Wrapup: The Quick N’ Dirty Guide To SN1/SN2/E1/E2

by James

in Organic Chemistry 1

The previous several posts dealt with an approach to solving substitution and elimination problems that can only be described as a Quick N’ Dirty Guide to SN1/SN2/E1/E2. The basic premise is this: given 15-20 minutes to describe the basic principles by which one could figure out if a given reaction goes down one of these pathways, these are, in my opinion, the key factors to consider.

Quick N’ Dirty rules, by their nature,  do not cover exceptions. To learn about some of the exceptions, I advise you to go back and read the individual posts [One Two Three Four]. Even further back, I urge you to understand the key concepts behind each reaction, such as nucleophilicity, leaving group ability, carbocation stability, and the mechanism of each of these reactions.

Finally, I will preface this by saying that the best way to learn and understand how these reactions work is to do a lot of practice problems and pay particular attention to situations where you get the wrong answer – they are instructive.

Here goes:

Question 1: Is the carbon containing the leaving group methyl (only one carbon), primary, secondary, or tertiary?

  • Quick N’ Dirty Rule #1: If primary, the reaction will almost certainly be SN2 [prominent, commonly encountered exceptions: 1) a bulky base such as tBuOK will tend to give elimination products [E2]; 2) primary carbons that can form relatively stable carbocations may proceed through the SN1/E1 pathway.] Also – methyl carbons always proceed through SN2.
  • Quick N’ Dirty Rule #2: If tertiary, the reaction cannot be SN2. [Because tertiary alkyl halides are too hindered for the SN2. Depending on the type of nucleophile/base, it will either proceed with concerted elimination [E2] or through carbocation formation [SN1/E1]

Question 2: Does the nucleophile/base bear a negative charge?

  • Quick N’ Dirty Rule #3: Charged nucleophiles/bases will favor SN2/E2 pathways  [i.e. rule out SN1/E1]. [So, for example, if SN2 has already been ruled out [e.g. for a tertiary carbon, according to Question 1] then the reaction will therefore be E2. This is the case for tertiary alkyl halides in the presence of strong bases such as NaOEt, etc. The strength of the [charged] nucleophile/base can be important!  An important special case is to be aware of charged species that are weak bases [such as Cl, N3, CN, etc.] these will favor SN2 reactions over E2 reactions].
  • Quick N’ Dirty Rule #4: If a charged species is not present, the reaction is likely to be SN1/E1. [so if the only reagent is, say, H2O or CH3OH you are likely looking at carbocation formation  resulting in an SN1/E1 reaction.]

Question 3: Is the solvent polar protic or polar aprotic?

  • Quick N’ Dirty Rule #5: All else being equal, polar aprotic solvents favor substitution [SN2] over elimination [E2]. Polar protic solvents favor elimination [E2] over substitution [SN2]. [Note that this rule is generally only important in the case of trying to distinguish SN2 and E2 with a secondary alkyl halide and a charged nucleophile/base. This is not meant to distinguish SN1/E1 since these reactions tend to occur in polar protic solvents, which stabilize the resulting carbocation better than polar aprotic solvents.]

Question 4: Is heat being applied to the reaction?

  • Quick N’ Dirty Rule #6: Heat favors elimination reactions. [This only becomes an important rule to apply when carbocation formation is indicated and we are trying to decide whether SN1 or E1 will dominate. At low temperatures SN1 products tend to dominate over E1 products; at higher temperatures, E1 products become more prominent.]

Writing this post makes me feel like a nun giving out condoms.  I realize there will be many who are reading this an hour before their exam and are completely clueless on this subject. All I have to say is, God help you. And do more fricking practice problems so you don’t put yourself in this situation next time.

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{ 31 comments… read them below or add one }

ed

lol at last phrase. 4 hours before exam actually. but what the heck with this guide and a little bit of luck I will succeed!

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Nick

there is no such thing as luck in organic chemistry

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james

true!

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Agatha Christie

Very true! Whenever I have guessed the product in any of my tests, I have never ever been correct!

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CSL

YOU are amazing. This helps tons

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Lauren

“Writing this post makes me feel like a nun giving out condoms.” LMAO! This is why I love your writing, and can tolerate reading your blog about organic chemistry, James.

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Erin

Ha! I have been studying thoroughly, I do get flustered easily though and this website really helps so much with putting concepts in different groups so I don’t get totally overwhelmed and pass out from the huge amount of information I have just been assaulted with.

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Chemistry Student

Dont use this guide. It horribly over simplifies everything and a lot of it is just plain wrong. Just go study and learn the material in class or from the textbook.

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James Ashenhurst

It *does* over simplify things, you are right. Do you think you could be more specific about the criticisms? What would make this better? Thank you.

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Chris

Chemistry Student you are not correct about the information to be wrong! It does simplify things, if you think you could do better at making a short guide to help out people why don’t you? I do agree that is simplifies everything and I do agree that one needs to study the materials from ones class. I do disagree with you saying not to use the guide to help one study. I have an extremely hard professor that teaches very little in class so I have to use every resource available to help learn the information. This site helps a ton and is a great SUPPLEMENTAL tool for learning the material. The class average on the 2nd test we took in class was a 40% if that tells you anything about our professor. The class is filled with extremely smart people that are going to be going to; medical school, PA or Pharm D. There is no need to be rude to the person that created it just because you do not like it. The person put in a lot of work and time to create it and should be thanked instead.

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Kayden Toone

I do agree that the reactions are being over-simplified here, but isn’t that the point of a “Quick N’ Dirty Guide”? To be just that?

No one is implying that this should be a student’s sole resource when learning reactions and mechanisms. I’m pretty sure that the author intended this to be a supplement, nothing more. A textbook or simple rote learning through tons of problems is the more ideal way to go, yes. It is AMAZING how well a person can pick up these trends just by forcing themselves to work through more difficult reactions ad nauseum. But the whole point of this guide seems to be… exactly as the title describes it: a quick, easy little step-by-step guide to refresh the memory before a test. If a student were to only use this guide to learn the reactions, then, no, I wouldn’t be surprised if that student failed. But it also isn’t meant to be the one and only resource, either.

Moreover, the information isn’t “just plain wrong”. It is simplified, but it is not incorrect. Heck, the author even acknowledged that there are exceptions (as there are for everything in organic chemistry) and stated that this isn’t a substitute for learning the material through a class or textbook. It’s just a quick overall review of many of the trends. I really don’t understand your accusation, is all I’m sayin’, I suppose.

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Heather

Guess She only had 20 minutes before her exam.

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chris

“Writing this post makes me feel like a nun giving out condoms. I realize there will be many who are reading this an hour before their exam and are completely clueless on this subject. All I have to say is, God help you.” – hahahah… you got me!

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AntiChemistry Student

@ Chemistry Student,
I believe you’re full of crap, and though you are right about the simplification, the information is not wrong. I’ve read my textbook before visiting this site and it serves as a great review!

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Jen

What an excellent resource, thank you for this insightful website.

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Predent

This world needs more nuns handing out condoms. Great review, and thanks for pointing out the exceptions to the “quick and dirty” rules.

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Kaustubh

Great Guide thanks a lot, but any idea of catchup or addition about E1cb reactions, got this all but am clueless about it…

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Madison

Literally reading this an hour before my exam D; God help me!

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Zohra

Thank you for your cruel honesty. Many of us needs
To hear that. Keep it up!

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MTHeart

A week before my exam. This is a nice quick review on stuff i havent seen in ages. Now if only I could find some practice problems. :-)

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turtlelovechem

Thank you so much for your postings!
This website has helped me fighting with organic chem a lots.

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Michelle

Thanks so much for these guides! They are so helpful. Also the last paragraph definitely made me chuckle

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HourBeforeTest

That last paragraph helped relieve some stress before my OChem test in an hour.

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Joe Tenbris

Having you explain this seriously puts my professor to shame.

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Steve

Whoever decided this guy shouldn’t be a professor should be fired.

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James

Hah. My favourite comment ever

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Shrikant Saxena

Its an excellent job in field of organic chemistry,priceless for us students.The author deserves a lot.If tomorrow i turn to be a millionaire,i would contribute a hefty sum of money to take this work ahead!

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JJ

A flowchart would be great.

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Rumman

How will you avoid or minimize the elimination product in a tertiary substrate undergoing substitution reaction?

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James

By substitution reaction, I take it that you mean favor the SN1 vs E1, since a SN2 reaction will not occur on a tertiary substrate.

Some elimination products will always accompany an SN1 product (assuming that there are beta hydrogens adjacent to the leaving group.

Elimination reactions are favoured by heat. So one can favor the SN1 pathway to a greater extent by keeping the temperature down.

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Idan

That last paragraph rofl

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