O'Chem help: E1 E2 Zaitsev Rule

[bballrules]

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Hi guys. I was wondering if you could help me clarify when which Elimination rxn is used between E1 and E2. I'm confused on which goes to the more stable, less stable molecule. Also, I don't understand when strong bases or weak bases are used and how bulky bases play a role in this one. I've been reading the Destroyer problems and it seems like all the problems are E2. Some go to the more stable, some go to the less stable, so I was wondering if anyone can explain the differences and when each one is used. Thanks a lot.

 

[klutzy1987]

A reaction will undergo E2 when one of the strong bases mebtioned in the destroyer somewjere are used. These bases include small unhindered ones such as CH3O- and CH3CH2O-. Bases like these will make the double bond on the inside because they are small and are able to get into the molecule. When the double bond forms on the inside(the more substituted alkene) it is known as the Zaitchev. However when a big bulky base such as terbutoxide- ([CH3]3O-) is used It is too sterically hindered to get inside and make the zaitchev. Therefore it will cut to the outside and make the less subtituted and less stable hoffman form. The E2 happens when you use one of the 5 bases mentioned in the destroyer. ( i forget the others offhand). If the halide is primary and you use one of the smaller cutting tools, it will compete with SN2. However a secondary or tertiary with the strong bases forms E2 always.
E1 occurs when you use a different base like KOH. If the halide is secondary or tertiary it will undergo an E1 elimination. Again if the halide is primary it will likely form a substitution.so be careful. Yu have to familiarize yourself with the laws of E/E1 and SN2/SN1. I would suggest first going through a textbook and reading the guidelines there carefully. It is impossible to teach the entire chapter in a single post.

 

[drgreen]

there are many factors involved in the determination of E1 and E2:

1) the substrate
-when the substrate is primary (1 R group attatched at site of attack) only E2 is observed
-when the substrate is secondary (2 R groups attatched at site of attack) E2 is also mainly observed
-when the substrate is tirtiary (3 R groups attatched at site of attack) E1 and E2 are observed depending on the other factors

2) the base
-strong basses favor E2

3) the leaving group
-Better leaving groups favor quicker elimination reactions but E1 is more sensitive to the leaving group factor

4) the solvent
polar solvents tend to support E1 because the carbocation formed from the elimination of a proton is stabilized by the