Hammett correlation in competition experiments in dissociation of ionised substituted benzophenones and dibenzylideneacetones.

A convenient method of applying competition experiments to devise a Hammett correlation in the dissociation by α-cleavage of 17 ionised 3- and 4-substituted benzophenones, YCHCOCH [Y=F, Cl, Br, CH, CHO, NH, CF, OH, NO, CN and N(CH)] is reported and discussed. The results given by this approach, which rely on the relative abundance of [M-CH] and [M-CHY] ions in the electron ionisation spectra of the substituted benzophenones, are compared with those obtained by previous methods. Various refinements of the method are considered, including reducing the ionising electron energy, making allowance for the relative abundance of ions such as CH and CHY, which may be formed to some extent by secondary fragmentation, and using substituent constants other than the standard σ constants.

Proximity effects in the electron ionisation mass spectra of substituted cinnamamides.

The electron ionisation mass spectra of an extensive set of 53 ionised monosubstituted and disubstituted cinnamamides [XCHCH=CHCONH, X = H, F, Cl, Br, I, CH, CHO, CF, NO, CHCH, (CH)CH and (CH)C; and XYCHCH=CHCONH, X = Y = Cl; and X, Y = F, Cl or Br] are reported and discussed. Particular attention is paid to the significance of loss of the substituent, X, from the 2-position, via a rearrangement that is sometimes known as a proximity effect, which has been reported for a range of radical-cations, but is shown in this work to be especially important for ionised cinnamamides. When X is in the 2-position of the aromatic ring, [M - X] is formed to a far greater extent than [M - H]; in contrast, when X is in the 3-position or 4-position, [M - H] is generally much more important than [M - X].

Proximity Effects in Mass Spectra of Benzanilides.

The analytical value of peaks arising by a proximity effect in the electron ionization mass spectra of benzanilides has been established by examining the spectra of numerous examples of general structure XCHNHCOCHY. Significant [M-X] signals are observed only when X = Cl, Br, I or CHO in the 2-position. The presence of strong [M-X] signals, but negligibly weak [M-Y] peaks, even when the C-Y bond would be expected to break more readily than the C-X bond, indicates that these diagnostically useful signals do not arise by simple cleavage.

Hammett Correlation in the Accelerated Formation of 2,3-Diphenylquinoxalines in Nebulizer Microdroplets.

The accelerated formation of 2,3-diphenylquinoxalines in microdroplets generated in a nebulizer has been investigated by competition experiments in which equimolar quantities of 1,2-phenylenediamine, CH(NH), and a 4-substituted homologue, XCH(NH) [X = F, Cl, Br, CH, CHO, COCH, CF, CN or NO], or a 4,5-disubstituted homologue, XCH(NH) [X = F, Cl, Br, or CH], compete to condense with benzil, (CHCO). Electron-donating substituents (X = CH and CHO) accelerate the reaction; in contrast, electron-attracting substituents (X = F, Cl, Br and particularly COCH, CN, CF and NO) retard it. A structure-reactivity relationship in the form of a Hammett correlation has been found by analyzing the ratio of 2,3-diphenylquinoxaline and the corresponding substituted-2,3-diphenylquinoxaline, giving a ρ value of -0.

Structure reactivity relationship in the accelerated formation of 2,3-diarylquinoxalines in the microdroplets of a nebuliser.

Competition experiments in which 1,2-phenylenediamine, CH(NH), condenses with equimolar quantities of benzil, (CHCO), and a 3,3'- or 4,4'-disubstituted benzil (XCHCO) (X = F, Cl, Br, CH or CHO) to form a mixture of 2,3-diphenylquinoxaline and the corresponding 2,3-diarylquinoxaline (Ar = XCH) in the microdroplets produced in a nebuliser allow a Hammett relationship with a ρ value of 1.8 to be developed for this accelerated condensation in the nebuliser. This structure reactivity relationship reveals that an appreciable amount of negative charge builds up on the carbon of the carbonyl group of the benzil during the rate-limiting step of the reaction, thus confirming that this process involves nucleophilic addition of the 1,2-phenylenediamine to the benzil.