Basic Organic Nomenclature

Aromatic Compounds - Benzene

prefix: phenyl

Aromatic compounds refer to a group of chemicals which rather than having a strong odor, have a fully conjugated cyclic (or polycyclic) structure. This arrangement of sp2 hybridized carbon atoms gives the compound a unique stability, making them less reactive than their alkene counterparts. All aromatic compounds obey Huckel's rule, possessing 4n + 2 p electrons (n=1,2,3, ...) in a cyclic arrangement. The simplest carbon example of an aromatic system is benzene (C6H6, or named as an alkene, 1,3,5-cyclohexatriene). All of the carbon atoms are sp2 hybridized which results in the planar structure common to aromatic compounds. There are a number of equivalent ways to draw benzene:



The stability of these compounds is a result of the 3 p bonds interacting with one another. As can be seen in the left and central structures above, the double bonds can be drawn between any pair of adjacent carbon atoms. In fact the carbon - carbon bond lengths in benzene are all the same, with a bond length between that of a carbon - carbon double bond and single bond. The structure on the right emphasizes this by the circle in the center which is meant to indicate that all C -C bonds are equivalent. Remember benzene (C6H6) is different from cyclohexane (C6H12), compare the structures below.
 

Compound Name
Line Drawing
3D Model
Cyclohexane
Benzene

Polycyclic Aromatics:

There are a number of polycyclic aromatic compounds, several of the more common ones are (including the numbering scheme for substituents):
 
Compound Name
Line Drawing
3D Model
Naphthalene
Anthracene
 Phenanthrene

Hetro-aromatics:

There are also a large number of compounds which contain hetro-atoms (O, N, S) which are also aromatic. This is because the hetro-atom is either involved in a double bond in the ring, or it can make use of a lone pair of electrons to interact with the p electrons in the double bonds within the ring to satisfy Huckel's rule. Several of the simpler and more common ones are:
 
Compound Name
Line Drawing
3D Model
Furan
Pyrrole
 Pyridine
Indole
Purine *

* Purine is a precursor to the DNA bases Adenine and Guanine.

Benzene as a substiuent:

Benzene can of course be a substituent on a molecule. When this occurs the benzene ring is referred to as a phenyl group. Commonly it is written as C6H5-R or Ph-R or in a line drawing as a benzene ring (see top of this page) or . Another common substituent is a R-CH2-C6H5 group which is called a benzyl substituent. A general short hand for any aromatic system is Ar-.


Next Page: Aromatic - II

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