Electrophiles Electron-poor reactants nucleophiles

Both electron richness and electron accessibility lead to the prediction that a carbon-carbon double bond should be nucleophilic. That is, the chemistry of alkenes should involve reactions of the electron-rich double bond with electron-poor reactants. This is exactly what we find The most important reaction of alkenes is their reaction with electrophiles. 

A full description of how a reaction occurs is called its mechanism. There are two general kinds of mechanisms by which reactions take place radical mechanisms and polar mechanisms. Polar reactions, the most common type, occur because of an attractive interaction between a nucleophilic (electron-rich) site in one molecule and an electrophilic (electron-poor) site in another molecule. A bond is formed in a polar reaction when the nucleophile donates an electron pair to the electrophile. This movement of electrons is indicated by a curved arrow showing the direction of electron travel from the nucleophile to the electrophile. Radical reactions involve species that have an odd number of electrons. A bond is formed when each reactant donates one electron. 

Electrophiles are electron-poor reactants they seek electrons. Nucleophiles are electron-rich reactants they form bonds by donating electrons to electrophiles. 

Novv, what about the second reactant, HBr As a strong acid, HBr is a powerful proton (H ) donor. Since a proton is positively charged and electron-poor, it is a good electrophile. Thus, the reaction between HBr and ethylene is a typical electrophile-nucleophile combination, characteristic of all polar reactions. 

Polar reactants can be classified as either electrophiles or nucleophiles. Electrophiles (literally, electron lovers) are electron-poor reagents in reactions with some other molecule, they seek electrons. They are often positive ions (cations) or otherwise electron-deficient species. Nucleophiles (literally, nucleus lovers), on the other hand, are electron rich they form bonds by donating electrons to an electrophile. 

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