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Identifing Aromatic and Anti-Aromatic Compounds

It was first devised by Huckel in 1931. The present study will be an innovative method1-8 involving two formulae by just manipulating the no of π bonds within the ring system and delocalized electron pair (excluding π electron pair within the ring system) with one (01).

Predicting Aromatic behavior

In the first case, the compound must be cyclic, planar (i.e. all the carbon atoms having same state of hybridization) with even number of A value, where

\[A = \pi b + e^-p + 1(constant) \tag{1}\]

, here πb = number of π bonds with in the ring system and e-p = number of electron pair outside or adjacent to the ring system i.e. if the ring contains hetero atoms (atoms containing lone pair of electrons) which can undergo delocalization and each negative charge if present may be treated as one pair of electrons.

If the value of ‘A’, for a certain organic compound comes out as even number then this compound will be treated as aromatic compound.

Predicting Anti-aromatic behavior

In the second case, the compound must be cyclic, planar (i.e. all the carbon atoms having same state of hybridization) with odd number of A value, where

\[A = \pi b + e^-p + 1(constant)\]

here πb = number of π bonds with in the ring system and e-p = number of electron pair outside or adjacent to the ring system i.e. if the ring contains hetero atoms which can undergo delocalization and each negative charge if present, may be treated  as one pair of electrons.

If the value of ‘A’, for a certain organic compound comes out as odd number then this compound is anti-aromatic.

General Conditions for Non-Aromatic Behavior

Any compound that lacks one or more of the above features i.e. it may be acyclic / non-planar, is to be treated as non aromatic. But in this case, ‘A’ value may be even or odd number. It is always to be noted that if the ring contains hetero atom like N, O, S etc, in this case we must count that electron pair in the evaluation of ‘A’ value which can undergo delocalization. We never count localized electron pair. Examples have been illustrated in Table 1.

Table 1: Aromatic, anti-aromatic and non-aromatic behavior of organic compounds

Organic Compound

(Cyclic, Planar/Cyclic, non-planar)

πb value

[number of π bonds with in the ring system]

e-p value

[ number of delocalized electron pair outside or adjacent to the ring system]

A value

[A = πb + e-p + 1(constant)]

(even no/odd no)

Nature

of compound

( aromatic/anti-aromatic/non aromatic)

Benzene or [6] annulene

(Cyclic, Planar)

3 π bonds

0

3 + 0+1 = 4

(even no)

Aromatic

Naphthalene

(Cyclic, Planar)

5 π bonds

0

5 + 0 +1 = 6

(even no)

Aromatic

Anthracene

(Cyclic, Planar)

7 π bonds

0

7 + 0 + 1 = 8

(even no)

Aromatic

Cyclopropene

(Cyclic, non planar due to one sp3 hybridized carbon atom)

1 π bond

0

1 + 0 + 1 = 2

(even no)

Non-aromatic

Cyclopropenyl cation

(Cyclic, Planar)

1 π bond

0

1 + 0 + 1 = 2

(even no)

Aromatic

 

 

Cyclopropenyl anion

(Cyclic, Planar)

1 π bond

1

(For one negative charge on carbon which undergoes delocalization)

1 + 1+ 1 = 3

(odd no)

Anti-aromatic

Cyclobutadiene or

[4] annulene

(Cyclic, Planar)

2 π bonds

0

2 + 0 + 1 = 3 (odd no)

Anti aromatic

Cyclopentadiene

(Cyclic, non planar due to one sp3 hybridised carbon atom)

2 π bonds

0

2 + 0 + 1 = 3 (odd no)

Non-aromatic

Cyclopentadienyl cation

(Cyclic, Planar)

2 π bonds

0

2 + 0 + 1 = 3 (odd no)

 

Anti-aromatic

Cyclopentadienyl anion

(Cyclic, Planar)

2 π bonds

01(For one negative charge on carbon which undergo delocalization)

2 + 1 + 1 = 4

(even no)

Aromatic

Cyclooctatetraene or

[8] annulene

(Cyclic, Planar)

4 π bonds

0

4 + 0 + 1 = 5

(odd no)

Anti-aromatic

 

 

 

Cyclooctatrienyl cation

(Cyclic, non-planar due to one sp3 hybridized carbon atom adjacent to positive charge)

3 π bonds

0

3 + 0 + 1 = 4

(even no)

Non aromatic

 

Pyridine

(Cyclic, Planar)

3 π bonds

0

( Here lone pair on N does not take part in delocalization)

3 + 0 + 1 = 4

(even no)

Aromatic

Pyrrole

2 π bonds

1

( Here lone pair on N take part in delocalization)

2 + 1 + 1 = 4

(even no)

Aromatic

Furan

2 π bonds

1

( Here out of two lone pairs on O only one LP take part in delocalization)

2 + 1 + 1 = 4

(even no)

Aromatic

 

There are some compounds which do not follow the above rule. Huckel’s also cannot explain the aromatic or non aromatic behavior of these compounds. These compounds have been represented in the Table 2.                                                       

 

Table 2 : Omission behavior of aromatic and non aromatic organic compounds

Organic Compound

(Cyclic, Planar/Cyclic, non-planar)

πb value

[number of π bonds with in the ring system]

e-p value

[ number of delocalized electron pair outside or adjacent to the ring system]

A value

[A = πb + e-p + 1(constant)]

Nature

of compound

Remarks on Exception Behavior

Cyclodecapentaene 

or [10]annulene 

(C10H10)

5 π bonds

0

5 + 0 + 1 = 6 (even no.)

Not aromatic

Due to the interaction of the hydrogen of 1 and 6 compound become non planar.

(combination of steric and angular strain)

Pyrene

(C16H10)

8 π bonds

0

8 + 0 + 1 = 9 (odd no.)

Aromatic

Because double bonded C15-C16 do not take part in resonance.

 

If we easily predict the nature of organic compound i.e. aromatic, anti aromatic or non aromatic then we can resolve different kind of problems regarding stability, reactivity, acidity etc. by using the following supposition.

  1. Order of stability is aromatic > non aromatic > anti aromatic
  2. Order of reactivity just follows the reverse order of stability as Anti-aromatic > non aromatic > aromatic
  3. Acidity: Stability of Conjugate base α acidity

eg: cyclopentadienyl anion(aromatic) > cyclopentadiene (non-aromatic) > cyclopentadienyl cation (anti aromatic). Hence, cyclopentadiene (its conjugate base i.e. Cyclopentadienyl anion is aromatic in nature) is much more acidic than cycloheptatriene (its conjugate base i.e. Cycloheptatrienyl anion is anti-aromatic in nature).

References

  1. I.L. Finar, Organic Chemistry, Vol-1, Pearson, 6th ed., 2004, 1, 532, 577-580
  2. R.T. Morrison and R.N. Boyd, 7th ed., Organic Chemistry, Pearson, 7th ed. 208
  3. T.W.Graham Solomons and C.B. Fryhle, Organic Chemistry, Wiley: India 10th ed., 2009, 2012, 150, 632-639
  4. J.G. Smith, Organic Chemistry, 2nd ed., 2008, 616-619
  5. Jerry March, Advanced Organic Chemistry: Reaction, Mechanisms and Structure, 3rd ed.,Willy: New York, 1985, ISBN 0-471-85472-7
  6. Arijit Das, Suman Adhikari, Bijaya Paul, R. Sanjeev and V. Jagannadham, ‘A rapid and innovative method for the identification of  aromatic and anti-aromatic nature of organic compounds’,WJCE, 2013, 01(01), 6-8,  doi :10.12691/wjce-1-1-2
  7. Arijit Das, R.Sanjeev and V.Jagannadham, “Innovative And Time Economic Pedagogical Views In Chemical Education – A Review Article”, World Journal of Chemical Education, 2014, 2(3), 29-38, Science and Education Publishing , USA, DOI:10.12691/wjce-2-3-1
  8. Arijit Das, ‘Simple Thinking Makes Chemistry Metabolic And Interesting- A Review Article’, IOSR-JAC, 2013, 6(4), 8-15, e-ISSN: 2278- 5736, doi:10.9790/5736-0640815

Contributor

  • Dr. Arijit Das, Ph.D. (Inorganic Chemistry), MACS ( Invited,USA ), SFICS, MISC, MIAFS (India), Assistant Professor, Department of Chemistry, Ramthakur College, Agartala, Tripura(W), Tripura, India, Pin-799003.