Stereochemistry of Radical Halogenation Reactions. An ab Initio Molecular Orbital Study

Abstract

The stereochemistry of radical halogenation of alkyl halides has been studied by ab initio molecular orbital theory. Two key elementary reactions, hydrogen abstraction reaction [XCH2CH3 + Y• → XCH2CH2 • + HY (R1)] and halogen abstraction reaction [XCH2CH2 • + Y2 → XCH2CH2Y + Y• (R2)], as well as rotational barrier of XCH2CH2 • radical, with X = H, F, Cl, and Br and Y = F, Cl, and Br, were studied using the G2(MP2,SVP) theory. Reactions R1 and R2 with X = F, Cl, and Br were found to be stereoselective. For X = F, both reactions prefer a gauche abstraction, whereas for X = Cl and Br, both reactions prefer a trans pathway. The high rotational barriers of ClCH2CH2 • and BrCH2CH2 • radicals and the low abstraction barriers of their reactions with Cl2 and Br2 are the two main factors that guarantee the retention of their radical configuration during the abstraction reactions. Thus, radical chlorination and bromination of alkenes and chlorine- and bromine-substituted alkanes are predicted to be stereospecific, in good accord with experimental results. We show that the stereochemical control observed in radical halogenation reactions can be explained without the use of Skell hypothesis. The trends of the calculated energy differences between the gauche and trans transition states of reactions R1 and R2, the rotational barriers of XCH2CH2 • radicals, and the gauche effect of XCH2CH3 can be rationalized in a uniform manner in terms of hyperconjugation interaction.

Conformational analysis of trans‐2‐halocyclohexanols and their methyl ethers: a 1H NMR, theoretical and solvation approach

“…The energy difference between the conformations should not be explained only in terms of dipole-dipole and steric effects, but also as being due to an attractive interaction between the two heteroatoms, known as the 'gauche effect.' 1,11,[22][23][24][25][26][27][28] For the halohydrins, the main factor governing the conformational equilibrium is intramolecular hydrogen bonding, mainly when the compounds are diluted in non-polar solvents, such as in CCl 4 solution.…”

Revisiting the Bromination of CH Bonds with Molecular Bromine by Using a Photo‐Microflow System

“…The ability of Br 2 to convert CH bonds into CBr bonds is much less than that of Cl 2 or F 2 for CH/CCl or CH/CF conversion, respectively 10. However, Br 2 provides site selectivity.…”

“…The ability of Br 2 to convert CÀH bonds into CÀBr bonds is much less than that of Cl 2 or F 2 for CÀH/CÀCl or CÀH/CÀF conversion, respectively. [10] However, Br 2 provides site selectivity. The site selectivity of the photoinduced CÀH/CÀBr conversion is tertiary alkyl > secondary alkyl > primary alkyl.…”

Additivity of conformational energy: Quantum mechanical investigation of fluoro- and bromoalkanes

“…The energy difference E G − E A was obtained being equal to 2.20±0.14 kcal/mol and the geometry of anti form was reported as well. Calculations performed in [18,19] using G2/MP2 method led to the value 1.95 kcal/mol.…”