Book Cover Organic Chemistry 4e Carey
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Chapter 22: Amines



Overview | Alkylation of Amines by Alkyl Halides | Reaction of Amines with Aldehydes and Ketones | Preparation of Amides | Hofmann Elimination | Electrophilic Aromatic Substitution of Aryl Amines | Nitrosation of Amines |


Hofmann Elimination

Chapter 22: Amines

Hofmann elimination of quaternary amines
Reaction type: Elimination

Summary

completely methylate then convert to the ammonium hydroxide salt
1,2-elimination of the ammonium salt gives mainly the less highly substituted alkene
Related reactions
MECHANISM OF THE HOFMANN ELIMINATION
The initial steps are an example of the alkylation of an amine by methyl iodide. The mechanism of the elimination step is shown. 

When heated, the hydroxide removes the more accessible proton, the p bond of the alkene forms and the leaving group, a neutral amine departs.

the Hofmann elimination  of ammonium salts

Selectivity of the Hofmann Elimination

In general E2 reactions occur most rapidly when the H-C bond and C-LG bonds at 180o with respect to each other. This is described as an antiperiplanar conformation. This conformation positions the s bonds that are being broken in the correct alignment to become the p bond.
 
 

antiperiplanar arrangement in E2 reaction
anti orbital alignment in the E2 reaction The staggered, antiperiplanar alignment is preferred because it aligns the two s bonds that become the p bond. Newman projection of antiperiplanar alignment in the E2 reaction

Lets' look at the Hofmann elimination and the selectivity of the reaction:

1,2-elimination of the ammonium salt gives mainly the less highly substituted alkene
Highlight leaving group, N(CH3)3
Show antiperiplanar H 
Reset colors
The CHIME image to the left shows the calculated energy minima of the sec-butyltrimethylammonium compound. 

Since these reactions are E2, we need to look at the bonds that are antiperiplanar to the C-N bond that is broken. 

Orient the molecule so that you can see this. 

The only C-H bond antiperiplanar to the C-N bond is in the C1 methyl group and so leads to the formation of 1-butene. 
 

Highlight leaving group, N(CH3)3
Show antiperiplanar H 
Reset colors
The CHIME image to the left shows the of the sec-butyltrimethylammonium compound in the conformation that leads to the formation of trans-2-butene. 

Again look at the bonds that are antiperiplanar to the C-N bond that is broken, this time there are two. Can you see them ? 

One is on the -CH2-  and leads to the formation of 2-butene, the other is in the C1 methyl group and so leads to the formation of 1-butene. 

Note that this conformation puts the large N(CH3)3 and the C4 methyl group gauche to each other,  this destabilizes the conformation compared to the one that leads to 1-butene. Using the space filling model to check..... 
 



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