Nucleophiles and Electrophiles

Once a bond has broken -- homolytically into free radicals, or heterolytically into a carbocation-type or carbanion-type fragment -- the reacting species that come together to form new bonds fall into two broad, electronically opposite camps: nucleophiles and electrophiles.

Nucleophiles ('nucleus-loving') are reagents with a strong affinity for electro-positive centres. What makes a species a nucleophile is that it possesses an atom with an unshared (lone) pair of electrons, and it is effectively 'searching' for an electron-poor (electropositive) centre with which it can share that pair to form a new covalent bond -- a process that stabilises the nucleophile itself. Nucleophiles are usually either negatively charged ions or electron-rich neutral molecules carrying one or more lone pairs. Since a Lewis base is, by definition, an electron-pair donor, every Lewis base is a nucleophile.

Nucleophiles fall into two broad groups:

• Neutral molecules with an unshared pair of electrons -- e.g. ammonia ($NH_3$) and amines ($RNH_2$) donating through nitrogen's lone pair; water ($H_2O$), alcohols (ROH) and ethers (R-O-R) donating through an oxygen lone pair; hydrogen sulphide ($H_2S$) and thiols (RSH) donating through a sulphur lone pair.
• Negatively charged nucleophiles -- e.g. the halide ions chloride, bromide and iodide ($Cl^-$, $Br^-$, $I^-$); the oxygen nucleophiles hydroxide ($HO^-$), alkoxide ($RO^-$) and carboxylate ($RCOO^-$); and cyanide ($CN^-$).

Electrophiles ('electron-loving') are the electronic opposite: reagents that are attracted toward a negative charge or an electron-rich centre. They are either positively charged ions or electron-deficient neutral molecules. Since a Lewis acid is, by definition, an electron-pair acceptor, every Lewis acid is an electrophile. Neutral molecules can be electrophilic even without a formal positive charge if they have an empty or accessible orbital to accept electrons into -- for instance $SnCl_4$, which has vacant d-orbitals capable of accommodating an incoming electron pair.

Electrophiles likewise fall into two broad groups:

• Neutral electrophiles -- e.g. carbon dioxide ($CO_2$) and dichlorocarbene ($:CCl_2$), electron-deficient at carbon; and the classic Lewis-acid trio aluminium chloride ($AlCl_3$), boron trifluoride ($BF_3$) and ferric chloride ($FeCl_3$), electron-deficient at the metal/metalloid centre.
• Positively charged electrophiles -- e.g. carbocations ($R^+$); the proton ($H^+$) itself; alkyl halides acting through their electrophilic halogen end ($X^+$); the oxonium ion ($H_3O^+$) and nitrosonium ion ($NO^+$); and the nitronium ion ($^+NO_2$), the active electrophile in nitration. …