Many polar organic reactions can be understood as an interaction between an electron-rich site and an electron-poor site.

A nucleophile is a species or molecular site that donates an electron pair to form a bond. An electrophile is a species or site that accepts that electron pair.

Nucleophilic sites commonly include:

• negatively charged atoms such as $\mathrm{OH^-}$ or $\mathrm{Cl^-}$;
• neutral atoms with available lone pairs, such as nitrogen in $\mathrm{NH_3}$ or oxygen in water;
• electron-rich multiple bonds such as C=C.

Electrophilic sites commonly include:

• positively charged atoms;
• electron-deficient atoms with incomplete valence shells;
• positively polarized atoms in polar covalent bonds.

For example, in chloromethane,

$$\mathrm{CH_3Cl},$$

chlorine is more electronegative than carbon, so the C-Cl bond is polarized toward Cl. The carbon is partially positive and can act as an electrophilic center even though the molecule is neutral overall.

A carbonyl group provides another important pattern:

$$\mathrm{C=O}.$$

Because oxygen pulls electron density toward itself, the carbonyl oxygen is electron-rich while the carbonyl carbon is electron-poor. The same functional group therefore contains a potential nucleophilic site at oxygen and an electrophilic site at carbon, although which behavior occurs depends on the reaction conditions.

Example

Consider cyanide reacting with chloromethane:

$$\mathrm{CN^-+CH_3Cl\rightarrow CH_3CN+Cl^-}.$$

The negatively charged carbon end of cyanide has an electron pair available and acts as the nucleophile. The polarized methyl carbon acts as the electrophile. A new C-C bond forms while the C-Cl bond is broken.

The terms nucleophile and electrophile describe roles in a reaction, not permanent identities of whole molecules. Water, for example, can donate a lone pair as a nucleophile in one reaction and contain an electrophilic hydrogen in another.

Steric accessibility, solvent interactions and the stability of charge that develops during reaction can strongly influence how reactive a particular site is. The first step in polar-reaction reasoning is nevertheless simple: identify where electron density is available and where an electron pair can be accepted.