For All Those In Need Of Clarifications Between E1/e2 And Sn1/sn2:

[Kkae]

Overall summary of Sn1, Sn2, E1 and E2 rxns:

1. Methyl Halides (CH3X): Gives Sn2 rxns
NB: Methyl halides do not form stable carbocations therefore sn1 rxns are unlikely. They cannot undergo eliminiation rxns therefore no E1 and E2.

2. Primary Alkyl Halides (RCH2X): Gives mainly Sn2 rxns. Also, with bulky hindered bases such as (CH3)3CO-K gives mainly E2
NB: Primary alkyl halides do not form stable carbocations therefore no Sn1 and E1.

3. Secondary Alkyl Halides (R2CHX): Gives mainly Sn2 with weak bases such as CN-, N3-, CH3COO-, SH-, RS-, I-.
Gives maily E2 with strong bases such as RO- (CH3CH2O-, CH3o- etc)

4. Tertiary Alkyl Hailides (R3C-X): No Sn2 rxns (alkyl halides too crowded)
Solvolysis gives Sn1/E1 rxns (tertiary alkyl halides form very stable carbocations).
At lower temperatures Sn1 is favored raising the temperature, increases E1
NB: When strong bases such as RO- is used with tertiary alkyl halides E2 elimination predominates.

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[doc3232]

nice explanations.

 

[pidbull]

I am getting confused with whats a strong base and weak one. Also the nomenclature with nucelophile has made it even more confusing. In the other SN1/SN2/E1/E2 thread there was a lot of mention of big bulky bases so I am a little confused. Please help me understand. BTW thanks for the guide.

 

[ZenoVT]

awesome guide

 

[Mamona]

Overall summary of Sn1, Sn2, E1 and E2 rxns:

1. Methyl Halides (CH3X): Gives Sn2 rxns
NB: Methyl halides do not form stable carbocations therefore sn1 rxns are unlikely. They cannot undergo eliminiation rxns therefore no E1 and E2.

2. Primary Alkyl Halides (RCH2X): Gives mainly Sn2 rxns. Also, with bulky hindered bases such as (CH3)3CO-K gives mainly E2
NB: Primary alkyl halides do not form stable carbocations therefore no Sn1 and E1.

3. Secondary Alkyl Halides (R2CHX): Gives mainly Sn2 with weak bases such as CN-, N3-, CH3COO-, SH-, RS-, I-.
Gives maily E2 with strong bases such as RO- (CH3CH2O-, CH3o- etc)

4. Tertiary Alkyl Hailides (R3C-X): No Sn2 rxns (alkyl halides too crowded)
Solvolysis gives Sn1/E1 rxns (tertiary alkyl halides form very stable carbocations).
At lower temperatures Sn1 is favored raising the temperature, increases E1
NB: When strong bases such as RO- is used with tertiary alkyl halides E2 elimination predominates.

Thanks for the table
I am having problems identifying strong bases from weak bases. I though alkoxides were strong bases, so why CH3COO- is consider a weak base. Pleaaaase help me with this!!!!!
Also I have in my notes the weaker the acid the more the reverse rxn is favored, the stronger its conjugate base. By following this I was thinking HCN is a weak acid its conjugate base should be a strong base????? I need help I am getting crazyyyy

 

[Mamona]

 

[Tina324]

haha yea i second the strong base, weak base question. Anyone?

Can someone put up a list of strong bases and weak bases?

Isn't CN a strong base btw?

 

[Optimism Smiles]

haha yea i second the strong base, weak base question. Anyone?

Can someone put up a list of strong bases and weak bases?

Isn't CN a strong base btw?

To my knowledge, CN isn't a strong base.

Here are the strong bases I look for:
KOH
NaOH (Hydroxides of Alkali and Alkali Earths are STRONG bases)
CH3CH2O- (sodium ethoxide)
(CH3)3COK - This one is BULKY remember so it does Hoffman Elimination
LDA
NaNH2
NaH

Weak bases:
NH3
Pryidine (C5H5N)
And basically (haha get it?..BASICALLY ) anything NOT on the strong list.

 

[ucla2134]

Thanks for the table
I am having problems identifying strong bases from weak bases. I though alkoxides were strong bases, so why CH3COO- is consider a weak base. Pleaaaase help me with this!!!!!
Also I have in my notes the weaker the acid the more the reverse rxn is favored, the stronger its conjugate base. By following this I was thinking HCN is a weak acid its conjugate base should be a strong base????? I need help I am getting crazyyyy

Ch3cooh is a weak base because its conjugate acid is strong becuase of the resonance, whether the the alkoxides, its conjugate acid is alcohol, which is a very weak acid, thus the alkoxides is a strong conjugate base.

 

[water88]

CN is not a strong base; its a strong nucleophile.

 

[Mamona]

This is a very confusing subject, but I went and read my general chemistry book (Ebbing - Gammon 8th edition) and in page 668 (in case you have it) there is a list of relative strengths of Acid and Bases HCN is a weak acid and it shows CN- being a strong base!!!

 

[ucla2134]

from what i know is mostly used in SN2 reaction since the strength of the nucleophile is important.

 

[Mamona]

from what i know is mostly used in SN2 reaction since the strength of the nucleophile is important.

I am not sure about this because I am still reading and putting everything together, but it seems that a weak base would be a strong nucleophile....

 

[Night_Tears]

Awesome Summary, very helpful. 🙂

 

[rak173]

Had this from my Ochem class. Sorry about the scribbles and poor lighting.