Effects of Structure on Reactivity

The effects of structure on reactivity can be divided into three major types field, resonance (or mesomeric), and steric. In most cases two or all three of these are operating, and it is usually not easy to tell how much of the rate enhancement (or decrease) is caused by each of the three effects. 

TABLE 9.1 Some Groups with +Af and in Order of Strength of Effect 

On the other hand, groups that have a multiple-bonded electronegative atom directly connected to an unsaturated system are —A/ groups. In such cases, we can draw canonical forms in which electrons have been taken from the unsaturated system into the group, for example. 

It must be emphasized once again that neither by the resonance nor by the field effect are any electrons actually being donated or withdrawn, though these terms are convenient (and we shall use them). As a result of both effects, the electron-density distribution is not the same as it would be without the effect (see pp. 17, 42). One thing that complicates the study of these effects on the reactivity of compounds is that a given group may have an effect in the transition state which is considerably more or less than it has in the molecule that does not react. 

An example will show the nature of electrical effects (resonance and field) on reactivity. In the alkaline hydrolysis of aromatic amides (10-11), the rate-determining step is the attack of hydroxide ion at the carbonyl carbon  

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We will describe the effects of structure on reactivity during hydrosi-... 

The solute-solvent-polymer (membrane material) interactions, similar to those governing the effect of structure on reactivity of molecules (20,21,22,23,24) arise in general from polar-, sterlc-, nonpolar-, and/or ionic-character of each one of the three components In the reverse osmosis system. The overall result of such interactions determines whether solvent, or solute, or neither is preferentially sorbed at the membrane-solution Interface. 

The Bronsted correlation for five-membered rings shows that effects of structure on reactivity and on acidity are related. Variations in rate constants for quaternization and in pKa values (Table III) are understandable in terms of resonance and inductive effects of the heteroatom X.120 The effects on the energy of a transition state leading to quaternized product are similar but smaller than those on the energy of protonated material. The following considers in more detail the influence of benzo-fusion. 

The published data give a clear picture of effect of structure on reactivity in the cracking of alkylaromatic compounds. The reactivity increases with the size of the alkyl group, and with its branching, as the studies of... 

Effects of structure on reactivity have been studied several times. The sulphides are more stable than the thiols [248,250], In both series of thiols and of sulphides, the reactivity increases with the inductive effect of the alkyl group [248,251,252], in accordance with other elimination reactions. A linear relation between the logarithm of the rate coefficient and the enthalpy change on carbonium ion formation from the corresponding alkanes has been observed [248]. As Fig. 9 shows, linear correlation of the same rate data by means of the Taft equation is also possible. 

It is in this area of dienone photochemistry that the effect of structure on reactivities of excited states seems to be especially complex. For example, Kropp has shown that the presence of the 4-methyl group in santonin is responsible for preferential formation of the fused 5,7 ring system in photosantonic lactone.403 An analogous cross-conjugated dienone with a 2-methyl substituent yields only a spiro compound in acidic media,404 while a dienone with neither 2- nor 4-substituents yields mixtures of the two types of products.409... 

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