Reactions

CH 106 - Lesson 2

Reactions

In this section we will focus on the reactivity of the pi portion of the double bond.

Addition Reactions

The most common kind of reaction that alkenes undergo are called addition reactions.

Here we have just the active the portion of an alkene, the double bond. The double bond in an alkene will react with a wide variety of compounds. Bromine is one of them.

| |
- C = C -

Br-Br

|   |
- C - C -
|   |
Br  Br

Here is one method by which you might consider bromine to be added into the structure of the molecule at the place where the double bond now exists. The pi bond in the double bond starts breaking and new bonds start forming to the two bromine atoms, while at the same time the bond between the two bromine atoms is breaking. The net result is that where we formerly had two carbon atoms that were double bonded to one another, those two carbon atoms are now single bonded, and each of them is also bonded to a bromine atom.

| |
-C = C-

Br-Br

| |
-C - C-
· ·
Br··Br

| |
-C - C-
| |
Br Br

This is called an addition reaction. Bromine is said to be added to the double bond. If this diagram and explanation make sense to you, good -- continue. If not, stop and clarify the process using the slightly different diagrams in your workbook (Example 7) or textbook or with the instructor in the lab.

Lab Test

There are many similar addition reactions. I chose to discuss this one for a particular reason. This reaction is quite often used to indicate the presence of a double bond when you are trying to identify the nature of organic compounds. In choosing reactions that are used to indicate what kinds of compounds might be present, it is best to choose a reaction that will result in some easily observed change. Bromine, Br₂, as a pure liquid or dissolved in solution (which is the way that we will use it) has a deep reddish-brown color. As the bromine reacts and is taken into the structure of a molecule, it is no longer in the form of Br₂ and we lose the color associated with that form. The loss of color indicates that bromine is reacting. We can use this loss of color to infer the presence of a double bond. An advantage to this particular reaction is that it is easy to do.

That is what I would like you to do now or when you are in the lab. Part I of the experiment for this lesson includes performing a reaction of this type. If you are in the lab, do Part I of the experiment and then continue with the lesson.

Synthesis of Alkyl Halides

I want you to note that these addition reactions fit in with what you were doing earlier in Lesson 1. When bromine is added to an alkene you end up with an alkyl halide. You can convert one kind of compound into another through a chemical reaction.

Other Addition Reactions

Here we have another addition reaction. (This is also shown in Example 8 in your workbook.) Here, propene is shown reacting with hydrogen chloride to form 2-chloropropane. It would also seem possible for the chlorine to attach to the first carbon atom. It can, but for reasons that we won't go into here, it prefers to bond to the carbon atom that has the most carbon atoms bonded to it. Put another way, the hydrogen from HCl prefers to bond to the carbon atom that has the most hydrogen atoms already bonded to it.

H CH₃
| |
H - C = C - H

H - Cl

   H   CH₃
|   |
H - C - C - H
|   |
H   Cl

The purpose of this example is to show you that we are not limited to adding Br₂ to double bonds and also to lead into the topic of polymerization.

Top of Page

E-mail instructor: Eden Francis