Inductive effects substituents

Organochlorines Aromatic Aliphatic 105-103 Few polar moieties hydrophobic and Van der Wasls interaction induction effects substituents, non-conjug. double-bonds)... 

This work already showed that substituent constants of one reaction can only be transferred to another reaction when similar effects are operating and when they are operating to the same extent. In order to find a broader basis for the transfer-ability of substituent constants, they were split into substituent constants for the resonance effect and those for the inductive effect. 

The TT-inductive effect describes how an inductive substituent might selectively influence the electron distribution at the o- and -positions of the aromatic nucleus. A familiar example is represented by the... 

Excluding the phenomenon of hyperconjugation, the only other means by which electronic effects can be transmitted within saturated molecules, or exerted by inductive substituents in aromatic molecules, is by direct electrostatic interaction, the direct field effect. In early discussions of substitution this was usually neglected for qualitative purposes since it would operate in the same direction (though it would be expected to diminish in the order ortho > meta > para) as the cr-inductive effect and assessment of the relative importance of each is difficult however, the field effect was recognised as having quantitative significance. ... 

In unsaturated molecules electronic effects can be transmitted by mesomerism as well as by inductive effects. As with the latter, the mesomeric properties of a group are described by reference to hydrogen. Groups which release electrons to the unsaturated residue of the molecule are said to exert a +Af effect, whereas groups which attract electrons are said to exert a —Af effect. In aromatic structures the important feature of an M-substituent is that it influences the 0- and p-positions selectively. 

A familiar feature of the electronic theory is the classification of substituents, in terms of the inductive and conjugative or resonance effects, which it provides. Examples from substituents discussed in this book are given in table 7.2. The effects upon orientation and reactivity indicated are only the dominant ones, and one of our tasks is to examine in closer detail how descriptions of substituent effects of this kind meet the facts of nitration. In general, such descriptions find wide acceptance, the more so since they are now known to correspond to parallel descriptions in terms of molecular orbital theory ( 7.2.2, 7.2.3). Only in respect of the interpretation to be placed upon the inductive effect is there still serious disagreement. It will be seen that recent results of nitration studies have produced evidence on this point ( 9.1.1). 

The model adopted by Ri and Eyring is not now acceptable, but some of the more recent treatments of electrostatic effects are quite close to their method in principle. In dealing with polar substituents some authors have concentrated on the interaction of the substituent with the electrophile whilst others have considered the interaction of the substituent with the charge on the ring in the transition state. An example of the latter method was mentioned above ( 7.2.1), and both will be encountered later ( 9.1.2). They are really attempts to explain the nature of the inductive effect, and an important question which they raise is that of the relative importance of localisation and electrostatic phenomena in determining orientation and state of activation in electrophilic substitutions. 

Ingold introduces the terms substrate field effect and reagent field effect to describe those aspects of the direct field effect numbered (z) and (3) in 9.1.2. His description of the substituent effect of the trimethylammonio group is thus given substantially in terms of the substrate field effect and the TT-inductive effect, i.e. it is an isolated molecule description. The reagent field effect is seen to be significant in nitration and to produce qualitatively the same 226... 

The greater positive character hence the increased acidity of the O—H proton of 2 2 2 tnfluoroethanol can be seen m the electrostatic potential maps displayed m Figure 1 8 Structural effects such as this that are transmitted through bonds are called indue tive effects A substituent induces a polarization m the bonds between it and some remote site A similar inductive effect is evident when comparing acetic acid and its trifluoro derivative Trifluoroacetic acid is more than 4 units stronger than acetic acid... 

Inductive effects depend on the electronegativity of the substituent and the num ber of bonds between it and the affected site As the number of bonds increases the inductive effect decreases... 

The decreased shielding caused by electronegative substituents is primarily an inductive effect and like other inductive effects falls off rapidly as the number of bonds between the substituent and the proton increases Compare the chemical shifts of the pro tons m propane and 1 mtropropane... 

Closely related to the inductive effect and operating in the same direction is the field effect In the field effect the electronegativity of a substituent is communicated not by successive polarization of bonds but via the medium usually the solvent A substituent m a molecule polarizes surrounding solvent molecules and this polarization is transmit ted through other solvent molecules to the remote site... 

Carboxylic acids are weak acids and m the absence of electron attracting substituents have s of approximately 5 Carboxylic acids are much stronger acids than alcohols because of the electron withdrawing power of the carbonyl group (inductive effect) and its ability to delocalize negative charge m the carboxylate anion (resonance effect)... 

Each of these intermediates can be hthiated in the 2-position in good yield. The reactivity toward hthiation is due to the inductive effect of the nitrogen atom and coordination by oxygen from the N-substituent. A wide variety of electrophiles can then carry out substitution at the 2-position. Lithiation at other positions on the ring can be achieved by halogen—metal exchange 3-hthio and 5-hthioindoles have also been used as reactive intermediates. 

The alkyl and alkoxy substituents of phosphate or phosphonate esters also affect the phosphorylating abiUty of the compound through steric and inductive effects. A satisfactory correlation has been developed between the quantitative measure of these effects, Tafts s O, and anticholinesterase activity as well as toxicity (33). Thus long-chain and highly branched alkyl and alkoxy groups attached to phosphoms promote high stabiUty and low biological activity. 

Steric and inductive effects determine the rate of formation of the pentacovalent siUcon reaction complex. In alkaline hydrolysis, replacement of a hydrogen by alkyl groups, which have lower electronegativity and greater steric requirements, leads to slower hydrolysis rates. Replacement of alkyl groups with bulkier alkyl substituents has the same effect. Reaction rates decrease according to ... 

If the reactions of the same substituents on heteroaromatic azoles and on benzene rings are compared, the differences in the reactivities are a measure of the heteroatoms influence. Such influence by the mesomeric effect is smaller when the substituent is /3 to a heteroatom than when it is a or y. The influence by the inductive effect is largest when the substituent is a to a heteroatom. 

The observed acidities in the gas phase are interpreted in terms of the negative induction effect of the halo substituents however, the microscopic picture of the solvent effects in addition to such induction effects of the solute have not been clarified. 

One underlying physical basis for the failure of Hammett reaction series is that substituent interactions are some mixture of resonance, field, and inductive effects. When direct resonance interaction is possible, the extent of the resonance increases, and the substituent constants appropriate to the normal mix of resonance and field effects then fail. There have been many attempts to develop sets of a values that take into account extra resonance interactions. 

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