Effect of X Substituents

The enolate anion may be considered as an alkene with a very good powerful X -type substituent, the alkoxide oxygen. The HOMO of the n bond is strongly raised in energy 

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Table 1 The Effects of X Substituents of the Anilide Functionality on the Absorption and Sensitivity of a Series of Dual Layer Photoreceptors Using a Diaminofluorenone bisazo Pigment in the Generation Layer. (After Hashimoto, 1986)...

Further manifestations of stabilisation by the overlap of a filled with an unfilled orbital are the effects of X-substituents on an empty p orbital on a metal. Thus trimethylborate 2.6 is much less Lewis acidic than boron halides 2.7, because the oxygen lone pairs overlap more efficiently with the empty p orbital on the... 

These and other studies of the relative substituent effects of X and CH X in nitration were considered in terms of the transmission factor a of the methylene group. To avoid complications from conjugative interactions, attention was focussed mainly on substitution at the meta-position, and ct was defined in terms of partial rate factors by the equation ... 

The reactivity of the individual O—P insecticides is determined by the magnitude of the electrophilic character of the phosphoms atom, the strength of the bond P—X, and the steric effects of the substituents. The electrophilic nature of the central P atom is determined by the relative positions of the shared electron pairs, between atoms bonded to phosphoms, and is a function of the relative electronegativities of the two atoms in each bond (P, 2.1 O, 3.5 S, 2.5 N, 3.0 and C, 2.5). Therefore, it is clear that in phosphate esters (P=0) the phosphoms is much more electrophilic and these are more reactive than phosphorothioate esters (P=S). The latter generally are so stable as to be relatively unreactive with AChE. They owe their biological activity to m vivo oxidation by a microsomal oxidase, a reaction that takes place in insect gut and fat body tissues and in the mammalian Hver. A typical example is the oxidation of parathion (61) to paraoxon [311-45-5] (110). 

When the multipathway scheme (Scheme 3) was applied to the first series, we obtained (ref. 9) a highly negative p-value (-4.6) for the kinetic effect of the substituent X in 1 and a significantly smaller value (p = -1.7) for the effect of Y in 2, at least as long as Y is less electron-donating than a p-methyl group. From the p-ratio, we were able to demonstrate... 

Taft (21) has suggested that the electrical effect of a substituent is composed of localized (inductive and/or field) and delocalized (resonance) factors. Thus we may write the substituent constant of the group X as... 

MRx is the calculated molar refractivity of X-substituents, whereas I is an indicator variable taking the value of 1 and 0 for the presence and absence of a phenyl ring in the X-substituents. The negative sign of MRx brings out a steric effect for the X-substituents that do not appear to reach a hydrophobic surface for ttx (calculated hydrophobicity of X-substituents), r = 0.470. The indicator variable (I) with positive coefficient suggests that the presence of a phenyl ring in the X-substituents would be favorable. 

Thus crx can be regarded as a measure of the overall polar effect exerted by a substituent, X, on the reaction centre. Its sign indicates the direction (-ve = electron-donating +ve = electron-withdrawing), and its magnitude the extent, of the effect that X exerts—compared, of course, with the effect exerted by H. Indeed, the assumed constancy of a substituent s absolute polar effect of X always remains constant, but only that its effect relative to H remains constant. 

Introduction of first one, and then two, CH2 groups between the benzene ring and C02H progressively reduces the susceptibility of the acid s ionisation to the polar effect of the substituent X in the benzene ring. The susceptibility, as revealed by the value of p, rises again for (16), however, as CH=CH is a markedly better transmitter of electronic effects than is CH2—CH2. 

For stilbene bromination, a markedly non-linear structure-reactivity relationship is observed (Fig. 5). Detailed analysis of the kinetic effects of two substituents, X and Y, on each aromatic ring shows that the three pathways leading to the C+ and carbocations and to the bromonium ion can... 

The ring size (degree of oligomerization x) and conformation of M-E heterocycles strongly depends on steric effects of the substituents (repulsive interactions), ring strain effects and on entropy factors. This was shown for instance by Beachley and Racette for several heterocyclic aminoalanes [R2A1NR2]X97 and confirmed by our own results. However, predictions whether four- or six-membered heterocycles will be formed are... 

Quantum yields for the formation of 141 from 138 in TFE-MeCN were estimated by transient absorption actinometry (Table l).62 The data refer to solvated carbocations (141) since ion pairs (140) are too short-lived for detection on the ns time scale. The modest to poor yields of 141 could be due to predominant ion-pair recombination (140 -> 142), or to parallel protonation (139 — 140) and insertion (139 — 142). Picosecond LFP studies on photoheterolyses of A CH-X in MeCN revealed that the ratio of collapse to escape (k /ki) for [Ar2CH+ X-] is slightly affected by p-substituents (H, Me, OMe) and by X (Cl, Br).66 In contrast, 4>M1 was found to increase by a factor of 17 as p-H (138d) was replaced with p-OMe (138a).62 Hence the ion-pair hypothesis seems difficult to reconcile with the effect of p-substituents on unless the strong nucleophile RO in 140 behaves differently from the weakly nucleophilic halide ions. 

Replacement of a substituent X by its second row counterpart should lead to better a-a interactions. Once more, a substituent effect exerted principally on the minimum will give rise to the orders Cl > F, SR > OR, PR2 > NR2, SiR3 > CR3. Here, unfortunately, the same orders are expected if the principal effect of the substituent is exerted on the maximum. The experimental results collected in Table 51 clearly show that the predicted unequivocal trend is observed. 

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