Electrophilic substituents

Aikene chemistry is dominated by electrophilic addition reactions. When HX reacts with an unsymmetrically substituted aikene, Markovnikov s rule predicts that the H will add to the carbon having fewer alky) substituents and the X group will add to the carbon having more alkyl substituents. Electrophilic additions to alkenes take place through carbocation intermediates formed by reaction of the nucleophilic aikene tt bond with electrophilic H+. Carbocation stability follows the order... 

Diphenyl cyclopropenone has been subjected to nitration217 and bromina-tion218 in sulphuric acid. According to the second order functionality of the cyclopropenone substituent electrophilic substitution of the phenyl residues took place at the m-position giving rise to 313. 

To sum up, the rate retardation attributed to steric effects of bulky alkyl groups can arise from substituent-electrophile, substituent-substituent and substituent-solvent interactions in the first ionization step of the reaction and also from substituent-nucleophile interactions in the product-forming step. It is therefore not surprising that the usual structure-reactivity correlations or even simpler log/log relationships cannot satisfactorily describe the kinetic effects of alkyl groups in the electrophilic bromination of alkenes. 

Tetrazole type or 5-substituent Electrophile Main site (if any) Ref... 

For alkyl substituents, electrophilic substitution (chlorination, bromination, acetylation, trifluoroacetylation, formylation, hydrogen exchange) of 2-alkylthiophenes happens mainly... 

Electrophilic substitution at nitrogen can occur in either ring, and the preference may depend on the nature of the substituents. Electrophilic substitution at carbon proceeds readily in the presence of electron-releasing substituents. 

These latter effects of substituent, electrophile and medium on rate processes are huge—they invert the normal order ... 

It is interesting to see completely different regiochemistries in nucleophilic and electrophilic reactions to 1,1-difluoroethene as a model. Scheme 1.55 shows a typical electronic effect of the fluorine atom as a substituent. Electrophiles mostly attack the (3-carbon of 1,1-difluoroethene generating a-difluorinated carbocations as intermediates in contrast to regioselective nucleophilic additions on the a-carbon of 1,1-difluoroethene generating (3-fluorocarbanions. 

If the 3-position of indole carries a substituent, electrophilic attack usually occurs first on the 2-position and subsequently on the 6-position [132],... 

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