Nucleophiles bromides

The secondary carbon bears more of the positive charge than does the primary carbon and attack by the nucleophilic bromide ion is faster there Hence the major product is the secondary bromide... 

Is the location of positive charge in the more stable cation also where the lowest-unoccupied molecular orbital (LUMO) is most concentrated Rationalize what you observe. Does attack by a nucleophile (bromide) lead to the Markovnikov or anti Markovnikov product ... 

We call the carbocation, which exists only transiently during the course of the multistep reaction, a reaction intermediate. As soon as the intermediate is formed in the first step by reaction of ethylene with H+, it reacts further with Br in a second step to give the final product, bromoethane. This second step has its own activation energy (AG ), its own transition state, and its own energy change (AG°). We can picture the second transition state as an activated complex between the electrophilic carbocation intermediate and the nucleophilic bromide anion, in which Br- donates a pair of electrons to the positively charged carbon atom as the new C-Br bond starts to form. 

Reversible formation of ionic intermediates in halogenated solvents has been suggested to be due to the weakly nucleophilic character of the counteranion, the tribromide ion, which should dissociate into nucleophilic bromide and free bromine before reacting with the bromonium ion (refs. 11,25,26). In order to check this hypothesis the product distribution of the c/s-stilbene bromination in chloroform was investigated (ref. 27). In the latter solvent the formation constant of Br3 is considerably lower than in DCE, Kf = 2.77 (0.13) x 10 against > 2 x 107 M 1. (ref. 28). As a consequence, at 10 3 M [Br2] relevant amounts of bromide ions are present as counteranion of the bromonium intermediate. Nevertheless, the same trend for the isomerization of cis- to rran -stilbene, as well as an increase of... 

In pyrazolium salts a ring chlorine can be displaced by the more nucleophilic bromide, and nucleophilic substitutions of diazonium groups by halogen are also relatively common (especially for fluorination) with yields frequently 80% or higher [61CB1036 66CB3350 90JAP(K)02/304064],... 

The bromination of cinnamic acid leads to two intermediate enantiomeric cyclic bromonium ions which are subsequently attacked by a nucleophilic bromide ion. Since this attack can occur both at positions 2 and 3, and because the reaction proceeds stereospecifically with inversion, both bromonium ions will give rise to mutually identical pairs of enantiomeric products. This is easier to see from the Fischer projection formulae, which are readily obtained from the zigzag projections by rotation around the C2-C3 bond. 

The bromonium ion resembles a protonated epoxide (see Section 10.10) in both structure and reactions. Therefore, when the nucleophilic bromide attacks to open the three-membered ring, it approaches from the side opposite the other bromine. 

The chiral tertiary alcohol (/ )-3-methyl-3-hexanol reacts with HBr by an SnI pathway. HBr protonates the hydroxyl group, which dissociates to yield a planar, achiral carbocation. Reaction with the nucleophilic bromide anion can occur from either side of the carbocation to produce ( )3-bromo-3-methylhexane. 

The hydrogen and the electrons in its bond to carbon, highlighted in 4-1, move to the adjacent carbon. Now the carbon from which the hydride left is deficient by one electron and is, thus, a carbocation. Because the new carbocation, 4-2, is tertiary, the molecule has gone from a relatively unstable primary carbocation to the much more stable tertiary carbocation. In the final step, the nucleophilic bromide ion reacts with the positive electrophilic tertiary carbocation to give the alkyl halide product. 

Compared with results obtained for ethyl diazoacetate, reactions of diazomethane with allyl bromide produce a dramatic reversal in the relative reactivities of the carbenoid species towards the nucleophilic bromide and the carbon-carbon double bond. These results are in accord with a greater electrophilic selectivity of the dialkoxycarbonyl carbenoid intermediate relative to the ethoxycarbonyl carbenoid. That increasing the electrophilicity of the carbenoid intermediate can reverse its reactivity towards the nucleophilic heteroatom relative to the olefin is consistent with the nature of this... 

Once the bromonium ion is formed, it can be attacked by either nucleophile, bromide or chloride, leading to the mixture of products. 

A typical reaction that illustrates Markovnikov addition is the reaction of HBr with 2-methyl-2-butene to give 2-bromo-2-methylbutane (1, sec. 2.10.A). This reaction proceeds by formation of the more stable carbo-cation, which reacts with the nucleophilic bromide ion. If the anti-Markovnikov bromide (the bromine resides on the less substituted carbon) is desired, a different mechanistic pathway must be followed. A typical anti-Markovnikov addition reaction is addition of borane to the alkene, giving primary alcohol (2) after oxidation of the intermediate alkylborane (sec. 5.4.A). This alcohol can be converted to the anti-Markovnikov bromide, 3, by treatment with PBr3. The key to controlling such reactions is a fundamental... 

Now Sn2 opening of the protonated lactone with the soft nucleophile (bromide ion) gives the y-bromoketone that cyclizes through its enolate. The formation of three-membered rings is favoured kinetically. 

The mechanism for the reaction of phosphorus tribromide with alcohols is similar to that for the reaction of alcohols with thionyl chloride. The alcohol reacts with PBr3 giving an intermediate of the type ROPBr2 which reacts with the nucleophilic bromide ion. 

The carbocation formed from the alkene then reacts with the nucleophilic bromide ion to form the addition product. 

A method for introducing halogens by use of triflates and tetrabutylammonium halides has been published. Excellent yields were obtained for many primary chlorides, bromides, and iodides and for some secondary compounds. However, unfavoured displacements, for example, at C-3 of D-allofuranose compounds gave poorer yields, especially with the weaker nucleophiles bromide and chloride. Synthesis of glycosyl chlorides has been achieved by sequential use of... 

Cyclopentene is a symmetrical alkene, which takes on meaning when it is compared to the reaction of an unsymmetrical alkene such as 2-methyl-2-butene. An unsymmetrical alkene will have different atoms or groups attached to the carbons of the C=C unit. In 2-methyl-2-butene, one carbon of the C=C unit has a methyl and a hydrogen, and the other carbon has two methyl groups. If 2-methyl-2-butene reacts with HBr in the same way as cyclopentene, the intermediate is a carbocation, and subsequent reaction with the nucleophilic bromide ion will form an alkyl bromide. However, the reaction may generate two different products—4 and 5— via two different carbocation intermediates. When the percentage yield of products from this reaction is determined experimentally, it is clear that 5 is the mqjor product. Why An analysis of the mechanism for formation of 4 and also for formation of 5 will provide a prediction of the major product based on rmderstanding each carbocation intermediate. 

If J-2-hexene (13) reacts with HCl, there are two possible alkyl bromide products because there are two carbocation intermediates regioisomers 14 and 15. Both 14 and 15 are secondary carbocations, so there is no energy difference to drive the reaction to favor one intermediate over the other. It is therefore anticipated that both will be formed, in approximately equal amounts, in the absence of any other information. Carbocation intermediate 14 reacts with the nucleophilic bromide ion to give 2-bromohexane as the product, and... 

In accord with the principles just discussed, 3-methyl-l-pentene should react with HBr to form the more stable secondary carbocation 20. Based on the mechanism presented in Section 10.2.1, the reaction of carbocation 20 and the nucleophilic bromide ion should give 2-bromo-3-methylpentane. It does not When the experiment is done, the m or product isolated from this reaction is 21, 3-bromo-3-methylpentane. If this reaction proceeds via a carbocation 20, something has happened to 20 before the bromide ion could react. 

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