Selasa, 18 Desember 2012

Hoffman Product Versus Zaitsev Product In Elimination Reactions

As an organic chemistry student you will come across the topic of beta-elimination in your first semester. When performing these reactions you will often be faced with the question of where to place the carbon to carbon double bond. Do you go for the Hoffman product or the Zaitsev product in elimination reactions?

Before you can understand which products to choose, you have to understand two concepts:

Stability of Alkenes
  •     What is the Zaitsev Product
  •     What is the Hoffman Product

Stability Of Alkenes

A carbon to carbon double bond is stabilized by alkyl groups or carbon chains attached to the double bond. The more carbon groups that are attached, the more substituted the double bond will be. In other words, the greater the alkene substitution, the greater the stability of the alkene.

Thus the order of stability for substituted alkenes are as follows (assuming we are disregarding resonance possibility or conjugation)
  •     tetra-substituted = 4 groups attached
  •     tri-substituted = 3 groups attached
  •     di-substituted = 2 groups attached
  •     mono-substituted = 1 group attached
  •     non-substituted = no groups attached

These trends must be taken into account when determining which type of double bond you can and should form in your beta elimination reaction.

But how does this relate to the Zaitsev and Hoffman Products?

Zaitsev's Rule

Zaitsev's rule simply states that when you have the option to form more than one form of a double bond, you should choose the double bond that is most stable. Translating the above trends, this means you will form the more substituted double bond.

In most cases of elimination, this means that you will remove the beta-hydrogen that will give the most substituted product. This applies to any strong base that is small enough to reach this hydrogen atom, regardless of the size of the molecule.

However, this changes when you are dealing with a big and bulky base. If the base is really large, steric hindrance of the alkyl group will play a big factor. In this situation you will follow Hoffman's Rule instead

Hoffman Products

The Hoffman product in an elimination reaction is one that forms the less stable or less substituted double bond. While this is obviously not the desired product, this is the only option in certain situations. As mentioned above, a big bulky base may sometimes have a difficult time reaching the most stable hydrogen atom due to the size and bulk of the base getting in the way

When this happens, the big bulky base will reach for the most accessible hydrogen atom, usually one that is sticking out at the end of the molecule. While this forms a less stable product, it's really a choice between Hoffman or no product

To better understand the idea of a big and bulky base, picture the following scenario. Have you ever walked on the street while it was raining, next to a person carrying a huge and obnoxious umbrella? I've done it. The person under the umbrella stays nice and dry, but because of the umbrella size, everyone walking nearby has to keep at least a foot distance between themselves and the other person, as the obnoxious umbrella will keep poking you in the face if you get too close.

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