How to determine substitution or elimination besides heat?

[tooth_hurty]

If a rxn uses a reagent that may act as a strong base or nucleophile (ex. NaOCH3), a good leaving group is tertiary and heat is NOT shown in the rxn, how can you determine if it will prefer E1 or SN1? I've seen cases where heat is not shown but elimination is still the major product - and even some cases where heat is shown and substitution is preferred...so what else can be looked at to determine if it will prefer elimination or substitution? Thanks in advance

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[w4nt2baDDS]

I don't think E1 or SN1 require a anti periplanar beta hydrogen, that is for (E2)

E1 and SN1 reactions are never exclusive reactions and both will happen at the same time (at least that is what I was taught). Still, there is typically a major product and I think the stability of the resulting compounds is a key factor to consider. If the elimination reaction creates a conjugated system or puts a bulky structure is more suitable trans position then that could be advantages for E1 to occur. SN1 reactions often add -OH or -OR to a compound which could inductively improve or disable the resulting compounds stability. For instance, the oxygen can make the carbon its bound to more partial positive and if that carbon is attached to a carbonyl then you would have two partially positive carbons next to one another which wouldn't be ideal (but still not really all that bad...).

I'm just shooting from the hip and hope I'm not throwing anybody off, these are just a few things I try to consider.

 

[DAT Destroyer]

I will assume you are talking about a six membered ring. The anti-periplanar conformation is only required in the E2 process. If the halide is treated with a strong base such as NaOCH3 or NaOC2H5......this is E2.....and both halogen and hydrogens must depart in axial position...ie. antiperiplanar. Having groups in this position allows for better orbital overlap. If the base is weaker or diluted,,,,,,the E1 reaction can occur,,,,,or perhaps the SN1...and we dont need the anti-periplanar departion since a carbocation is formed. Heat will indeed favor the elimination process. Tertiary halides also love to do elimination. Take these rules...and look at a text book or two to convince you of these trends.

Hope this helps

Dr. Jim Romano

 

[tooth_hurty]

I will assume you are talking about a six membered ring. The anti-periplanar conformation is only required in the E2 process. If the halide is treated with a strong base such as NaOCH3 or NaOC2H5......this is E2.....and both halogen and hydrogens must depart in axial position...ie. antiperiplanar. Having groups in this position allows for better orbital overlap. If the base is weaker or diluted,,,,,,the E1 reaction can occur,,,,,or perhaps the SN1...and we dont need the anti-periplanar departion since a carbocation is formed. Heat will indeed favor the elimination process. Tertiary halides also love to do elimination. Take these rules...and look at a text book or two to convince you of these trends.

Hope this helps

Dr. Jim Romano

That clears things up. In my question, I meant to ask how to determine between between E2 and SN1 if heat is not shown. But now I see that if a good leaving group is tertiary and a hydrogen is antiperiplanar, a strong base will do E2 and not SN1 as SN1 I believe only occurs with weak or moderate nucleophiles...but if I'm mistaken with any of this please let me know. Thank you for the help!

 

[DAT Destroyer]

That clears things up. In my question, I meant to ask how to determine between between E2 and SN1 if heat is not shown. But now I see that if a good leaving group is tertiary and a hydrogen is antiperiplanar, a strong base will do E2 and not SN1 as SN1 I believe only occurs with weak or moderate nucleophiles...but if I'm mistaken with any of this please let me know. Thank you for the help!

Yes.....You have it correct...now work hard !

Dr. Romano