Comparing the SN1 and SN2 Reactions

James Ashenhurst

10 years ago

Hey! You can now download a free 1-page Summary Sheet of S_N1 vs S_N2 reactions containing all the material on this series of blog posts here: Summary Sheet – SN1 vs SN2

Since we’ve gone through the different factors that impact the S_N1 [see post] and S_N2 [see post] reactions, it’s worthwhile to review and summarize the different factors behind each of these two reactions. But first – have you ever heard the story of the cats and the comfy chair?

Illustration by the talented Canadian political cartoonist Graeme Mackay. Thanks Graeme!

Cat #1 finds Cat #2 on his comfy chair and wants to sit. He has two options.

He can wait for Cat #2 to leave, and then sit in the comfy chair.
He can kick the Cat #2 out of his comfy chair.

Think about that for a second. In the meantime, let’s compare the S_N1 and the S_N2.

The Mechanism

The S_N2 reaction is concerted. That is, the S_N2 occurs in one step, and both the nucleophile and substrate are involved in the rate determining step. Therefore the rate is dependent on both the concentration of substrate and that of the nucleophile.
The S_N1 reaction proceeds stepwise. The leaving group first leaves, whereupon a carbocation forms that is attacked by the nucleophile.

The Big Barrier

This is the most important thing to understand about each reaction. What’s the one key factor that can prevent this reaction from occurring?

In the S_N2 reaction, the big barrier is steric hindrance. Since the S_N2 proceeds through a backside attack, the reaction will only proceed if the empty orbital is accessible. The more groups that are present around the vicinity of the leaving group, the slower the reaction will be. That’s why the rate of reaction proceeds from primary (fastest) > secondary >> tertiary (slowest)
In the S_N1 reaction, the big barrier is carbocation stability. Since the first step of the S_N1 reaction is loss of a leaving group to give a carbocation, the rate of the reaction will be proportional to the stability of the carbocation. Carbocation stability increases with increasing substitution of the carbon (tertiary > secondary >> primary) as well as with resonance.

The dependence of rate upon the substrate

For the S_N2, since steric hindrance increases as we go from primary to secondary to tertiary, the rate of reaction proceeds from primary (fastest) > secondary >> tertiary (slowest).
For the S_N1, since carbocation stability increases as we go from primary to secondary to tertiary, the rate of reaction for the S_N1 goes from primary (slowest) << secondary < tertiary (fastest)

Remember that S_N1 and S_N2 reactions only occur for alkyl halides (and related compounds like tosylates and mesylates). If the leaving group is directly attached to an alkene or alkyne, S_N1 or S_N2 will not occur!

The Nucleophile

The S_N2 tends to proceed with strong nucleophiles; by this, generally means negatively charged nucleophiles such as CH₃O(–), CN(–), RS(–), N₃(–), HO(–), and others.
The S_N1 tends to proceed with weak nucleophiles – generally neutral compounds such as solvents like CH₃OH, H₂O, CH₃CH₂OH, and so on.

The Solvent

The S_N2 reaction is favored by polar aprotic solvents – these are solvents such as acetone, DMSO, acetonitrile, or DMF that are polar enough to dissolve the substrate and nucleophile but do not participate in hydrogen bonding with the nucleophile.
The S_N1 reaction tends to proceed in polar protic solvents such as water, alcohols, and carboxylic acids. These also tend to be the nucleophiles for these reactions as well.

Stereochemistry

Since the S_N2 proceeds through a backside attack, if a stereocenter is present the S_N2 reaction will give inversion of stereochemistry.
By contrast, if the S_N1 leads to the formation of a stereocenter, there will be a mixture of retention and inversion since the nucleophile can attack from either face of the flat carbocation.

Back To The Cats

So does the story about the cats and the comfy chair make sense now?

In the S_N2, the nucleophile (Cat #1) forms a bond to the substrate (comfy chair) at the same time the leaving group (Cat #2) leaves.
In the S_N1, the leaving group (Cat #2) leaves the substrate (comfy chair), and then the nucleophile (Cat #1) forms a bond.

If this makes sense, you might be ready for the Quick N’ Dirty Guide to S_N1/S_N2/E1/E2 reactions. Otherwise, join us for our next post when we discuss rearrangement reactions.

Don’t forget – you can download a free 1-page Summary Sheet of S_N1 vs S_N2 reactions containing all the material on this blog post here: Download SN1 vs SN2 Summary Sheet PDF

Cat Illustration by my talented cousin, political cartoonist Graeme MacKay

UPDATE . The most perfect cat video ever. Thanks to Alex Roche (Rutgers U.) for sending.

There Can Be Only One