Unsymmetrical Alkenes

Unsymmetrical Alkenes

Chemistry in Action

Alkenes are unsymmetrical when the two carbon atoms which make up the double bond are attached to different groups. However, if the carbon atoms are attached to the same group the alkenes are symmetrical.

If an unsymmetrical alkene reacts with an unsymmetrical electrophile, like H-X or H2SO4, two possible products are formed. For example, propene with hydrogen bromide can form 2-bromopropane or 1-bromopropane. The quantity of each product is not the same. Which product will be formed depends on the carbocation intermediate’s stability.

  • When 2-bromopropane is produced: the intermediate is a secondary carbocation because the carbon with the positive charge is attached to the other two carbon atoms.
  • When 1-bromopropane is produced: the intermediate is a primary carbocation because the carbon with the positive charge is attached to only one other carbon atom

A secondary carbocation has more stability than a primary carbocation while a tertiary carbocation is even more stable than a secondary carbocation. This means that it is more probable that 2-bromopropane will be produced making 2-bromopropane a major product and 1-bromopropane a minor product.

Generally, the higher the stability of a carbocation the higher the chances it will be substituted. Therefore, the part of the electrophile with the most electronegativity will also attach itself to the carbon atom with the most substitution. This is called Markownikoff’s rule:

Remember for Alkenes – The more electronegative part of the electrophile will usually attach itself to the more highly substituted carbon atom.

The major product is known as the Markownikoff product while the minor product as the anti-Markownikoff product.

For two products to form, the alkene and the electrophile must be unsymmetrical. A symmetrical alkene gives only one product as does an unsymmetrical alkene with a symmetrical electrophile.

Other addition reactions of alkenes

Addition steam: hydration

If an alkene is treated with 300