7.1.4 Organic Chemistry Terminology

• Organic reactions are often associated with terminology students should be familiar with
• Two of these important terms include:
  • Electrophilic substitution
  • Addition-elimination

Electrophilic substitution

• Electrophiles are species that are electron deficient and can act as an electron pair acceptor
  • Electrophiles are ‘electron loving’ species

• Substitution reactions are reactions that involve the replacement of one atom or group of atoms by another
• Electrophilic substitution reactions are therefore reactions in which an atom or group of atoms are replaced by an electrophile after initial attack by the electron-deficient species
• An example of an electrophilic substitution reaction is the reaction of benzene with bromine in the presence of anhydrous aluminium bromide catalysts
  • The bromine acts as an electrophile and attacks the electron-rich benzene ring
  • A hydrogen atom is substituted by a bromine atom to form bromobenzene and hydrogen bromide

• Benzene undergoes substitution reactions rather than addition reactions because of the stability of the benzene ring

The hydrogen atom in benzene is substituted by the bromine which acts as an electrophile

Addition- elimination

• Other common organic reactions include addition and elimination reactions
• In an addition reaction, two or more molecules combine to give a single product only
• An example of an addition reaction is the hydrogenation of alkenes
  • Hydrogen (H₂) is reacted with an alkene to form an alkane only with no by-products

Hydrogenation of an alkene to give an alkane is an example of an addition reaction

• Elimination reactions are the reverse of addition reactions; a small molecule (such as H₂O or HCl) is removed or ‘eliminated’ from an organic molecule
• An example of such a reaction is the elimination of HCl from an alkane to form an alkene

Elimination of HCl from an alkyl halide results in the formation of an alkene