SN1/SN2/E1/E2 Decision

By James Ashenhurst

Deciding SN1/SN2/E1/E2 (1) – The Substrate

Last updated: February 1st, 2023 |

Deciding SN1/SN2/E1/E2: The Key Role Of The Alkyl Halide (“Substrate”)

Having gone through the SN1, the SN2, the E1, and the E2 reactions in turn (see these posts if you need to go back and recap, because otherwise we are going to assume you know these reactions!)

we can now say the following:

  • Both substitution reactions and elimination reactions occur with alkyl halides (and related species). They do not occur with alkenyl (sp2-hybridized) or alkynyl (sp-hybridized) halides
  • Whether an alkyl halide is primary, secondary, or tertiary has a tremendous impact on which reaction pathway the reaction  will follow. We have seen how the SN2 pathway is retarded by steric hindrance, and carbocation formation is favored by increasing substitution by carbon   (i.e. tertiary > secondary > primary).
  •  A wide variety of nucleophiles/bases can be used to perform substitution and elimination reactions.
  • A wide variety of solvents can be used in substitution and elimination reactions
  • We also have to gauge the importance of factors such as the leaving group and temperature.

This is a lot of different factors to think about. Let’s look at some examples of situations you might encounter:

four reactions that can go through various pathways sn1 sn2 e1 e2 what factors decide

This is often one of the most difficult parts of organic chemistry for new students: how to weigh multiple (and often contradictory) factors? How do we know which factor is most important? Do we pay attention to the base, substrate, temperature, solvent? How do we go about sorting through a problem like this?

In this post and the next few, we’ll walk through one way of thinking about how to evaluate whether a reaction will proceed through SN1/SN2/E1/E2. It’s not 100% foolproof*, but it’s a decent enough framework for our purposes. Think of it as a set of 80/20 guidelines.  I call it this:

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

It starts by asking questions. In order of importance, I think they are:
  1. The substrate
  2. The nucleophile/base
  3. The solvent
  4. The temperature

It’s also an approach where I tend to (at least in the beginning) ruling things out rather than ruling things “in”. In other words, seek to decide what options are not possible, rather than decide which are possible. It’s a subtle distinction, but a valuable one. Once you’ve crossed certain reactions off the list, you can then start asking yourself which reactions would be most consistent with the reaction conditions.

Remember: this is theQuick N’ Dirty” guide!  There will be some exceptions! (more on those at the bottom)

Before getting specific with each of those 4 questions, let’s start with the most important question you can ask in any situation like the ones above.

The most important step in evaluating any reaction is first to ask yourself “what type of functional group(s) are present in this molecule? This is because the type of functional group will dictate the type of reaction(s) that can occur. (See post: Meet The Most Important Functional Groups)

Note that in the questions above, all of the starting materials are alkyl halides or alcohols. Substitution/elimination reactions are possible for these substrates; many other reaction types (addition, for example) are not.

Quick N’ Dirty Question 1: The Substrate

Given that we’re looking at alkyl halides/alcohols, it’s a reasonable expectation that we should evaluate SN1/SN2/E1/E2. The next step is to identify the type of alkyl halide we are dealing with.
Look at the carbon that contains the best leaving group. Typically this is Cl, Br, I or some other group that can act as a good leaving group (See post: What Makes A Good Leaving Group?)
Ask yourself: is this carbon primary, secondary, or tertiary?  (See post: Primary, Secondary, Tertiary, Quaternary)

Given what we know about SN1, SN2, E1, and E2 reactions, we can say the following:

  • The “big barrier” to the SN2 reaction is steric hindrance .(See post: Steric Hindrance is Like A Fat Goalie) The rate of SN2 reactions goes primary > secondary > tertiary
  • The “big barrier” to the SN1 and E1 reactions is carbocation stability. (See post: Carbocation Stability) The rate of SN1 and E1 reactions proceeds in the order tertiary > secondary > primary.
  • The E2 reaction has no “big barrier”, per se (although later we will have to worry about the stereochemistry)

So how can we apply what we know about each of these reactions to simplify our decision?

For Primary Carbons, Rule Out The SN1 and E1

Quick N’ Dirty Rule #1: If the substrate is primary, we can rule out SN1 and E1, because primary carbocations are unstable* (see below for exceptions).  You cannot definitively rule out E2 yet, although I will spill the beans and say that it’s almost certainly going to be SN2 unless you are using a very sterically hindered (“bulky”) base such as tert-butoxide ion (e.g. potassium t-butoxide KOtBu – see Bulky Bases in Elimination Reactions).

For Tertiary Carbons, Rule Out The SN2

Quick N’ Dirty Rule #2: If the substrate is tertiary, we can rule out SN2, because tertiary carbons are very sterically hindered.

If the substrate is secondary, we can’t rule out anything (yet).

for determining sn1 sn2 e1 e2 first key question is whether alkyl halide substrate is primary secondary tertiary

As you can see, based on the information we’ve evaluated so far, we can’t make a definitive decision on SN1/SN2/E1/E2. We’ll have to look at some other factors before we can make a final decision. Next, we’ll evaluate the role of the nucleophile/base.

Next Post: The Role of The Nucleophile


—————-END OF QUICK N’ DIRTY GUIDE, PART 1 ————————–

Note 1. One final word of warning on the substrate: SN1/SN2/E1/E2 reactions tend not to occur on alkenyl or alkynyl halides. So if you see one of the substrates below, it is highly likely that no reaction will occur. 

sn1 sn2 e1 e2 can never occur with vinyl or aryl or alkynyl substrates no reaction

Why are alkenyl and alkynyl halides so bad? Well, the SN1, SN2, and E1 mechanisms all involve considerable build-up of positive charge on the carbon bearing the leaving group, and the stability of sp2 and sp hybridized carbocations is much lower than that for sp3 hybridized carbocations [for the same reason that sp and sp2 anions are more stable than sp3 carbanions!].

E2 reactions are also more difficult due to the stronger C-H bonds of alkenes.

[We’ll see later that there is one example of an E2 that can occur on alkenyl halides [See post: Double Elimination of Dihalides To Give Alkynes] , but the point remains that they are very rare!]

Note 2.  One question that comes up a lot is this: are there exceptions? Keeping in mind the two themes of “steric hindrance” and “carbocation stability”, there are edge cases where we can have a particularly sterically hindered primary alkyl halide, or a particularly stable primary carbocation.

For instance, the alkyl halide below (“neopentyl bromide”) is indeed primary, but is so crowded on the carbon adjacent to the primary alkyl halide that it is essentially inert in SN2 reactions. On the SN1/E1 side, the allyl halide below, while primary, can undergo SN1/E1 reactions because the resulting carbocation is stabilized through resonance. As long as you keep in mind the “big barriers” for each reaction, you should be fine.

one primary alkyl halide that does not undergo sn2 very well is neopentyl bromide


Struggling with SN1/SN2/E1/E2?  Our Org 1 Summary Sheets (PDF) contain a full-page flowchart on deciding SN1/SN2/E1/E2, as well as two more pages summarizing substitution and elimination reactions, in addition to many other Org 1 topics. 

Check them out now! 



Comment section

42 thoughts on “Deciding SN1/SN2/E1/E2 (1) – The Substrate

  1. How can you tell when a compound is stabilized by resonance, and it only important in Sn1 or E1 reaction mechanisms and not the other two?

    1. The compound is stabilized by resonance if the formation of resonance structures is possible on the carbocation. This only matters in SN1 and E1 because those are the only two reactions where carbocations are formed. For example look at the isoprenyl bromide he has drawn above. If you remove the Br leaving group you’re left with a positive charge on the carbon it was previously attached to. If you move the double bond over to the right, making a resonance structure, the positive charge is now on a tertiary carbon, making it a more stable carbocation. If the reaction is SN2 or E2 don’t worry about resonance because its all happening in one step and no carbocation is formed.

    1. Order of carboctaion stability is Triphenylcarbocation > tropilium > biphenylcarboction > cylopropenylcarb > 3°C > phenylcarb > Allylcarb > 2° > 1° > methylcarb > doublebondedcarb > benzylcarb > tripplebondedcarb generally prensence of phenyl, branching & saturation increases stability of a carbocation which is a planar sp2 charged species

    1. I believe carbocation rearrangement can be considered, but the reaction is going to prefer SN2/E2 over rearrangement + SN1/E1. If I am mistaken, please correct me

      1. In “real life” SN2 pathways are more favourable but textbook/exam questions do not always represent real life. Often cases with secondary alkyl halides are opportunities for instructors to test rearrangements, etc. That’s why it’s important to be aware of these types of situations. You’re right though – it can be instructor dependent.

  2. hi , is it possible for SN2 to occur at a primary alcohol ? i know that a primary carbocation is unstable , but what if a methyl/hydride shift is possible ? and it’ll form a secondary carbocation which is more stable than a primary carbocation ?

    1. Yes, you’d have to protonate the alcohol to give the conjugate acid, and then the leaving group would be water. One example is the brute force formation of diethyl ether from heating ethanol with acid.

  3. I dont understand something, and nowhere does anyone bother to explain it.
    when there is a primary carbocation, and it can go through rearrangement why won’t it? and then you cant rule out Sn1.
    like the example of cyclohexane ring, to which is connected CH2Cl it would be primary at first but with rearrangement it can become a teritary which is very stable….so what would happen?

    1. Mechanisms involving carboction formation will always prefer more stable carbocation which can be formed by rearrangement via hydride shift, methyl shift or ring expansion except in HydroborationOxidation rxn and Oxymercuration Demercuration rxn where carboction cant undergo any kind of rearrangement

  4. Could you explain *why* SN2 is favored over E2 for primary alkyl halides? I always thought acid-base reactions were fastest…

    Thank you for an incredibly valuable resource!

    1. Acid-base reactions on *heteroatoms* are fast, but that rule of thumb doesn’t really apply to the E2 reaction, where you’re breaking a C-H bond with accompanying change of hybridization on the carbon…

  5. Is there a chance that a primary alkyl halide undergoes a SN1 reaction? Yesterday at uni I was given this example: Hydrolisis of (Bromomethyl)cyclopentane.
    First the primary carbocation was formed and later a hydride shift occured to form a tertiary carbocation. Is it possible? My concern is:would the first primary carbocation intermediate form even if it’s later on rapidly stabilized? But I also think about allylic AHs being primary and first the primary carbocation being formed and rapidly stabilized.. Thanks in advance

    1. Primary allylic, yes. Primary alkyl halide, formation of the primary carbocation as an intermediate is unlikely. It’s possible to have concerted rearrangement reactions where migration of a neighboring group accompanies displacement of a leaving group, which is generally how these types of things happen.

  6. Hi James, at the bottom of your post you wrote “For instance, the alkyl halide below (“neopentyl chloride”) is indeed primary…” but the image shows neopentyl bromide. Just FYI.

  7. what happens if there is one factor that favors Sn1 but another factor that favors Sn2.

    Like what if it’s a primary substrate with a weak nucleophile, or a tertiary substrate in a polar aprotic solvent?

  8. In the note section, can it also be explained by partial double bond character on the halogen in vinyl and aryl halide?

  9. Sir what product should I expect on treating ethylbromide with t butoxide. By the way great job on creating such a fabulous blog

  10. hi, i was just wondering if a compound like (R) 2-bromopropan-1-ol was to undergo a substitution reaction with NaCN, and it resulted in the (R) configuration of the product with the bromine being substituted by the cyanide, which reaction mechanism does it do it by? SN1 or SN2?
    can the compound undergo frontside attack by any mechanism because it is a secondary compound?

  11. ok so i have a question about what if in differentiating either an SN1 or an SN2 reaction lets say the substrate is a 6 carbon single chain on the 2nd carbon with a wedge of H and ladder on that same carbon with a Cl and a attacking nucleophile of Br-. how can you tell that this is an SN2?

    For an Sn1 imagine a 5 carbon chain with carbon 2 has wedge of Br and ladder of H with nucleophile of Cl- how can you tell if its a SN1 reaction. like that what I’m having problem with. please let me know asap i have my exam on Thursday!

    1. Here would be the thought process:
      1) Secondary carbon. Can’t rule anything out at this stage.
      2) Identity of nucleophile/base: Br(-). Good nucleophile (negatively charged), weak base. Rule out E2 because E2 needs a strong base. Rule out SN1/E1 because they operate with weak (generally neutral) bases. That leaves SN2.
      3) So draw product of SN2 with inversion of configuration.

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