A nucleophile (literally meaning ‘nucleus-loving’) is an electron-rich species that can donate a pair of electrons to form a covalent bond. A nucleophile can be either a negative ion or a molecule with a lone pair of electrons. Nucleophiles are attracted to electron-deficient parts of other ions or molecules, i.e. parts that are positively charged or carry a partial positive charge. When a nucleophile attacks an electron-deficient portion of a molecule and displaces another group, such as an OH group or a halide ion, the reaction is described as nucleophilic substitution.
Electrophiles (literally meaning ‘electron-loving’) are either positively charged ions or electron-deficient molecules that have a δ+ atom at one end of a polar bond. An electrophile tends to attack a negatively charged (electron-rich) part of another molecule, forming a new covalent bond by accepting an electron pair from the molecule that it attacks. In an electrophilic substitution reaction, an electrophile replaces another atom or group of atoms. A benzene ring can undergo electrophilic substitution as it is electron-rich, due to its delocalised p electrons. By replacing an atom or functional group on a benzene ring (rather than just adding an extra group to the ring), an electrophilic substitution reaction maintains the aromatic ring, with its delocalised electrons.
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