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.