Saturated hydrocarbons undergo few other reactions. The halogen derivatives are not important for their own sake, but because they are a bridge to other, more useful compounds. Once formed, halogen compounds can react to form alcohols, acids, amine bases, and other types of molecules to be discussed in Chapter 20.

Unsaturated hydrocarbons are considerably more reactive than alkanes; their reactions take place at moderate temperatures with the help of catalysts.

The Achilles' heel of alkanes is the double bond, and the main alkene reaction is the addition across this bond of a variety of reagents.

Notice that only one product is formed when an asymmetric reagent such as HCl or H20 is used. 2-Chloropropane is formed to the exclusion of  1-chloropropane.

This is known as Markovnikov's Rule: When a substance HX adds across a double bond, the H always goes to the carbon that already has the greater number of H atoms. For many years organic chemists used Markovnikov's rule without understanding why; now we know that it arises because of the mechanism of reaction, which involves intermediates known as carbonium ions. We shall only note that the rule works, without going into the details of the mechanism.