1.13 An Overview of Organic Compounds: Functional Groups

Certain types of bonds and arrangements of atoms tend to appear quite frequently in organic chemistry. The most common types of bonds we encounter are C—C and C—H single bonds. Compounds consisting of nothing but C—C and C—H single bonds are called alkanes. Examples include methane, 2-methyloctane, and ethylcyclohexane (Fig. 1-32).

Alkanes tend to be among the most unreactive organic compounds. For this reason, liquid alkanes (e.g., pentane and hexane) are often used as solvents in which to carry out organic reactions.

Alkanes are relatively inert in part because C—C and C—H bonds are so strong; and, for a reaction to occur, these bonds must break. In fact, the data in Table 1-2 (p. 10) show that these are among the strongest single bonds we will encounter.

Alkanes also tend to be unreactive because C—C and C—H bonds are either nonpolar or are only very slightly polar. In Chapter 7, we explain that a large driving force behind many chemical reactions depends on the existence of substantial bond dipoles.

When other types of bonds and other types of atoms are introduced, then organic molecules tend to be more reactive. In some cases, this may be due to the introduction of weaker bonds, and in other cases, to the presence of large bond dipoles.

As we will learn throughout the remainder of this book, specific arrangements of atoms connected by specific types of bonds tend to react in characteristic ways. In fact, even though such groups may consist of only two or three atoms, they dictate the function (i.e., the chemical behavior) of the entire molecule. Hence these structural components are called functional groups. Consequently:

The functional groups that appear most often are listed in Table 1-6, along with the compound classes in which they are found. Take the time to commit these functional groups and compound classes to memory and review them frequently. Pay particular attention to the fact that some bonding arrangements, such as the hydroxyl group (O—H) and the carbonyl group (C═O), appear in multiple functional groups.

There are two important features in Table 1-6 that you must understand. The first is “R,” the symbol used to represent an alkyl group, which is an attached group that consists only of carbon and hydrogen. As with alkanes, alkyl groups tend to be unreactive. The benefit of replacing large, unreactive alkyl groups with R is that it allows us to focus on just the reactive portion of the molecule (i.e., the functional group).

We will learn more about alkyl groups in the first nomenclature unit (Interchapter A), following this chapter, which pertains to the naming of molecules. For the purpose of Table 1-6, a bond to R specifically indicates a bond to a carbon atom.

The second important feature in Table 1-6 is the absence of atoms at the ends of specific bonds. This indicates that the bond may be either to an alkyl group (R) or to a hydrogen atom.

With these in mind, the following conclusions can be drawn from Table 1-6:

Connections Cyclohexanone is an important industrial compound because it is a precursor in the synthesis of nylon, the material used to make this parachute.

Solved Problem 1.31

Will the chemical reactivity of these compounds be similar or significantly different? Explain.

problem 1.32 Will the chemical reactivity of δ-valerolactone and pentanoic acid be similar or significantly different? Explain.

YOUR TURN 1.14

SHOW ANSWERS

Circle the functional groups present in cyclohexanone and hexan-3-one in Solved Problem 1.31 and use Table 1-6 to verify that they are both ketones.

Each compound has a carbonyl (C═O) functional group and is classified as a ketone.

Solved Problem 1.33

Circle each of the functional groups present in ebalzotan, which was developed as an antidepressant and an antianxiety agent. Also, name the compound class that each functional group characterizes.

The functional groups are circled at the right and the compound classes they characterize are labeled. There are three C═C double bonds alternating with single bonds in a complete ring, characteristic of an arene. These are not separate C═C groups that characterize alkenes. There are two O atoms and two N atoms present, each providing the potential for another functional group. The N atom at the right is part of a N—C bond, characteristic of an amine. The O atom at the top is part of a C—O—C functional group, characteristic of an ether. The O atom and the N atom at the bottom left are part of the O═C—N functional group, characteristic of an amide.

problem 1.34 Circle each of the functional groups present in ciprofloxacin, a powerful antibiotic commonly sold under the brand name Cipro. Also, name the compound class that each functional group characterizes.