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