Reactivity and Substitution Effect

The intrinsic or induced (substitution effect) difference in reactivity of epoxy groups in polyepoxides and amino groups in polyamines can greatly affect the network for- 

The same and independent reactivity of epoxy groups of diglycidylether of Bisphe-nol A 

Moreover, the position of the pair of glycidyl groups in DGA and TGDDM enhances the probability of an internal etherification under formation of a morpholine 

In the reaction with amine, the proximity of glycidyl groups also makes the ring formation more probable 

Moreover, the analysis by LC and NMR is complicated by the existence of well distii ishable stereoisomers in the amine-DGA adducts These complications make the determination of the substitution effect difficult. At present, no studies are available concerning the possible substitution effect in another polyfunctional epoxide — tris(hydroxyphenylmethane) but the reactivity of epoxy groups can be expected to be independent. 

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This model has been extended by Gordon et al. by taking into account unequal reactivity and substitution effects within monomer units and by a mean field treatment of very restricted cyclization. However, all these models are limited to systepis which can be described by Markovian statistics without any long range correlations affecting the apparent reactivity of a functional group. 

A nitro group behaves the same way m both reactions it attracts electrons Reaction is retarded when electrons flow from the aromatic ring to the attacking species (electrophilic aromatic substitution) Reaction is facilitated when electrons flow from the attacking species to the aromatic ring (nucleophilic aromatic substitution) By being aware of the connection between reactivity and substituent effects you will sharpen your appreciation of how chemical reactions occur... 

The last topic to be treated is unequal reactivity by substitution effects. As a first example, the effect of an infinitely negative substitution effect in C due to a reaction with an h group (so I CD Kqj = 0) is compared with the case of equal (random) reactivity of the two functional groups in C for formulation F40. This is suggested as an example of polyesterification with an anhydride and a carboxylic acid, respectively. Figure 15 gives the dramatic effect on... 

Silyl enol ethers offer both enhanced reactivity and an effective termination step. Electrophilic attack is followed by desilylation to give an a-substituted carbonyl compound. The carbocations can be generated from tertiary chlorides and a Lewis acid, such as TiCl4. This reaction provides a method for introducing tertiary alkyl groups a to a carbonyl, a transformation which cannot be achieved by base-catalyzed alkylation because... 

The kinetic theory can also be used for polyfunctional systems with unequal reactivities of groups and substitution effects, but an explicit solution of the partial differential equation corresponding to Eq. (23) derived for the equireactive system is not possible. One can use, however, the method of moments for derivation of certain averages as was explained in... 

The reactivity and directing effects of common substituted benzenes... 

One consequence of three-dimensionality is the existence of stereoisomers, namely cis and trans isomers of substituted cycloalkanes (Section 4-1). We shall see in Chapter 5 that the phenomenon of stereoisomerism is more general and occurs in acyclic molecules as well. These concepts influence such diverse areas as relative reactivities and biological effectiveness. Because of its fundamental importance and its powerful utility in biological applications, stereochemistry constitutes a recurring theme through the remainder of this book. 

However, the electronic theory also lays stress upon substitution being a developing process, and by adding to its description of the polarization of aromatic molecules means for describing their polarisa-bility by an approaching reagent, it moves towards a transition state theory of reactivity. These means are the electromeric and inductomeric effects. 

The selectivity of an electrophile, measured by the extent to which it discriminated either between benzene and toluene, or between the meta- and ara-positions in toluene, was considered to be related to its reactivity. Thus, powerful electrophiles, of which the species operating in Friedel-Crafts alkylation reactions were considered to be examples, would be less able to distinguish between compounds and positions than a weakly electrophilic reagent. The ultimate electrophilic species would be entirely insensitive to the differences between compounds and positions, and would bring about reaction in the statistical ratio of the various sites for substitution available to it. The idea has gained wide acceptance that the electrophiles operative in reactions which have low selectivity factors Sf) or reaction constants (p+), are intrinsically more reactive than the effective electrophiles in reactions which have higher values of these parameters. However, there are several aspects of this supposed relationship which merit discussion. 

The same situation is observed in the series of alkyl-substituted derivatives. Electron-donating alkyl substituents induce an activating effect on the basicity and the nucleophilicity of the nitrogen lone pair that can be counterbalanced by a deactivating and decelerating effect resulting from the steric interaction of ortho substituents. This aspect of the reactivity of thiazole derivatives has been well investigated (198, 215, 446, 452-456) and is discussed in Chapter HI. 

When a benzene ring bears two or more substituents both its reactivity and the site of further substitution can usually be predicted from the cumulative effects of its substituents In the simplest cases all the available sites are equivalent and substitution at any one of them gives the same product... 

Dinitrochlorobenzene can be manufactured by either dinitration of chlorobenzene in filming sulfuric acid or nitration ofy -nitrochlorobenzene with mixed acids. Further substitution on the aromatic ring is difficult because of the deactivating effect of the chlorine atom, but the chlorine is very reactive and is displaced even more readily than in the mononitrochlorobenzenes. 

A tertiary carbonium ion is more stable than a secondary carbonium ion, which is in turn more stable than a primary carbonium ion. Therefore, the alkylation of ben2ene with isobutylene is much easier than is alkylation with ethylene. The reactivity of substituted aromatics for electrophilic substitution is affected by the inductive and resonance effects of a substituent. An electron-donating group, such as the hydroxyl and methyl groups, activates the alkylation and an electron-withdrawing group, such as chloride, deactivates it. 

These effects can be attributed mainly to the inductive nature of the chlorine atoms, which reduces the electron density at position 4 and increases polarization of the 3,4-double bond. The dual reactivity of the chloropteridines has been further confirmed by the preparation of new adducts and substitution products. The addition reaction competes successfully, in a preparative sense, with the substitution reaction, if the latter is slowed down by a low temperature and a non-polar solvent. Compounds (12) and (13) react with dry ammonia in benzene at 5 °C to yield the 3,4-adducts (IS), which were shown by IR spectroscopy to contain little or none of the corresponding substitution product. The adducts decompose slowly in air and almost instantaneously in water or ethanol to give the original chloropteridine and ammonia. Certain other amines behave similarly, forming adducts which can be stored for a few days at -20 °C. Treatment of (12) and (13) in acetone with hydrogen sulfide or toluene-a-thiol gives adducts of the same type. 

In this section three main aspects will be considered. Firstly, the basic strengths of the principal heterocyclic systems under review and the effects of structural modification on this parameter will be discussed. For reference some pK values are collected in Table 3. Secondly, the position of protonation in these carbon-protonating systems will be considered. Thirdly, the reactivity aspects of protonation are mentioned. Protonation yields in most cases highly reactive electrophilic species. Under conditions in which both protonated and non-protonated base co-exist, polymerization frequently occurs. Further ipso protonation of substituted derivatives may induce rearrangement, and also the protonated heterocycles are found to be subject to ring-opening attack by nucleophilic reagents. 

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