Substituent types

As Table 12 2 indicates a variety of substituent types are meta directing and strongly deactivating We have already discussed one of these the trifluoromethyl group Several of the others have a carbonyl group attached directly to the aromatic ring... 

Ill of Chart 3), as is shown in Fig. 4. This is of interest because both of these reactions involve direct attack on the ring and, in spite of the fact that they are of widely different types, they are both highly selective. The correlation, however, is limited to alkoxy, methyl, and halogeno groups, but its value would certainly be increased by extension to other substituent types. 

In a most recent paper [418] the preparation of corn fiber arabinoxylan esters by reaction of the polymer with C2-C4 anhydrides using methanesul-fonic acid as a catalyst is described. The water-insoluble derivatives with high molecular weight showed glass-transition temperatures from 61 to 138 °C, depending on the DS and substituent type. The products were thermally stable up to 200 °C. Above this temperature their stability rapidly decreased. 

A number of tetracyanoethylene (TCNE) n complexes of mono-substituted [2.2]paracyclophanes were examined both spectroscopically and kinetically. The position of max of the longest-wavelength charge-transfer band in the UV spectrum of the complexes, and in some cases estimated equilibrium constants 76> were used as a measure of the relative 7t-base strengths of the substituted cycles 15>. From the results of this investigation it was at once clear that complexes of tetracyanoethylene and [2.2]paracyclophane with electron-releasing substituents (Type A,... 

Allenyne represents an interesting substrate for the intramolecular Pauson-Khand(-type) reaction, where an allene moiety acts as an ene component. Here, there are two possible reaction pathways (Scheme 11.21) (i) the reaction of an external tr-bond of allene moiety gives a bicyclic dienone (type A) or (ii) the reaction of an internal 7i-bond gives a bicyclic cyclopentenone with an alkylidene substituent (type B). 

On the condition that two subsets have a structural feature whose parameterization is essentially the same, they can be combined into a single data set. As an example consider set 0X13, anfi-Ak—iyw-X—C=NOH, where Ak is either Me or Et. The electrical effects of these groups for Me and Et, respectively, are a , —.01, —,Q ad, —.14, —.12 ae, —.030, —. 036 the values of the steric parameter v are. 52 and. 56. A significant difference is found only in the polarizability parameter a, where the values for Me and Et are. 046 and. 093, respectively. Combination of oxime pXa values for Ak = Me or Et results in set 0X13 the best correlation was with the LDR equation. As only three substituent types are present in this data set and is 0.33, this data set cannot be considered as proof of anything. The only acceptable conclusion is that it is in accord with the combination of the two subsets. 

For pXa values of 4-XPnC(NH2)=NOH in MeOH at 25 °C, set 0X15, best fit was obtained with the LD equation. The range of substituent type is good but the value is too small for reliability. The correlation is significant at the 99.0% confidence level. 

Substituent type MO of cyclopropane involved Change in vicinal bonds C1C2 Change in distal bond C2C3 Favoured substituent conformation... 

Fig. 9 Comparison of selected examples of compounds combining one semiperfluorinated chain with a benzene ring, showing the effects of substituent type and position (77°C) [113, 115, 117, 120]...

Resonance donor ( + M) and acceptor (-M) substituents may produce substantially larger effects than they do in benzene because they are able to become more coplanar with the heteroaromatic ring. A proper analysis requires a range of substituent types to detect such anomalies. These need, in turn, to be considered carefully because hydrogen bonding may reduce the reactivities of more reactive compounds relative to less reactive ones, so producing attenuated p values. 

Most cyclic organosiloxanes have only a very small ring strain the highest, 12—20 kJ mol-1, occurs in the trimer [54], Their polymerization is thermodynamically controlled, by the entropy factor. Substituent type and ring size determine the polymerization mechanism and kinetics. Practically all polymerizations are ionic [55], The most frequent substituents, R, are —CH3, —C6H5, —CH=CH2, and for special applications —CH2CH2CN, —CH2CH2—CF3 and others. 

Taft [109] postulated that the rate of aliphatic ester hydrolysis is a function of substituent type. Using methyl as a standard substituent, he compared, for example, the effects of larger alkyls, and interpreted the results by means of two constants characterizing the steric and polar behaviour of the alkyls. In copolymerization theory, the Taft equation was used many times to describe the relative reactivities (See Chap. 5, Sect. 5.2) in homologous polymer series... 

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