Orientation, substituent effects

The particular array of chemical shifts found for the nuclei of a given polymer depends, of course, on such factors as bond orientation, substituent effects, the nature of nearby functional groups, solvation influences, etc. As a specific example, derivatives of the carbohydrate hydroxyl moieties may give rise to chemical shifts widely different from those of the unmodified compound, a fact that has been utilized, e.g., in studies (l ) on commercially-important ethers of cellulose. Hence, as illustrated in Fig, 2, the introduction of an 0-methyl function causes (lU,15) a large downfield displacement for the substituted carbon. This change allows for a convenient, direct, analysis of the distribution of ether groups in the polymer. Analogously, carboxymethyl, hydroxyethyl and other derivatives may be characterized as well... 

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). 

Decide if ion-dipole interactions are responsible for the observed substituent effects. Obtain the charge on carbon and nitrogen in each cyano group. What evidence is there for a polar CN bond Should the ion (O )-dipole (CN) interaction be stabilizing or destabilizing Can these interactions explain the trends in electrostatic potential (Hint Focus on changes in O—CN distance and in orientation of the cyano group.)... 

According to Roberts et al. the direction of addition of ammonia to 3-substituted benzynes might be predicted by considering the amide ion to add so as to provide the most favorable location of the negative charge with respect to the inductive effect of the orienting substituent. Thus, ammonia adds to 3-methoxybenzyne (39) producing chiefly n-aminoanisole (40). 

Hydroboration is highly regioselective and stereospecific. The boron becomes bonded primarily to the less-substituted carbon atom of the alkene. A combination of steric and electronic effects works to favor this orientation. Borane is an electrophilic reagent. The reaction with substituted styrenes exhibits a weakly negative p value (-0.5).156 Compared with bromination (p+ = -4.3),157 this is a small substituent effect, but it does favor addition of the electrophilic boron at the less-substituted end of the double bond. In contrast to the case of addition of protic acids to alkenes, it is the boron, not the hydrogen, that is the more electrophilic atom. This electronic effect is reinforced by steric factors. Hydroboration is usually done under conditions in which the borane eventually reacts with three alkene molecules to give a trialkylborane. The... 

There are two different relative orientations of X and a y-carbon atom in staggered n-propyl derivatives. Accordingly, we have to make a distinction between y-gauche (yg-SCS) and y-antiperiplanar (ya-SCS) substituent effects. 

Much work has been done to determine sets of substituent effects in specific conformationally mobile or rigid frameworks, since a knowledge of such effects often furnishes valuable conformational information. For instance, it was concluded from yg- and 8-SCS values that substituents (X = OH, I) on the C(19) methyl group in some cholest-S-enes prefer an antiperiplanar orientation [X-C(19)H2-C(10)-C(1)] with respect to C(l) (81). In the following, SCS information for various classes of cyclic systems is discussed, with particular emphasis on configurational and conformational analysis. 

Six-membered rings containing one nitrogen atom resemble cyclohexanes in their geometry, and the stereochemical dependence of substituent effects is often similar. However, there is a complication involving nitrogen inversion of neutral amines which results in different orientations of the lone pair with respect to the ring621 for such effects on chemical shifts see Section 4.1.1.1. and on /H C and 2/HH values see Section 4.1.1.2. 

Substituent Reactivity Orientation Inductive effect Resonance effect... 

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