SN1 vs. SN2 vs. E1 vs. E2

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heyitscyndi

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What is the easiest way to remember how to determine if a reaction is SN1, SN2, E1, or E2? Here's what I have from class, but where do E1 rxns fit? How do you distinguish between SN1 and E1?


Alkyl halide 1 2 3
Nucleophile

Strong SN2 E2 (mostly) E2
SN2

Weak No rxn SN1 SN1


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SN1 and E1 have the same conditions (highly polar solvents, stable carbon (Tertiary > secondary > primary), good leaving groups, and weak nucleophiles in low concentration)

Since E1 is a elimination reaction, usually the reaction has to be accompanied by heat. I think this is the only difference.

For primary:
- SN2 and E2 occurs, but E2 requires a strong base since it removes the proton. SN2 also requires a strong nucleophile.

For secondary:
- E2 mostly occurs and SN2 (few occurs) because SN2 require nucleophilic attack and will be hindered by steric hindrance.
- SN1 and E1 also occurs, but they are very slow because they form carbocation intermediates which requires stability (3 > 2 > 1)

For tertiary:
- SN1 and E1 occurs (weak nucleophile)
- E2 occurs (Require Strong base)

I hope this helps.
 
Comet208 said:
Thats E2, as you said, but SN2 needs a weak lewis base.


One other note... E1 and Sn1 ALWAYS happen together and it is very difficult to tell the difference between the two. My gess is you won't see a q that says E1 or Sn1.
 
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look at the reagent first. It seems intuitive to decide between SN and E first, but it's not a good idea. Start by deciding between 1 step and 2 step and two step mechanisms.

If it's a strong base, start thinking bi-molecular. If you have something that really wants to attack, then there won't be time for carbocation formation. Also note the size of the base. Is it bulky like t-butoxide or small like Ethyl oxide? Bulky is elimination.

Next look at the substrate. If you have methyl or primary and a good leaving group, it's SN. If it's tertiary and If you can get an anti orientation from one of the Beta Hydrogens, go with E2, especially at high temp.

If you have a weak reagent, then it's going to sit there and chill until something is really attractive like a positive charge draws it to attack. If it's a tertiary substrate, look for heat to be the giveaway for elimination.

Also with reagents, some can be nucleophiles but not bases. Know which ones these are.
 
smildoc2 said:
look at the reagent first. It seems intuitive to decide between SN and E first, but it's not a good idea. Start by deciding between 1 step and 2 step and two step mechanisms.

If it's a strong base, start thinking bi-molecular. If you have something that really wants to attack, then there won't be time for carbocation formation. Also note the size of the base. Is it bulky like t-butoxide or small like Ethyl oxide? Bulky is elimination.

Next look at the substrate. If you have methyl or primary and a good leaving group, it's SN. If it's tertiary and If you can get an anti orientation from one of the Beta Hydrogens, go with E2, especially at high temp.

If you have a weak reagent, then it's going to sit there and chill until something is really attractive like a positive charge draws it to attack. If it's a tertiary substrate, look for heat to be the giveaway for elimination.

Also with reagents, some can be nucleophiles but not bases. Know which ones these are.


Is it general chemistry or organic chemistry question?
 
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