Electron density withdraw

Of the three C-X bonds that might break in IV, the C-N linkage stands as the best choice, owing to the amide environment of the nitrogen atom, which experiences electron density withdrawal by the neighboring carbonyl. The subsequent incorporation of the benzylic fragment follows the course of a bimo-lecular nucleophilic displacement. 

That leads ns to the following explanation Carbon is more electronegative than phosphorus, but less so than oxygen. In conseqnence, carbon snbstitution on oxygen increases the electron density at oxygen and resnlts in less electron density withdrawal from phosphorus in the case of 0=P(0R)3. That translates into an npfield shift with increased carbon substitution. For 0=PR3, the sitnation is different. Since carbon is more electronegative than phosphorus, increased carbon snbstitntion on phosphorus results in decreased electron density on phosphorus, and thns a downfield shift. 

A decrease of a- and tt-electronic density in both adjacent positions. For the a system this decrease is approximately the same at the 2- and 4-positions, which expresses an equivalent electron withdrawing from nitrogen in both positions. On the other hand, the decrease in tr-electronic density is twice as large at C-2 as at C-4. 

The carbonyl group withdraws rr electron density from the double bond and both the carbonyl carbon and the p carbon are positively polarized Their greater degree of charge separation makes the dipole moments of a p unsaturated carbonyl compounds signifi cantly larger than those of comparable aldehydes and ketones... 

Because carbon is sp hybridized m chlorobenzene it is more electronegative than the sp hybridized carbon of chlorocyclohexane Consequently the withdrawal of electron density away from carbon by chlorine is less pronounced m aryl halides than m alkyl halides and the molecular dipole moment is smaller... 

Fig. 4.59. Raman spectrum of methyl mercaptan (a) and SERS spectrum of methyl mercaptide (b) formed by adsorption ofthe mercaptan on a silver surface. The surface reaction is proven by the disappearance ofthe S-H stretching and bending bands at 2575 cm" and 806 cm", respectively. The Raman shift ofthe C-S stretching band at approximately 700 cm" is reduced during adsorption by withdrawal of electron density from the C-S, because of bonding to the silver. The symmetric methyl stretching appears above 2900cm" [4.303].

Similarly, carboxylic acid and ester groups tend to direct chlorination to the / and v positions, because attack at the a position is electronically disfavored. The polar effect is attributed to the fact that the chlorine atom is an electrophilic species, and the relatively electron-poor carbon atom adjacent to an electron-withdrawing group is avoided. The effect of an electron-withdrawing substituent is to decrease the electron density at the potential radical site. Because the chlorine atom is highly reactive, the reaction would be expected to have a very early transition state, and this electrostatic effect predominates over the stabilizing substituent effect on the intermediate. The substituent effect dominates the kinetic selectivity of the reaction, and the relative stability of the radical intermediate has relatively little influence. 

Because the fluorine substituents both inductively and hyperconjugatively withdraw electron density from the C(2)-C(3) tt bond, the LUMO is located there, and Diels-Alder reactions take place exclusively with this bond [25] 1,1 -Difluoro allene and fluoroallene reaet readily with a large selection of cyclic and acyclic dienes, and acyclic dienes, [2+2] cycloadditions compete with the Diels-Alder processes As shown in the example in equation 79, a significantly different regiochemistry is observed for the [2+4] cycloaddition compared with the [2+2]... 

This phenomenon is not possible in p-nitrobenzoic acid hence, p-nitrophenol is a stronger acid with respect to p-nitrobenzoic acid than is expected on the basis of a comparison of substituents in which this resonance delocalization is not an important factor. It was, therefore, recommended that Op = 1.27 be used for p-nitro derivatives of phenols and anilines, rather than the Op = 0.78 given in Table 7-10. These enhanced sigma constants, symbolized a, apply primarily to electron-withdrawing groups in reactions aided by low electron density at the reaction site. 

When written in this way it is clear what is happening. The mechanisms of these reactions are probably similar, despite the different p values. The distinction is that in Reaction 10 the substituent X is on the substrate, its usual location but in Reaction 15 the substituent changes have been made on the reagent. Thus, electron-withdrawing substituents on the benzoyl chloride render the carbonyl carbon more positive and more susceptible to nucleophilic attack, whereas electron-donating substituents on the aniline increase the electron density on nitrogen, also facilitating nucleophilic attack. The mechanism may be an addition-elimination via a tetrahedral intermediate ... 

Naphthyridines are (just as pyridines) characterized as 7r-deficient systems. Introduction of an electron-withdrawing group such as the nitro group further depletes the ring of its 7r-electrons and lowers its electron density. On account of this low electron density, nitronaphthyridines show a high reactivity to nucleophilic reagents and low reactivity to electrophiles several characterictic examples of this behavior are shown in this chapter. 

Taking into account the high structural similarity of dppf and cdpp, their different influence on the reaction s selectivity has to be attributable to electronic effects. The electron density at the phosphorus atoms is significantly lower in the case of cdpp, due to the electron-withdrawing effect of the formal cobalt(III) central atom... 

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