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Lone Pair of Electrons Discriminate Hybridization with Aromaticity in the Heterocyclic Compounds - Innovative Mnemonics

Arijit Das

Department of Chemistry, Ramthakur College, Agartala -799003, Tripura West, Tripura, India

Abstract: In this approach, formulae based mnemonics by counting lone pair of electrons (localized or delocalized) have been highlighted by innovative and time economic way to enhance interest of students’ who belong to paranoia zone of chemistry for the prediction of aromatic behavior and hybridization state of heterocyclic compounds. In order to ameliorate the field of chemical education, I have taken an enterprise to hub two economic mnemonics including two different formulae. This article encourages students to solve multiple choice type questions (MCQs) at different competitive examinations for a time being in an economic ground for the prediction of aromaticity and hybridization state of heterocyclic compounds containing two or more hetero-atoms. Educators can use these mnemonics in their teaching style in the classroom lectures after discussing conventional methods to make chemistry intriguing because the use of mnemonics in classroom lectures is an essential tool to become a distinguished educator.

Keywords: high school, under graduate student, graduate student, post graduate student, chemical education research, heterocyclic compound, DLP, LLP, aromaticity and hybridization

Citation: Arijit Das, Lone Pair of Electrons Discriminate Hybridization with Aromaticity in the Heterocyclic Compounds - Innovative Mnemonics. World Journal of Chemical Education, 6 (3). 107-112. January 2018. doi:10.12691/wjce-6-3-1

1. Introduction

The conventional methods [1,2,3,4,5,6,7] for determination of hybridization state and aromatic nature of heterocyclic compound is time consuming. Keeping this in mind, in this article, I have introduced two innovative mnemonics, economically sound by using two formulae to make heterocyclic chemistry metabolic and inspiring for students.

Time Economic Innovative Mnemonics in Heterocyclic Chemistry

Classification of Lone Pair of Electrons

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 delocalized lone pair of electron (DLP).

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).

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.

Hybridization state theory

Pauling (1931) first developed the Hybridization state theory in order to explain the structure of molecules such as methane (CH₄). Pauling’s 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 to excuse the structures of organic and inorganic compounds along with their related problems.

Innovative Mnemonics on the Prediction of Hybridization state in Heterocyclic Compounds with LLP

Formula used for the determination of hybridization state:

Power on the Hybridization state of the hetero atom = (Total no of σ bonds around each hetero atom - 1).

All single (-) bonds are σ bond, in double bond (=) there is one σ and 1π. In addition to these each localized lone pair of electron (LLP) can be treated as one σ bond. Hybridization state of hetero atom along with the help of LLP in heterocyclic compounds has been shown in Table 1 below.

Table 1. (Hybridization state of hetero atom in heterocyclic compounds with the help of LLP)

2. Aromaticity

It was first devised by Hückel in 1931. According to Hückel’s rule of aromaticity, a molecule will be aromatic if it is a

1. Cyclic molecule,

2. Planer molecule in which all bonded atoms lie in same plane

3. Conjugated molecule with conjugated π-electron system,

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

Innovative Mnemonics on the Prediction of Aromatic behavior of Heterocyclic Compounds with DLP:

The present study asserts an innovative mnemonic involving calculation of ‘A’ value by just metamorphosing 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.

Formula used for the prediction of ‘A’ value:

Evaluation of A Value:

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

DLP = Delocalized lone pair of electron.

For aromatic behavior of heterocyclic compounds A will be even number.

If a multi hetero atom based on the heterocyclic compound, containing both DLP and LLP hetero atoms, aromaticity should be predicted with respect to DLP based hetero atom only. But when both hetero atoms are in the same mode of classification (DLP or LLP) then aromaticity should be predicted with respect to that hetero atom which contains lowest possible position number as per IUPAC.

Eg. Imidazole 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

Eg. Pyrimidine is a multi hetero atom based hetero cyclic compound in which, both N1 & N3 are in same environment (LLP based hetero atom) based hetero atom and N3 is LLP based hetero atom. In this case aromaticity should be predicted with respect to N1 (lowest possible position number).

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

Aromaticity of heterocyclic compounds has been illustrated in Table 2.

Table 2. Aromatic behavior of heterocyclic compounds with the help of DLP

3. Conclusions

It might be expected that these two innovative mnemonics will help the students of chemical education at Undergraduate, Senior Undergraduate and Post-Graduate level to discriminate aromaticity and hybridization quite easily only by giving a look on the classification of lone pair of electrons (DLP or LLP). Experiment in vitro on 100 students showed that by using these formulae students can save up to 5-7 minutes time in the examination hall to find out the aromatic character and state hybridization for heterocyclic compounds. On the basis of this, I can strongly recommend to use these two time economic innovative mnemonics in the field of heterocyclic chemistry for a better comprehension and up gradation.

Acknowledgements

I am grateful to the SERB, DST, New Delhi, Govt. of India, for their financial assistance (Sanction no – SERB / F / 5537 / 2013-14 dated 27/11/2013 and D.O. No. SB / EMEQ - 014 / 2013).

In this special day (41th birth day – D.O.B.: 14/03/1978), I want to delegate my this innovative research article in the field of Chemical Education to my beloved father Late Anil Ranjan Das, who was a chemistry teacher and also founder of my chemistry life.

I would like to give my cordial thanks to Mr.Santanu Chakraborty, Lecturer, Department of English, Ramthakur College, Agartala, Tripura, for his helping manner in recasting statements in this article.

References

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[2]   | Erich Hückel , Quantentheoretische Beiträge zum Benzolproblem I. Die Elektronenkonfiguration des Benzols und verwandter Verbindungen. Z.Phys. 70 (3-4), 1931, 204-86;
[3]   | I.L. Finar, Organic Chemistry, Vol-2, 5th ed., Pearson, 2002, pp. 619-620.
[4]   | R.T. Morrison and R.N. Boyd, 6th ed., Organic Chemistry, 6th ed., 1992, pp. 1059-1061.
[5]   | T.W.Graham Solomons and C.B. Fryhle, Organic Chemistry, 9th ed., Wiley: India, 2012, pp. 655-658.
[6]   | J.G. Smith, Organic Chemistry, 2nd ed., 2008, pp. 616-619.
[7]   | Jerry March, Advanced Organic Chemistry: Reaction, Mechanisms and Structure, 4th ed. A Willy-Interscience Publication, JOHN WILEY & SONS, New York, 2005, p. 45, ISBN: 0-471-85472-7.
[8]   | A.Bahl & B.S.Bahl, A Text Book of Organic Chemistry, 22nd ed., S.Chand & Company Pvt.Ltd., New Delhi, India, 2016, pp. 900-920.