Saytzeff Rule, Zaitsev Rule: How to predict Major Product for Elimination Reaction

Elimination Reaction for some alcohols and alkyl halides will result in different alkene products, and Saytzeff or Zaitsev Rule is used to determine the major product.

Saytzeff or Zaitsev Rule states that the more substituted alkene will be the major product.

So by looking at the number of alkyl groups attached to the alkene, the degree of substitution and hence major and minor products can be determined.

Let's take the elimination of 2-bromobutane as an example:

saytzeff rule example using elimination of 2-bromobutane

 

We notice that when bromine is eliminated from carbon-2, we can either remove a neighbouring hydrogen from carbon-1 or carbon-3.

Therefore 2 different alkene products, but-1-ene and but-2-ene can be formed.

2 possible alkene products from elimination of 2-bromobutane

To determine which is the major product, we have to figure out the degree of substitution for each alkene.

To make it easier to visualise, we can box up the alkene and count the number of alkyl groups attached to the alkene.

For but-1-ene we notice it has only 1 alkyl group attached to the alkene. It is less substituted hence the minor product.

but-1-ene has only 1 alkyl group substituted to the alkene hence minor product

For but-2-ene, it has 2 alkyl groups attached to the alkene. It is more substituted therefore the major product.

but-2-ene has more substituted group hence major product

For a step-by-step guide to predict the major product using Saytzeff Rule, check out this video lesson now!

Topic: Alkenes, Organic Chemistry, A Level Chemistry, Singapore

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