Definition, characteristics of aromatic compounds, Huckel's rule, structure of benzene, isomerism and orientation of benzene derivatives

Aromatic compounds

These carbocylic compounds contain at east one benzene ring i.e., a ring of six carbons atoms with alternate double and single bonds. Aromatic compounds are called so because many of them possess a fragrant smell.

Typical examples of aromatic compounds are given below:

Benzene

The aromatic compounds may have a side-chain or a functional group attached directly to the ring. For example,

Toluene Phenol Nitrobenzene Benzaldehyde

The aromatic compounds may also contain more than one benzene rings fused together

Naphthalene Anthracene

There are two classifications of aromatic compounds.

Nuclear Substituted Compounds

When the functional group or any substituent, in aromatic compounds is directly attached to the benzene ring, it is a called nuclear substituted compound. Such compounds are named as the derivatives of benzene under the IUPAC system. However, the common names of many such compounds are retained by IUPAC.

Sidechain Substituted Compounds

Aromatic compounds where the functional group is present in the sidechain of the ring are called sidechain substituted compounds. Sidechain substituted compounds are named as the phenyl derivatives of the corresponding aliphatic compounds.

Naming Benzene Derivatives

Only one kind of monosubstituted derivatives are possible in benzene rings as all six hydrogen atoms are equivalent. For example, there is only one toluene. It does not matter where the methyl group is attached because all the following arrangements are equivalent.

When two same or different monovalent substituents, are present on a benzene ring, the following three arrangements are possible.

For the same substituent (A)

ortho (or, 1,2-) meta-(or, 1,3-) para- (or, 1,4)

For different substituents (A and B)

ortho (or, 1,2-) meta-(or, 1,3-) para- (or, 1,4)

These arrangements are named as follows:

The compound containing the two groups on the adjacent sites is called 'ortho'; it is denoted as 'o-'. In the IUPAC system, the ortho position is designated as 1,2-.

The compound containing the two groups on alternate sites is called 'meta': it is denoted as 'm-'. In the IUPAC system,the meta position is designated as 1,3-.

The compound containing the two groups diagonally opposite to each other is called 'para': denoted as 'p-'. In the IUPAC system, the para position is designated as 1,4-.

For example, the three xylenes are named as,

o-xylene m-xylene p- xylene

(1,2-dimethylbenzene) (1,3-dimethylbenzene) (1,3-dimethylbenzene)

In the case of trisubstituted derivatives, the nature of the substituted groups determine the number of arrangements. When the three substituent groups are identical (say, A), three arrangements are possible. These are termed as follows.

vicinal (vic-) unsymmetrical (unsym-) symmetrical (sym-)

For a trisubstituted product, if the two substituents are identical and the third different, then six products are possible. When all the three groups are different, ten products are possible. Since, naming each individual compound is not possible, it was found convenient to indicate the position of any substituent by the numeral indicating the serial number of the carbon atom in the ring, to which that substituent is attached.

Numbering the Carbon Atoms in the Ring

The numbering of carbon atoms in the ring (or nucleus) is done as follows.

• When there is only one substituent on the ring, there is only one compound possible. Thus, numbering of the carbon atoms of the nucleus does not arises.
• If there are two or more substituents, then numbering is in the alphabetical order of the substituents on the carbon atoms. The prefixes such as 'di', 'tri', 'cyclo', 'iso', etc., are ignored while arranging the substituents alphabetically.
• When two or more functional groups are present, then the principal functional group is assigned the number 1. The order of priority of the functional groups is the same as done for aliphatic polyfunctional compounds.

For the sake of convenience, the ring is oriented in such a way that position 1 is at the top and numbering is done in a clockwise or anticlockwise manner whichever gives lower numbers to the other substituents.

This is illustrated through the following example. The IUPAC names are written in bold letters.

(o-xylene) (m-xylene) (p-xylene)

1,2-dimethylbenzene 1,3-dimethylbenzene 1,4-dimethylbenzene

Names of some typical aromatic compounds are given below:

Aromatic hydrocarbons (arenes)

toluene o-xylene m-xylene p-xylene

Methylbenzene 1,2-dimethylbenzene 1,3-dimethylbenzene 1,4-dimethylbenzene

1,3,5-trimethylbenzene 2-phenylpropane(cumene) phenylethene (styrene)

Anthracene Phenonthrene

Halogen derivatives

2-chlorotoulene 1,2-dichlorobenzene phenyl chloromethane

Hydroxy derivatives:

Phenols and aromatic alcohols

2-methyl phenol phenylmethanol 1,4-dihydroxy benzene 2,4,6-trinitrophenol(o-cresol) (picric acid)

Aldehydes and Ketones

Nuclear substituted

Benzaldehyde Methyl phenyl ketone Diphenyl ketone(Acetophenone) (Benzophenone)

Carboxylic acids

Nuclear substituted

benzoic acid 2-methylbenzoic acid 2-hydroxybenzoic acid 1,4-benzenedicarboxylic acid(o-toluic acid) (o-salicylic acid) (terephthalic acid)

Acid derivatives

Benzoyl chloride Benzamide Phenyl benzoate

Benzoic anhydride Ethyl-4-bromobenzoate

Alkoxy derivatives

Methoxy benzene 1-methoxy- phenoxybenzene4-nitrobenzene

Amines

Aminobenzene 2-Amino toluene Benzyl amine

Nitro derivatives

nitrobenzene 1,3-dinitrobenzene 2,4,6-trinitrotoluene(m-nitrobenzene)

Nitriles and Carbylamines

Benzonitrile Phenylcarbylamine(phenyl isocyanide) (phenyl cyanide)

Sulphonic acids

benzenesulphonic acid benzene-1,3-disulphonic acid

Aromaticity

Aromatic compounds are those, which resemble benzene in chemical behavior. These compounds contain alternate double and single bonds in a cyclic structure. They undergo substitution reactions rather than addition reactions. This characteristic behavior is called aromatic character or aromaticity the criteria for which are as follows:

• Contains a cyclic cloud of delocalized p electrons above and below the plane of the molecule.

• Electrons cloud must contain a total of (4n+2) a electrons, where p is an integer equal to 0,1,2,3 ...........

This is known as 'Huckel rule' according to which the aromatic compounds have delocalized electron cloud of p electrons of 2 or 6 or 10 or 14 electrons.

For example, benzene (6p electrons), naphthalene (10p electrons) and anthracene (14p electrons) are aromatic compounds.

Benzene Naphthalene Anthracene

6p electrons 10p electrons 14p electrons

Similarly, cyclopentadienyl anion and cycloheptatrienyl cation (tropylium ion) are also aromatic because these contain 6p electrons.

Cyclopentadienyl anion Cycloheptatrienyl cation

6p electrons (Tropylium ion) 6p electrons

Heterocyclic compounds such as pyrrole, furan, thiophene and pyridine also behave as aromatic because all have 6p electrons.

Furan Thiophene Pyrrole Pyridine

Structure of benzene

The molecular formula of benzene has been found from analytical data, to be C₆H₆. Relatively higher proportion of carbon and addition of chlorine to benzene molecule indicate it to be an unsaturated compound. Depending on the various facts available to scientists from time to time, many structures for benzene had been proposed. Some are described below.

Open Chain Structure

Based upon observable facts given above and the tetravalency of carbon, the following open chain structures were proposed for benzene.

Drawbacks of open chain structure

The open chain structure for benzene was rejected due to the following reasons:

• Addition reactions usually given by alkenes and alkynes are not given by benzene.

• Benzene forms only one kind of mono- substituted product. An open chain structure however, can form more than one kind of monosubstituted product as shown below:

• The open chain compounds do, not give reactions such as Friedel-Craft reaction, nitration, sulphonation.

• On reduction with hydrogen in the presence of Ni at 200°C, actually a cyclic compound cyclohexane is obtained.

These facts suggest a ring structure for benzene.

Ring structure of benzene

After taking into account account the above observed facts, Kekule (1865) suggested a ring structure for benzene. According to him, six carbon atoms occupied six corners of a regular hexagon in benzene and each carbon carried one hydrogen atom. To satisfy the tetravalency of carbon, the system consisted of alternate single and double bonds. Kekule's formula is shown below.

Defects in Kekule's formula

While Kekule's formula explained most of theory served facts for benzene, it could still not explain the following facts,

• The saturated nature of benzene.

• In actuality only one kind of ortho derivatives are known, but according to Kekule's formula, there can be two ortho positions.

The defect of having two ortho positions was explained by proposing that the positions of the double bonds in benzene are not fixed. Instead, the double bonds in the benzene molecule keep changing their positions and thus all positions in benzene molecule become identical.

Chemists generally used the Kekule's structure as late as 1945. Many ring structures for benzene have been proposed after Kekule's structure. Some of them are,

Claus diagonal Dewar'sformula(1867) formula(1867)

Resonance hybrid structure of benzene

The currently accepted structure was developed by the application of the theory of resonance proposed in 1933. This theory states that benzene is a resonance hybrid of the following canonical forms.

Since, the forms I and II are the most contributing, hence benzene is represented as a hybrid structure of these two structures, i.e.,

Resonance hybrid

Evidences which support resonance structure of benzene

The following facts support the resonance structure of benzene:

• The carbon-carbon bond length in benzene is identical at 139 pm, for all bonds. This value is intermediate between the bond lengths for C-C bond (154 pm) and C = C (134 pm).

• A regular hexagonal structure for benzene is obtained by X-ray diffraction, which gives a C-C bond length of 139 pm.

Large resonance energy

Due to resonance, the p-electron charge in benzene gets distributed over greater area, i.e., gets delocalized. Delocalization results in the energy of the resonance hybrid decreasing relative to the contributing structures, by about 150 kJ mol^-1. This decrease in energy is called resonance energy. The unusual stability of benzene is due to this resonance stabilization.

Orbital structure of benzene

X-ray studies show that a

• Benzene molecule is a flat (planar) molecule. All carbon and hydrogen atoms lie in the same plane.

• It has a regular hexagon structure with all six carbon atoms lying at the corners; each carbon atom is bonded to three other atoms.

• All carbon-carbon bond lengths are equal at 139 pm.

• All CC angles (or CH angles) are equal at 120°.

These results indicate that each carbon atom in benzene molecule is sp² hybridized. All sp² hybrid orbitals lie in the same plane (the plane of the carbon atoms) and are oriented towards the corners of an equilateral triangle. Thus, each carbon in benzene has three sp² hybrid orbitals lying in the same plane and one -unhybridized 'p' orbital.

(a) Formation of a planar hexagonal structure due to overlapping of the sp² hybrid orbital of each carbon atom with its neighboring carbon atoms and hydrogen atoms.

(b) A unhybridized 2p orbital on each carbon lies perpendicular to the carbon-carbon plane.

Out of the three hybrid orbitals, two overlap axially with the orbitals of the neighboring carbon atoms on either sides to form C-C 's' bonds. The third, sp² hybridized orbital of the carbon atom overlaps with the half-filled '1s' orbital of the hydrogen atom forming a 's' C-H bond.

A planar hexagonal structure is formed when six carbons are placed in a hexagonal geometry, the orbital overlapping leads to the structure (a).

In (b), each carbon is left with one unused '2p' orbital at right angle, to the hexagon. These unused '2p' orbitals of carbon atoms overlap each other sideways, and form carbon-carbon p-bonds. As the system is completely symmetrical, the '2p' orbitals can overlap sideways equally well with either of the neighboring carbon atoms. Hence sideways overlapping of '2p' orbitals of carbon atoms can form two sets of

p-bonds as shown.

Sideways overlap of 2p orbitals leading to formation of two sets of p-bonds.

All the 'p' orbitals on the six C atoms in benzene molecule are equidistant from each other. Thus 'p' orbitals of any one carbon atom are able to overlap equally well with the similar orbitals of both the carbon atoms on either sides. A continuous ring of electron cloud covering all the six carbon atoms results because of such overlap. Since a 'p' orbital consists of two equal lobes, one lying above and the other below the plane of the ring, the sideways overlapping of the p orbitals in benzene molecule leads to a molecular orbital consisting to two continuous rings, one lying above, and the other below the plane of the ring as shown.

The continuous rings of the p molecular orbital of benzene. One lying above and the other below the plane of the ring.

The shape and size of benzene molecule.

Thus, each bond in benzene has a character intermediate between a single and a double bond.

orientation of benzene derivatives

The substituent already present on the benzene ring directs the incoming substituent to occupy ortho (2 or 6), meta (3 or 5) or para (4) position. This direction depends on the nature of the first substituent and is calleddirective or the orientation effect.

The substituent already present can increase or decrease the rate of further substitution, i.e., it either activates or deactivates the benzene ring towards further substitution. These effects are called activity effects.

There are two types of substituents which produce directive effect are,

(i) Those which direct the incoming group to ortho- and para-positions simultaneously (Neglecting meta all together).

(ii) Those which direct the incoming group to meta-position only (Neglecting ortho- and para-positions all together).

ortho para directors

Strong activating NH2, NHR, NR2, OH, O:-

moderately activating NHCOCH3, NHCOR, OCH3, OR

weakly activating CH3, C2H5, R, C6H5

Meta directors

Strong deactivating CN, SO3H, COOH, COOR, CHO, COR

Moderately deactivating NO2, NR3, CF3, CCl3

Weakly deactivating F, Cl, Br, I