Substituent groups electron-attracting

When substituents are present, they may influence the regioselectivity of the Birch reduction. The product is determined by the site of the first protonation, since the second protonation is nearly always opposite (para to) the first. Electron-donating substituents such as ethers and alkyl groups favour protonation at an unoccupied site meta to the substituent whereas electron-attracting substituents such as carboxyl favour para protonation. This can be explained by the stability of the intermediates 6.29 and 6.30 formed in both cases. 

The rate of hydrogenation is determined by the precious metal profile and the electronic properties of the substituent in the 4-position of the aromatic aldehyde. Unlike the electron-donating methyl group, electron-attracting groups, such as chlorine or carboxyl, increase the reaction rate. This is in agreement with similar observations made comparing... 

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. 

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)... 

It has long been known that the Diels-Alder reaction is particularly efficient and rapid when the dienophile contains one or more electron-attracting groups. These substituent effects are illustrated by the data in Table 11.3. In the case of the diene, reactivity is increased by electron-releasing substituents. Some illustrative data are given in Table 11.4. 

The stabilizing role of other functional groups can also be described in resonance terms. Both electron-attracting groups such as carbonyl and cyano and electron-donating groups such as methoxy and dimethylamino have a stabilizing etfect on a radical intermediate at an adjacent carbon. The resonance structures which depict these interactions indicate delocalization of the unpaired electron onto the adjacent substituents ... 

Like the 5/) -hybridized carbons of carbocations and free radicals, the sp -hybridized carbons of double bonds are electron attracting, and alkenes are stabilized by substituents that release electrons to these carbons. As we saw in the preceding section, alkyl groups are better electron-releasing substituents than hydrogen and aie, therefore, better able to stabilize an alkene. 

A nucleophile can interact reversibly with an electron-attracting substituent (e.g., —CN or —CHO) in the course of nucleophilic displacement of another group (e.g., halo), thereby producing a spurious substituent effect (cf. 96). The interaction has been found to vary with the nucleophile (see Section II, E). 

There is a difference of opinion about the net effect of resonance between the leaving group and an electron-attracting heterocycle, carbocycle, or substituent- This conjugation (101, 102) has been regarded as a deactivating influence on nucleophilic substitution since the C— Le bond is lower in polarity and higher in... 

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