ISSN:
0894-3230
Keywords:
Organic Chemistry
;
Physical Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
,
Physics
Notes:
A mechanistic investigation of the gas-phase reactivity of the halomethyl anions XCH2- (X = Cl, Br) towards a mixture of the corresponding halomethane and ammonia was performed using Fourier transform ion cyclotron resonance mass spectrometry. The interpretation of the experimental data is supported by high-level density functional theoretical (DFT) calculations for the chlorine-containing systems (X = Cl). When the specific isotopomer AXCH2- (AX = 35 Cl, 79Br) is allowed to react in an atmosphere of CH3X and NH3, the exclusive formation of the isotope cluster of the halide anions AX- and BX- (BX = 37Cl, 81Br) is observed. However, the intensity ratio I(AX-)/I(BX-) exceeds significantly the value expected from the natural relative isotope abundances and depends linearly on the pressure ratio p(NH3)/P(CH3X). The experimental results are interpreted in terms of three competing reaction mechanisms: (i) The by far dominating process is the more than 70 kcal mol-1 exothermic one-step SN2 substitution of AXCH2- on CH3X, generating haloethane AXCH2CH3 and X- isotopomers, the latter in the proportion of their natural abundances (direct SN2). The experimentally observed excess of AX- stems from two minor reaction pathways: (ii) in a secondary reaction, the halide X- in the primary product anion-molecule complex [AXCH2CH3 … X-] * of the SN2 substitution induces a 1,2-elimination, leading to the formation of the AX- isotopomer (two-step SN2/E2). (iii) Finally, ACH2- can react with ammonia by consecutive endothermic proton transfer (PT) from NH3 to AXCH2- and a very exothermic SN2 substitution of the resulting amide on AXCH3 leading to CH3NH2 and an excess of AX- which depends linearly on p(NH3)/p(CH3X) (PT/SN2). Theoretical calculations show that in the case of [ClCH2-…NH3] *, the PT/SN2 reaction has no stable intermediate. Therefore, it is concluded that this reaction is not a two-step but a one-step process.
Additional Material:
4 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/poc.610050403
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