Compounds with Additional Elements
The discussion of organic chemistry to this point has described only compounds of carbon and hydrogen. 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
CH 4 CH 3F CH 2F 2 CHF 3 CF 4
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 previously mentioned ethylene molecule is planar; that is, all six 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.
Figure 2. Ethylene.
If a reaction substitutes a different atom such as a bromine atom for one or more hydrogen atoms, the resulting compound can exist in either of two different structural configurations. The configuration with the bromines adjacent is called cis (from the Latin derivative for “on this side”), whereas the configuration 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 C 2H 5– 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.
If you compare the carbon‐oxygen bonding, you will observe that oxygens may be bonded to carbon by either single or double bonds.
Both alcohols and carboxylic acids have a single hydrogen bonded to an oxygen in the functional group. In aqueous solution, such hydrogens can become detached, producing slightly acidic solutions.
The amines contain nitrogen bonded to one, two, or three carbon chains. These compounds are derivatives of ammonia, hence the name of the class, as shown in Figure 5.
Figure 5. Ammonia.
Consider three possible amines created by replacing hydrogen with the –CH 3 methyl group. See Figure 6.
Figure 6. Methyl derivatives of ammonia.
Of course, more complex carbon groups can be attached at any of the three bonds to nitrogen. Notice that the nitrogen atom is truly the core atom in an amine, in contrast to the functional groups in alcohols, aldehydes, and carboxylic acids, in each of which the functional group must be at the end of the molecule.
- The oxidation of methyl alcohol produces a substance that has the composition of CH 2O. Draw the structure of this molecule and classify it on the basis of its functional group.