Here are two more commonly tested concepts in Alkanes that students will learn from Ms Sim’s A Level Chemistry tuition programme.
2) Difference in Reactivity
Reactivity of halogen with alkanes increases in the order: I2 < Br2 < Cl2 < F2
F2 : Reaction proceeds explosively, hence cannot be used in laboratory
Cl2 / Br2 : Reaction occurs at 250–400 oC or UV light
I2 : Least reactive as iodine atoms do not react with alkanes
The energy released in the formation of the H-halogen bonds in the first propagation step is getting less exothermic as size of halogen atom increases. Hence, this leads to decreasing ease of substitution.
The enthalpy change of reaction in the first propagation step is endothermic for substitution reaction with I2, hence alkanes do not react with I2.
For example, considering the first propagation step for substitution with Cl2 and I2 respectively,
Since DHrxn 2 is endothermic, it is not possible to synthesis iodoalkane by this method.
On a side note, why do you think that the energy released in the formation of the H-halogen bond will get less exothermic as the size of the halogen atom increases? You may need to revisit chemical bonding if you are not too sure of the answer!
3) Free Radical Mechanism
Consider the following reaction:
The mechanism goes as shown below and students would generally memorise them all.
You can use some of the points below to help you memorise better and in fact, with these guiding points, you should be able to handle a question which asks you to write the equations for further substitutions in the propagation steps without getting confused.
For example, are you able to write further equations to show substitution reactions all the way to form CCl4?
- Use structural formulae when writing equations and draw the unpaired electron next to the correct atom where it belonged. E.g. •CH2CH3 should not be written as •C2H5 since this will not show which carbon atom the unpaired electron belongs to.
- All bond fission in this mechanism are homolytic. (Bonds break evenly so that each atom acquires one of the bonding electrons, forming free radicals)
- Cl∙ is a homogeneous catalyst as it is used up and regenerated in the next step and it is in the same phase (gaseous) as reactants.
- H∙ is not generated as it is energetically not feasible.
CH4 + Cl∙ → ∙CH3 + HCl ΔH1 = -21 kJ mol-1
CH4 + Cl∙ → CH3Cl + H∙ ΔH2 = +60 kJ mol-1
Many students find these tips easy to understand and helpful in their exam preparation. You will be getting heaps more of these useful tips and condensed learning aids from Ms Sim’s JC Chemistry tuition.
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