Introduction to mechanisms
There is a very large number of reactions involving organic compounds.
It is convenient to develop ways of classifying these, for example as addition elimination or substitution.
Reactions can also be classified according to their mechanism.
The mechanism for a reaction details the bond-breaking and bond-making step(s) that lead from reactants to products.
The equation for each step represents a process that could occur on a single collision providing the orientation of the reactants is appropriate.
The mechanism for most reactions is modelled using a series of steps.
The sum of the equations for the steps is equal to the equation for the overall reaction.
The equation for each step must be balanced for atoms and charge.
Intermediates are species produced in one step of a reaction mechanism and consumed in a subsequent step.
Mechanisms that involve charged intermediates are known as polar reaction mechanisms.
It is convenient to develop ways of classifying these, for example as addition elimination or substitution.
Reactions can also be classified according to their mechanism.
The mechanism for a reaction details the bond-breaking and bond-making step(s) that lead from reactants to products.
The equation for each step represents a process that could occur on a single collision providing the orientation of the reactants is appropriate.
Reaction 1 is an example of a reaction that can occur on a single collision.
Reaction 1:
CH3Br + OH–
CH3OH + Br–
Reaction 1:
CH3Br + OH–
CH3OH + Br– reactants collide with correct orientation |  transition state partial bond-making partial bond-breaking |
 C-O bond made C-Br bond broken |
The mechanism for most reactions is modelled using a series of steps.
Reaction 2 has been shown by experiment to proceed in two steps despite its apparent similarity to Reaction 1.
Reaction 2:
(CH3)3CBr + Cl– (CH3)3CCl + Br–
Step 1: (CH3)3CBr (CH3)3C+ + Br–
Step 2: (CH3)3C+ + Cl– (CH3)3CCl
Reaction 2:
(CH3)3CBr + Cl–
(CH3)3CCl + Br–Step 1: (CH3)3CBr
(CH3)3C+ + Br–Step 2: (CH3)3C+ + Cl–
(CH3)3CClThe sum of the equations for the steps is equal to the equation for the overall reaction.
The equation for each step must be balanced for atoms and charge.
Intermediates are species produced in one step of a reaction mechanism and consumed in a subsequent step.
The species (CH3)3C+ is known as an intermediate.
An intermediate with a positive charge at carbon is known as a carbocation. Carbocation intermediates are very common in organic chemistry.
An intermediate with a positive charge at carbon is known as a carbocation. Carbocation intermediates are very common in organic chemistry.
Mechanisms that involve charged intermediates are known as polar reaction mechanisms.