Untitled

Polyhalomethanes Combined with Lewis Acids in Alkane Chemistry

“…20 kcal/mol for the transformation of these complexes into the final complex of CH 3 Br and HBr with AlBr 3 . The reaction of propane with Br 2 •AlBr 3 occurs via a similar mechanism …”

“…Reactions of Methane with Br 2 • AlBr 3 . The systems Br 2 • n AlBr 3 for n ≥ 1 have been found to behave as superelectrophiles to initiate, under mild conditions, the cracking of linear alkanes and the selective monobromination of other saturated hydrocarbons more stable to fragmentation. 146a,, The nature of the complexes, which can be generated in the simplest system Br 2 •AlBr 3 , as well as the mechanisms of their reactions with methane and propane have been investigated. 146b,− A detailed study of the PES for the system Br 2 •AlBr 3 by the semiempirical MNDO/PM3 method revealed five local minima, two of which respond to intermediates carrying positive charges of 0.35 and 0.45 au on the Br atoms. Only those two structures, both of which exhibited a pronounced ionic character, turned out to be important for methane activation.…”

“…Polyhalomethyl cations are likely generated from polyhalomethanes in the presence of strong Lewis acids, as is suggested by the spectroscopy data for the polyhalomethane−SbF 5 systems in solution 122 and in the solid state, as well as by the established fact of aluminum halide-induced polyhalomethane ionization. , Both semiempirical and nonempirical calculations of the CCl 4 • n AlCl 3 and CBr 4 • n AlBr 3 ( n = 1−3) complexes and related CX 3 + , CHX 2 + , CHX 2+ , CX 2 2+ ions have been carried out. These studies indicated that the polyhalomethyl cations (X = Cl, Br, I, but not F), both free and incorporated in cationic CX 3 + Y - and dicationic CX 2 2+ Y 2 - (Y = AlBr 4 or Al 2 Br 7 ) complexes, are not carbenium ions but rather are halenium cations or cationic complexes containing positively charged halogen atoms directly bonded to the carbon, that is, X 2 CX + or X + CX + . , In other words, electron density transfer from the lone electron pairs of the halogen to the carbocation center ( Scheme ) is significant to the extent that a large positive charge is localized on the halogen atoms, with the C-atom being electroneutral or negatively charged. Similar results have been obtained by DFT and ab initio calculations. 122b,− Selected data on the charge distribution and bond lengths in the polyhalomethyl cations are presented in Tables and .…”

“…These studies indicated that the polyhalomethyl cations (X = Cl, Br, I, but not F), both free and incorporated in cationic CX 3 + Y - and dicationic CX 2 2+ Y 2 - (Y = AlBr 4 or Al 2 Br 7 ) complexes, are not carbenium ions but rather are halenium cations or cationic complexes containing positively charged halogen atoms directly bonded to the carbon, that is, X 2 CX + or X + CX + . , In other words, electron density transfer from the lone electron pairs of the halogen to the carbocation center ( Scheme ) is significant to the extent that a large positive charge is localized on the halogen atoms, with the C-atom being electroneutral or negatively charged. Similar results have been obtained by DFT and ab initio calculations. 122b,− Selected data on the charge distribution and bond lengths in the polyhalomethyl cations are presented in Tables and . These halenium cations differ fundamentally from the classical carbenium (e.g., CF 3 + , CHF 2 + , CF 2 2+ ) and halonium ions…”

“…These halenium cations differ fundamentally from the classical carbenium (e.g., CF 3 + , CHF 2 + , CF 2 2+ ) and halonium ions 4 Calculated Atomic Mulliken Charges of CX 3 + , a CX 2 2+ , and Related Ions b q, auq, auspeciesXCspeciesXCH CF 3 + −0.04 ÷ −0.18 1.12 ÷ 1.55 HCF 2 + 0.07 0.54 0.31 CCl 3 + 0.34 ÷ 0.39 −0.02 ÷ −0.17 CBr 3 + 0.43 ÷ 0.51 −0.29 ÷ −0.53 HCCl 2 + 0.35 0.04 0.25 CI 3 + 0.51 ÷ 0.63 −0.52 ÷ −0.90 CF 2 2+ 0.47 1.06 HCBr 2 + 0.46 −0.17 0.25 CCl 2 2+ 0.94 0.12 CBr 2 2+ 1.04 −0.07 HCI 2 + 0.53 −0.29 0.23 CI 2 2+ 1.04 −0.08 a MP2/6-31G*, MP2/LANL2DZ 122b , MP2/VDZ+P, DFT, , SCF-RHF/6-31G, AM1 AM1 …”