The reaction of HCl with but-2-ene
is shown. Why can this reaction only possibly
produce a single product?
But-2-ene is an alkene because it
contains a carbon-carbon double bond. It is being reacted with hydrogen
chloride, a hydrogen halide. When an alkene reacts with a
hydrogen halide, hydrohalogenation 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 with a 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 between the hydrogen atom of the hydrogen halide
and one of the carbon atoms that was a part of the alkene and the second bond being
formed between the other carbon atom that was a part of the alkene and the halogen
from the hydrogen halide. This produces a product in which
the carbon-carbon double bond has become a carbon-carbon single bond and a halogen
has been added. This is a haloalkane.
With this information in mind,
let’s consider the hydrohalogenation of but-2-ene. To make this reaction slightly
easier to discuss, we’ll rotate the drawing of but-2-ene slightly and we’ll number
the carbon atoms from left to right. We know that when an alkene, like
but-2-ene, reacts with a hydrogen halide, like hydrogen chloride, the 𝜋 bond
between the carbon atoms is broken, along with the bond between the hydrogen atom
and the chlorine atom. And to make up for this loss of
bonds, two new bonds are formed.
In this example, we’ve shown carbon
number two forming a new bond with a hydrogen atom and carbon number three forming a
new bond with a chlorine atom. This produces a product in which
the carbon-carbon double bond is now a carbon-carbon single bond and a chlorine atom
has been added. But in this example, we chose to
add the hydrogen atom to carbon number two and the chlorine atom to carbon number
three. So let’s try this the other way
around. Two bonds will break, and two new
bonds will form. This time, carbon number two has
formed a new bond with a chlorine atom and carbon number three has formed a new bond
with a hydrogen atom. The product produced contains a
chlorine atom. And the carbon-carbon double bond
of the alkene is now a carbon-carbon single bond.
At first glance, these two products
may appear different. But if we flip the bottom product
over, we can see that the two product molecules are in fact the same. This has occurred because but-2-ene
is a symmetrical molecule. So, no matter which carbon atom of
the alkene the chlorine atom adds to, only one product will be produced. Therefore, this reaction can only
produce a single product because the alkene is symmetric. And so addition to either side of
the double bond gives the same product.