Compounds with Additional Elements

Although all organic compounds contain carbon, and almost all have hydrogen, most of them contain other elements as well. The most common other elements in organic compounds are oxygen, nitrogen, sulfur, and the halogens.

The halogens resemble hydrogen because they need to form a single covalent bond to achieve electronic stability. Consequently, a halogen atom may replace any hydrogen atom in a hydrocarbon. Figure 1 shows how fluorine or bromine atoms proxy for hydrogen in methane.

Figure 1 Methane and two derivatives.

Halogens can replace any or all of the four hydrogens of methane. If the halogen is fluorine, the series of replacement compounds is

Such halogenated compounds are called organic halides or alkyl halides. The substituted atoms may be fluorine, chlorine, bromine, iodine, or any combination of these elements.

The ethylene molecule is planar; that is, all 6 atoms lie in a single plane because the double bond is rigid. In Figure 2 , the stiff double bond prevents the molecule from being “twisted” around the axis between the carbon atoms. If a reaction substitutes a different atom—like a bromine atom—for two of the hydrogens, the resulting compound can exist in either of two different structural configurations.

Figure 2 Ethylene derivatives.

The configuration with the bromines adjacent is called cis (from the Latin derivative for “on this side”), whereas that with bromines opposite is called trans (which means “on the other side”). The two configurations are different substances with unique chemical and physical properties. They are described as being geometric isomers. (See Figure 3 .)

Figure 3 Geometric isomers.

Figure 4 lists some common classes of organic compounds containing oxygen or nitrogen. The main carbon-bearing part of the compound attaches to the bond extending leftward in the second column. The examples use the ethyl unit as the carbon chain attached to the functional group, but the immense number of organic compounds arises from the fact that virtually any carbon chain can be attached at that site.

Figure 4 Common functional groups.

The amines contain nitrogen bonded to 1, 2, or 3 carbon chains. These compounds are derivatives of ammonia, hence the name of the class, as shown in Figure 5 .

Figure 5 Ammonia.

Consider 3 possible amines created by replacing hydrogen with the -CH₃ methyl group. (See Figure 6 .)

Figure 6 Methyl derivatives of ammonia.

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