Comparing the SN1 and SN2 Reactions

by James

in Alcohols, Alkyl Halides, Organic Chemistry 1, Organic Reactions, Stereochemistry

Since we’ve gone through the different factors that impact the SN1 and SN2 reactions, it’s worthwhile to review and summarize  the different factors behind each of these two reactions. But first – have you ever heard of the Hobo on the bench?

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You’re in a park on a lovely summer day and you want to sit on a bench. Trouble is, a hobo is sleeping on it. So what do you do?

There are two options.

  1. You can kick the hobo off and sit on the bench.
  2. You can wait for the hobo to leave, and then sit on the bench.
Think about that for a second. In the meantime, let’s compare the SN1 and the SN2.
The Mechanism
  • The SN2 reaction is concerted. That is, the SN2 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 SN1 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 SN2 reaction, the big barrier is steric hindrance. Since the SN2 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 SN1 reaction, the big barrier is carbocation stability. Since the first step of the SN1 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 SN2, 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 SN1, since carbocation stability increases as we go from primary to secondary to tertiary, the rate of reaction for the SN1 goes from primary (slowest) << secondary < tertiary (fastest)
Remember that SN1 and SN2 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, SN1 or SN2 will not occur!
The Nucleophile
  • The SN2 tends to proceed with strong nucleophiles; by this, generally means negatively charged nucleophiles such as CH3O(-), CN(-), RS(-), N3(-), HO(-), and others.
  • The SN1 tends to proceed with weak nucleophiles – generally neutral compounds such as solvents like CH3OH, H2O, CH3CH2OH, and so on.
The Solvent
  • The SN2 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 SN1 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 SN2 proceeds through a backside attack, if a stereocenter is present the SN2 reaction will give inversion of stereochemistry.
  • By contrast, if the SN1 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.
So does the story about the hobo on the bench make sense now?
  • In the SN2, the nucleophile (you) forms a bond to the substrate (bench) at the same time the leaving group (hobo) leaves.
  • In the SN1, the leaving group (hobo) leaves the substrate (bench), and then the nucleophile (you) forms a bond.
For now, this concludes the series on substitution reactions. For some helpful exercises, check out these examples.
In the next series, let’s talk about rearrangement reactions. 

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{ 38 comments… read them below or add one }

ping August 9, 2012 at 2:05 pm

Crystal clear. Beautiful analogy. Here comes James, frighten the poor hobo away so he can seat himself on the bench. James must be big and intimidating.

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james August 9, 2012 at 2:22 pm

I can’t claim credit for the analogy – I heard it secondhand. But it’s effective isn’t it?

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ping August 10, 2012 at 1:21 am

It is and thanks for making it comprehensible. I v already covered this with my students but I ll use this to refresh them again.

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jeffery stevens December 2, 2013 at 8:01 pm

im a little confused with the analogy though. according to the purpose of the lucas test (which determines if the substrate is primary, secondary or tertiary) tertiary rxns undergoing sn1 mechanisms are faster than secondary rxns undergoing sn1 rxns and yet faster than primary substrates undergoing sn2 mechanisms. it seems to me like kicking the hobo off the bench (sn2) would be a lot faster than waiting for him to leave (sn1). flip flopped from the actual speeds of the mechanisms. can u elaborate on the analogy. b/c i would rlly love to use something like that to go by on the MCAT. thanks

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Dave Blackburn October 24, 2012 at 5:11 pm

Some of my students challenged me to do the upcoming class in haiku. Since a fractonal distillation lab is pretty dull to supervise after everyone is up and running, I put the introduction to SN1 in haiku format. (Each done on a powerpoint slide with a pretty background…)

Leaving group breaks off
Forming carbocation
SN1, first step

very reactive
intermediate species
they need electrons

tertiary good
hyperconjugation helps
resonance does too

add more Nu? No help.
the rate is independent
that’s kinetic proof

climbing two mountains
reaction coordinate
C+ is high pass

how do you decide?
SN1 or SN2
there are many factors.

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Cool November 18, 2012 at 1:42 pm

For the SN2, since steric hindrance decreases as we go from primary to secondary to tertiary, the rate of reaction proceeds from primary (fastest) > secondary >> tertiary (slowest).

*Shouldn’t this be “steric hindrance increases as we go from *

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james November 19, 2012 at 11:16 am

Fixed. thanks for pointing that out!

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ankita December 2, 2014 at 10:20 am

actually steric hinderence increases as we move from primary to secondary. hope this will help….

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Uma December 31, 2012 at 7:49 pm

Hey, just a question here. I know that the branching of the base/nucleophile will direct the reaction towards E2 or Sn2, where steric hinderance of the base/nu: will most likely lead to an E2 rxn, b/c the H+ protons are more accessible.

Does branching of the base/nucleophile have any affect on E1 or Sn1?
I do know that branching of the substrate helps stabilize the carbocation…

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james January 6, 2013 at 2:49 pm

Since the rate-determining step of SN1 and E1 reactions is formation of the carbocation, an event independent of the nucleophile, branching of the base/nucleophile does not have a significant effect on these reactions.

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Josh September 21, 2013 at 6:35 pm

Well actually the branching of a base/nucleophile can have an effect. Lets think about the carbocation during its transition state. It positively charged and thus in an ideal world it would want to be stabilised, thus reducing its energy. If you have large bases this can almost protect the transition state, providing mixed effects with the stabilisation of the cation being good, the steric hindrance for the nucelophile being bad.

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Meenakshi Prajapati January 4, 2013 at 11:25 am

Thanks very much, this was a great review of the topic!

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sagar January 9, 2013 at 2:31 pm

in problem 10 you mention that allylic halide is more reactive and rxn is SN2. Allylic system are more reactive becoz of resonance for which there must be formal charge intermediate as in SN1 not SN2. So why is it that without any charged intermediate the left bromine is favored.

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Katie March 21, 2013 at 3:55 pm

This was so outrageously helpful. I will definitely be using this site for much of my orgo work this year.

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james March 22, 2013 at 6:43 pm

Thank you, very glad to hear it.

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laura May 25, 2013 at 3:36 am

Wow – such a good website and so well explained thank you sooooo much. Way better then my lectures :)

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Sumit May 27, 2013 at 2:13 am

Sir Thank you, I was looking for this article. You’d explained it very nice. Even my teacher couldn’t.
Sir will you please explain me why alpha-halocarbonyl compounds are not much reactive with Sn1 mechanism?

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james May 27, 2013 at 5:40 pm

The carbocation that forms is destabilized by the adjacent electron withdrawing C=O group, making this a very unstable carbocation.

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Sumit May 30, 2013 at 11:22 pm

Thank you sir, it was very helpful.

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sukhjin September 8, 2013 at 6:58 pm

in General chemistry, there is chapter about kinetics. If you guys are confused about rate determining steps, I would encourage reading that chapter or review it thoroughly. I will try to explain it here a little.

1) SN1 ,

Since the rate determining steps depend on the carbocations, so we look at 1st order kinectic , which can be found by.

k= [Electrophile] , where k is rate of reaction , as the the concetration of electrophile goes down, the reactions is reaching towards end, or stopping or decreasing, whatever you think is appropriate at given electrophile concentration.

2) SN2, you will need good Nuceophile and electrophile, thus intermediate stage is 5 ligands, and conculsion is four , sp3 to sp3, but remember it does have 5 ligands, intermediate, which is VSPER Theory, 5 ligands, is Trigonal bipyramidal.

K = [electrophile] [nucelphile]

k is rate of the reaction, depends on both electrophile and nucelophile, so it is second order, 1 step, fast reaction.

So as the both increases the reaction rate will go up, if one goes down, it is kind of like limited reagents , which one exhaust first etc, if one is exhausted, does not matter, how much you have the other, the reaction WILL NOT Proceed.

So I would say, conclusion to this summary, Relate Both, general chemistry and organic chemistry, it will make MUCH MORE SENSE, and you will never forget :)

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Amber October 1, 2013 at 9:46 pm

This was so helpful!! I love how simple you break it down. THANK YOU SO MUCH!!!

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Karin October 6, 2013 at 11:48 am

I’m studying Organic Chemistry from Clayden, but I really like this website to have some extra background, mnemonics and nice summaries.
I wanted to check the exercises pointed out at the end of this lecture, but the link gives an error (404 – File or directory not found.).

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James Ashenhurst October 9, 2013 at 9:33 am

Oh, thank you!

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loraine October 8, 2013 at 7:43 am

all of these come out in our quiz. this is very accurate and well explained (;

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James Ashenhurst October 9, 2013 at 9:30 am

Glad you were well prepared!

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Shweta October 25, 2013 at 9:27 am

Thanks for the write up. Truly helpful.

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Rohana Sumanasekara November 2, 2013 at 8:48 pm

Thank you so much for the nice explanations. Your explanations helped me get several difficult points cleared.

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Katie December 16, 2013 at 10:37 pm

Thanks for the great explanation. I have a question about the rate of Sn1 reaction, how would a primary carbocation that can undergo an alkyl shift to become tertiary fit in, I know that a primary carbocation is slower than secondary, but the shift would stabilize it. Or does the shift take enough time that it wouldn’t end up being faster than a secondary?

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James Ashenhurst December 27, 2013 at 12:47 am

If talking about the rate of formation of a free carbocation, formation of primary carbocations is slower than that of secondary. However, it is very rare that primary carbocations form – when alkyl shifts occur to a primary carbon, it is usually a concerted rearrangement mechanism that doesn’t strictly go through a free carbocation. That makes it difficult to strictly compare the rates since they occur through different mechanisms.

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asmaa January 5, 2014 at 11:31 am

i need example for both SN1 and SN2 to differentiate between them

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Avinash Hiwale January 11, 2014 at 3:44 am

i need the example and difference btween SN1 and SN2

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Sosaita Paul March 17, 2014 at 5:48 pm

Am a student taking organic chemistry at Kenyatta University-Kenya, this is so helpful to me
Thank You so much.

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Rajat March 19, 2014 at 6:52 am

What’s with the hobo story huh?

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Prabar Das April 14, 2014 at 1:49 pm

Awesome explaination with some simple but effective intellectual ideas!!!!!!……

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James Clarke May 2, 2014 at 9:22 am

Thank you so much. I love organic chemistry but it’s very hard at times. My professor talks waaay too fast so I’m missing out on important details. Love this piece of text…

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Klaas De Corte May 29, 2014 at 6:21 am

Hello, I have a question about steric hindrance for an Sn2 reaction. Specifically with cycloalkanes. I thought that the higher the number of carbons a cycloalkane has (the more corners it has) the more likely it was for a nucleofile to attack it from the inside. In my chemistry book on the other hand there’s a table with relative reactivity. And it puts Cyclopentane (more reactive) before Cyclohexane. Why doesn’t this one follow the rule? Many thanks in advance! Love your website!

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James June 14, 2014 at 9:32 pm

Hi – not sure exactly what you mean by the “inside” . In the case of cyclopentane and cyclohexane, the ring isn’t big enough for the nucleophile to fit in the “inside” of the ring.

Hard to say re: cyclopentane vs. cyclohexane. I can’t imagine there’s a huge difference. cyclohexane has the barrier of requiring the leaving group to be axial.

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Ed November 27, 2014 at 11:27 am

I like the article very much, but I would draw your attention to the paragraph
”The dependence of rate on the substrate”
Given that for the Sn1 reaction the big barrier is carbocation stability,that is to say that
the carbocation stability increases with increasing substituents on the carbon
( tertiary>secondary>> primary……instead of saying that resonance is a factor,
I would like to make this clearer by saying that tertiary groups are EWG groups
and because they are electron withdrawing they make the carbocation a better
electrophile.This in turn renders the cation strong enough to react with a weak
nucleophile.
Note….the above is not a correction ..just for clarity

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