Reaction Mechanism. Part 3 of 8

Classification of Reagents:The presence of an electron-attracting or electron-repelling group in amolecule develops an electron deficient or electron-rich site. Thiswill predict the type of reagent with which the compound will reactmore rapidly.

Thus on the basis of the above principle, most of the reagents can be classified into following to types.

1. Electrophilic Reagents (Electrophiles):As the name implies electrophilic (electro-electron, phile- love)reagents are electron seeking or loving thus attack the substrate atthe point of maximum electron density. Thus an electrophilic reagent oran electrophile is a species having electron-deficient atom or centre.

Electrophilic reagents can be of two types:

(i)Positive Electrophile

(ii)Neutral Electrophile

(i) Positive Electrophiles:The positive electrophiles are those species which carry a positivecharge. These include proton, cation and the carbon radicals carrying apositive charge (carbonium ions). Some of the important examples ofpositive electrophiles are

Thepositive electrophiles attacks the substrate rich in electrons andaccepts an electron pair for sharing and form neutral molecule.  

(ii) Neutral Electrophiles: The neutral electrophiles are those electron deficient species which don’t carry any charge. These include BF₃, AlCl₃, ZnCl₂ and carbon radicals having six electrons in the outermost orbit (Carbenes).

Allthese substances have six electrons in the outer most orbits and thusshort of a pair of electrons to attain stable configuration andtherefore behave as electron seeking reagent.

Some of the neutral electrophiles are

Neutral electrophiles attack the electron rich substrate to produce a negatively charged molecule.

Itis interesting to note that since the electrophiles are capable ofaccepting electron pair, they are considerer as Lewis acids.

(Note – According to Lewis, An acid is a species capable of accepting electrons pair; while a base is a species prepared to donate electron pair.)

Nucleophilic Reagents (Nucleophiles):The reagents possessing at least one lone pair of electron are known asnucleophilic reagents or nucleophiles (nucleo-nucleus; phile-to love).Since they posses higher electron density, they attack the substrate atthe point of minimum electron density.

Nucleophile can be of two types:

(i) Negative nucleophile

(ii) Neutral nucleophile

(i) Negative nucleophilesare those species which carry an excess of electron pair and arenegatively charged. These include halide ions, alkoxy ions, hydroxylion, and the carbon radicals carrying a negative charge (carbanion).Some of the important examples of negative nucleophiles are.

Negative nucleophiles attack on the positively charged substrate to form a neutral molecule.

(ii) Neutral Nucleophilesare those species which are although rich in electron (because ofpresence of unshared electron pair), yet electrically neutral.

Examples:

Neutral Nucleophiles attack on the positively charged substrates to form positively charged products.

It is interesting to note that since the nucleophiles are referred to as nucleophilic reactions.

Difference between Electrophiles and Nucleophiles

Ambiphiles: Ambiphilic reagents have electron attracting (electrophiles) and electron-repelling (nucleophile) sites hence, have dual nature.    

Nucleophile attack electrophiles during the course of the reaction

The reaction of electrophiles with a nucleophile is the same as the reaction of a Lewis acid with a Lewis base and is termed as Lewis-acid base association reaction.As a result of this association reaction, each atom in the productcompletes its octet, and completion of the octet provides the majordriving force for this type of Lewis acid-base reaction.

Theformation of a chemical bond is described by a flow of electrons fromthe electron donor (Lewis base) to the electron acceptor (Lewis acid).This electron flow is indicated by arrow.