Functional Groups

FUNCTIONAL GROUPS

There are over 12 million known compounds of which more than 80% are organic compounds. To make sense out of the nearly 9 million organic compounds and be able to manipulate them and make new compounds, there must be some system of organization whereby organic compounds can be categorized by a particular property or group of properties. A natural method utilized by early practitioners was to group organic compounds by the reactions that they underwent. Thus there developed a whole variety of qualitative tests called classification tests which could be used to systematically categorize the reactivity of a compound and thus allow it to be grouped with others of similar chemical reactivity. These tests are still very useful to practicing organic chemists and collectively are known as organic qualitative analysis.

Classification tests are used to distinguish organic compounds and segregate them into different functional classes based on their chemical properties. Orig-inally a group of compounds that showed similar chemical behavior based on the classification tests were named for a property or behavior (e.g., acids from acer meaning “sour,” aromatic compounds from their odors). With the evolu-tion of the science of chemistry and the development of more modern views of atoms and molecules, a different definition of functional classes is possi-ble. The behavior of organic compounds is now organized into patterns that are based on recurrent groups of atoms — functional groups. The sites in molecules at which chemical reactions occur are localized at the functional groups in the molecule; the rest of the molecule is the same after the reaction as before. Thus, instead of thinking of the whole molecule in terms of its chemical reac-tivity, it is only necessary to recognize what functional group or groups are present in the molecule. It is then possible to predict the chemical behavior of the molecule based on the known chemistry of the functional groups that it contains.

This turns out to be a huge simplification. Since the numbers of functional groups are relatively small, it is possible to classify a very large number of individual compounds by a relatively small number of functional groups. So the first step to enlightenment in organic chemistry is to realize the key role that functional groups play in simplifying the subject, and the second step is to learn the functional groups by name, structure, and formula. While a great number of them may have already been encountered in the introductory organic course, it is helpful to review them. Table 1.1 is a list of the most common functional groups. While there are quite a few other functional groups that are not shown, those found in Table 1.1 are the most common and are present in the vast majority of organic compounds. Notice that not all functional groups contain only carbon atoms (e.g., the nitro group and the carbodiimide groups), and some functional groups differ at atoms other than carbon (compare the nitro and nitroso groups and the sulfoxide and sulfone groups). Since functional groups are reference points for predicting and understanding the reactions of individual organic molecules, it is very important to be able to recognize these functional groups (and others that might be encountered in the future). It is also useful to learn normal structural abbreviations that are used to indicate functional groups that are present in chemical structures. The abbreviations in Table 1.2 correspond to the groups that are shown in Table 1.1.

A major reason that the behavior of organic compounds can be generalized in terms of the functional groups they contain is because the bonds holding a given functional group together are the same regardless of the compound which contains that functional group. The four compounds shown below all contain the carboxylic acid functional group, which is highlighted within the boxes. Thus all four contain the bonding pattern characteristic of the –COOH functional group which is independent of the bonds found in rest of the molecule!

Since most organic reactions involve the conversion of one functional group to another, it follows that most organic reactions quite simply involve bond changes involving functional groups. If one knows the bonds found in the reactant functional group and the bonds found in the product functional group, then one automatically knows what bonding changes are required to effect the desired chemical change. Thus, in addition to being able to recognize functional groups, it is also important to be able to describe the numbers and types of bonds found in functional groups.

Bonds in functional groups can first be described by Lewis structures, which are merely formalisms for denoting numbers of shared and unshared electron pairs, formal charges, and types of bonds (numbers of shared pairs, single, double, and triple). Chemistry students learn to write Lewis structures in virtually all of their early chemistry courses. How to write Lewis structures will not be reviewed here, but knowing the correct Lewis structures for molecules and functional groups in molecules is an indispensable first step in being able to describe the structure and bonding of functional groups.

The next level of insight into functional groups comes from the translation of Lewis structures into more accurate bonding descriptions based on modern bonding theories. Structural details including geometries also result from the proper description of the bonding in the functional group. The ideas of structure and bonding currently in use had their origins in the late 1920s. It is again beyond the scope of this book to trace the developments which were seminal in the development of current theories; however, early studies were all rooted in the quest to understand and be able to describe the behavior of electrons in atoms. The development of quantum mechanics and the particle – wave duality of the electron and the uncertainty principle led to mathematical descriptions of the behavior of electrons in the electric field of the nucleus. The solution of those equations resulted in a new conceptual framework for understanding chemical bonding.