New Insights into the Bromination Reaction for a Series of Alkenes A Computational Study

Abstract

Ab initio calculations were carried out for the reaction of Br2 with ethene, propene, isobutene, fluoroethene, chloroethene, (E)-1,2-difluoroethene, and (E)-1,2-dichloroethene. For ethene the calculations were also carried out for the reaction with 2Br2. Geometries were optimized at the HF, MP2, and B3LYP levels using the 6-31G(d) and 6-31+G(d) basis sets where for Br both the standard 6-31G and the Binning-Curtiss bromine basis sets were used. Energies were also calculated at the G3MP2 and G3MP2B3 levels. For a single Br2 one mechanism involves a perpendicular attack by Br2 to the C=C bond, and a second mechanism consists of sidewise attack by Br2. Alkenes can react with 2Br2 via several mechanisms, all leading to the dibromo product. The most likely pathway for the reaction of ethene and 2Br2 involves a trans addition of a Br atom from Br3- to one of the bromonium ion carbons. Activation energies, free energies, and enthalpies of activation along with thermodynamic properties (DeltaE, DeltaH, and DeltaG) for each reaction were calculated. We have found that the reaction of ethene with 2Br2 is favored over reaction with only Br2. There is excellent agreement between the calculated free energies of activation for the reaction of ethene and 2Br2 and experimental values in nonpolar aprotic solvents. However, the free energies of activation for the reaction with a single Br2 agrees well with the experimental results for polar protic solvents only when the reaction is mediated by a solvent molecule. A kinetic expression is proposed that accounts for the difference between bromination of alkenes in protic and nonprotic solvents. Some previously unknown heats of formation are reported.

The Inherent Competition between Addition and Substitution Reactions of Br2 with Benzene and Arenes

“…Polybenzenoid hydrocarbons (PBHs) were recognized to undergo addition reactions in the 19th century, and the very common addition versus substitution competition of arenes has been investigated for decades. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] Both phenanthrene and anthracene add Br 2 to give isolable 9,10-dibromo-9,10-dihydro products (the latter even in the presence of FeCl 3 ); [12] subsequent facile HBr elimination (rather than direct substitution) is a preparative route to both 9-bromo arenes. [5][6][7] Some higher PBHs also favor addition-elimination routes to substituted products.…”

“…[8,9] Even the bromination of naphthalene gave 15 % addition (in CCl 4 at 20-25 8C in the dark); [10] its chlorination yielded a 34 % total of various addition products and was 32 000 times faster than the analogous reaction with benzene in acetic acid. [11] These fairly spectacular experimental findings and the unambiguous results of careful investigations, particularly of de La Mare et al, [4,[11][12][13] conflict with the overwhelmingly popular current viewpoint that while alkenes undergo doublebond addition (e.g., of bromine), [14,15] this reaction is disfa-vored for arenes generally since aromaticity is retained in the substitution but not in the initially formed addition products. The direct evidence provided by the preparative additionelimination routes to substitution products discussed above reveal problems with the generality of such conventional electrophilic mechanistic interpretations.…”

Electrophilic Addition to Alkenes: The Relation between Reactivity and Enthalpy of Hydrogenation: Regioselectivity is Determined by the Stability of the Two Conceivable Products

“…Addition of Br 2 to various alkenes was studied by Islam and Poirier by theoretical calculations at the level of MP2/G3 MP2 Large//MP(FULL)/6‐31G(d) 13. The reported theoretical gas‐phase activation energies, E a , for the hydrocarbon alkenes were plotted versus Δ H hyd (Figure 9).…”

“…The electron availability of the double bond, as quantified by Δ H hyd , is the controlling factor for energies of activation. The first step in the reaction was found to be the formation of a weakly bound π‐complex between the alkene and Br 2 13. Thus, the reaction rate is also dependent on the concentration of the π‐complex precursor, which constitutes the reactant leading to the transition state.…”

Bromine photoelimination from organoplatinum(IV) complexes

“…Addition of Br 2 to alkene proceeds through a cyclic bromonium ion mechanism and anti-products are favored. [60][61][62] The mixed alkene bromination competition experiment also support elimination of molecular bromine (Br 2 ), whilst photolysis of 2(BrNap). Only 3,4-transdibromohexane addition product is observed on photolysis of 2(BrNap) with excess 1:1 mixture of trans-3-hexene and 1-hexene .…”