Ci

FeCl,

FeCl,

Notice that the aromatic C6 ring survives intact. Notice also that in these structures the lone pairs on the chlorine atoms are not shown, as they were in the previous structures for alcohols and amines. Chemists use both types of structures—those that show, the lone pairs and those that do not-—for such compounds.

If the reaction is allowed to continue, i.e., if excess Cl2 is available, one or more hydrogen atoms of the chlorobenzene molecules will in turn be replaced by chlorine. There are three isomeric dichlorobenzenes, all of which could in principle be produced in such a reaction:

1,2-dichlorobenzene 1,3 -dichlorobenzene 1,4-dichlorobenzene

The numbering scheme begins at one of the "substituted" carbons; the direction of numbering around the ring is chosen to yield the smallest possible number for the second substituent. In older nomenclature, 1,2-disubstituted benzene is called the artho substituted isomer, 1,3-disubstitution calls for the prefix meta, and the 1,4 isomer is termed para. Thus the compound 1,4-dichlorobenzene, shown above at the right, is also called para-dichlorobenzene or p-dichlorobenzene.

When using the Kekule structure for benzene in which double and single bonds are displayed, it is important to remember that the choice of a structure (A or B on page AP-11) for the positions of the double bonds is an arbitrary one. This has the consequence that there are only three, not five, isomeric dichlorobenzenes. For example, 1,6-dichlorobenzene is not different from the 1,2 isomer; they represent the same molecule viewed from different perspectives. (To avoid any such complications, many chemists use only the circle-containing hexagon symbol shown on page AP-12.)

In some derivatives of benzene, there are two or more types of substituents, even other benzene rings. In such cases, the carbon having the most important substituent is called C-I, and the numbering continues in the direction that gives the smallest number to the first substituent of the second kind. Many examples of multiply substituted benzenes are encountered in Chapters 10-12.

PROBLEM 8

Deduce the structures and names for the three chemically different trichlorobenzenes.

Continue reading here: Review Questions

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