Major Product in Hoffman Elimination

[bnleong]

http://en.wikipedia.org/wiki/Hofmann_elimination So I reading about the Hoffman Elimination on Wikipedia. Did they make a mistake by making that trans alkene as the major product. I thought that double bond should be connected to the R as the final product. Am I wrong?

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[IamSuperDoctor]

Its an R not a C ?

 

[Entadus]

Obviously the product will be the trans alkene (as opposed to the cis alkene)

The OH- grabs a proton, yielding water (which leaves). The electrons associated w/ that hydrogen then form the double bond, kicking the leaving group (the amine)

 

[BerkReviewTeach]

http://en.wikipedia.org/wiki/Hofmann_elimination So I reading about the Hoffman Elimination on Wikipedia. Did they make a mistake by making that trans alkene as the major product. I thought that double bond should be connected to the R as the final product. Am I wrong?

There are a couple typos in their listing. First, if you look at the cyclooctene, they should not show stereochemistry for the Hs, because they are on PLANAR carbons (they have sp2-hybridization), so the two Hs and C3 and C8 must all be coplanar.

The other typo is that they completely missed the point of the Hoffman elimination. It is carried out in order to make the non-Zaitsev product in highest yield. The example they show completely misses that point. Without showing what the R-group is, it's hard to say what should form. But if the R-group is a straight chain alkyl group, then the double bond would fom in the R-group (as you stated).

A better example of Hoffman elimination is starting with 2-aminobutane and doing exhaustive methylation followed by AgOH, after which you'd get predominantly 1-butene over 2-butene. Then there is no issue with cis versus trans.

Obviously the product will be the trans alkene (as opposed to the cis alkene)

Not sure why you have the obvious at the start of your sentence, because the example they show is a case where the Hoffman elimination is acting like a standard E2 reaction. Like any E2 reaction, formation of the product depends on the H being anti to the leaving group, which in their example shows a trans product. But with a different stereogenic center for the eliminated H or the leaving group, it could have been a cis product. With an E2 reaction, it is not obvious whether it'll be cis or trans when dealing with specific stereocenters.

Overall, this Wikipedia listing is pretty bad (the rare 0.1% case it seems). Hoffman elimination is used to generate terminal alkenes which don't have any cis vs. trans issue.

 

[SN2ed]

Thread moved. These types of questions belong in the Study Question Q&A forum.

 

[chemtopper]

With Zaitsav's rule you are getting most stable alkenes which are most substituted.
Hofmann elimination rule is for the least substituted alkene and not only for terminal alkene .

Both the products are formed --cis and trans .Trans is going to be major because it is more stable .

 

[chemtopper]

Hofmann elimination rule is for the least substituted alkene and not only for terminal alkene .
Generally with Zaitsav's rule you are getting most stable alkenes which are most substituted.

Both the products are formed --cis and trans .Trans is going to be major because it is more stable .