Sn1, e1, sn2, e2

[JP2740]

Ok I get when to use SN1 vs SN2 and E2 vs E1, but looping all 4 together, which mechanism takes place in every circumstance w/ all competing?

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[homestar]

I agree with that statement. My gut tells me that they wouldn't ask something like that directly: What would be the major product of this reaction? I don't believe that AAMC is out to deceive you, and that would be a tricky question indeed. When substitution and elimination are directly competing, they are yielding the products in different proportions based on multiple variables: substrate, nucleophile, leaving group, sterics, solvent, base strength, etc. You can't know those proportions without experimental data.

I think that I saw a question in Kaplan MCAT Advanced that described a reaction that yielded multiple products, but they provided the reaction for you.

As an addendum, I do actually remember seeing a blurb in Kaplan Organic Chemistry that stated that competition between SN1 v. E1 cannot be controlled well, but competition between SN2 v. E2 can be somewhat directed.

That's my guess.

 

[MDub1]

It's not always easy to tell, and in most cases it's impossible to tell on Sn1 vs. E1 but it is possible to tell with Sn2 vs. E2. The key is to remember that when a strong nucleophile/base is present and the group is a secondary or tertiary carbon then E2 will occur because the it's too hindered for direct attack as in Sn2. If the group is primary, then Sn2 will likely predominate because the rate of Sn2 reactions increases with a decreasing number of substituents and the rate of E2 reactions decreases with a decreasing number of substituents. Make sure to look at the base/nucleophile as well. If it is strongly basic but only weakly nucleophilic (because it's too bulky, like tert-butoxide) then it will do E2.

Hope this helps a bit, good luck!

 

[JP2740]

Ok but what about E2 vs SN2 vs SN1 vs E1 all 4 in every situation. Is this a little too much for the MCAT? Like you compared E2 vs SN2, but SN1 and E1 could compete too in some situations?

 

[MDub1]

If there's ever a strong base/nucleophile, will do one of E2 or Sn2. If it's a weak base/nucleophile, it will do both E1 and Sn1.

 

[RogueUnicorn]

Ok but what about E2 vs SN2 vs SN1 vs E1 all 4 in every situation. Is this a little too much for the MCAT? Like you compared E2 vs SN2, but SN1 and E1 could compete too in some situations?

it should be completely possible.. generally speaking, you can rule out E or S based on your product, which is often given to you, and you're asked to identify the reaction pathway. once this has been narrowed down, it's relatively simple to compare whether it's uni (1) or bi(2)molecular based on the reaction condtions. questiosn to ask yourself:

1) a) polar solvent? b) protic solvent?
2) sterics (bulkiness) of the nucleophile?

there are others, but these two are the main ones that will guide you.

 

[MLT2MT2DO]

it should be completely possible.. generally speaking, you can rule out E or S based on your product, which is often given to you, and you're asked to identify the reaction pathway. once this has been narrowed down, it's relatively simple to compare whether it's uni (1) or bi(2)molecular based on the reaction condtions. questiosn to ask yourself:

1) a) polar solvent? b) protic solvent?
2) sterics (bulkiness) of the nucleophile?

there are others, but these two are the main ones that will guide you.

Also if the temperature of the reaction is given, this is a dead give away for E vs S

 

[JP2740]

Does high temperature go w/ e or s?

 

[MLT2MT2DO]

High temperature favors elimination reactions