zeolites can facilitate the shape-selective bromination of olefins. When a mixture of cyclohexene and oct-2-ene are reacted with bromine the two alkenes are brominated to a similar extent. However, when the zeolite catalyst silicalite-1 is present the reaction becomes selective [143]. This selectivity depends upon the order in which the reactants are introduced to the catalyst. If the alkene mixture is stirred with the zeolite prior to the addition of bromine then the straight chain octene enters the zeolite pores. The more bulky cyclohexene remains in solution and is halogenated in preference to the octene when the bromine is introduced. If the bromine is pre-absorbed on the zeolite before the alkene mixture is added then the selectivity of the process is reversed [144]. [Pg.106]

If we take the addition of hydrogen bromide to an alkene as a model for the alkene halogenation reaction, we can build a mechanism quite quickly. Just as the alkene reacts with hydrogen bromide to produce a carbocation and bromide (Br ), so reaction with bromine-bromine might give a brominated cation and bromide (Fig. 10.9).The n bond of the alkene acts as a nucleophile, displacing Br from bromine. Addition of bromide to the carbocation would give the dibromide product. [Pg.414]

PROBLEM 13.64 We answered questions about the StabiUzed alkene halogenation reaction in Chapter 10. This reaction is at the top of the Semester IB page. Watch it again to remind yourself about the pathway. Bromination in this animation is shown to occur via a syn addition. Bromination usually proceeds through an anti addition. Why would syn addition be favored over anti addition for this reaction Notice the benzyUc-Uke intermediate in the reaction. Do you suppose the napthylmethyl cation (Uke the one in this animation) is more stable than a benzyl cation Why ... [Pg.622]

Many of the features of the generally accepted mechanism for the addition of halogens to alkenes can be introduced by referring to the reaction of ethylene with bromine... [Pg.256]

Table 6 3 shows that the effect of substituents on the rate of addition of bromine to alkenes is substantial and consistent with a rate determining step m which electrons flow from the alkene to the halogen Alkyl groups on the carbon-carbon double bond release electrons stabilize the transition state for bromonium ion formation and increase the reaction rate... [Pg.258]

In aqueous solution chlorine and bromine react with alkenes to form vicinal halohy drins, compounds that have a halogen and a hydroxyl group on adjacent carbons... [Pg.259]

Addition of halogens (Sections 6 14-6 16) Bromine and chlorine add to alkenes to form vicinal dihalides A cy clic halonium ion is an intermediate Stereospecific anti addition is observed... [Pg.273]

When treated with bromine or chlorine in aqueous solution alkenes are con verted to vicinal halohydrins A haloni um ion IS an intermediate The halogen adds to the carbon that has the greater number of hydrogens Addition is anti... [Pg.273]

Although alkenes typically react with chlorine and bromine by addition at room tern perature and below (Section 6 14) substitution becomes competitive at higher tempera tures especially when the concentration of the halogen is low When substitution does occur It IS highly selective for the allylic position This forms the basis of an industrial preparation of allyl chloride... [Pg.396]

N Bromosuccimmide provides a low concentration of molecular bromine which reacts with alkenes by a mechanism analogous to that of other free radical halogenations... [Pg.397]

The fact that the bromine concentration remains at veiy low levels is important to the success of the allylic halogenation process. The allylic bromination of alkenes must... [Pg.705]

Perfluorinated organic bromides can be oxidatively fluonnated with elemental fluorine to derivatives containing tn- [124] and pentavalent [/25 126 127] bromine in yields up to 42% Perfluoroheptylbromine tetrafluoride has been used to fluonnate double bonds in halogenated alkenes [127]... [Pg.48]

Electrophilic addition of hydrogen bromide to alkenes follows Markovnikov s rule, leading to the product with halogen on the more-substituted position. However, trace amounts of hydroperoxides (among other impurities ) may initiate a reaction that gives rise to the anti-Markovnikov product, with bromine in the less-substituted position. [Pg.241]

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. [Pg.246]

This allylic bromination with NBS is analogous to the alkane halogenation reaction discussed in the previous section and occurs by a radical chain reaction pathway. As in alkane halogenation, Br- radical abstracts an allylic hydrogen atom of the alkene, thereby forming an allylic radical plus HBr. This allylic radical then reacts with Br2 to yield the product and a Br- radical, which cycles back... [Pg.339]

Simple alkyl halides can be prepared by radical halogenation of alkanes, but mixtures of products usually result. The reactivity order of alkanes toward halogenation is identical to the stability order of radicals R3C- > R2CH- > RCH2-. Alkyl halides can also be prepared from alkenes by reaction with /V-bromo-succinimide (NBS) to give the product of allylic bromination. The NBS bromi-nation of alkenes takes place through an intermediate allylic radical, which is stabilized by resonance. [Pg.352]


See other pages where Halogenations alkenes, bromine is mentioned: [Pg.364]    [Pg.298]    [Pg.90]    [Pg.279]    [Pg.533]    [Pg.533]    [Pg.288]    [Pg.291]    [Pg.533]    [Pg.913]    [Pg.288]    [Pg.301]    [Pg.364]    [Pg.21]    [Pg.22]    [Pg.195]    [Pg.72]   
See also in sourсe #XX -- [ Pg.112 ]




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Alkene bromination

Alkenes halogenation

Alkenes halogens

Brominations alkenes

Halogenated Alkenes

Halogenations bromine

Halogens bromine

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