Video Transcript
Consider the reaction of but-1-ene
with HCl. What major product is formed from
this reaction?
Looking at the structure of
but-1-ene, we see that this molecule contains a carbon-carbon double bond, so
but-1-ene is an alkene. It is being reacted with hydrogen
chloride, a hydrogen halide. When alkenes are combined with
hydrogen halides, a hydrohalogenation reaction can occur. Hydrohalogenation is an addition
reaction in which a hydrogen atom and a halogen atom are added to a molecule. Let’s take a look at the
hydrohalogenation of a generic alkene using a generic hydrogen halide represented
here by HX.
Over the course of this reaction,
the 𝜋 bond between the two carbon atoms will break, as will the bond between the
hydrogen atom and the halogen atom. To make up for this loss of bonds,
two new bonds will be formed. One bond will be formed between the
hydrogen atom of the hydrogen halide and one of the carbon atoms of the alkene. And a second bond will be formed
between the other carbon atom of the alkene and the halogen of the hydrogen
halide. The product of this reaction
contains a new hydrogen atom and halogen atom. And the carbon-carbon double bond
of the alkene is now a carbon-carbon single bond.
This is a haloalkane. In the reaction as shown, the
halogen atom was added to the carbon on the right. But it could’ve been added to the
carbon on the left, possibly producing a different product. In most cases, we can predict how
the hydrogen halide will add to the alkene by using Markownikoff’s rule. Markownikoff’s rule states that the
acidic hydrogen atom will add to the carbon of the double bond with the greatest
number of hydrogen substituents. With this information in mind,
let’s return to the reaction given in the question.
We know that over the course of
this reaction, the 𝜋 bond between the carbon atoms will break, as will the bond
between the hydrogen atom and the chlorine atom. Following Markownikoff’s rule, the
acidic hydrogen atom will add to the carbon atom of the double bond that has the
greatest number of hydrogen substituents. If the leftmost carbon atom of the
double bond forms a new bond with the hydrogen of the hydrogen chloride, then the
rightmost carbon atom of the double bond will form a new bond with the chlorine of
hydrogen chloride. This produces a haloalkane. More specifically, this haloalkane
is 2-chlorobutane.
So the major product formed from
the reaction of but-1-ene with hydrogen chloride is 2-chlorobutane.
