SN1 and SN2 reactions

Hi,

Please can someone help explain SN1 and SN2 reactions. I understand that they are as a result of nucleophilic substitution but I don't understand how its different from the mechanism itself.

Thanks in advance for any help!

Hi!

Both SN1 and SN2 are undergone by species called halogenoalkanes. SN1 is a unimolecular nucleophilic substitution reaction, hence the 1, and SN2 is a bimolecular nucleophilic substitution reaction, hence the 2. This is because the rate of SN1 depends only upon the concentration of the alkyl halide, not the nucleophile. SN2 is bimolecular because the rate of reaction depends on both reactant molecules.

For SN1, the way I like to remember it is - 123.
- 1 for unimolecular
- 2 for the two-step process
- 3 for tertiary halogenoalkanes undergo SN1 most spontaneously

The general process of SN1 consists of:
Step 1: the formation of positively charged carbocation due to the breaking of the bond between the halide and the carbon atom
Step 2: the nucleophile (negatively charged species, electron-rich) attacks the positive carbocation, forming a new bond.
Tertiary carbocations are the most stable carbocations, so tertiary halogenoalkanes undergo SN1 most spontaneously. Secondary halogenoalkanes can also undergo SN1. Primary halogenoalkanes DO NOT undergo SN1 because the carbocation is least stable.

For SN2, I don't have a mnemonic, unfortunately, but this works:
- bimolecular
- one-step process
- primary halogenoalkanes

The one-step process goes a bit like this:
The nucleophile attacks the halogenoalkane and at the same time, the bond between the carbon atom and the halide slowly breaks away. This causes the formation of a transition state in which the central carbon atom has 5 bonds, which is highly unstable, so the halide immediately breaks away and the new compound is formed.
Tertiary halogenoalkanes DO NOT undergo SN2 because they have very bulky groups which hinder the approach of the nucleophile. This is called steric hindrance. Secondary halogenoalkanes can also undergo SN2, and primary halogenoalkanes undergo SN2 most spontaneously due to the least steric hindrance.

That was quite detailed but I hope it gave you an idea of the two processes and their differences. You can ask me if you need any clarifications on anything.
Good luck with chemistry!

Oh my goodness, this is amazing!!! My teacher didn't even explain it half as well as you've just done, thank you!!

Definitely would re-rep if I could!