OChem E2 vs SN2

[A6project]

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Hi, I always had troubles learning E2 vs SN2 when I took Ochem and now when I study for DAT, I still struggles. 🙁 I guess something never change. I have 3 quetions. I would deeply appreciate if someone answer these for me!

Q1)
bromo-cyclohexane ---C2H5ONa/Eto--------> hexene
why this rxn is E2 instead of SN2? I looked up the SN2/SN1/E1/E2 chart, but still I have not figured it out. Plz someone explain it to me!

Q2) what would be easy ways to distinguish E2 from SN2?


Q3) E1/SN1 usually happens when the molecule is unstable like primary???!? right?!?

Gracias!!!

 

[UndergradGuy7]

1. CH3CH2O- (Or C2H5O-) is a strong base that favors E2. So you have to look at the basicity/nucleophicity of the base. There are some rules/charts to determine which is strong/weak etc.

2. E2 tertiary>secondary>primary carbons. E2 strong base.
Sn2 good nucleophile. primary>secondary>tertiary.

3. False. E1 and Sn1 prefer tertiary because it makes a carbocation. Tertiary carbocation is more stable than secondary>primary. E2 also prefers tertiary. Only Sn2 prefers primary>secondary>tertiary. Sn2 wants an unhindered molecule and does not have a carbocation so primary is better.

 

[UndergradGuy7]

Watch Chad's videos at www.coursesaver.com for $20. He explains it pretty good.

 

[orgohacks]

Hi, I always had troubles learning E2 vs SN2 when I took Ochem and now when I study for DAT, I still struggles. 🙁 I guess something never change. I have 3 quetions. I would deeply appreciate if someone answer these for me!

Q1)
bromo-cyclohexane ---C2H5ONa/Eto--------> hexene
why this rxn is E2 instead of SN2? I looked up the SN2/SN1/E1/E2 chart, but still I have not figured it out. Plz someone explain it to me!

Q2) what would be easy ways to distinguish E2 from SN2?


Q3) E1/SN1 usually happens when the molecule is unstable like primary???!? right?!?

Gracias!!!

1) For E2 vs. SN2, one critical variable is HEAT. Heat favors elimination versus substitution (due to entropy) If you see the "delta sign" or a temperature above room temperature it means it is going to be E2.

2) A dead giveaway for E2 is a bulky base like tBuO- . That should induce a Pavlovian response for "E2". SN2 will be disfavored due to steric hindrance in this case. The other is heat, like I mentioned.

3) E1/Sn1 usually occurs for tertiary carbons (not primary). You're going through a carbocation. Distinguishing between the two is usually solvent - SN1 occurs in a weakly nucleophilic solvent like alcohol or water, E1 occurs when there is no nucleophile around to trap the carbocation.
Common acid for E1 is H2SO4 since the HSO4- anion is an extremely poor nucleophile and will not trap the carbocation.

hope this helps - James