Elimination Reactions

By James Ashenhurst

E1 vs E2: Comparing the E1 and E2 Reactions

Last updated: November 5th, 2020 |

E1 versus E2 : Comparing The E1 and E2 Reactions

Now that we’ve gone through the mechanisms of the E1 and E2 reactions, let’s take a moment to look at them side by side and compare them.

Table of Contents

  1. Comparing The Mechanism Of The E1 and E2 Reactions
  2. What Do The E1 and E2 Reactions Have In Common?
  3. How Are The E1 and E2 Reactions Different?
  4. E1 vs E2: Why Does One Elimination Give The “Zaitsev” Product, And The Other Elimination Does Not?
  5. The Key Requirements Of Stereochemistry In The E2 Reaction
  6. (Advanced) References and Further Reading

1. Comparing The Mechanism Of The E1 and E2 Reactions

Here’s how each of them work:

comparing e1 vs e2 mechanism e1 is two step e2 is one step

2. What Do The E1 and E2 Reactions Have In Common?

Here’s what each of these two reactions has in common:

  • in both cases, we form a new C-C π bond, and break a C-H bond and a C–(leaving group) bond
  • in both reactions, a species acts as a base to remove a proton, forming the new π bond
  • both reactions follow Zaitsev’s rule (where possible)
  • both reactions are favored by heat.

3. How Are The E1 and E2 Reactions Different?

Now, let’s also look at how these two mechanisms are different. Let’s look at this handy dandy chart:

table comparing e1 vs e2 reactions rate law big barrier strong base stereochemistry

The rate of the E1 reaction depends only on the substrate, since the rate limiting step is the formation of a carbocation. Hence, the more stable that carbocation is, the faster the reaction will be. Forming the carbocation is the “slow step”; a strong base is not required to form the alkene, since there is no leaving group that will need to be displaced (more on that in a second). Finally there is no requirement for the stereochemistry of the starting material; the hydrogen can be at any orientation to the leaving group in the starting material [although we’ll see in a sec that we do require that the C-H bond be able to rotate so that it’s in the same plane as the empty p orbital on the carbocation when the new π bond is formed].

The rate of the E2 reaction depends on both substrate and base, since the rate-determining step is bimolecular (concerted). A strong base is generally required, one that will allow for displacement of a polar leaving group. The stereochemistry of the hydrogen to be removed must be anti to that of the leaving group; the pair of electrons from the breaking C-H bond donate into the antibonding orbital of the C-(leaving group) bond, leading to its loss as a leaving group.

4. E1 vs E2: Why Does One Elimination Give The “Zaitsev” Product, And The Other Elimination Does Not? 

Now we’re in a position to answer a puzzle that came up when we first looked at elimination reactions. Remember this reaction – where one elimination gave the “Zaitsev” product, whereas the other one did not. Can you see why now?

e2 sometimes in rings does not follow zaitsev rule because no proton anti to leaving group

5. The Key Requirements Of Stereochemistry In The E2 Reaction

So what’s going on here?

  • The first case is an E2 reaction. The leaving group must be anti to the hydrogen that is removed.

e2 reaction on cyclohexane ring where deprotonation can only give anti zaitsev product

  • The second case is an E1 reaction.

e1 reaction on cyclohexyl ring will always follow zaitsev rule alignment of h with p orbital

  • In our cyclohexane ring here, the hydrogen has to be axial. That’s the only way we can form a π bond between these two carbons; we need the p orbital of the carbocation to line up with the pair of electrons from the C-H bond that we’re breaking in the deprotonation step. We can always do a ring flip to make this H axial, so we can form the Zaitsev product.
  • Here’s that deprotonation step:

e1 deprotonation and formation of double bond zaitsev in cyclohexane ring

As you can see, cyclohexane rings can cause some interesting complications with elimination reactions! In the next post we’ll take a detour and talk specifically about E2 reactions in cyclohexane rings.

Next Post: Elimination Reactions and Cyclohexane Rings


(Advanced) References and Further Reading

  1. Mechanism of elimination reactions. Part XI. Kinetics of olefin elimination from tert.-butyl and tert.-amyl bromides in acidic and alkaline alcoholic media
    M. L. Dhar, E. D. Hughes, and C. K. Ingold
    J. Chem. Soc. 1948, 2065-2072
    DOI:
    10.1039/JR9480002065
    The E1 reaction is not very useful synthetically for olefin synthesis, because the ratio of elimination to substitution products is substantially lower than in the E2 reaction. For example, solvolysis of t-butyl bromide in dry ethanol only yields 19% isobutylene, whereas 93% yield of the alkene is obtained with 2M ethoxide.
  2. Mechanisms of elimination reactions. XIII. Effect of base, solvent, and structure on product ratios in elimination reactions of some secondary tosylates
    Irving N. Feit and William H. Saunders
    Journal of the American Chemical Society 1970, 92 (6), 1630-1634
    DOI:
    1021/ja00709a035
    Table I in this paper shows that olefin yields under E1 conditions are lower than under E2 conditions. E2 conditions can be promoted by using a strong base, and base strength for the bases employed increases in the order n-butoxide < sec-butoxide < t-butoxide. The effect of reaction conditions on product stereochemistry (trans/cis olefin) is also investigated here.
  3. Mechanisms of Elimination Reactions. V. Sulfur Isotope Effects in Some Reactions of t-Butyldimethylsulfonium Iodide
    William H. Saunders and Stuart E. Zimmerman
    Journal of the American Chemical Society 1964, 86 (18), 3789-3791
    DOI: 1021/ja01072a038
    This paper features an example of a competition between E2 and E1 that strongly favors E2 when strong base present. In both cases, the reaction rate increases with increasing temperature.
  4. The mechanism and kinetics of elimination reactions
    D. Hughes and C. K. Ingold
    Trans. Faraday Soc. 1941, 37,657-685
    DOI: 10.1039/TF9413700657
    A review on early investigations of E1 and E2 reactions by Hughes and Ingold, who came up with the terms “E1, E2, SN1, SN2” – these are now called Hughes-Ingold symbols. This review also summarizes the conditions favoring E1/E2 reactions, which are taught to undergraduates the world over every year.
  5. Eliminations in Cyclic cis‐trans‐Isomers
    Dr. W. Hückel and Priv.‐Doz. Dr. M. Hanack
    Angew. Chem. Int. Ed. 1967, 6 (6), 534-544
    DOI:
    10.1002/anie.196705341
    Very interesting study comparing the rates of E1 and E2 reactions between cis and trans isomers in a cyclic system. Where E1 and E2 compete, the paper states: “In order that the E2 reaction may be favored as strongly as possible in relation to the El reaction, the alkoxide concentration must be high and the alkyl group of the alkoxide must be as large and as highly branched as possible. Thus for E2 reactions, the order of preference is methanol < ethanol < isoamyl alcohol ≈ isopropyl alcohol < t- butanol < 2-n-butylcyclohexanol.”

Comments

Comment section

17 thoughts on “E1 vs E2: Comparing the E1 and E2 Reactions

  1. “The rate of the E2 reaction depends on both substrate and base, since the rate-determining step is unimolecular (concerted).” Shouldn’t it be “bimolecular,” as in the table?

    1. Reactions taking place in single step via a high energetic transition state are usually referred as concerted,,,,where as unimolecular refferes to molecularity of the reaction,i.e.no. of molecules involved in rate limiting step

  2. Thank you for the helpful info. I had one question… do E1 reactions able to make cis and trans products like the ones that are common to E2 reactions?

    Like for example the 4-bromo-4-methylheptane according to my answer book says that it forms 3 products, 2 of which seem like a cis and trans but they don’t specify that, unlike in the answers for some of the E2 reactions in my book.

    And to clarify things E2 are the only ones that are truly regioselective due to the fact that they don’t have a carbocation intermediate step in which they hydrogen can be flipped to the right axial position?

  3. Hello James,
    I have been reading your articles for quite a while now. They are easy to comprehend and to-the-point. Do you have an article on E1cb reactions and it’s mechanism?

  4. Is isotopic effect is observed in E-1 Reaction ? as carbocation formation is the RDS in E-1 so the bond cleavage between C-H doesn’t have any impact on RDS , but can we say the reaction intermediate stability is affected by the isotopic effect ? as we know C-H bond can easily breaks as compared to C-D. http://bit.ly/2LFQNvn

    1. There is no primary isotope effect in the E1. The rate of reaction is the same whether the beta carbon has a C-H or a C-D. In the E2 reaction a typical value for kH/KD is from 2-8.
      In the E1 there is a significant difference in rates as the identity leaving group is changed.

  5. This is one of the best organic chem short notes website offering such amazing content I have ever seen, I’m preparing JEE and this is super handy! I really would love to appreciate and thank you for sharing this!!

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