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identifying Sn1,Sn2,E1,E2

Discussion in 'DAT Discussions' started by wkawk2416, Jun 18, 2008.

  1. wkawk2416

    wkawk2416 7+ Year Member

    Mar 14, 2007
    does anyone have a good method to identify those.

    i am having serious problems with those

    please help me out!
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    DRHOYA 2+ Year Member

    May 15, 2008
    Well, SN1 and E1 are going to involve Carbocations. SN1 and E1 are also more stable tertiary > secondary > primary. If double bonds form, its indication of an elimination reaction. SN2 is more stable for primary > secondary > tertiary. Also, E2 favors big bulky bases. Also, SN1 likes polar protic (H-bonds) solvents, and SN2 likes polar aprotic solvents. Those are some quick facts off the top of my head. Hope they help a little.:thumbup:

    Good practice is just repitition...........doing enough of these rxn questions.
  4. osimsDDS

    osimsDDS 5+ Year Member

    Jun 8, 2007
    arent E1/E2 always with strong base and heat....

    Sn1: protic solvents to neutralize the carbocation formed, good leaving group, nucleophile strength doesnt matter for Sn1.... prefers tertiary, secondary, primary, methyl

    Sn2: polar aprotic solvents so it doesnt react with nucleophile in solution, good leaving group, strong nucleophile because the rate is determined by [nucleophile] and [reactant]... prefers methyl, primary, secondary, tertiary...
  5. harrygt

    harrygt 2+ Year Member

    Jun 16, 2008
    Nice answer
  6. DCRedskinsRule

    DCRedskinsRule 7+ Year Member

    May 27, 2008
    yea I don't remember the exact reason behind the rule, but I think that you generally cannot form a carbocation in the presence of a strong base...but i could be wrong
  7. PreDent2009

    PreDent2009 5+ Year Member

    Dec 20, 2007
    E2/SN2- these reactions are always in competition with eachother. When you have a strong base and a reactant with a leaving group, you have to decide whether a substitution or elimination will preceed. To do this you alawys look at the E2 product first. If it is a stable alkene, with 3 or more C-C bonds around the double bond, then the major product is E2. If there is 2 C-C bonds around the double bond, you will get a mixture of sub and elim. In order to drive the reaction one way or the other you will heat it to get E2, or react at low temp to force SN2. You can force E2 with heat and concentrated base, even if the elimination product is weak.

    SN1- can either be a weak base with a leaving group, or it can be something like HBR, HCl, etc reacting with an alcohol. Always preceeds through a flat carbocation intermediate.
    E1-you will only do a E1 reaction when reacting a strong acid like sulfuric acid or phosphoric acid with an alcohol.

    Hope this helps. Let me know if you have any other questions. Also, you should prob know what solvent systems work best for each of these mechanisms.

  8. wkawk2416

    wkawk2416 7+ Year Member

    Mar 14, 2007
    thank all for replies,

    but strong base and heat are required to E1 to occur?

    is this true?
  9. harrygt

    harrygt 2+ Year Member

    Jun 16, 2008
    This is NOT true. Those are the conditions for E2, and not E1. E1 has the same condition that SN1 has, which is: a weak base like an alcohol, or CL- or Br- ions. No heat is needed.
    Whenever you see KOH, K t-butok, and heat in the reaction, that means the reaction is screaming, "I'M E2".
  10. Touchwood

    Touchwood Touchwood

    Jul 16, 2008
    I get the SN1 and Sn2 thing but can anyone explain why in SN1 the carbocation forms in the first place? What drives it?

  11. Thundercatz

    Thundercatz 7+ Year Member

    Jun 1, 2008
    For something to be Sn1, it usually has to be tertiary. So when the leaving group leaves, a tertiary carbocation is formed. A tertiary cabocation is stablized by the tertiary formation as usually the carbon substitutes stabilizes the carbocation intermediate by inductive effects.

    If a primary or 2nd carbon were to forum, it is less stable. Remember that radicals and carbocations stability is in the order of: 3>2>1.
  12. flycd05

    flycd05 5+ Year Member

    May 5, 2008
    also, dont forget that with a bulky base, u use E2 and that favors the least subustituted (mostly monosubstitued) product
  13. razzy719


    Mar 2, 2008
    What are some good resources where I can get some practice in with these type of questions? Thanks.
  14. condoleezarice

    condoleezarice 2+ Year Member

    Jul 9, 2008
    This is how I break it down. It's quite easy to find out which reaction to use; the only tricky situation is for secondary substrates. First see if the substrate is primary, secondary or tertiary.

    If it is primary...
    It will always be an SN2 reaction, unless the base is bulky. Bulky base= E2. No carbocations here because primary is unstable.

    If it is secondary...
    These are the toughest to figure out. If nucleophile is weak and is a weak base, it will be both SN1 and E1. Strong base, weaker nucleophile goes E2 (such as a hindered base). Strong nucleophile, weak base goes SN2(such as Iodide). Strong nucleophile and strong base will get a mess of elimination and substitution products.

    If it is tertiary...
    There is no SN2 on tertiary substrates. E2 if the base is strong, Sn1/E1 for everything else.

    *sidenote: E1 and Sn1 will almost always accompany each other. Add heat if you want more elimination, however.

    i hope this helps.
  15. herkulis

    herkulis 7+ Year Member

    May 16, 2008
    So you get combination of substitution and elimination products with 1) weak acid and weak base OR 2) strong acid and strong base?
  16. condoleezarice

    condoleezarice 2+ Year Member

    Jul 9, 2008
    I don't quite understand your question. Do you mean weak nucleophile/weak base or strong nucleophile/strong base?

    To answer that question...
    weak nucleophile/weak base will perform a E1/SN1 attack on a secondary substrate. This is because a secondary carbocation will form before a weak base/nucleophile can attack.

    strong nucelophile/strong base (this one I'm not totally sure) will lead to SN2 and E2, but mainly SN2 attack on secondary substrate. To differentiate: high temperature promotes E2.
    Last edited: Jul 18, 2008
  17. kush87

    kush87 2+ Year Member

    Jul 12, 2008

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