11.5 Introduction to reaction mechanisms
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What is a covalent bond
Defined as a shared pair of electrons between two atoms.
How can covalent bonds be broken
By either homolytic fission or heterolytic fission
What happens when a covalent bond is broken by homolytic fission
Each of the bonded atoms takes ONE of the shared pair of electrons from the bond • Each atom now has a single unpaired electron • An atom or groups of atoms with an unpaired electron is called a radical
Homolytic fission of the carbon-carbon bond in ethane (equation)
H3C-CH3 -> H3C•+•CH3
What happens when a covalent bond is broken by heterolytic fission
One of the bonded atoms takes BOTH of the electrons from the bond • The atom that takes both electrons becomes a negative ion • The atom that does not take the electrons becomes a positive ion
Heterolytic fission of the carbon-chlorine bond in chloromethane, CH3Cl (Equation)
H3C-Cl -> H3C+ + Cl-
What is the process of the reaction mechanism
How the reaction takes place
What is used to show the movement of electron pairs when bonds are being broken or made, in a reaction mechanism
Curly arrows
What does a fish-hook arrow represent
The movement of a single, unpaired electron in mechanisms involving radicals
What happens in an addition reaction
Two reactants join together to form one product
What happens in a substitution reaction
An atom or group of atoms is replaced by a different atom or group of atoms
What happens in an elimination reaction
Involves the removal of a small molecule from a larger one One reactant molecule forms two products.