** official Sn1/Sn2 vs E1/E2 clarification thread**

[Tina324]

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I am having big probs identifying these 4 mechanisms and would be super grateful if we could combine a list of characteristics and post weird substitution/elimination probs on this thread =)

This one is from Achiever:

(CH3)2CHCl + CH3O- --> ??
temp is 55 celsius
solvent=methanol

which reaction mechanism predominates?: Sn1,Sn2,E1, or E1

So here's how I narrow it down:
1. 2ndary carbon (so hmm can be Sn1,Sn2,E1, or E1 still)
2. solvent=polar protic (so Sn1 or E1)
3. temp is high...so E1

But...I am wrong =( the answer is E2! (because -OCH3 is a strong base grr)
so my logic goes out the window??

Can someone Paaalease explain this and help me make a clear distinction between all of these.

thank u
...sorry a little bummed, just took my 2nd achiever exam =*(

 

[Thundercatz]

I am having big probs identifying these 4 mechanisms and would be super grateful if we could combine a list of characteristics and post weird substitution/elimination probs on this thread =)

This one is from Achiever:

(CH3)2CHCl + CH3O- --> ??
temp is 55 celsius
solvent=methanol

which reaction mechanism predominates?: Sn1,Sn2,E1, or E1

So here's how I narrow it down:
1. 2ndary carbon (so hmm can be Sn1,Sn2,E1, or E1 still)
2. solvent=polar protic (so Sn1 or E1)
3. temp is high...so E1

But...I am wrong =( the answer is E2! (because -OCH3 is a strong base grr)
so my logic goes out the window??

Can someone Paaalease explain this and help me make a clear distinction between all of these.

thank u
...sorry a little bummed, just took my 2nd achiever exam =*(

Most of your reasoning is correct. Temp does play a role in determining E1 or E2, but in this case, it is a dead give away that its an E2. Firstly, like you mention, it is secondary. That alone should lead you to believe its an E2. Remember, that an E1 reaction will form a carbocation and it will prefer a tertiary carbocation.

 

[Tina324]

Woahh i always thought that 2ndary could be either bimolecular or unimolecular

 

[Mamona]

Woahh i always thought that 2ndary could be either bimolecular or unimolecular

The same thing here, now I am Officially confuse!!!!

 

[Thundercatz]

Woahh i always thought that 2ndary could be either bimolecular or unimolecular

Its not always E2 but it usually will be. With the given conditions of a strong base, polar solvent, and secondary carbon, its no doubt an E2 elimination. You have to ask yourself, which is more likely given the conditions.

 

[artanis]

Its not always E2 but it usually will be. With the given conditions of a strong base, polar solvent, and secondary carbon, its no doubt an E2 elimination. You have to ask yourself, which is more likely given the conditions.

Correct. With the conditions given, it is definately E2. They try to trick you with the "polar protic solvent", but actually, if there is a strong base present, -OCH3 in its usualy solvent CH3OH, the strong base takes precedence.

 

[doc3232]

Ya, that isn't really an acidic solution...It is secondary though, so it messes everything up and makes everything hard.

 

[Doctor PJ]

good post. i was going to ask about this myself. Another question though from what I've seen, lets say we have the CH3O-, vs a tert-butyl group, the big bulky group will always be E2 correct? I actually chose E1 for this question on achiever since it was a strong base but not bulky.

 

[doc3232]

good post. i was going to ask about this myself. Another question though from what I've seen, lets say we have the CH3O-, vs a tert-butyl group, the big bulky group will always be E2 correct? I actually chose E1 for this question on achiever since it was a strong base but not bulky.

Big bulky base makes it only Elimination for sure.

 

[soopasteve]

what I have found to work when differentiating between E1 and E2 is simply looking at how powerful the base is. A week base is used to force an E1 while a strong base will react much quicker than the ionization step which forces an E2 reaction. Your reasoning is correct in thinking E1 since the protic solvent will allow the Cl to ionize and break away, the only issue is that the methoxide ion is very strong base (it has a negative charge and the molecule is very unhindered).

 

[soopasteve]

good post. i was going to ask about this myself. Another question though from what I've seen, lets say we have the CH3O-, vs a tert-butyl group, the big bulky group will always be E2 correct? I actually chose E1 for this question on achiever since it was a strong base but not bulky.

a bulky base only ensures that it is elimination and not substitution. the bulkyness of the base hinders its speed in stealing a hydrogen and can lead to E1 over E2 due to the leaving group's ability to ionize quicker than the bases ability to attack.

 

[Deontologist]

Hope the attachment helps....!!

 

[Mamona]

Deontologist thank you for you table is very nice and helpful. But I have a question, still a little bit confuse about this. I am reading my notes and I was very happy understanding all the difference between SN1, SN2, E1, and E2, until I got to a note from bb kaplan that says: "A strong base favors E2 over SN2" perfect it makes sense, it's the base of this treat, but it continues and says " SN2 is favored over E2 by weak lewis bases (strong nucleophiles)". Is this a mistake or I don't understand this concept yet, I though it will be a weak nucleophile (weak lewis base) will favor an SN2 rxn.

 

[Mamona]

Anyone 🙄

 

[kush87]

Deontologist thank you for you table is very nice and helpful. But I have a question, still a little bit confuse about this. I am reading my notes and I was very happy understanding all the difference between SN1, SN2, E1, and E2, until I got to a note from bb kaplan that says: "A strong base favors E2 over SN2" perfect it makes sense, it's the base of this treat, but it continues and says " SN2 is favored over E2 by weak lewis bases (strong nucleophiles)". Is this a mistake or I don't understand this concept yet, I though it will be a weak nucleophile (weak lewis base) will favor an SN2 rxn.

Lewis base = electron pair donor
weak lewis base = weak electron donor (likes the electrons for it self...doesnt want to give them away that easily) for this reason they will be more negative than the strong lewis base

strong nucleophile= positive charge seeking ( they are them self negative and want to balance that out)

For this reason WEAK lewis base act like strong nu- than STRONG lewis base

i hope i didnt confuse you more....correct me if i am wrong guys

 

[Electrons]

http://www.brynmawr.edu/Acads/Chem/mnerzsto/SN2vsE2_flowchart.htm

Is this one correct?

 

[MattEvers]

The first thing my professor said when talking about carbocations and strong bases: they are not found together. Strong base ---> no SN1 or E1

Furthermore, the use of a strong base usually points to E2, especially if 2ndary. If that "proton acceptor" has the choice between an open proton and a hindered carbon atom, it's going to choose the proton.

I highly recommend for those who have the money to spend and need clarification about organic to buy the Organic Odyssey (same Dr. who created destroyer). It is an excellent resource; probably a bit of overkill for the DAT, but you won't have any problems with basic reactions after you get through with it. The substitutions and eliminations chapters worked wonders for me clearing up the SN2/E2 and SN1/E1 reactions.

 

[Mamona]

Lewis base = electron pair donor
weak lewis base = weak electron donor (likes the electrons for it self...doesnt want to give them away that easily) for this reason they will be more negative than the strong lewis base

strong nucleophile= positive charge seeking ( they are them self negative and want to balance that out)

For this reason WEAK lewis base act like strong nu- than STRONG lewis base

i hope i didnt confuse you more....correct me if i am wrong guys

Thank you very much I think I got it now, weak lewis bases strong nucleophiles and strong lewis bases weak nucleophiles???? I hope I'm right...

 

[pidbull]

I thought that Nucleophile and Base are the same thing...?

 

[Mamona]

I thought that Nucleophile and Base are the same thing...?

I though the same thing, but there is more into it read what kush87 wrote and I think it makes sense

 

[pidbull]

I though the same thing, but there is more into it read what kush87 wrote and I think it makes sense

ah ok thanks...it sort of makes sense, i have to go over my general chemistry now. I saw your post in the other thread, I am also having problems identifying strong bases from weak bases. I think we are both missing a basic concept, let me know if you figure it out (vice versa) or hopefully someone can help us here.