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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 σ 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 σbonds that become the π bond. Newman projection of antiperiplanar alignment in the E2 reaction

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


The JMOL 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 destabilises the conformation compared to the one that leads to 1-butene. Using the space filling model to check..... 
 


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Dr. Ian Hunt, Department of Chemistry, University of Calgary