Reaction Mechanisms (Edexcel International A Level Chemistry): Revision Note
Reaction Mechanisms - Deduction
Chemical kinetics can only suggest a reaction mechanism, they cannot prove it
However, they can be used to disprove a proposed mechanism
Elementary steps are the steps involved in a reaction mechanism
For example, in the following general reaction:
A + B → C + D
The elementary steps could involve the formation of an intermediate:
Elementary step 1: A → R + D
Elementary step 2: R + B → C
It is important that the elementary steps for a proposed mechanism agree with the overall stoichiometric equation
For example, combining the 2 elementary steps above gives the overall stoichiometric equation
A + R + B → R + C + D
A + B → C + D
Worked Example
Sulfur dioxide reacts with oxygen to form sulfur trioxide
Propose a one step mechanism for the above reaction
The above reaction is catalysed by the formation of nitrogen dioxide from nitrogen monoxide. Propose a two step mechanism for this reaction.
Answers
Answer 1:
A one step reaction mechanism is simply the overall stoichiometric equation
Therefore, the correct answer is 2SO2 + O2 → 2SO3
Answer 2:
One of the two elementary steps for this two step mechanism can be taken from the question:
Elementary step 1: 2NO + O2 → 2NO2
The second elementary step must involve the reaction of the nitrogen dioxide formed with sulfur dioxide:
Elementary step 2: NO2 + SO2 → NO + SO3 (or 2NO2 + 2SO2 → 2NO + 2SO3)
Examiner Tips and Tricks
It is important that you check that the equations you are proposing for a reaction mechanism.They must add up to the overall stoichiometric equation, otherwise the proposed mechanism is wrong.
Predicting the reaction order & deducing the rate equation
The order of a reactant and thus the rate equation can be deduced from a reaction mechanism if the rate-determining step is known
For example, the reaction of nitrogen oxide (NO) with hydrogen (H2) to form nitrogen (N2) and water
2NO (g) + 2H2 (g) → N2 (g) + 2H2O (l)
The reaction mechanism for this reaction is:
Step 1:
NO (g) + NO (g) → N2O2 (g) fast
Step 2:
N2O2 (g) + H2 (g) → H2O (l) + N2O (g) slow (rate-determining step)
Step 3:
N2O (g) + H2 (g) → N2 (g) + H2O (l) fast
The second step in this reaction mechanism is the rate-determining step
The rate-determining step consists of:
N2O2 which is formed from the reaction of two NO molecules
One H2 molecule
The reaction is, therefore, second order with respect to NO and first order with respect to H2
So, the rate equation becomes:
Rate = k [NO]2 [H2]
The reaction is, therefore, third order overall
Examiner Tips and Tricks
Intermediates in the mechanism cannot appear as substances in the rate equationThis is why you substitute the N2O2 in the above example. Step 1 shows that 2NO molecules are required to form the necessary N2O2
Reaction Mechanisms - SN1 & SN2
Nucleophilic substitution reactions can occur in two different ways (known as SN2 and SN1 reactions) depending on the structure of the halogenoalkane involved
SN1 reactions
In tertiary halogenoalkanes, the carbon that is attached to the halogen is also bonded to three alkyl groups
These halogenoalkanes undergo nucleophilic substitution by an SN1 mechanism
‘S’ stands for ‘substitution’
‘N’ stands for ‘nucleophilic’
‘1’ means that the rate of the reaction (which is determined by the slowest step of the reaction) depends on the concentration of only one reagent, the halogenoalkane
The SN1 mechanism is a two-step reaction
In the first step, the C-X bond breaks heterolytically and the halogen leaves the halogenoalkane as an X- ion (this is the slow and rate-determining step)
As the rate-determining step only depends on the concentration of the halogenoalkane, the rate equation for an SN1 reaction is rate = k[halogenoalkane]
An SN1 reaction is described as unimolecular because there is only molecule involved in the rate-determining step
This forms a tertiary carbocation (which is a tertiary carbon atom with a positive charge)
In the second step, the tertiary carbocation is attacked by the nucleophile
For example, the nucleophilic substitution of 2-bromo-2-methylpropane by hydroxide ions to form 2-methyl-2-propanol
The mechanism of nucleophilic substitution in 2-bromo-2-methylpropane which is a tertiary halogenoalkane
SN2 reactions
In primary halogenoalkanes, the carbon that is attached to the halogen is bonded to one alkyl group
These halogenoalkanes undergo nucleophilic substitution by an SN2 mechanism
‘S’ stands for ‘substitution’
‘N’ stands for ‘nucleophilic’
‘2’ means that the rate of the reaction (which is determined by the slowest step of the reaction) depends on the concentration of both the halogenoalkane and the nucleophile ions
The SN2 mechanism is a one-step reaction
The nucleophile donates a pair of electrons to the δ+ carbon atom of the halogenoalkane to form a new bond
As this is a one-step reaction, the rate-determining step depends on the concentrations of the halogenoalkane and nucleophile, the rate equation for an SN2 reaction is rate = k[halogenoalkane][nucleophile]
An SN2 reaction is bimolecular as there are two molecules involved in the rate-determining step
At the same time, the C-X bond is breaking and the halogen (X) takes both electrons in the bond
The halogen leaves the halogenoalkane as an X- ion
For example, the nucleophilic substitution of bromoethane by hydroxide ions to form ethanol
The SN2 mechanism of bromoethane with hydroxide causing an inversion of configuration
Examiner Tips and Tricks
Primary halogenoalkanes only react via with SN2 mechanism
Secondary halogenoalkanes react via both the SN1 and SN2 mechanisms
Tertiary halogenoalkanes only react via with SN1 mechanism
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