http://www.ck12.org Chapter 25. Organic Chemistry
Introduction
With over twenty million organic compounds to keep track of, we need some way to organize information other
than simply memorizing reactions for each substance. The presence of functional groups allows us to group together
similar modes of chemical reactivity. Instead of learning the reactions that can be performed with a given compound,
we learn the reactions that can be performed on a given functional group. Although there are always special cases,
specific sets of reaction conditions tend to cause the same changes to a certain functional group regardless of what
the rest of the reactant molecule looks like.
Although there are thousands of different types of organic reactions, many of the more complex ones can be broken
down and understood in terms of simpler processes, such as addition, elimination, and substitution reactions. We
will look at a small sampling of the most common processes that occur during organic reactions. In each case, we
will focus on how one functional group is changed into another under a certain set of conditions.
Additions Across a Double Bond
There are two types of double bonds we will look at in terms of reactions, the C=C double bond and the C=O double
bond. Because one of these bonds is polarized and the other is not, they differ quite a bit in terms of the conditions
under which they are reactive. However, the overall change from reactant to product has the same form in either
case; the double bond becomes a single bond, and a new atom or group of atoms is attached to each of the two atoms
originally involved in the double bond.
Addition of Hydrogen
FIGURE 25.22
Hydrogenationis a very common addition reaction that involves the net addition of H 2 across a double bond.
Although the actual reaction mechanism for such a process can be quite complicated, the overall transformation is
relatively simple. Each atom involved in the double bond is attached to a new hydrogen atom, and the double bond
becomes a single bond.
For alkenes and alkynes (unpolarized multiple bonds), this process is generally accomplished by mixing the starting
material and H 2 gas in the presence of a metal catalyst (often Pt or Pd, although some cheaper metals can also be
effective under certain conditions). An alkene can combine with one equivalent of H 2 gas to form an unfunctionalized
alkane. Similarly, an alkyne can combine with two equivalents of H 2 (one for each pi bond) to make an alkane.
C=O bonds can also undergo a net addition of H 2 , but the reactants are often quite different. In fact, many conditions
for adding hydrogen to an alkene will not affect a C=O bond at all, and vice versa. Complementary reactivity patterns
like this allow chemists to selectively change one portion of a molecule without altering the rest.
In our chapter onOxidation-Reduction Reactions, we learned that one definition of reduction is the addition of
hydrogen atoms. The addition of H 2 across a double bond is an example of this type of reduction reaction.