An electrophile is an electron pair acceptor. The name means 'electron loving': electrophiles are attracted to regions of high electron density, and they form a new covalent bond by accepting a pair of electrons from the species they attack.Electrophiles are usually either positive ions or molecules containing an atom with a partial positive charge (δ+). Examples include:• H+ ions
• the nitronium ion, NO2+
• the H atom of HBr, which is δ+ because bromine is more electronegative than hydrogen
• a bromine molecule, Br2, once it has been polarisedBr2 is non-polar on its own. When it approaches the π electrons of a C=C bond, the electron density of the π bond repels the electrons in the Br-Br bond. This induces a dipole: the bromine atom nearer the double bond becomes δ+ and can act as an electrophile.
Why alkenes react with electrophiles: the π electrons of the C=C bond lie above and below the plane of the molecule, exposed and away from the nuclei. This gives a region of high electron density that attracts electrophiles. Alkanes contain only σ bonds and have no π bond, so they do not react with electrophiles in this way.When an electrophile attacks a C=C bond, a bond breaks heterolytically, with both electrons going to one atom, and ions are formed.
• the nitronium ion, NO2+
• the H atom of HBr, which is δ+ because bromine is more electronegative than hydrogen
• a bromine molecule, Br2, once it has been polarisedBr2 is non-polar on its own. When it approaches the π electrons of a C=C bond, the electron density of the π bond repels the electrons in the Br-Br bond. This induces a dipole: the bromine atom nearer the double bond becomes δ+ and can act as an electrophile.
Why alkenes react with electrophiles: the π electrons of the C=C bond lie above and below the plane of the molecule, exposed and away from the nuclei. This gives a region of high electron density that attracts electrophiles. Alkanes contain only σ bonds and have no π bond, so they do not react with electrophiles in this way.When an electrophile attacks a C=C bond, a bond breaks heterolytically, with both electrons going to one atom, and ions are formed.