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Question Video: Determining What Product Is Produced When the But-2-ene Hydrocarbon Is Combined with Hydrogen Chloride Reactants Chemistry

The reaction of HCl with but-2-ene is shown. Why can this reaction only possibly produce a single product?

03:35

Video Transcript

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.

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