Addition reactions occur when two reactants add together to form a single product with no leftover atoms. Common addition reactions in organic chemistry involve reagents adding across a carbon-carbon multiple bond to give a product containing two new single bonds for alkenes or four new single bonds for alkynes. Alkenes react with the halogens, such as Br2, to give dihaloalkane products in a type of reaction called halogenation. The reactivity of halogenation reaction depends on the nature of the carbon-carbon double bond. In this carbon-carbon double bond, we have a sigma bond and a pi bond. When a halogen reacts with the double bond, the weaker pi bond breaks to form two stronger sigma bonds to result in a saturated carbon compound like this. We say that the halogen has added to the double bond, so these reactions are categorized as addition reactions. To illustrate this type of reaction, lets take 1-octene, which contains a double bond and add liquid Br2 to it. As you can see, the liquid Br2 is brown in color due to the diatomic bromine that is present. What will happen when I add a few drops of the liquid bromine to the solution of 1-octene? As you can see, the resulting solution is colorless. This colorless product indicates that this bromine bond had to break. So, the bromine bond breaks and adds across the double bond to form a 1,2-dibromohexane product. If we continue to add bromine, the solution will eventually turn a light brown color, indicating that the bromine is the excess reactant. Now lets see what will happen when we add bromine to a hexane solution. Hexane is an alkane, meaning it does not contain a double bond. What will happen when I add a few drops of bromine to the hexane solution? In this case, the mixture of the two solutions is brown, indicating that no reaction has occurred. Why did the bromine not react with the hexane? The carbons in an alkane are saturated, while the carbons in an alkene (or an alkyne) are unsaturated. In an unsaturated alkene or alkyne, the bromine is able to add across the multiple bond, while in this case, we have an alkane, no reaction occurs. Now let's observe a similar example involving tomato juice and chlorine water. Tomato juice contains beta carotene, which is a polyunsaturated molecule. Note that it has several double bonds. Beta carotene is also responsible for the deep red color of tomato juice. If the double bonds are converted to single bonds in beta carotene, the tomato juice will turn colorless or light pink. What will happen when I add chlorine water, which contains Cl2, to the tomato juice, which contains beta carotene? As you can see, the Cl2 adds across the double bond in beta carotene, converting the double bonds to single bonds and thus destroying the conjugated system, which decolorizes the tomato juice. This gives us visual evidence of an addition reaction for alkenes since they are unsaturated.