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 bond breaks with the bonding pair of electrons both moving to one atom creating a positive and negative ion).
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.
