Inductive withdrawal

The way in which various substituents affect the polarization of a carbonyl group is similar to the way they affect the reactivity of an aromatic ring toward electrophilic substitution (Section 16.5). A chlorine substituent, for example, inductively withdraws electrons from an acyl group in the same way that it withdraws elections from and thus deactivates an aromatic ring. Similarly, amino, methoxvl, and methylthio substituents donate electrons to acyl groups by resonance in the same way that they donate electrons to and thus activate aromatic rings. 

The nucleophilicity of oxacarbenes can be fine-tuned by adjusting the oxa substituent s donor potential.91 For instance, due to its inductively withdrawing CF3 unit, the trifluoioethoxy group o = —0.56) is less electron donating... 

Applications of these rules may be demonstrated using formaldehyde as an example. Inductive withdrawal of electrons by the directly bonded oxygen from the symmetric orbital l/t should produce a positive contribution to VHH(Rule 1). Concurrent contribution of electrons from the doubly occupied nonbonding 2py orbital of the oxygen back into the antisymmetric orbital ij/2 should also produce a positive contribution to VHH (Rule 2). These shifts are illustrated schematically in Fig. 5. Thus, VH H for formaldehyde is predicted to be large and positive, as is in fact the case 4S>, Entirely analogous considerations may be developed for the compounds studied in this paper. 

The superbases are similar in that they avoid hindered positions next to coordinating groups, and prefer to deprotonate ortho to small, powerfully inductively withdrawing OMe, or even at benzylic positions. Superbases prefer to deprotonate both 647 and 648 ortho to OMe rather than the usually more powerfully directing anilide group (Scheme 251) . ... 

Talcing ECHO as standard, we can say that Cl destabilises by inductive withdrawal, Me stabilises more by n-delocalisation, and NH2 and OEt by lbne pair donation (nH2 is more effective at this compare ammonia and water as bases). Our order is ... 

Since the carbonyl group has gone in R2C(OH)CN, we would expect very little effect from any of these substitutents. There can t be any conjugation, and inductive effects on the distant 0 atom will be small. What effect would an inductively withdrawing substituent have on the equilibrium for cyanohydrin formation ... 

Thus, the net impact of a substituent such as fluorine, which is inductively withdrawing and 7r-donating, will result from a complex interplay of these disparate interactions. To make matters even more complicated, die combined influences of multiple fluorine substituents is not additive, and cannot be readily derived from an understanding of the effect of a single fluorine substituent. 

OR —SR o o. . II. .II —nhcch3 —OCCH3 Moderately activating ortho and para Resonance donating effect is stronger than inductive withdrawing effect, but not as much so as above... 

In contrast to NH2 and OH, Table 13-1 shows that F, CN and N02 are primarily inductive withdrawers of electronic charge. Thus, while they also cause a tube-long delocalization, it is toward them, and their ends become the strongly negative ones. 

The positively charged nitrogen inductively withdraws electron density from the aromatic ring. This aromatic ring is less electron-rich than benzene, so it is deactivated toward reactions with electrophiles. 

The halogen atom of an acyl halide inductively withdraws electron density from the carbonyl carbon, enhancing its electrophilic nature and making acyl halides particularly reactive toward nucleophilic acyl substitution. The halide ion also serves as a good leaving group. 

Protonation, and likewise alkylation, leads to weakening of the respective S —> Fe a bonds and an inductive withdrawal of electron density from the Fe centers. The newly formed thiol or thioether donors have 71-acceptor capability and partial Fe—S n back-bonding leads to a further decrease of electron density at Fe. However, the weakening of the Fe— S a bonds and formation of the Fe —> S 7t back-bonds compensate each other so that the Fe—S distances remain invariant. 

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