Video: EC17-18-S1-Q02A

Explain why Markownikoff’s rule is not applicable for all alkenes when hydrogen bromide is added to them.

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Video Transcript

Explain why Markownikoff’s rule is not applicable for all alkenes when hydrogen bromide is added to them.

Markownikoff’s rule is that, for the addition of a protic acid, HX, to an asymmetric alkene, the hydrogen adds to the carbon with more hydrogen substituents, and the halide adds to the carbon with more alkyl substituents. You may have seen Markownikoff written differently. Either version is okay.

So what is an asymmetric alkene? An asymmetric alkene is an alkene where the groups on one side of the carbon–carbon double bond are not equivalent to the groups on the other side. These groups could be hydrogens, alkyl groups, or something else. On the other hand, a symmetric alkene has groups on both sides equivalent to each other.

Here’s an example of an asymmetric alkene. On one side, we have two hydrogen groups. And on the other, we have two methyl groups. And here we have an example of a symmetric alkene, where the two groups on either side of the carbon–carbon double bond are equivalent.

When looking at the addition of protic acids, like hydrogen bromide, to a symmetric alkene, it does not matter which way round the addition occurs. Because the carbons are equivalent, the final molecule, whichever way round the hydrogen bromide adds, will be equivalent. So Markownikoff’s rule does not apply to symmetrical alkenes.

Now we have to put this into full sentences. Alkenes can either be symmetrical or asymmetrical. Markownikoff’s rule is only applicable to asymmetric alkenes.

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