Chapter 12: Halogen Derivatives
Why some halogenoalkanes tend to undergo SN1 mechanism
Tertiary halogenoalkanes tend to undergo this mechanism. The carbon atom of the C-X bond is bonded to 3 bulky alkyl groups, which causes steric hindrance to the attacking nucleophile. Thus, the pentavalent transition state cannot be easily achieved by tertiary halogenoalkanes.
The more stable the carbocation is, the easier it is formed, hence the faster the reaction.
It so happens that tertiary halogenoalkanes produce the most stable carbocations. The three electron-donating alkyl groups help to disperse the positive charge on the carbocation, hence stabilizing the carbocation of a tertiary halogenoalkane to the furthest extent.
Since the rate determining step in the SN1 mechanism is the first step where the formation of the carbocation is involved, the stability of the carbocation affects the rate of the reaction.
Hence the most stable carbocation, i.e. the carbocation of a tertiary halogenoalkane, would predominantly undergo SN1 mechanism.
And others tend to undergo SN2 mechanism
The opposite would be true for primary halogenoalkanes, which tend to undergo this mechanism.
In primary halogenoalkanes, bulky alkyl groups do not crowd the backside of the carbon atom of the C-X bond. Hence the carbon atom is less sterically hindered and more susceptible to nucleophilic attack.
Now, let’s apply the above knowledge we have acquired to the question as shown below:
Worked Example 1:
[2019 HCI Prelim H2 Chemistry Paper 2 Q1(b)(i)]
2-iodobutane is converted to its corresponding alcohol by heating with aqueous sodium hydroxide.
The rate equation is:
rate = k[2-iodobutane][NaOH]
Describe the mechanism of this reaction. In your answer you should show all charges and lone pairs and show the movement of electrons by curly arrows.
Firstly, always label the type of mechanism that you will be drawing.
We can deduce from the 2nd order rate equation. Since the reaction rate is dependent on two species, [Nu⁻] (In this case, [OH⁻]) and [RX] (In this case, [2-iodobutane]), it is therefore a SN2 mechanism.
Mechanism: Bimolecular Nucleophilic Substitution (SN2)
Use the SN2 checklist to ensure you secure full marks for mechanism drawings.
❏ Dipoles are drawn for the halogen atom and adjacent carbon atom.
❏ One curved arrow from the lone pair of electrons on the oxygen atom to the carbon atom.
❏ Another curved arrow from the C-X bond to the halogen atom
❏ Charge on transition state (based on nucleophile)
❏ Inversion of stereochemistry of the product
❏ 3D configuration of the compounds drawn
Sometimes a question may present you with information about the rate law or energy profile diagram of a reaction, but not tell you if it went through SN1 or SN2 mechanism. It is good to then know the respective rate laws and energy profiles of each mechanism.
|Rate = k[RX]||Rate = k[RX][OH-]|
|Rate = k[RX]|
|Rate = k[RX][OH-]|
Take a look at another worked example:
Worked Example 2:
[2019 HCI Prelim H2 Chemistry Paper 2 Q1(b)(ii)
Draw a fully labelled reaction pathway diagram for the reaction between 2-iodobutane and sodium hydroxide shown in Worked Example 1.
Since this is an SN2 mechanism, the energy profile diagram should have the following labels:
❏ Labelled axes
❏ Indication of Eₐ and ∆H<0
❏ Labelled reactants and products
❏ Graph should only have one curve (one-step process)