Substitutions electrophilic aliphatic

For aliphatic electrophilic substitution, we can distinguish at least four possible major mechanisms,2 which we call SeI, Se2 (front), Se2 (back), and SEi. The SeI is unimoiecular the other three are bimolecular. 

The bimolecular mechanisms for electrophilic aliphatic substitution are analogous to the Sn2 mechanism in that the new bond forms as the old one breaks. However, in the Sn2 

Both the Se2 (front) and Se2 (back) mechanisms are designated DFAE in the IUPAC system. With substrates in which we can distinguish the possibility, the former mechanism should result in retention of configuration and the latter in inversion. When the electrophile attacks from the front, there is a third possibility. A portion of the electrophile may assist in the removal of the leaving group, forming a bond with it at the same time that the new C—Y bond is formed  

This mechanism, which we call the SEi mechanism3 (IUPAC designation cyclo-DtAEDnAn), also results in retention of configuration.4 Plainly, where a second-order mechanism involves this kind of internal assistance, backside attack is impossible. 

4lt has been contended that the Ei mechanism violates the principle of conservation of orbital symmetry (p. 846). and that the Se2 (back) mechanism partially violates it Slack Baird J. Am. Chem. Soc. 1976, 98, 5539. 

Since a carbanion is what remains when a positive species is removed from a carbon atom, the subject of carbanion structure and stability (Chapter 5) is inevitably related to the material in this chapter. So is the subject of very weak acids and very strong bases (Chapter 8), because the weakest acids are those in which the hydrogen is bonded to carbon. 

It is evident that these three mechanisms are not easy to distinguish. All three give second-order kinetics, and two result in retention of configuration. In fact, although much work has been done on this question, there are few cases in which we can unequivocally say that one of these three and not another is actually taking place. 

Peer-reviewed journals [110, 111] proceedings [112,113] sections in reviews [90, 99,114,115]. 

1 Drivers Ibr Performing Isomerization of Allyl Alcohols in Micro Reactors 

Allyl alcohol isomerization is typically conducted as a single-phase reaction, needing efforts for separation of the catalyst [110, 113], One driver was to exploit a catalyzed liquid/liquid route with aqueous (catalytic) and organic phases as commonly employed in the chemical industry. 

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