Benzene rings, naming compounds

When several substituents are present on a benzene ring, the ring is numbered in the same manner as the rings of cycloalkanes—that is, so that the numbers for the substituents are as low as possible. In addition, some special terms are used with disuhsti-tuted benzenes only. Two substituents on adjacent carbons (positions I and 2) are said to be ortho, or o-. Two substituents on positions 1 and 3 are meta, or m-. And two substituents on positions 1 and 4 are para, or p-. Finally, if an alkyl group with six or more carbons is attached to a benzene ring, the compound is named as an alkane with a phenyl substituent. Some examples are as follows ... 

A special group of aromatic compounds are ketones where the carbonyl is attached to at least one benzene ring. Such compounds are named as "phenones", the prefix depending on the size and nature of the group on the other side of the carbonyl. These are the common examples ... 

It is simplest to name the compound as a derivative of benzoic acid. The substituent, Cl, is attached to carbon-4 of the benzene ring. This compound is 4-chlorobenzoic acid or p-chlorobenzoic acid. 

If alkyl groups containing fewer than six carbon atoms are bonded to a benzene ring, the compound is named as an alkyl-substituted benzene. For more complex molecules, the term phenyl is named as a substituent on the parent chain of carbon atoms, as in 3-phenylheptane. 

Later in this chapter we 11 explore the criteria for aromaticity in more detail to see how they apply to cyclic polyenes of different ring sizes The next several sections intro duce us to the chemistry of compounds that contain a benzene ring as a structural unit We 11 start with how we name them... 

All compounds that contain a benzene ring are aromatic and substituted derivatives of benzene make up the largest class of aromatic compounds Many such compounds are named by attaching the name of the substituent as a prefix to benzene... 

Biphenyl is Ihe accepted lUPAC name for Ihe compound m which Iwo benzene rings are connected by a single bond... 

Aromatic compounds are named by giving the substituents on the benzene ring the lowest numbers. When the benzene ring is itself named as a substituent, it is called a phenyl group. 

The chemistry of cycloproparenes is characterized by the interplay of two contradictory effects, namely aromaticity which is generally known to stabilize compounds, and strain which destabilizes them. The fusion of a cyclopropene to a benzene ring results in geometrical distortions and strain which has consequences on the properties of the resulting cycloproparene. It perturbs the aromatic Ji-electron... 

Our discussion of pesticides has focused primarily on insecticides. In the United States the primary use of pesticides is in the form of herbicides, those pesticides used to control weeds. Approximately 70% of the pesticides used in the United States are herbicides and 20% are insecticides. The use and development of herbicides parallels that of insecticides. The first herbicides were inorganic metal compounds and salts. During World War II organic herbicides were synthesized and their use increased dramatically. One of the first major classes of herbicides synthesized in the mid-1940s was phenoxyaliphatic acids. As this name implies, the phenoxyaliphatic acids contain the benzene ring, oxygen, and an aliphatic acid. The two most common phenoxyaliphatic acids are 2,4 dichlorophenoxyacetic acid, called 2,4-D and 2,4,5 trichlorophenoxyacetic acid, known as 2,4,5-T (Figure 18.10). The numbers in these... 

The nucleus of the ACE inhibitor perindopril (17-6) consists, as the name suggests, of a perhydroindole the compound also differs from the foregoing examples by omitting the benzene ring on the side chain terminus. The side chain... 

In certain polyfunctional compounds, an aldehyde or ketone group can also be named as a substituent on a molecule with another functional group as its root. The aldehyde carbonyl is given the prefix formyl-, and the ketone group is named 0x0- with a number to show its position in the molecule. Compounds with both an aldehyde and ketone are named as aldehydes, because aldehydes have functional group priority over ketones. A ketone containing a benzene ring is named as a -phenone. 

Aromatic The name originally given to cyclical compounds containing the benzene "ring" because the first benzene-type compounds isolated smelled "good". 

Unfortunately, a completely systematic method of naming aromatic compounds is not in use. The system used is a combination of trivial names and the IUPAC system. Sometimes- compounds that contain benzene rings are considered to be substituted benzenes, in which case the word benzene appears in the name of the compound along with the name of the substituent. 

According to the IUPAC rules for naming polycyclic compounds, when a benzene ring is ortho-fused to another ring the prefix benzo is attached to the name of the parent ring. This is contracted to benz" when preceding a vowel, as in benzanthracene. 

Substituted aromatic compounds are named using the suffix -benzene. Thus, C6H5Br is bromobenzene, C6H5CH3 is methylbenzene (also called toluene), C6H5N02 is nitrobenzene, and so on. Disubstituted aromatic compounds are named using one of the prefixes ortho-, meta-, or para-. An ortho- or o-disubstituted benzene has its two substituents in a 1,2 relationship on the ring a meta- or m-disubstituted benzene has its two substituents in a 1,3 relationship and a para- or p-disubstituted benzene has its substituents in a 1,4 relationship. When the benzene ring itself is a substituent, the name phenyl (pronounced fen-nil) is used. 

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