Most reactions take place via two or three steps with short-lived, unstable, intermediate ions bridging the steps from reactants to products. A reaction mechanism shows the details – bonds breaking, new bonds forming and the movements of electrons.
An intermediate ion can be a positively charged carbocation or a negatively charged carboanion, formed in either case by the heterolytic fission of a C-X bond.
The bonding pair of electrons moves to the more electronegative atom in the bond.
The product is formed by heterolytic bond formation.
The hydroxide ion is an example of a nucleophile.
Nucleophile: a negatively charge ion or neutral molecule that donates a pair of electrons to an electron acceptor forming a covalent bond
e.g. OH–, NH3, Cl–, Br–, H2O
The strength of a nucleophile depends on its ability to donate a pair of electrons.
Amines (R-NH2) are stronger nucleophiles than alcohols (R-OH) because nitrogen is less electronegative than oxygen and so is holding on to its lone pair less strongly. Nitrogen finds it easier to donate the lone pair and form a new bond with a carbocation.
The opposite of a nucleophile is an electrophile.
Electrophile: a positively charged ion or neutral molecule that accepts a pair of electrons from an electron donor to form a covalent bond
e.g. carbocations, molecules with a 𝛅+ carbon
Halogenoalkanes react by nucleophilic substitution – this label describes the first step in the reaction mechanism where the 𝛅+ carbon is attacked by a nucleophile.
Alkenes react by electrophilic addition. In the first step an electrophile is added to the double bond.
