New Insights into the Bromination Reaction for a Series of Alkeness A Computational Study Shahidul M. Islam and Raymond A. Poirier* Department of Chemistry, Memorial UniVersity, St. John’s, Newfoundland A1B 3X7, Canada ReceiVed: July 19, 2007; In Final Form: September 11, 2007 Ab initio calculations were carried out for the reaction of Br 2 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 2Br 2 . 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 Br 2 one mechanism involves a perpendicular attack by Br 2 to the CdC bond, and a second mechanism consists of sidewise attack by Br 2 . Alkenes can react with 2Br 2 via several mechanisms, all leading to the dibromo product. The most likely pathway for the reaction of ethene and 2Br 2 involves a trans addition of a Br atom from Br 3 - to one of the bromonium ion carbons. Activation energies, free energies, and enthalpies of activation along with thermodynamic properties (ΔE, ΔH, and ΔG) for each reaction were calculated. We have found that the reaction of ethene with 2Br 2 is favored over reaction with only Br 2 . There is excellent agreement between the calculated free energies of activation for the reaction of ethene and 2Br 2 and experimental values in nonpolar aprotic solvents. However, the free energies of activation for the reaction with a single Br 2 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. 1. Introduction The electrophilic addition of Br 2 to alkenes is a well-known organic reaction. 1,2 The reaction mechanism has been extensively studied experimentally, and the generally accepted reaction scheme consists of several steps. 2-7 Studies of this reaction go back to as early as 1937 from the work of Roberts and Kimball. 8 Their work suggested the existence of a cyclic bromonium ion intermediate, which was shown in the late 1960s using NMR, by Olah and co-workers, 9,10 to be the actual reactive species. However, there was no structural evidence for the existence of a cyclic bromonium ion because the reaction is too fast. There have been many experimental attempts by a variety of tech- niques to confirm the occurrence of cyclic bromonium ions in the gas phase, including photoionization, 11 ion cyclotron resonance, 11-14 radiolytic technique, 15 and conventional mass spectrometry. 16,17 Although some experiments suggest the formation of a bromonium ion, no conclusive evidence could be provided for its actual structure. Strating et al. 18 first produced a bromonium ion tribromide in the lab by reacting adamantyl- ideneadamantane (AddAd) with Br 2 in CCl 4 . Slebocka-Tilk et al. 19 for the first time obtained the X-ray structure of adaman- tylideneadamantane bromonium ion with a Br 3 - counterion. In this case, because back-side attack by Br - is sterically hindered, bromination stops at the adamantylideneadamantane bromonium ion. (E)-2,2,5,5-Tetramethyl-3,4-diphenylhex-3-ene is the first reported example of an olefin whose interaction with bromine is limited to π complex formation. 20 Similarly, tetraneopentyl- ethylene does not react with bromine in CCl 4 , and on the basis of the 13 C NMR spectrum there is no evidence of formation of a π complex. 21 Thus the reactivity of olefins toward bromine depends on their steric hindrance. A study of the product of bromination of ethene in dichloroethane by 1 H and 2 H NMR spectra indicates that the addition gives trans-1,2-dibromo- ethane. 22 Chretien et al. 23 studied the selectivity of alkene bromination by using stereo-, regio-, and chemoselectivity. It is believed that the electrophilic bromination of alkenes follows a mechanism that has three successive steps: (i) fast-equilibrated formation of an olefin-bromine charge-transfer complex, (ii) rate-limiting ionization of this π complex into a σ complex, the so-called bromonium ion, and, finally, (iii) fast product formation by nucleophilic trapping of the ionic intermediate. 24-26 In comparison to experimental studies, there have been a limited number of theoretical studies on the bromination of alkenes. Yamabe et al. 27 studied the electrophilic addition reactions X 2 + C 2 H 4 f C 2 H 4 X 2 (X ) F, Cl, and Br) at the MP3/ 3-21G//RHF/3-21G level of theory. Their study shows that the fluorination of ethene occurs via a four-center transition state, while chlorination and bromination give zwitterionic three-center transition states. The activation energies were found to be 212.5, 212.1, and 256.9 kJ mol -1 for X ) F, Cl, and Br, respectively. Hamilton and Schaefer 28 in their study on the structure and energetics of C 2 H 4 Br + isomers proposed that the transition state for the bromination reaction is a three-membered bromonium ion with a nearby counter bromide ion. Later, Cammi et al. 29 conducted a detailed MP2 study from the charge-transfer complex (CTC) to the transition state (TS) for the addition of Br 2 to ethene both in the gas phase and in water. At the MP2/ CEP-121G(aug) level the structures of the CTC and the TS were found to have C 2V symmetry both in the gas phase and in water. They also found that the structure of the CTC is not very * Author to whom correspondence should be addressed. Phone: (709) 737-8609. Fax: (709) 737-3702. E-mail: rpoirier@mun.ca. 13218 J. Phys. Chem. A 2007, 111, 13218-13232 10.1021/jp075674b CCC: $37.00 © 2007 American Chemical Society Published on Web 11/15/2007 Downloaded by CKRN CNSLP MASTER on July 28, 2009 Published on November 15, 2007 on http://pubs.acs.org | doi: 10.1021/jp075674b