The Effect of Substituent Groups

Substituent groups have a marked effect on the fluorescence quantum yield of many compounds. Electron-donating groups such as -OH, -NH2 and -NR.2 enhance the fluorescence efficiency, whereas electron-withdrawing groups such as -CHO, -C02H and -N02 reduce the fluorescence quantum yield, as shown by naphthalene and its derivatives in Table 4.3. 

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Relative values of the rate constants are useful in themselves by measuring such values for the reactions of a series of compounds with the same reactant, one is able to determine the rank order of reactivity within the series. Such determinations are useful in the development of correlations of the effects of substituent groups on the... 

Table 4.3 The effect of substituent groups on fluorescence efficiency of naphthalene and its derivatives. Fluorescence quantum yields measured in fluid solution at room temperature...

The effect of substituent groupings upon the first ionization potential of, at least, the aliphatic aldehydes and ketones does not seem to provide clear evidence for a choice between loss of a 7r-electron from the C=0 double bond or one of the oxygen lone-pair electrons. Concordant data for the homologous series of aliphatic ketones have been obtained by... 

It is known that the effects of substituent groups on a diene or dienophile vary between different types of parents [23]. A function, r(Y), has been determined for several functional groups, with Y corresponding to their electron donating or withdrawing capability such that a reasonable estimate of the HOMO energy could be obtained by use of the equation [15] ... 

The effects of substituent groups in the reactions of the cationic heterocycles with nucleophiles are entirely as expected. Appropriately situated substituent groups which can function as leaving groups can be displaced (e.g. equation 41), and ANRORC reactions are very common (e.g. equations 42 and 43). The latter type of reaction is of particular... 

The effects of substituent groups in the benzo-fused ring on the ease of electrophilic aromatic substitution are essentially identical to those of the same substituent groups in benzene, so electron-donating groups facilitate reaction while electron-withdrawing groups... 

Structure and stability of the ions and the effects of substituent groups on that stability. 

The structural aspects which determine the inherent capability of these acids to undergo ring closure, and which consequently control the course of the cyclization, are (a) the size of the ring formed (b) the reactivity of the aromatic nucleus, including the effect of substituent groups (c) steric considerations other than ring size. 

The behavior of pyridine, its i T-oxide, and its quaternary salts has been the subject of a number of recent theoretical treatments.1-4 The general conclusions will be summarized here to serve as a guideline on which to superimpose the effect of substituent groups. 

As previously described, the effect of substituent groups on the silyi function is an important factor in terms of determining the reaction course. Table 4 shows the ratio of the products in the Pummerer reaction using various SKAs.33 These results suggest that carbon-carbon bond formation preferentially occurs when using a small silyi function such as the trimethylsilyl function. This tendency was observed in another substrate which has asymmetric carbon at the (3-position of the sulfur atom (Table 5).33 Interestingly, the syn-selectivity of the rearrangement product... 

The effect of substituent groups was also studied. In a series of N-substituted hydrazones, PhNRN CMeEt, cyclization proceeded with increasing difficulty in the series R = H, Me, Et, Ph this was ascribed to the effect of increasing steric hindrance [12]. From acetaldehyde phenylhydrazones with methoxy substituents on the aromatic ring, yields were poor, which was attributed to decomposition of the substrate to non-volatile products on the alumina catalyst surface [13]. In contrast, with an acidic ion-exchange resin as catalyst, higher yields were obtained with the 4-methoxy derivative than with the 4-nitro this was interpreted as the effect of methoxy in facilitating adsorption on the resin [14]. 

We report here a QSAR study of the in vitro cytotoxicity of simple substituted aniline mustards, as part of a programme (5) directed towards the rational design of nitrophenyl mustards as hypoxia-selective agents. In order to study the effects of substituent groups, a carefully-selected set of derivatives... 

Relative values of the rate constants are useful in themselves by measuring such values for the reactions of a series of compounds with the same reactant, one is able to determine the rank order of reactivity within the series. Such determinations are useful in the development of correlations of the effects of substituent groups on the rates of a given class of reactions. By measuring a series of rate constant ratios, one eventually is able to arrive at one reaction that is amenable to investigation by conventional procedures. A study of this reaction provides the key numerical value that permits one to convert the relative rate constants into absolute values for each parameter. Illustrations 5.3 and 5.4 indicate how one utilizes the concepts developed in this section in the determination of kinetic parameters for competitive parallel reactions. 

In Chapter 1, Section 1.2 (p. 10), the effect of substituent groups on the acid strength of acetic acid and derivatives was discussed qualitatively. It was noted in particular that the presence of groups more electronegative than hydrogen increased the acid strength relative to acetic acid. A number of important relationships between substituent groups and chemical properties have been developed. Some are very useful both for interpretation of reaction mechanisms and for prediction of reaction rates and equilibria. 

Sommer et al. [106] have looked at the effect of substituent group structure of the bisphenol groups on the Tg and the y-relaxation temperature of the following four polyetherke-tones (PEKs) ... 

Considering Zisman s order of the effect of substituent groups [14] developed from solid surface tension measurements, when we go from PDMS to PMTFPS, we... 

X = Y = CO M = Mo, X = CO, Y=T] -allyl), the K -N,N,N coordination pattern has been observed. The complexes [M(Tm )Me] (M = Zn, Cd, Hg) have been recently reported and investigated for their ability to exchange alkyl and sulfur donor hgands between the Group 12 metals. A number of cationic Pt(IV) carbene complexes [Pt(=C(OMe)(NHR)) (Tp )(Me)2] (OTf) have been synthesized (R=Et, Pr, Pr, Bn OTf=trifluoromethanesulfonate) by methylation with MeOTf of the Pt-carboxamido precursors [Pt(C(=0)(NHR))(Tp )(Me)2]. A computational study on the mechanism of trimerization of aUcynes in the presence of a hydrotris(pyrazolyl)borate iridium catalyst [lr(Tp)(Ti -HC=CH)2] and the effect of substituent groups in R-C=C-R (R = Me or OCOMe) has been reported, demonstrating that two mechanisms for the formation of the benzene complex, including the intramolecular [4 + 2] cycloaddition and Schore mechanisms, are possible in this reaction. 

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