No abstract

H and 13C n.m.r. data are reported for a series of ortho and para nitro-substituted diphenylmethanes and related diphenylmethyl anions. In the case of the neutral molecules, a predominance of the skew conformation is suggested for mono-and di-o nitro-substituted derivatives, e.g. 2,4,4'-trinitro-and 2,2'4,4'-tetranitro-diphenylmethanes, while a perpendicular conformation is preferred by the tri-ortho nitro-su bstituted derivative, e.g. 2,2',4,4',6-pentanitrodiphenylmet hane. 13C shifts and lJCXHl coupling constants indicate that, in all carbanions studied, the exocyclic m-carbon is sp2 hybridized and carries only a small fraction of the negative charge. For carbanions with different substituents on the aromatic rings, e.g. 2,4,4'-trinitro-and 2,2',4,4',6-pentanitrodiphenylmethyl carbanions, the negative charge appears to be preferentially delocalized in the more substituted aromatic ring.From arguments based upon the observed n.m.r. and U.V. spectroscopic data, the two rings in diphenylmethyl anions with up to two ortho nitro substituents are essentially coplanar, whereas steric interactions prevent such coplanarity in those with three or four such substituents.

SYNOPSISUsing organic peroxide initiators in the melt grafting of monomers containing amino groups onto polyethylenes is known to cause serious discoloration of the resulting polymers. To eliminate the discoloration while preserving the controlled degree of grafting, a nonoxidative and hydrogen-abstracting initiator with appropriate thermal stability is needed. In this study, the hydrogen-abstracting capability of three azo initiators with suitable decomposition rates in the melt grafting temperature ranges was evaluated using polyethylene cross-linking and polypropylene degradation experiments ( called "hydrogen-abstraction experiments").Among the three azo initiators tested, only a phenylazo initiator, Z-phenylazo-2,4-dimethyl-4-methoxyl-valeronitrile (V-191, demonstrated strong hydrogen-abstracting capability from polymer backbones. These three azo initiators were used in the melt grafting of 2-(N,Ndimethylamino) ethyl methacrylate (DMAEMA) and 2-(N-t-butylamino) ethyl methacrylate (tBAEMA) onto a linear low-density polyethylene (LLDPE) . In agreement with the results obtained from the hydrogen-abstraction experiments, only V-19 led to a considerable amount of grafting of both DMAEMA and tBAEMA onto LLDPE. As expected, polymers grafted with V-19 showed significantly reduced discoloration compared with those grafted with peroxide initiators. Further examination of the grafting results indicated that the initiator efficiency, defined as the number of grafted monomer units per radical generated from initiator decomposition, was higher with the phenylazo initiator than with peroxide initiators. The hydrogen-abstracting capability of the phenyl free radical generated from the decomposition of V-19 was estimated to be higher than peroxide and methyl free radicals produced in the decomposition of peroxide initiators.

Rate and equilibrium data for the reversible deprotonation of 2,2 ',2",4,4',4"-hexanitro-, 2,2',4,4',4"pentanitro-, 2,4,4',4"-tetranitro-, 4,4',4",-trinitro-, 4,4'-dinitro-and 4-nitro-triphenylmethanes (2a-f ) by hydroxide ion have been measured in various H,O-dimethyl sulphoxide (DMSO) mixtures at 25 "C. The increase in acidity (pK,) brought about by the introduction of a first p-nitro group in triphenylmethane (29) to give (2f) is very large and equal to about 12 pK units while the acidstrengthening influence of each of the second and third p-nitro groups is only of the order of 2 pK units. This suggests that only one p-nitrophenyl ring of the 4,4'-dinitro-and 4,4',4"-trinitrotriphenylmethyl anions [(C-2e), (C-2d)l is in a favourable position for effective conjugation with the exocyclic sp2 carbon atom of these carbanions at any given time. Accordingly, the effects exerted by the additional p-nitrophenyl ring(s) are mainly inductive in nature. Support for these ideas is the observation that the increases in acidity observed on going from (2f) to (2e) to (2d) are essentially the result of the corresponding increases in the kinetic acidity (k,OH). Addition of a first o-nitro group to (2d) to form (2c) further increases the kinetic acidity but it also results in a large decrease in

The effect of solvent composition on the acidity of substituted phenols in DMSO-H 2 O systems was examined. It is shown that in DMSO-H 2 O mixtures with up to 35 mol% DMSO there is a linear relationship between the pK a values of monosubstituted phenols in the binary solvent mixtures of different compositions and the corresponding pK a values in water. This leads to an equation relating the slope and intercept terms, pK a (DMSO-H 2 O) = slope[pK a (H 2 O)] intercept. The basis of this relationship in the Kirkwood-Westheimer equation is examined. The significance of a possible 'supersolvent' is discussed. The applicability of the relationships derived herein to other rate-equilibria relationships and solvent mixtures is considered.