7 Benzene Aroma

17
Br
3
4 2 1
CH3
4-Bromo-1,2-dimethylbenzene
NO2
CH3
4
1 2 3
CH3 O2N
1 6 2 3 4
O2N
NO2
Cl
2-Chloro-1,4-dinitrobenzene
5
NO2
2,4,6-Trinitrotoluene (TNT)
18


Note in the third example shown that -toluene is used as the parent name rather than -benzene. Any of the monosubstituted aromatic compounds shown in Table 7.1 can serve as a parent name, with the principal substituent (–CH3 in toluene) assumed to be on C1.
4
O H3C
HO
H H HO H
O N CH3
Estrone
CH3 O
N
HO
Morphine
Cl
N
Diazepam (Valium)
5


Benzene itself has been found to cause bone marrow depression and consequent leucopenia (lowered white blood cell count) on prolonged exposure. Benzene should therefore be handled cautiously if used as a laboratory solvent.
Br NO2 CH2CH2CH3
Bromobenzene
Nitrobenzene
Propylbenzene
9



Alkyl-substituted benzenes, sometimes referred to as arenas, are named in different ways depending on the size of the alkyl group. If the alkyl substituent is smaller than the ring (six or fewer carbons), the arene is named as an alkyl-substituted benzene. If the alkyl substituent is larger than the ring (more than six carbons), the compound is named as a phenylsubstituted alkane.
Chapter 7
Benzene and Aromaticity
1


In the early days of organic chemistry, the word aromatic was used to describe such fragrant substances as benzaldehyde, and benzene. It was soon realized, however, that substances grouped as aromatic differed from most other organic compounds in their chemical behavior.
Benzaldehyde
O C H
Benzene
Toluene
CH3
2



Today, we use the word aromatic to refer to benzene and its structural relatives. We’ll see in this and the next chapter that aromatic compounds show chemical behavior quite different from the aliphatic compounds we’ve studied to this point. Thus, chemists of the early 19th century were correct about there being a chemical difference between aromatic compounds and others, but the association of aromaticity with fragrance has long been lost.
Table 7.1
8


Monosubstituted benzenes are systematically named in the same manner as other hydrocarbons, with -benzene as the parent name. Thus, C6H5Br is bromobenzene, C6H5NO2 is nitrobenzene, and C6H5CH2CH2CH3 is propylbenzene.




Note that two of the bonding orbitals, ψ2 and ψ3, have the same energy, as do the antibonding orbitals ψ4* and ψ5*. Such orbitals are said to be degenerate. Note also that two of the orbitals, ψ3 and ψ4*, have nodes passing on these carbons. The six p electrons of benzene occupy the three bonding molecular orbitals and are delocalized over the entire conjugated system, leading to the observed 150 kJ/mol stabilization of benzene.
10



The name phenyl, sometimes abbreviated as Ph or , is used for the –C6H5 unit when the benzene ring is considered as a substituent. The word is derived from the Greek pheno (―I bear light‖), commemorating the fact that benzene was discovered by Michael Faraday in 1825 from the oily residue left by the illuminating gas used in London street lamps. As mentioned previously, the C6H5CH2– group is called benzyl.
3



Many compounds isolated from natural sources are aromatic in part. In addition to benzene, benzaldehyde, and toluene, compounds such as the steroidal hormone estrone and the wellknown analgesic morphine have aromatic rings. Many synthetic drugs are also aromatic in part; the tranquilizer diazepam (Valium) is an example.
13
Cl
Cl
Ortho-Dichlorobenzene
1,d
H3C
CH3
O C H
para-Dichlorobenzaldehyde
1,3-disubstituted
meta-Xylene
Cl
1,4-disubstituted
14


The ortho, meta, para system of nomenclature is also useful when discussing reactions. For example, we might describe the reaction of bromine with toluene by saying, ―Reaction occurs at the para position‖ —in other words, at the position para to the methyl group already present on the ring.
15
X
Ortho Meta
Ortho Meta
Para
CH3
CH3
Br2 FeBr3
Br
Toluene
p-Bromotoluene
16


Benzenes with more than two substituents are named by numbering the position of each substituent so that the lowest possible numbers are used. The substituents are listed alphabetically when writing the name.