World Journal of Chemical Education, 2018, Vol. 6, No. 2, 95-101 Available online at http://pubs.sciepub.com/wjce/6/2/4 ©Science and Education Publishing DOI:10.12691/wjce-6-2-4 Lone Pair Electron Discriminate Hybridization with Aromatic and Anti Aromatic behavior of Heterocyclic Compounds - Innovative Mnemonics Arijit Das* Department of Chemistry, Ramthakur College, Agartala -799003, Tripura West, Tripura, India *Corresponding author: [email protected], [email protected] 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 Hybridization state of hetero atom and Aromatic, Anti aromatic, non aromatic behavior of different heterocyclic compounds. Here, I have tried to hub three (03) time economic mnemonics by including three (03) 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 ground. Keywords: high school, under graduate student, graduate student, post graduate student, chemical education research, heterocyclic compound, DLP, LLP, hybridization, aromatic, anti aromatic and non aromatic Cite This Article: Arijit Das, “Lone Pair Electron Discriminate Hybridization with Aromatic and Anti Aromatic behavior of Heterocyclic Compounds - Innovative Mnemonics.” World Journal of Chemical Education, vol. 6, no. 2 (2018): 95-101. doi: 10.12691/wjce-6-2-4. through conjugation then it is to be treated as delocalized lone pair of electron (DLP). Hetero atom (atom containing 1. Introduction lone pair of electron) which is directly attached with single bonds only from all ends is to be considered as DLP The conventional methods [1-7] for determination of containing hetero atom and its lone pair is to be treated as hybridization state of hetero atom, prediction of aromatic DLP. 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 N using three formulae for the prediction of hybridization state of hetero atom, aromatic and anti aromatic nature of Eg. In Pyrrole H lone pair of N atom is to be heterocyclic compounds containing one, two or more treated as DLP because it is directly attached with three number of hetero atoms to make heterocyclic chemistry single bonds only. metabolic and interesting for students. This study also shows ii) Localized lone pair of electron (LLP): When lone how LP electrons discriminate prediction of hybridization pair of electron of hetero atom does not undergo state of hetero atom with identification of its Aromatic and delocalization through conjugation then it is to be treated Anti Aromatic nature of heterocyclic compounds. 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 2. Time Economic Innovative Mnemonics system is to be considered as LLP containing hetero atom in Heterocyclic Chemistry and its lone pair is to be treated as localized lone pair of electron (LLP). 2.1. Classification of Lone Pair of Electrons Loan Pair of electrons can be generally classified into two types as Delocalized lone pair of electron (DLP) and N Localized lone pair of electron (LLP) as follows: Eg. In Pyridine lone pair of N atom is to be i)Delocalized lone pair of electron (DLP): When lone treated as LLP because it is directly attached with double pair of electron of hetero atom undergo delocalization and single bonds with the ring system. World Journal of Chemical Education 96 2.2. Hybridization State Theory Innovative Mnemonics for the prediction of hybridization state of hetero atom in the heterocyclic compounds Prof. Linus Pauling (1931) first developed the with LLP: Hybridization state theory in order to explain the structure Formula used for the determination of hybridization of molecules such as methane (CH4). This concept was state: developed for simple chemical systems but this one Power on the Hybridization state of the hetero atom = applied more widely later on and from today’s point of (Total no of σ bonds around each hetero atom - 1). view it is considered an operative empirical for excusing This formula should be applicable up to 4 σ bonds. the structures of organic and inorganic compounds along If the power of the hybridization state will be 03, 02 with their related problems. and 01 then the hybridization state will be sp3, sp2 and sp Conventional method for prediction of hybridization state: respectively. All single (-) bonds are σ bond, in double Hybridization state for a molecule can be calculated by bond (=) there is one σ and one π. In addition to these each the formula 0.5 (V+H−C+A), Where, V = Number of localized lone pair of electron (LLP) can be treated as one valance electrons in central atom, H = Number of σ bond. Hybridization state of hetero atom with the help surrounding monovalent atoms, C = Cationic charge, A = of LLP in heterocyclic compounds are shown in Table 1 Anionic charge. below. Table 1. (Hybridization state of Hetero atom in Heterocyclic Compounds with the help of LLP) Power of the Number of σ bonds Total Number of σ bonds Hybridization state around hetero atom LLP around hetero atom of the hetero atom Heterocyclic Compounds (from single and double (localized Lone Pair of e-s) (A+B) (Corresponding bonds) (B) Hybridization state) = (A) (A+B)-1 02 0 (sp2 N) 03 (lone pair of electron N 03 undergo delocalization,DLP with the ring system) H Pyrrole 01 (out of two lone pair of electrons, one undergo 02 O 02 delocalization,DLP and other 03 (sp2 O) remain as LLP) Furan 01 (out of two lone pair of 02 02 electrons, one undergo 03 (sp2 S) S delocalization,DLP and other Thiophene remain as LLP) 02 01 02 03 (sp2 N) N Pyridine 02 03 0 N 03 (sp2 N) H Indole 02 01 02 03 (sp2 N) N Quinoline 5 4 10 6 3 02 2 7 N 02 01 03 (sp N) 9 2 8 1 Isoquinoline 97 World Journal of Chemical Education Power of the Number of σ bonds Total Number of σ bonds Hybridization state around hetero atom LLP around hetero atom of the hetero atom Heterocyclic Compounds (from single and double (localized Lone Pair of e-s) (A+B) (Corresponding bonds) (B) Hybridization state) = (A) (A+B)-1 3 4 N 02 03 0 5 03 (sp2 N1) (N1) (N1) 2 N 1 02 02 01 03 (sp2 N3) (N3) (N3) H Imidazole 4 3 02 5 N 02 01 (sp2 N1) 03 (N1) (N1) 6 2 02 N 02 01 (sp2 N3) 03 1 (N3) (N3) Pyrimidine 02 01 02 7 2 6 (N1) (N1) 03 (sp N1) N 1 5 02 01 02 N 2 8 (N3) (N3) 03 (sp N3) 2 4 N N9 02 01 02 (N7) (N7) 03 (sp2 N7) 3 H Purine 03 0 02 (N9) (N9) 03 (sp2 N9) 01 (N) 02 N 02 (sp2 N) 03 (N) 01 (S) 02 S 02 (out of two lone pair of (sp2 S) 03 Thiazole (S) electrons on S, one undergo delocalization, DLP and other remain as LLP) 01 (N) 02 N 02 (sp2 N) 01 03 (N) (S) 02 (out of two lone pair of 02 (sp2 S) S electrons on S, one undergo 03 (S) delocalization, DLP and Benzothiazole other remain as LLP) 4 N 02 5 3 02 01 03 (sp2 N1) (N1) (N1) 6 2 N 02 02 01 1 03 (sp2 N4) (N1) (N1) Pyrazine (p-diazine) N 02 02 01 (sp2 N1,N3,N5) N N 03 (N1,N3 and N5) (N1,N3 and N5) Cyanidine 0 H (N) N 01 03 02 (S) (N) 03 (sp2 N) (out of two lone pair of electrons on S, one undergo S 02 03 02 delocalization, DLP and (S) (sp2 S) Phenothiazine other remain as LLP) World Journal of Chemical Education 98 Power of the Number of σ bonds Total Number of σ bonds Hybridization state around hetero atom LLP around hetero atom of the hetero atom Heterocyclic Compounds (from single and double (localized Lone Pair of e-s) (A+B) (Corresponding bonds) (B) Hybridization state) = (A) (A+B)-1 N 02 02 01 03 (sp2 both N) N (both N) (both N) Phenazine N N 02 02 01 (sp2 All N) N 03 N (N1,N2,N3,N4) (N1,N2,N3,N4) 1,2,3,4-tetrazine N 02 01 02 03 (sp2 N) Azocine N 01 02 02 03 2 Azetine (sp N) H N 03 01 03 04 (sp3 N) Aziridine O 02 03 02 04 3 Oxetan (sp O) 2.3. Aromaticity The present study will be an innovative mnemonic involving calculation of ‘A’ value by just manipulating It was first devised by Hückel in 1931. the no of π bonds within the ring system and delocalized Conventional method for prediction of Aromatic nature lone pair of electron (DLP) with one (01). of organic compound: The heterocyclic compound having cyclic, planar, 1. Cyclic molecule, conjugated (i.e. all the carbon atoms having same state of 2. Planer molecule in which all bonded atoms lie in hybridization, sp2) with even number of ‘A’ value will be 2 same plane (having sp hybridized) treated as aromatic in nature and with odd number of ‘A’ 3. Conjugated molecule with conjugated π-electron value will be treated as anti aromatic in nature.