Summary of Substituent Effects

The conclusions reached in the preceding discussion of substituent effects can be summarized in a simple way. Table 4.3 indicates the stabilizing effect of a substituent at an active position in various types of odd AHs relative to the effect of the substituent in an even AH or at an inactive position in an odd AH. Here + implies stabilization, — destabilization, the relative magnitude of the effect being indicated by the number of plusses or minuses. 

One point should perhaps be cleared up. The only types of substituents 

In the preceding discussion, the effects of -type substituents on the properties of odd AHs have been deduced by examining the substituent effect on the properties of the NBMOs of odd AHs. If an -type substituent R is attached to an inactive position in an odd AH, HS, it will not affect the NBMO of the latter. In the resulting derivative RS, R will form an inactive segment. Since the specific effects of -type substituents arise entirely from their influence on the NBMOs of adjacent odd AHs, such a substituent at an inactive position will show no -type activity. It will to all intents and purposes behave as an /-type substituent. In this sense, there will be no conjugation between R and S in RS. Such a molecule is called cross-conjugated. 

Another type of cross-conjugation is observed in the case of odd conjugated substituents attached to an adjacent substrate through an inactive atom. Consider for example the substituent 

This is derived from an odd AH anion by loss of a hydrogen atom, but the hydrogen atom is lost from an inactive position. The NBMO of such a substituent cannot therefore interact with the NBMO of an adjacent odd AH, so none of the first-order effects indicated in Fig. 4.13 can operate. Such a substituent is therefore not of — type. 

Although these results were very promising, we desired further improvements in catalyst activity and enhanced stability toward dioxygen. Moreover, we wished 

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A Summary of Substituent Effects in Aromatic Substitution A summary of the activating and directing effects of substituents in electrophilic aromatic substitution is shown in Table 16.2. 

Summary of Substituent Effects in Electrophilic Aromatic Substitution (18.6-18.9)... 

Scheme 3.3. Summary of Substituent Effects on Carbonyl Substitution Reactions... 

The fourth chapter in this volume, contributed by Helmut Duddeck, is an exceptionally thorough survey of substituent effects on carbon-13 nuclear magnetic resonance (NMR) chemical shifts. Organic chemists and others who are routinely dependent on 13C NMR for structure elucidation and for information about stereochemistry will welcome the summary presented here. Although... 

When substituted benzene undergoes electrophilic attack, groups already on the ring affect the reactivity of the benzene ring as weU as the orientation of the reachon. A summary of these effects of substituents on reachvity and orienta-hon of electrophihc substituhon of substituted benzene is presented below. 

Cf. the Chemistry Award Lecture of the Academy of Science in Gottingen 1969 by H. Bock, The Perturbation of n Systems as a Molecular Probe of Substituent Effects , Jahrb. Akad. Wiss. Gottingen, 1969, pp. 13-25. For a summary of silicon compounds investigated see Ref. 40. 

The Effect of Substitution (Xn) on Phenyl. A summary of the effect on phenyl is given in Table VI. The dividing line roughly separates substituents and substitution patterns that give herbicidally active N-(benzylideneamino) heterocycles (above the dotted line) from combinations that are herbicidally uninteresting (below the dotted line). 

A summary of the effects of some substituents on reactivity and orientation is provided in Table 15.2. 

With a theoretical understanding now in hand of substituent effects on orientation and reactivity, we refer you back to Table 15.2 for a summary of specific groups and their effects. 

Substituent effect, additivity of, 570 electrophilic aromatic substitution and, 560-563 summary of. 569 Substitution reaction, 138 Substrate (enzyme), 1041 Succinic acid, structure of, 753 Sucralose, structure of. 1006 sweetness of, 1005 Sucrose, molecular model of. 999 specific rotation of, 296 structure of, 999 sweetness of, 1005 Sugar, complex, 974 d, 980 L, 980... 

In summary we think that, on a superficial basis, a comparison of the effects of different nucleophilic species added covalently at the (3-nitrogen atom of an arenedi-azonium ion yields results that are almost trivial. Of more interest are unexpected results such as those of Exner and Lakomy for the substituent -N = CHC6H5. A possible explanation for the latter results emerged when the twisted structure of the substituent became known. We emphasize, however, that definitive explanations on the basis of Hammett or related substituent constants are not found very frequently. 

In summary then, the kinetics and related data are most consistent with protonated acetyl nitrate as the reagent in this medium. It is unfortunate that there is doubt as to the nature of the electrophile, as this medium combines high reactivity with good solvent properties, which has made it popular for studying substituent effects in nitration. Some relative reactivities (mostly obtained under competition conditions) are given in Table 20. 

As a summary of these considerations we must conclude that on the basis of polar effects most substituents are captors and that it is the resonance effect which leads to the discrimination of two classes of substituents. 

Figure 2 Diagrammatic summary of selected structural, substituent, and solvent effects on rate constants (kj, at 298 K) for base hydrolysis of low spin iron(II)-diimine complexes. Ligand abbreviations not appearing in the list at the end of this chapter are apmi = (73) with = Me BOH cage = (78) with X = OH ...

The same group studied the lithium cation basicities of a series of compounds of the general formula R R R PO, i.e. phosphine oxides, phosphinates, phosphonates and phosphates, by using Fourier Transform Ion Cyclotron Resonance (FTTCR) mass spectrometry. A summary of their results is shown in Figure 4. The effect of methyl substitution on LCA as well as the correlation between LCA and PA was also investigated by Taft, Yanez and coworkers on a series of methyldiazoles with an FTICR mass spectrometer. They showed that methyl substituent effects on Li binding energies are practically additive. 

In summary, substituent effects on the structure and electronic properties of m-benzyne are rather small in many cases. One aspect of forthcoming investigations may be the search for derivatives of 13, in which the distance of the radical centers is reduced drastically, which should lead to markedly different properties and reactivity. 

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