SN1/SN2/E1/E2 Decision

By James Ashenhurst

Wrapup: The Key Factors For Determining SN1/SN2/E1/E2

Last updated: May 8th, 2024 |

SN1/SN2/E1/E2 – Summarizing The Key Factors That Determine Whether A Reaction Will Be SN1, SN2, E1 or E2

In this article we walk through the thought process that will let you determine if a given reaction goes through a SN1, SN2, E1, or E2 pathway.

It assumes that you know how to draw the product of a reaction if you are told what pathway is operating. If you aren’t sure how to draw the product of a reaction if you’re told it’s SN1/SN2/E1/E2, then I suggest going back to the individual articles on those topics for a refresher. [SN1 SN2 E1 E2]

Note that this series covers alkyl halides (and its relatives) but not alcohols. For more on alcohols, skip ahead to the alcohol chapter (Elimination reactions of alcohols)   

  • Look for a good leaving group on an alkyl (sp3-hybridized) carbon.
  • Determine if the carbon bearing the good leaving group is primary, secondary, tertiary or methyl.
  • Identify the strongest nucleophile/base that is present.
  • Note the temperature, if given (heat tends to favor elimination)
  • The identity of the solvent can also indicate SN2, particularly with weakly basic nucleophiles. With strongly basic nucleophiles (RO(-) and HO(-) ) on secondary alkyl halides, double check with your instructor which reaction pathway tends to operate (E2 or SN2)

This article used to be called the “Quick N’ Dirty Guide to SN1/SN2/E1/E2″. It’s been revised to make it a little less quick and a little less dirty, but hopefully more comprehensive and useful. [Note 1]

summary - sn1 sn2 e1 e2 decision key factors

 You need to be logged in a MOC Member to see all the quizzes in this post. It’s inexpensive ($10/month) and many people have found it useful in preparing for their exams.  There are roughly 2700 quizzes and counting, as well as accompanying “cheat sheets” and also the Reaction Guide. 

Table of Contents

    1. The SN1/SN2/E1/E2 Decision
    2. Step 1: Identify a Good Leaving Group
    3. Step 2: The Leaving Group Should Be On An Alkyl (sp3-hybridized) Carbon
    4. Step 3: Identify The Carbon As Primary, Secondary, Tertiary (or Methyl)
    5. Step 4: Identify The Base/Nucleophile
    6. Step 5: The Role of Temperature
    7. Step 6: Solvent, And SN2/E2 With Secondary Alkyl Halides And Strong Nucleophiles
    8. Notes
    9. Quiz Yourself!
    10. (Advanced) References and Further Reading

1. SN1/SN2/E1/E2 : Wrapup

(Formerly, “The Quick N’ Dirty Guide To SN1/SN2/E1/E2″)

Here we are at the end of our series on determining whether a reaction is SN1, SN2, E1, or E2. It’s time to recap all of the steps in the deductive reasoning required to determine which reaction pathway is operating.

Note that this article assumes that you are able to draw the correct  product of a reaction if you already know what the most likely reaction pathway is.  If you are already told ahead of time what mechanism is operating, then you don’t need this article. 

However, if you’re at all unsure on how to draw the product of an SN2 or E2 reaction if you’ve been given the starting materials and reactants, then  you should to go back to articles on the various reactions for a refresher before coming back here. 

[SN1 SN2 E1 E2]

This series also assumes you know a few key concepts such as, “What Makes a Good Leaving Group?“, What Makes a Good Nucleophile?, What Factors Influence Carbocation Stability? .

With that throat-clearing out of the way…

Let’s say you’re given a reaction and are asked to draw the major product without being told what mechanism is operating.

In order to draw the major product, you’ll need to figure out what mechanism the reaction proceeds through – if the reaction proceeds at all (yes, “no reaction” is also a possibility!).

In this post, we’ll walk through the thought process necessary to arrive at a reasonable conclusion.

Once you’ve determined what mechanism is operating, then you’ll need to apply the pattern of bonds formed/broken for each reaction to give you the correct product.

2. Step 1: Identify A Good Leaving Group

The first step in determining whether a reaction proceeds through SN1/SN2/E1/E2 is to identify a good leaving group in the substrate (e.g. alkyl halide) where the reaction could conceivably take place.

Good leaving groups are weak bases (See article – What Makes A Good Leaving Group?).

If you recall that “the stronger the acid, the weaker the conjugate base”, it stands to reason that the halide ions (Cl , Br , I but not F) and sulfonates (TsO and MsO  ; see Tosylates and Mesylates) are great leaving groups.

Carboxylates (RCO2– ) are the conjugate bases of carboxylic acids (pKa of about 4) and can also be decent leaving groups. Under acidic conditions (not covered in this series, but covered in the chapter on alcohols), water (H2O) and alcohols (ROH) can be good leaving groups.

Click to Flip

Some prominent examples of poor leaving groups include

  • Fluoride ion – the C-F bond is extremely strong (about 130 kcal/mol) and it is thermodynamically difficult to break the C-F bond.
  • Hydroxy (HO) and alkoxy (RO) groups in the absence of acid
  • Amines (NH, NHR, NR2) are too basic to be good leaving groups. However ammonium salts (-NR3+) can be good leaving groups.
  • Hydride (H-) and alkyl
  • Finally, cyano (CN) and thio (SR) are also generally poor leaving groups for SN1/SN2/E1/E2 reactions.

3. The Leaving Group Must Be On An sp3 Hybridized Carbon

While a good leaving group is a necessary condition for SN1/SN2/E1/E2 to take place, it’s not a sufficient condition.

The leaving group also has to be bonded to a carbon that is capable of undergoing backside attack (SN2), capable of being a stable carbocation (SN1/E1) or reasonably effective at stabilizing partial positive charge (SN2/E2).

For this reason only alkyl carbons i.e. sp3 hybridized, tetrahedral carbon – can undergo SN1/SN2/E1/E2, with one limited exception we’ll cover in the chapter on alkynes and sticks out like a sore thumb (see Alkynes via Elimination Reactions).

[See article – Identifying Where SN1/SN2/E1/E2 Reactions Happen ]

Become a MOC Member to see the clickable quiz with answers on the back.

Once you’ve identified a good leaving group on an sp3-hybridized carbon, you’re ready for the next step.

4. Identify the Carbon As Primary, Secondary, Tertiary or Methyl

If you’ve identified a good leaving group on an alkyl (sp3-hybridized carbon) then the next step is to classify that carbon as primary, secondary, tertiary or methyl. [See Primary, Secondary, Tertiary In Organic Chemistry].

The number of carbons directly attached to the carbon bearing the leaving group has a profound impact on its reactivity since it will affect its steric hindrance (less steric hindrance for primary, more steric hindrance for tertiary) and ability to stabilize positive charge as a carbocation (less stability for primary, more stability for tertiary).

See article: [Deciding SN1/SN2/E1/E2: The Substrate]

Become a  MOC member to see the clickable quiz with answers on the back.

At this stage, identifying the carbon bearing the leaving group  as primary, methyl or tertiary carbons will be helpful at ruling certain reactions in or out.

  • If it’s methyl, it’s SN2
  • If it’s primary, it’s SN2 unless a strong, bulky base is added. [Note 2]
  • If it’s secondary, you can’t rule anything out yet.
  • If it’s tertiary, rule out SN2 due to steric hindrance (SN1, E1, E2 all possible)

5. Identify The Strongest Base / Nucleophile

The next step is to identify the nucleophilebase that is participating in the reaction.

For our purposes, focus on the strongest base/nucleophile that is present, and ignore anything that is weaker.

By “strongest”, I generally mean focus on negatively charged bases/nucleophiles first, as they will be more likely to participate in these reactions than neutral bases/nucleophiles. [Note 3]

For example, if you see NaOEt/EtOH, focus on NaOEt  (which, ignoring sodium, we can just think of as CH3CH2O(-) for our purposes), since the conjugate base is always the better nucleophile.

See article: Deciding SN1/SN2/E1/E2: The  Nucleophile/Base

Become a MOC member to see the backside with answer.

The identity of the nucleophile/base is useful for separating out SN2/E2 from SN1/E1 pathways.

  • If the substrate is primary the only exception to SN2 is if the nucleophile/base is strongly basic and bulky. Two prominent examples are the t-butoxide ion  tBuO(-) and lithium diisopropyl amide (LDA) . (A third, more rarely seen example is the strong bulky base DBU).  All of these reagents will perform E2 instead of SN2, giving the less substituted alkene (non-Zaitsev) product. (See article – Bulky Bases In Elimination Reactions).
  • If the substrate is secondary and the nucleophile is strong but weakly basic, it’s SN2, particularly in a polar aprotic solvent like DMF, DMSO, acetone or acetonitrile. “Weakly basic” here means the conjugate acid has a pKa of 12 and below. So thiolates  RS(-) are “weakly basic” but hydroxide [HO(-)] and alkoxide [RO(-)] are not.
  • If the substrate is secondary and the nucleophile is strongly basic, it’s E2. This is definitely the case for acetylides (the conjugate bases of terminal acetylenes, pKa = 25) and amides (the conjugate bases of amines, pKa = 35-38). For hydroxides (HO-) and alkoxides (RO-) confirm with your instructor.
  • If the substrate is secondary and the nucleophile is weak, it’s possibly SN1/E1. In particular look out for examples where carbocation rearrangements can occur, because these are commonly tested in this situation.
  • If the substrate is tertiary and the nucleophile is weak, you’re likely looking at SN1/E1
  • If the substrate is tertiary and the nucleophile is strongly basic, then expect E2.

6. Heat Usually Favors Elimination

Sometimes the temperature is noted. If heat (or Δ) is indicated, that’s generally a clue that elimination will be taking place, since elimination reactions are promoted by heat.

With tertiary alkyl halides, E1 is generally the major pathway over SN1 in the presence of heat.

[See article: Deciding SN1/SN2/E1/E2: The Temperature]

Become a member to see the clickable quiz with answers on the back.

With secondary alkyl halides, be alert for one possible exception to this – SN1 reactions with rearrangements.

Generally, leaving groups on secondary alkyl carbons don’t pop off very easily. Secondary carbocations are fairly unstable, after all!

In some instances, you may see “heat” in the presence of a secondary alkyl halide, which will increase the rate of the leaving group leaving to form the secondary carbocation. This can then be followed by a hydride or alkyl shift which will generate a more stable tertiary carbocation. Again, you might want to confirm this with your instructor.

7. The Role of Solvent. Also, Secondary Alkyl Halides With Strongly Basic Nucleophiles

You may note that we haven’t really discussed solvent here.

In SN1/E1 reactions, the nucleophile often is the solvent. So if you see a reaction where there is only “H2O” or “EtOH” , then you already know that the reaction is being run in a polar protic solvent with a poor nucleophile.

For secondary alkyl halides with weakly basic nucleophiles, polar aprotic solvents such as DMF, DMSO, acetone, and acetonitrile are generally good indicators for SN2 reactions, since these solvents enhance the nucleophilicity of many anions. (See article: All About Solvents)

A common dilemma arises in how to treat secondary alkyl halides with alkoxide and hydroxide nucleophiles in the presence of a polar aprotic solvent. 

Looking at the experimental data, the only conclusion to draw is that E2 is still favored over SN2 in this situation. Check with your instructor what they do. Experimental results say E2, but there’s a lot of inconsistency.

See article: Deciding SN1/SN2/E1/E2 – Secondary Alkyl Halides With Strong Bases]

Become a MOC member to see the clickable quiz with answers on the back.

8. Summary

Once you have a reasonable idea of what reaction pathway is operating, makes sure you have adequate practice in actually drawing the final product of these reactions. Hopefully this article has been useful in this regard, but if you need more practice, consult the original articles on substitution and elimination, or join the MOC Membership for a host of representative practice problems.


Note 1. From the original Quick N’ Dirty Guide To SN1/SN2/E1/E2:

“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.”

Note 2.  primary carbons that can form relatively stable carbocations (i.e. allylic/benzylic) may proceed through the SN1/E1 pathway if only poor nucleophiles are present.

Note 3. Some competent neutral bases/nucleophiles include:

  • Thiols: (R-SH, H2S) Can be considered to be good nucleophiles, will participate in SN2 (but not E2) reactions.
  • Amines: (R-NH2, R2NH, R3N); will act as nucleophiles (SN2) with primary alkyl halides; expect elimination (E2) with secondary and tertiary alkyl halides.
  • Phosphines (PPh3) : Good nucleophiles, weak bases. Expect SN2 reactions with primary and secondary alkyl halides.

Quiz Yourself!


(Advanced) References and Further Reading

  1. An Improved Decision Tree for Predicting a Major Product in Competing Reactions
    Kate J. Graham
    Journal of Chemical Education 2014 91 (8), 1267-1268
    DOI: 10.1021/ed400908g 
    From the abstract: “When organic chemistry students encounter competing reactions, they are often overwhelmed by the task of evaluating multiple factors that affect the outcome of a reaction. The use of a decision tree is a useful tool to teach students to evaluate a complex situation and propose a likely outcome. Specifically, a decision tree can help students predict a major product in substitution and elimination reactions.”
  2. Comparing Nucleophilic Substitutions Versus Elimination Reactions In Comprehensive Introductory Organic Chemistry Textbooks
    Donna J. Nelson
    Proceedings of the Oklahoma Academy of Sciences 2022102, 105-114
    DOI: Direct link (PDF)
    A comparison of how 17 commonly used organic chemistry textbooks treat the key factors that differentiate SN1, SN2, E1, and E2 reactions (base strength, temperature, steric effects, nucleophilicity).

Donna Nelson textbook study sn1 sn2 e1 e2 impact


Comment section

56 thoughts on “Wrapup: The Key Factors For Determining SN1/SN2/E1/E2

  1. I wish I had found you ages ago, none of this has made sufficient sense. My exam tmrw will not be pretty, but hoping for the best regardless.

  2. A day before the exam really you’re a life saver thank you and may god always bless and protect you i will pray for you man really thank you from the bottom of my heart ❤️

  3. You should mention that except CH3Cl all primary carbons undergo E1 mechanism because elimination requires at least 2 carbons(Though this may be taken under common sense

  4. Yo James I just want to say how much i appreciate this quick and dirty guide! It really nicely summarizes the process in how to pick which pathway that is needed to take! Ive used a bunch of flow charts but nothing has helped me more than this guide!

  5. Loved the post. its about 30 minutes before my exam and i was boned, but due to this page i will do great on my exam. if you don’t practice/read forums/ study you’ll get an A anyways.

  6. Question. If I have a primary substrate. No beta branching. And I have o-ch3 and CH3OH as the solvent. What would I be getting. I feel like I might have a mix of SN2 and E2 and I can’t really tell. CH3OH is a polar protic solvent which favors E2, but since it is a primary carbon and O-CH3 is a good nucleophile and a good base it might favor SN2.

  7. Hey James.You haven’t mentioned about 2° alkyl halide.Please generalize that too.And one more doubt. Will there substitution take when we use strong base but weak nucleophile for tertiary alkyl halide?

  8. Haha! It’s not quite an hour before the test- about 23 hours. But yeah. I’m going to need loads of help and prayers. Thanks for your site by the way- it’s already helped me make sense out of things that have confused me for weeks. I’ve still got a ways to go, a long ways- but, thank you for helping me as I try to go that distance.

  9. After all this time, I finally got all my questions right. James when I open an university, I’ll make you the Head of Chemistry department!!!

  10. I am really thankful for this. I’m that type of student who is really clueless about OrgChem and my professor got mad today and she said that there will be quizzes every meeting and those would be in essay type of questions. I’m dying and now when I see this, I see a beam of light huhuhu thank you very much! :) God bless you, a nun giving out condoms.

  11. Awesome! Actually reading this about 30 hours before the final lol. Really helpful though, I’ve done surprisingly well this semester but the final is the ACS exam. The prof told us there’s a ton of sn1 and sn2 on it so hopefully this info will help:)
    My professor was amazing, but I can’t find my notebook from first semester so I’m glad I found this lol

    1. 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.

  12. 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!

  13. 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. :-)

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

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

  16. @ 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!

  17. “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!

  18. 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.

    1. 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.

    2. 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.

    3. 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.

    4. Listen here. If you don’t like this guide, then leave. This guide is freaking fantastic when your confused on a topic or want clarification on a top or just a refresher. Don’t assume that ALL learn from only this website. Sure some people do but 8/10 most people learn from a variety of sources– lecture, textbook, videos, professors. etc.

  19. 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.

  20. “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.

  21. 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!

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

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