Subject Chemistry
Paper No and Title Paper 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module No and Title Module 6: Aromaticity –Part-1
Module Tag CHE_P1_M6
CHEMISTRY PAPER 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) MODULE 6: Aromaticity Part-1
TABLE OF CONTENT
1. Learning outcomes 2. Introduction 3. Aromaticity 4. Huckel’s rule and aromaticity 5. Summary
CHEMISTRY PAPER 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) MODULE 6: Aromaticity Part-1
1. Learning Outcomes
After studying this module you shall be able to: Know criteria for aromaticity Be familiar with the terms aromatic and anti-aromatic Classify any organic compound into aromatic, anti-aromatic and aliphatic (or non- aromatic)
2. Introduction
Resonance arises due to conjugation, but what happens when the resonance is continuous in a circular manner? The property of continuous delocalization of π electrons (resonance) in a cyclic planar ring gives rise to stability in aromatic compound like benzene. But it can also lead to destability in systems like cyclobutadiene. Let us try to revisit aromaticity and the conditions required to classify any given compound into aliphatic, aromatic and anti-aromatic.
3. Aromaticity
You are already aware that the aromatic compounds apparently contain alternate double and single bonds in a cyclic structure and resemble benzene in chemical behaviour. They undergo substitution rather than addition reactions. This characteristic behaviour is known aromaticity.
There is more to know about them and aromaticity. These compounds exhibit significantly high diamagnetic susceptibility. Cyclic electron delocalization also results in bond length equalization, abnormal chemical shifts and magnetic anisotropies, as well as chemical and physical properties which reflect energetic stabilization. On the other hand, compounds with high paramagnetic susceptibility may be antiaromatic.
Following are the main criteria of aromaticity: 1. Chemical behavior: electrophilic aromatic substitution. 2. Structural: bond length equalization due to cyclic delocalization. 3. Energetic: enhanced stability (large resonance energy). 4. Magnetic: "ring current" effects. a) anomalous chemical shifts in NMR. b) large magnetic anisotropies c) high diamagnetic susceptibility.
CHEMISTRY PAPER 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) MODULE 6: Aromaticity Part-1
Let us begin with recalling the criteria for classifying any compound as aromatic, anti-aromatic and aliphatic or non-aromatic by examining its structure, i.e., by Huckel’s rule.
4. Huckel’s Rule and aromaticity
A molecule is aromatic if all the following conditions are fulfilled:
1. It is cyclic, planar and has continuous delocalization of electrons (electrons in p orbitals) with or without the participation of lone pair(s)/- charge/ + charge (i.e having electrons or vacant p orbital).
2. The delocalised -electron cloud must contain a total of (4n+2) electrons, where n is a whole number (i.e., n =0,1,2,3 and so on). Putting n= 0 in (4n+2) we get 2 electrons, similarly putting n=1, we get 6 electrons; n= 2 gives 10 electrons; n= 3 gives 14 electrons and so on..
Note that the p-orbitals in which the electrons are delocalising must be parallel to each other so that a continuous overlap of electrons is possible along the ring.
So whenever a cyclic, planar and (4n+2) electrons are continuously delocalized, it leads to the extra stability of the molecule referred to as aromaticity. On the other hand, whenever a cyclic, planar and (4n) electrons are continuously delocalized, it leads to the extra destability of the molecule referred to as anti-aromaticity. When either the molecule is not cyclic or is not planar or the continuous delocalization of electrons is not there, the molecule is aliphatic or non- aromatic.
In other words,
if first condition is fulfilled and if in the second condition there are (4n+2) electrons, it is aromatic. if first condition is fulfilled and if in the second condition there are 4n electrons, it is anti-aromatic. If the first condition is not fully met, it is aliphatic, the second condition need not be looked at all.
The term non-aromatic is applicable to both aliphatic as well as anti-aromatic.
Of these the stability is in the order : Aromatic > Aliphatic > Anti-aromatic
CHEMISTRY PAPER 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) MODULE 6: Aromaticity Part-1
In terms of the two required conditions, these three terms can be understood as
Condition for Aromaticity Aliphatic or Non- Aromatic Anti-aromatic aromatic 1. Cyclic, planar and Not fulfilled Fulfill the first Fulfill the first continuous delocalization of ( ) Condition Condition electrons ( ) ( ) 2. (4n+2) electrons No need to look for (4n+2) (4n) electrons this condition if first electrons condition is not (4n+2) fulfilled requirement not fulfilled N.A. ( ) ( ) ( )
Let us consider some examples to apply these conditions.
1. Three membered cyclic species
In case of cyclopropene, the first condition is not being fulfilled as the electrons are not continuously delocalized, although it is cyclic and planar. Second condition is not applicable and is thus aliphatic or non-aromatic.
In case of cyclopropenyl cation, the first condition is fulfilled. Due to the positive charge, there is a vacant p orbital in conjugation with the double bond and hence continuous delocalization of electrons is there. Counting the number of electrons taking part in delocalisation, there are 2 electrons in the double bond. Hence it fulfills the second condition also and is thus aromatic.
In case of cyclopropenyl anion, the first condition is fulfilled. Due to the negative charge, there are two electrons in the p orbital in conjugation with the double bond and hence continuous delocalization of electrons is there. Counting the number of electrons taking part in delocalisation, there are 2 electrons in the double bond and 2 electrons in the p orbital as negative charge taking part in the continuous delocalisation. Hence a total of 4 electrons. It hence does not fulfill the second condition and is thus anti-aromatic.
CHEMISTRY PAPER 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) MODULE 6: Aromaticity Part-1
2. Five membered cyclic species
In case of cyclopentadiene, the first condition is not being fulfilled as the electrons are not continuously delocalized, although it is cyclic and planar. Second condition is not applicable and is thus aliphatic or non-aromatic.
In case of cyclopentadienyl cation, the first condition is fulfilled. Due to the positive charge, there is a vacant p orbital in conjugation with the two double bonds and hence continuous delocalization of electrons is there. Counting the number of electrons taking part in the delocalistion, there are 2 electrons in each of the double bond. Hence, has a total of 4 electrons. It hence does not fulfill the second condition and is thus anti-aromatic.
In case of cyclopentadienyl anion, the first condition is fulfilled. Due to the negative charge, there are two electrons in the p orbital in conjugation with the double bond and hence continuous delocalization of electrons is there. Counting the number of electrons taking part in delocalisation, there are 2 electrons in the double bond and 2 electrons in the p orbital as negative charge taking part in the continuous delocalisation. It thus has total of 6 electrons. Hence it fulfills the second condition also and is thus aromatic.
CHEMISTRY PAPER 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) MODULE 6: Aromaticity Part-1
3. Six membered cyclic species
We already know that benzene is aromatic. But let us check whether it fulfills the two required conditions? In case of benzene, the first condition is fulfilled as it is cyclic, planar and due to 3 conjugated double bonds, there is hence continuous delocalization of electrons. Counting the number of electrons taking part in delocalisation, there are 2 electrons in each of the 3 double bonds and hence a total of 6 electrons taking part in the continuous delocalisation. Hence it fulfills the second condition also and is thus aromatic.
4. Seven membered cyclic species
As earlier, one can clearly understand that cycloheptatriene is non-aromatic, cycloheptatrienyl anion is anti-aromatic but cyclheptatrienyl cation (tropilium ion) is aromatic as shown below. Note that cycloheptatriene is cyclic and planar but one of the carbons is saturated and doesn't possess a p-orbital.
CHEMISTRY PAPER 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) MODULE 6: Aromaticity Part-1
5. Fused polynuclear hydrocarbons
Naphthalene and azulene have 10 electrons each, Anthracene has 14 electrons. All of them satisfy both the conditions and hence are aromatic.
6. Substituted cyclic species
Don’t get carried away by the conjugations present as open chain substituents on the cyclic ring while counting the number of electrons. Only count those electrons which are continuously delocalized in a cyclic manner.
For example, in the following two cases, both fulfill the first condition. Ethenyl-cyclobutadiene (I) is anti-aromatic as it has 4 electrons which are continuously delocalized in a cyclic manner (and not 6 electrons). On the other hand, ethenyl-benzene(II) is aromatic as it has 6 electrons which are continuously delocalized in a cyclic manner (and not 8 electrons).
CHEMISTRY PAPER 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) MODULE 6: Aromaticity Part-1
(I) (II)
7. Cyclodecapentaene
This molecule seems to be aromatic as it is cyclic, seems to be planar, conjugated system and has 10 electrons. But the fact is that it is not aromatic. Let us try to understand why? The reason is that if the molecule acquires planar geometry (I), there will be angular strain in the molecule as each angle will try to be 120o (due to sp2 hybridisation) but cannot do so, also it will lead to five cis double bonds. In order to become stable, it tries to exist as (II) where there are three cis and two trans double bonds. Looking closely we find that the two H atoms on the two trans double bonds come very close to each other and thus the molecule actually exists in a manner where the ring becomes puckered or non-planar so that the two half-cyclic rings are tilted in order to make these two hydrogen atoms slightly away from each other (III).
(I) (II)
CHEMISTRY PAPER 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) MODULE 6: Aromaticity Part-1
(III)
Due to non-planarity, the p orbitals on the two half-cyclic rings also become non-parallel and hence there is no continuous delocalisation also.
This molecule hence is not aromatic as it is non-planar as well as not having continuously delocalised electrons. Although, it is cyclic, has alternate double and single bonds and has 10 electrons.
8. Heterocyclic compounds
Let us consider pyridine and pyrrole:
Pyridine Pyrrole
The first condition seems to be satisfied in both. In pyridine, do not count the electrons as 8. Here, the lone pair electrons are in the sp2 orbital of nitrogen, which is out of plane of the p orbitals and cannot take part in delocalization, so no need to count them. The number of electrons delocalized in p orbitals are hence 6 and it is aromatic.
In case of pyrrole, the lone pair of electrons occupy p orbital with H bonding with electrons of sp2. Here the lone pair of electrons are counted as they are continuously delocalized and there are total of 6 electrons. Hence it is aromatic.
Similarly, furan and thiophene, both are aromatic as they have both conditions fulfilled.
CHEMISTRY PAPER 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) MODULE 6: Aromaticity Part-1
9. An interesting case:
Is the following compound aromatic?
You may conclude it is not-aromatic because of 10 electrons 1. 2. 10
But this compound has a very high dipole moment. This cannot be explained by the depicted structure as above. If we look carefully, the middle electrons, can move on either of the two carbpn, i.e either towards the 3 membered ring or towards 5 membered ring as shown below and both of these justify the high dipole moment.
But out of these, (I) is combination of 2 aromatic rings and (II) is combination of 2 anti-aromatic or non-aromatic rings. It is obvious that the molecule exists as (I).
CHEMISTRY PAPER 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) MODULE 6: Aromaticity Part-1
To sum up, aromaticity is the state of stability that any molecule would thrive to achieve. These molecules are cyclic, planar and have (4n+2) electrons continuously delocalised in a cyclic manner.
We shall study more about aromaticity in subsequent modules.
5. Summary
These compounds exhibit significantly high diamagnetic susceptibility. Cyclic electron delocalization also results in bond length equalization, abnormal chemical shifts and magnetic anisotropies, as well as chemical and physical properties which reflect energetic stabilization. On the other hand, compounds with high paramagnetic susceptibility may be antiaromatic. Huckel’s rule can be used to determine whether a molecule is aromatic or anti-aromatic by broadly two conditions o It is cyclic, planar and has continuous delocalization of electrons (electrons in p orbitals) with or without the participation of lone pair(s)/- charge/ + charge (i.e having electrons or vacant p orbital). o The delocalised -electron cloud must contain a total of (4n+2) electrons, where n is a whole number (i.e., n =0,1,2,3 and so on). If in the second condition there are 4n electrons, it is anti-aromatic. If the first condition is not fully met, it is aliphatic. The term non-aromatic is applicable to both aliphatic as well as anti-aromatic. Aromaticity is the state of stability that any molecule would thrive to achieve. These molecules are cyclic, planar and have (4n+2) electrons continuously delocalised in a cyclic manner.
CHEMISTRY PAPER 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) MODULE 6: Aromaticity Part-1