Prediction of Aromatic, Anti Aromatic and Non Aromatic Character of Heterocyclic Compounds along with their Omission Behavior- Innovative Mnemonics

In this article, formulae based mnemonics by classifying lone pair of electrons (localized or delocalized) have been highlighted in an innovative and time economic way to enhance interest of students’ on heterocyclic chemistry for determination of planarity by calculating Hybridization state of hetero atom and by prediction of Aromatic, Anti aromatic, non aromatic behavior of different heterocyclic compounds . Here, I have tried to hub three (03) time economic mnemonics by including two (02) formulae for the prediction of hybridization of hetero atom, aromatic and anti aromatic behavior of heterocyclic compounds. This article encourages students to solve multiple choice type questions (MCQs) on ‘Aromaticity of Heterocyclic compounds’ at different competitive examinations in a time economic way.

Introduction

The conventional methods1-7 for determination of hybridization state of hetero atom (planarity of molecule), prediction of aromatic and anti aromatic nature of heterocyclic compound is time consuming. Keeping this in mind, in this article, I have introduced three time economic innovative mnemonics by using two formulae for the prediction of hybridization state of hetero atom of heterocyclic compounds to determine its planarity and aromatic / anti aromatic / non aromatic nature of heterocyclic compounds containing one, two or more number of hetero atoms to make heterocyclic chemistry metabolic and interesting for students. This study also shows omission behavior of some heterocyclic compounds with respect to their aromatic/anti aromatic/non aromatic nature due to presence or absence of vacant d orbitals in DLP based hetero atoms and how lone pair electron discriminates prediction of hybridization state of hetero atom in heterocyclic compound with prediction of its Aromatic and Anti Aromatic nature.

Time Economic Innovative Mnemonics in Heterocyclic Chemistry

Classification of Lone Pair Electron

Lone Pair of electrons can be generally classified into two types as Delocalized lone pair of electron (DLP) and Localized lone pair of electron (LLP) as follows:

i)Delocalized lone pair of electron (DLP): When lone pair of electron of hetero atom undergo delocalization through conjugation then it is to be treated as delocalized lone pair of electron (DLP). Hetero atom (atom containing lone pair of electron) which is directly attached with single bonds only from all ends is to be considered as DLP containing hetero atom and its lone pair is to be treated as (DLP).

Example \(\PageIndex{1}\):

Eg. In Pyrrole lone pair of N atom is to be treated as DLP because it is directly attached with three single bonds only.

ii)Localized lone pair of electron (LLP): When lone pair of electron of hetero atom does not undergo delocalization through conjugation then it is to be treated as Localized lone pair of electron (LLP). Hetero atom (atom containing lone pair of electron) which is directly attached with single and double bonds with the ring system is to be considered as LLP containing hetero atom and its lone pair is to be treated as localized lone pair of electron (LLP).

Example \(\PageIndex{2}\):

Eg. In Pyridine lone pair of N atom is to be treated as LLP because it is directly attached with double and single bonds with the ring system.

Aromaticity

It was first devised by Hückel in 1931. Conventional method for prediction of Aromatic nature of organic compound:

1. Cyclic molecule,
2. Planer molecule in which all bonded atoms lie in same plane (having sp2 hybridized)
3. Conjugated molecule with conjugated π-electron system,
4. Contains (4n + 2) π electrons, where, n is a positive integer (n = 0,1,2,3 etc.)

Conventional method for Anti Aromatic nature of organic Compound:

Cyclic molecule,

Planer molecule in which all bonded atoms lie in same plane (having sp2 hybridized)

Conjugated molecule with conjugated π-electron system,

4nπ electrons, where, n is a positive integer (n = 0,1,2,3 etc.)

Conventional method for identification of Non Aromatic Nature of organic Compound:

If a compound violates any one of the above three conditions (1 or 2 or 3) then it is non aromatic in nature.

Planarity of Heterocyclic Compounds with the prediction of Hybridization State

Planarity of heterocyclic compounds depends on the nature of the hybridization state of carbon and hetero atoms present in it. When all atoms (carbon and hetero) in the heterocyclic compounds having sp2 hybridized then it is planar but when there is a mixing of sp2 and sp3 hybridization state then it is treated as non planar.

Hybridization state theory

Prof. Linus Pauling (1931) first developed the Hybridization state theory in order to explain the structure of molecules such as methane (CH4).This concept was developed for simple chemical systems but this one applied more widely later on and from today’s point of view it is considered an operative empirical for excusing the structures of organic and inorganic compounds along with their related problems.

Conventional method for prediction of hybridization state:

Hybridization state for a molecule can be calculated by the formula 0.5 (V+H−C+A), Where, V = Number of valance electrons in central atom, H = Number of surrounding monovalent atoms, C = Cationic charge, A = Anionic charge

Innovative Mnemonics for the prediction of hybridization state of hetero atom in the heterocyclic compounds with LLP:

Formula 1: Prediction of hybridization state of hetero atom

Power on the Hybridization state of the hetero atom =

(Total no of σ bonds around each hetero atom - 1)

This formula should be applicable up to 4 σ bonds.

If the power of the hybridization state will be 03, 02 and 01 then the hybridization state will be sp3, sp2 and sp respectively. All single (-) bonds are σ bond, in double bond (=) there is one σ and one π. In addition to these each localized lone pair of electron (LLP) can be treated as one σ bond. Hybridization State of Hetero atom with the help of LLP to find out the planarity in Heterocyclic Compounds are shown in Table-1 below.

Table-1 (Hybridization state of Hetero atom in Heterocyclic Compounds with the help of LLP)

Heterocyclic Compounds (Planar/non planar)	Number of σ bonds around hetero atom (from single and double bonds) (A)	LLP (localized Lone Pair of e-s) (B)	Total Number of σ bonds around hetero atom (A+B)	Power of the Hybridization state of the hetero atom (Corresponding Hybridization state) = (A+B)-1
Pyrrole (Planar)	03	0 (lone pair of electron undergo delocalization,DLP with the ring system)	03	02 (sp2 N)
Furan (Planar)	02	01 (out of two lone pair of electrons, one undergo delocalization,DLP and other remain as LLP)	03	02 (sp2 O)
Thiophene (Planar)	02	01 (out of two lone pair of electrons, one undergo delocalization, DLP and other remain as LLP)	03	02 (sp2 S)
Pyridine (Planar)	02	01	03	02 (sp2 N)
Indole (Planar)	03	0	03	02 (sp2 N)
Quinoline (Planar)	02	01	03	02 (sp2 N)
(Planar)	02	01	03	02 (sp2 N)
(Planar)	03 (N1) 02 (N3)	0 (N1) 01 (N3)	03 03	02 (sp2 N1) 02 (sp2 N3)
(Planar)	02 (N1) 02 (N3)	01 (N1) 01 (N3)	03 03	02 (sp2 N1) 02 (sp2 N3)
(Planar)	02 (N1) 02 (N3) 02 (N7) 03 (N9)	01 (N1) 01 (N3) 01 (N7) 0 (N9)	03 03 03 03	02 (sp2 N1) 02 (sp2 N3) 02 (sp2 N7) 02 (sp2 N9)
(Planar)	02 (N) 02 (S)	01 (N) 01 (S) (out of two lone pair of electrons on S, one undergo delocalization,DLP and other remain as LLP)	03 03	02 (sp2 N) 02 (sp2 S)
(Planar)	02 (N) 02 (S)	01 (N) 01 (S) (out of two lone pair of electrons on S, one undergo delocalization,DLP and other remain as LLP)	03 03	02 (sp2 N) 02 (sp2 S)
(Planar)	02 (N1) 02 (N1)	01 (N1) 01 (N1)	03 03	02 (sp2 N1) 02 (sp2 N4)
(Planar)	02 (N1,N3 and N5)	01 (N1,N3 and N5)	03	02 (sp2 N1,N3,N5)
(Planar)	03 (N) 02 (S)	0 (N) 01 (S) (out of two lone pair of electrons on S, one undergo delocalization, DLP and other is LLP)	03 03	02 (sp2 N) 02 (sp2 S)
(Planar)	02 (both N)	01 (both N)	03	02 (sp2 both N)
(Planar)	02 (N1,N2,N3,N4)	01 (N1,N2,N3,N4)	03	02 (sp2 All N)
(Planar)	02	01	03	02 (sp2 N)
(Planar)	02	01	03	02 (sp2 N)
(Non Planar)	03	01	04	03 (sp3 N)
(Non Planar)	02	02	04	03 (sp3 O)

Innovative Mnemonics for the Prediction of Aromatic, Anti Aromatic behavior of Heterocyclic Compounds with DLP:

The present study will be an innovative mnemonic involving calculation of ‘A’ value by just manipulating the no of π bonds within the ring system and delocalized lone pair of electron (DLP) with one (01).

The heterocyclic compound having cyclic, planar, conjugated (i.e. all the carbon atoms having same state of hybridization, sp2) with even number of ‘A’ value will be treated as aromatic in nature and with odd number of ‘A’ value will be treated as anti aromatic in nature.

Formula 2: Evaluation of A Value to predict Aromatic and Anti Aromatic Nature

A = πb+DLP+1(constant) = even no = Aromatic

A = πb+DLP+1(constant) = odd no = Anti Aromatic

where, πb = number of π bonds with in the ring system;

DLP = Delocalized lone pair of electron.

In case of a multi hetero atom based heterocyclic compound, containing both DLP and LLP hetero atoms,

Aromatic and Anti Aromatic behaviour should be predicted with respect to DLP based hetero atom only.

Example \(\PageIndex{3}\): Benzothiazole

Benzothiazole (Figure 1), is a multi hetero atom based heterocyclic compound, containing both DLP and LLP hetero atoms. Here, for N, DLP = 0 , LLP = 1 and for S, DLP = 1, LLP =1, so, in this case ‘A’ value should be calculated with respect to S only not N. Here, A = 4 + 1 + 1 = 6 (even no) = Aromatic.

Figure 1. Structure of Benzothiazole

But when heterocyclic compounds contain both LLP based hetero atoms then Aromaticity should be predicted with respect to that hetero atom which contains lowest possible position number as per IUPAC nomenclature or any one of the hetero atom.

Example \(\PageIndex{3}\): Imidazole

Imidazole (Figure 2) is a multi hetero atom based hetero cyclic compound in which, N1 is DLP based hetero atom and N3 is LLP based hetero atom. In this case Aromaticity should be predicted with respect to the DLP based hetero atom N1.

For N1, A = πb+DLP+1(constant) = 2+1+1 = 4 (even No) - Aromatic

Figure 2. Structure of Imidazole

Eg. Pyrimidine (Figure 3) is a multi hetero atom based hetero cyclic compound in which, both N1 & N3 are in same environment based hetero atoms (LLP based hetero atoms). In this case Aromaticity should be predicted with respect to N1 (lowest possible position number as per IUPAC nemenclature).

For N1, A = πb+DLP+1(constant) = 3+0+1 = 4 (even no) - Aromatic

Aromaticity of heterocyclic compounds have been illustrated in Table-2

Table-2 (Aromatic-Anti Aromatic and Non Aromatic behavior of heterocyclic compounds With the help of DLP)

Hetero Cyclic Compound (Cyclic, Planar, Conjugated)	πb value [πb =number of π bonds with in the ring system]	DLP	A value [A = πb + DLP + 1(constant)] (even No /odd No)	Remark on Nature of compound (Aromatic/Anti Aromatic)
Pyrrole	2	1	2 + 1 + 1 = 4 (even No)	Aromatic
Furan	2	1 ( Here out of two lone pairs on O only one LP take part in delocalization)	2 + 1 + 1 = 4 (even No)	Aromatic
Thiophene	2	1 (Here out of two lone pairs on O only one LP take part in delocalization)	2 + 1 + 1 = 4 (even No)	Aromatic
Pyridine	3	0	3 + 0 + 1 = 4 (even No)	Aromatic
Indole	4	1	4 + 1 + 1 = 6 (even No)	Aromatic
Quinoline	5	0	5 + 0 + 1 = 6 (even No)	Aromatic
	05	0	5 + 0 + 1 = 6 (even No)	Aromatic
	02	01 (N1)	2 + 1 + 1 = 4 (even No)	Aromatic
(m-diazine)	03	0 (N1)	3 + 0 + 1 = 4 (even No)	Aromatic
	04	01 (N9)	4 + 1 + 1 = 6 (even No)	Aromatic
	02	01 (S)	2 + 1 + 1 = 4 (even No)	Aromatic
	04	01 (S)	4 + 1 + 1 = 6 (even No)	Aromatic
	03	0	3 + 0 + 1 = 4 (even No)	Aromatic
	03	0	3 + 0 + 1 = 4 (even No)	Aromatic
	07	0	7 + 0 + 1 = 8 (even No)	Aromatic
	03	0	3 + 0 + 1 = 4 (even No)	Aromatic
	04	0	4 + 0 + 1 = 5 (odd No)	Anti aromatic
	02	0	2 + 0 + 1 = 3 (odd No)	Anti aromatic
Hetero Cyclic Compound (Cyclic, non-planar)	πb value [πb =number of π bonds with in the ring system]	DLP	A value [A = πb + DLP + 1(constant)] (even No/odd No)	Remark on Nature of compound
	-	-	-	Non Aromatic (non planar – sp3)
	-	-	-	Non Aromatic (non planar – sp3)

Omission behavior of some heterocyclic compounds with respect to their Aromatic / Anti Aromatic and Non Aromatic nature :

Aromatic Behavior of some heterocyclic compounds containing different DLP based hetero atoms (one contains vacant d orbitals) :

Figure 4. Structures of Phenothiazine

In Phenothiazine (Figure 4), there is two DLP based hetero atoms N and S. In between N and S, since S

having vacant d orbitals, so, in this case ‘A’ value will be predicted with respect to DLP based S

hetero atom which contains vacant d orbitals only.

Here, A = πb + DLP + 1(constant) = 6 +1+1 = 8 (even no) = Aromatic.

Non Aromatic Behavior of some heterocyclic compounds containing same DLP based heteroatom having no d orbitals:

Omission behavior of some heterocyclic compounds will be observed (Figure 5 and 6),when there, is at least two hetero atoms (same or different) but both the hetero atoms do not have any d orbitals (such as O,N etc.) and they are in DLP based environment in the ring system.

Figure 5. Structures of 1,2-dioxin and 1,4-dioxin

Figure 6. Structures of dibenzo-1,4-dioxin

These molecules have been studied with advanced molecular orbital techniques known as ‘ab initio calculations’. ‘Ab initio quantum chemistry methods’ are computational chemistry methods based on quantum chemistry8.

In the case of 1,2-dioxin, 1,4-dioxin and dibenzo-1,4-dioxin there is DLP based O atoms in all the molecules but still they will be non aromatic due to prevention of significant free electron delocalization (makes non conjugated). The π electrons from the carbon bonds and the lone pair electrons on the oxygen atoms do not overlap to a significant degree due to absence of vacant d orbitals in both O atoms in each case (pπ-dπ overlap is not possible here in conjugation). It makes these molecules non conjugated and thus allows the molecules to become non aromatic instead of aromatic (A value = even No).

In the heterocyclic compounds, where, there is two DLP based N atoms instead of two DLP based O atoms or there is one DLP N atom along with one DLP O atom, the same phenomena of non aromatic behavior will be observed. Because, both N and O atoms do not have any vacant d orbitals, and hence pπ-dπ overlap is not possible here in conjugation.

Anti Aromatic Behavior of some heterocyclic compounds containing same DLP based hetero atoms having vacant d orbitals:

Figure 7. Structures of 1,4-dithiin and 1,2-dithiin

These compounds (Figure 7) are anti aromatic, here both S atoms, having vacant d orbitals, contain one DLP and one LLP and here both DLP of both S atoms participate in the delocalization. Hence, for the prediction of ‘A’ value, consider both DLP (DLP = 2). Here, A = πb + DLP + 1 (Constant) = 2 + 2 + 1 = 5 (odd No) = Anti Aromatic.

Conclusions

It may be expected that these three time economic innovative mnemonics of heterocyclic chemistry will help the students of Undergraduate, Senior Undergraduate and Post-Graduate level to predict aromatic, anti aromatic and non aromatic character of heterocyclic compounds along with their omission behaviour. Experiment in vitro on 100 students showed that by using these two formulae students can save up to 5-10 minutes time in the examination hall to predict the aromatic, anti aromatic and non aromatic character of any heterocyclic compounds and their comparative study including omission behaviour with respect to the DLP and LLP based hetero atoms present on them. On the basis of this, I can strongly recommend to use these three time economic innovative mnemonics in the field of heterocyclic chemistry.

External Links: http://www.sciepub.com/WJCE/abstract/8888

References

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2. I.L. Finar, Organic Chemistry, Vol-2, 5th ed., Pearson, 2002, pp606-637, ISBN:81-7808- 164-4.
3. R.T. Morrison and R.N. Boyd, 6th ed., Organic Chemistry, 6th ed., 1992, pp1059-1061, ISBN: 0-87692-765-7.
4. T.W.Graham Solomons and C.B. Fryhle, Organic Chemistry, 9thed., Wiley: India, 2012, pp655-658, ISBN: 978-81-265-3687-0.
5. J.G. Smith, Organic Chemistry, 2nd ed., 2008, pp616-619.
6. Jerry March, Advanced Organic Chemistry: Reaction, Mechanisms and Structure, 4th ed. A Willy- Interscience Publication, JOHN WILEY & SONS, New York, 2005, p45, ISBN: 9971-51-257-2.
7. A.Bahl & B.S.Bahl, A Text Book of Organic Chemistry, 22nd ed., S.Chand & Company Pvt.Ltd., New Delhi, India, 2016, pp900-920, ISBN: 978-93-525-3196-7.
8. Levine, Ira N., Quantum Chemistry. Englewood Cliffs, New jersey: Prentice Hall, 1991, 455–544. ISBN 0-205-12770-3.

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.