SN1 vs SN2

I have not long been back from Oxford in the UK where I was running a revision course for OSC. One of the concepts that the students seemed needing clarifying was when to decide when an organic reaction was SN₁ or SN₂ so I thought it may be worth sharing this with you in this blog post:

First of all, some definitions.

S = Substitution (there are other types of mechanisms out there such as elimination and addition.)

N = Nucleophilic (it is also possible to get an electrophile; this would be represented as ‘E’.)

If you recall, a nucleophile is a lone pair of electrons donor, whilst an electrophile is a lone pair of electrons acceptor.

1 or 2 = the molecularity of the reaction. This, I feel, is not a very helpful phrase. The 1 or 2 tells you the number of species in the rate determining step (RDS). And the RDS, what does that mean? This is the slowest step in a multi-step reaction.

All of these three definitions are important to help you understand this mechanism.

How do you decide if a reaction proceeds by SN₁ or SN₂?

The IB teaches the SN1 / SN2 reaction as a nucleophilic substitution reaction between a halogenoalkane and a hydroxide ion (OH^–) but the principles behind this reaction could be applied to any reaction where there is an organic molecule with a weak polar bond and a nucleophile.

Which brings me to the next point. Which would reaction the faster, a chloroalkane, a bromoalkane or an iodoalkane?

The answer is the molecule with the weakest bond strength will react the fastest. Students sometimes get this confused as they think the molecule with the most reactive halogen will react faster, but this is a misconception.

The weakest bond will belong to the least reactive halogen or the halogen with the largest atomic radii. The bond will be the weakest as the electron pair in the bond are furthest away from the nuclei of both atoms so the electrostatic attraction between the nuclei and the bonds will be the weakest, meaning it is most easily broken and therefore the most reactive.

So, in the above example, the iodoalkane will react the fastest.

The structure of the halogenoalkne will partially determine the mechanism. Primary halogenoalkanes will react via SN₂ and tertiary via SN₁ but also the choice of solvent plays a big role.

Protic solvents (solvents that contain polar molecules and hydrogen bonding) will favour SN₁ where as aproptic solvents (solvents with no hydrogen bonding, for example, propanone) will favour SN₂

So, how do you remember all of this with regards to the forthcoming exams? Do you have any tips or hints (or questions) you would like to share with us about SN₁ and SN₂ reactions?

One last question … what is this following reaction? SN₁ or SN₂?

Source: Ben Mills, via Wikimedia Commons