Introduction to Mechanisms The bond has broken in a process called homolytic fission. each atom gets one electron from the covalent bond When a bond breaks by homolytic fission it forms two Free Radicals. Free Radicals do not have a charge and are represented by a DEFINITION A Free Radical is a reactive species which possess an unpaired electron The Mechanism: We use curly arrows in mechanisms to show the movement of an electron pair showing either breaking or formation of a covalent bond; A curly arrow will always start from a lone pair of electrons or the centre of a bond H C C + XH H H H X HO: – δ + δ – X Y X Y Y + X: – Y: – X + 2. HETEROLYTIC FISSION: (one atom gets both electrons) OR Heterolytic fission produces IONS Cl Cl xx xx xx x x Cl Cl xx xx xx + + – two headed arrow shows movement of pair of electrons To understand how the reaction proceeds we must first understand how bonds are broken in organic mechanisms There are two ways to break a covalent bond: 1.HOMOLYTIC FISSION: Most organic reactions occur via heterolytic fission, producing ions H C C H H H H OH X Y one headed arrow shows movement of one electron. To understand a reaction fully we must look in detail at how it proceeds step by step. This is called its mechanism.The breaking of a covalent bond is shown by a curly arrow starting from the bond. The formation of a covalent bond is shown by a curly arrow that starts from a lone electron pair or from another covalent bond.
188.8.131.52 Reaction mechanisms
Reactions of organic compounds can be explained using mechanisms.
• the unpaired electron in a radical is represented by a dot
• the use of curly arrows is not required for radical mechanisms.
Students should be able to write balanced equations for the steps in a free-radical mechanism.
• the formation of a covalent bond is shown by a curly arrow that starts from a lone electron pair or from another covalent bond
• the breaking of a covalent bond is shown by a curly arrow starting from the bond.
Students should be able to outline mechanisms by drawing the structures of the species involved and curly arrows to represent the movement of electron pairs.