How to predict major product for hoffmann elimination?
Answers
Explanation:
With asymmetrical amines, the major alkene product is the least substituted and generally the least stable, an observation known as the Hofmann rule. This is in direct contrast to normal elimination reactions where the more substituted, stable product is dominant (Zaitsev's rule).
Explanation:
As discussed earlier, the process of producing tertiary amines and alkenes from quaternary ammonium is called Hofmann elimination. This process can also be referred to as exhaustive methylation. The Hofmann rule states that the major alkene product is the least substituted and least stable product when it comes to asymmetrical amines. The Hofmann elimination process is named after its discoverer, the German chemist August Wilhelm Von Hofmann. The Hofmann elimination can be illustrated as follows.
Hofmann Elimination Example
It can be noted that methyl iodide is used in excess because it has no beta hydrogens and therefore cannot compete in the elimination reaction. If the given alkyl group contains two different sets of beta hydrogens which are available for the elimination process, the alkene isomer having less substituted double bond is formed as the major product. An important example of the Hofmann elimination process or the exhaustive methylation process is the synthesis of trans-cyclooctene which is shown below.
Hofmann Elimination Example 2
It is important to note that the neutral NR3 group acts as a better leaving group than the anionic NH2– or the NR2– groups. The outcome of the reaction is also affected by steric effects caused by large leaving groups.
