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σ+ Substituent constants for the ortho- and para-vinyl group have been determined by the application of the linear free-energy relationship to the nitration of the β-substituted styrene derivatives.Energy changes (relative to benzene system) for the proton and hydride ion transfer to individual positions in the styrene molecule have been calculated. Both approaches indicate that the vinyl group is capable of stabilizing both positively and negatively charged transition states. The interactions of the vinyl group with other substituents in the phenyl ring are also determined. Again, stabilizing effects with respect to both π-donor and π-acceptor substituents have been demonstrated for the vinyl group.

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Partial rate factors for the nitration of carboxylic and phosphonic acids PhCH2X and PhCH=CHX (X = C02H or P03H2) have been determined in acetic anhydride and in strongly acidic media. For P-styrylphosphonic acid the rate of bromine addition has been measured and compared with that for cinnamic acid. 13C N.m.r. spectra of the PhX, PhCH2X. and PhCH'CHX systems have been recorded. Substituent effects of the carboxy and phosphonic groups upon the reactivity and upon the shielding of ring carbon atoms are compared and discussed. Close similarity of substituent effects of C02H and P03H2 has been demonstrated. Good correlation between the partial rate factors and 13C chemical shifts has been found for the systems PhX and PhCH2X; in a-styryl derivatives this parallel breaks down owing to the versatile nature of the vinyl group. 270 nm (log E 2.76); for X = CO,H, A , , , , . 273 nm (log E = 2.87) 9]. The available data also allow a comparison of the substituent effects of phosphonic and carboxy groups in aromatic nitration, both in acidic and in aprotic media. Such a comparison, expressed in terms of the relative changes (with respect to benzene) of the free energy of substitution at orthoand meta-positions, is presented in Table 1. The data show that the two substituents produce almost identical selectivity with respect to the orthoand uneta-positions of the ring. This observation rules out immediately any specific ' ortho ' effect in nitration of phenylphosphonic acid, since no decrease in the relative change of the free t The similarity of the electron-acceptor abilities of P(0) (OR), and CO,R, expressed in terms of Taft's o* constants, has been pointed