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Aliphatic vs. Aromatic Compounds

By definition, aliphatic compounds are open-chain compounds and those cyclic compounds which resemble open-chain compounds. Except for the occasional appearance of a phenyl (C6H5) group, the hydrocarbon compounds that we have studied so far have been aliphatic.

Aromatic compounds include benzene and compounds that resemble benzene in their chemical behavior. Simple aromatic rings are aromatic compounds consisting only of conjugated planar ring systems with delocalized pi electron clouds and resonant structures instead of discrete alternating single and double bonds.

Simple aromatic rings can be heterocyclic if they contain non-carbon ring atoms, such as O, N or S. The nitrogen containing aromatic rings can be separated into non-basic and basic aromatic rings.

Non-basic rings: The lone pair of electrons if the N atom is delocalized and contributes to the aromatic pi electron system. In these compounds the nitrogen atom is connected to a hydrogen atom (e.g. pyyrole and indole).

Basic rings: The lone pair of electrons is not part of the aromatic system and extends in the plane of the ring. This lone pair is responsible for the basicity of these nitrogenous bases, similar to the nitrogen atom in amines. In these compounds the nitrogen atom is not connected to a hydrogen atom (e.g. pyridine or quinoline). Several rings contain both basic and non-basic nitrogen atoms (e.g. imadazole and purine). Under acidic conditions, these compounds get protonated and form aromatic cations.

In the oxygen (O) and sulfur (S) containing aromatic rings, one electron pair of the heteroatoms contributes to the aromatic system (like non-basic N), while the second lone pair extends in the plane of the ring (like basic N).

Types of Reaction

Aliphatic compounds - alkanes, alkenes, alkynes, and their cyclic analogs - undergo chiefly addition and free-radical substitution. This includes addition at multiple bonds and free radical substitution at other points along the aliphatic chain. 

Aromatic compounds, on the other hand, are characterized by their tendency to undergo heterolytic substitution. Furthermore, these same substitution reactions are characteristic of aromatic rings wherever they appear, regardless of other functional groups the molecule may contain.

Structure of Benzene

Benzene has the molecular formula C6H6. But how are these atoms arranged ?

All C-C bond lengths in the benzene molecule are equal and are intermediate in length between single and double bonds. Most C=C double bonds in alkenes are found to be about 1.34 angstroms long. C-C single bonds range anywhere from 1.53 angstroms (ethane) to 1.50 angstroms (propylene) to 1.48 angstroms (1,3-butadiene). X-ray diffraction studies show that the six C-C bonds in benzene are equal and have a length of 1.39 angstroms. This value is intermediate between that of typical single and double C-C bonds.

As in ethylene, the p orbital of one C atom can overlap the p orbital of an adjacent C atom, thus permitting the electrons to pair and an additional pi bond to be formed. But the overlap here is not limited to a pair of p orbitals as it was in ethylene. The p orbital of any one C atom overlaps equally well the p orbitals of both C atoms to which it is bonded. The result is two continuous torus (or doughnut) shaped electron clouds, one lying above and one lying below the plane of the atoms. 

As we shall see, the chemical properties of benzene are just what we would expect of this structure. Despite delocalization (which maximizes the stability of this structure) the pi electrons are nevertheless more loosely held than the sigma electrons. The pi electrons are thus particularly available to a reagent that is seeking electrons. The typical reactions of the benzene ting are those in which it serves as a source of electrons for electrophilic (acidic) reagents. Because of the resonance stabilization of the benzene ring, these reactions lead to substitution, whereby the aromatic character of the benzene ring is preserved.  

Aromaticity & the Huckel Rule

Besides the compounds that contain benzene rings, there are many other substances that re called aromatic. Yet some of these superficially bear little resemblance to benzene.

What properties do all aromatic compounds have in common ?

Experimentally, aromatic compounds are those whose molecular formulas would lead us to expect a high degree of unsaturation, and yet which are resistant to the addition reactions generally characteristic of unsaturated compounds. Instead of addition reactions, we often find that these aromatic compounds undergo electrophilic substitution reactions like those of benzene. Along with this resistance toward addition - and presumably the cause of it - we find evidence of unusually high stability (low heats of hydrogenation and low heats of combustion). Aromatic compounds are cyclic - typically containing 5, 6 or 7 membered rings. When examined by physical methods, they are found to be molecularly flat (or nearly so). Their protons show the same sort of chemical shift in NMR spectra as the protons of benzene and its derivatives.

Theoretically, an aromatic molecule must contain cyclic clouds of delocalized pi electrons above and below the plane of the molecule. Furthermore, the pi clouds must contain a total of (4n + 2) pi electrons.  Thus, simple delocalization is not enough to provide the particularly high degree of stability that characterizes an aromatic compound. There must be a particular number of electrons: 2 , 6, 10, etc. This Huckel rule is based on quantum mechanics, and has to do with the filling up of the orbitals that constitute the pi cloud.

For example, consider the following compounds, for each of which just one contributing structure is shown. Each molecule is a hybrid of either 5 or 7 equivalent structures, with the charge or odd electron on each carbon. Yet, of these six compounds, only two give evidence of unusually high stability: the cyclopentadienyl anion and the cyclkoheptatrienyl (tropylium) cation.

Consider the electronic configuration of the cyclopentadienyl anion. Each C atom, trigonally hybridized, is held by a sigma bond to tow other C atoms and one H atom. The ring is regular pentagon, whose angles (108 degrees) are not a bad fit for the 120 degree trigonal angle.

Any instability due to imperfect overlap (angle strain) is more than made up for by the delocalization that is to follow. Four C atoms have one electron each in p orbitals. The fifth C atom (the "one" that lost the proton, but of course, indistinguishable form the others) has two electrons.

Overlap of the p orbitals gives rise to pi clouds containing a total of six electrons - the aromatic sextet. 

Similarly, we arrive at the configuration of the tropylium ion. it has a regular heptagon (angles 128.5 degrees). Six C atoms contribute one p electron each, and the seventh contributes only an empty orbital. Thus, the aromatic sextet.

Six is the Huckel number most often encountered - and for good reason. In order to provide p orbitals, the atoms of the aromatic ring must be trigonally (sp2) hybridized. This means, ideally, bond angles of 12o degrees. To permit the overlap of the p orbitals that give rise to eh pi cloud, the aromatic compound must be flat (or nearly so). The number of trigonally hybridized atoms that fit a flat ring without undue angle strain (i.e. with reasonably good overlap for pi bond formation) is 5 , 6 or 7. Six is the Huckel number of pi electrons that can be provided by these numbers of atoms (benzene being the perfect specimen).  

Now, what evidence is there that other Huckel numbers 2, 10, 14, etc. are also magic numbers ? In such cases, the rings will be either too small or too large to accommodate trigonally hybridized atoms very well, so that any stabilization due to aromaticity may be largely offset by angle strain or poor overlap of p orbitals, or both.

We must therefore look for stability on a relative basis, and may find evidence of aromaticity only in the fact that one molecular species is less unstable than its relatives. The net effect of a great deal of elegant work is strongly to support the 4n + 2 rule. The question now seems rather to be: over how many unfavorable a combination of angle strain and multiple charge can aromaticity manifest itself ?