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Aromaticity- Aromatic compounds, Non-Aromatic compounds, Anti aromatic compounds, Rules for Aromaticity, Quasi-aromatic compounds Practice Problems & FAQs

Does aromaticity have something to do with the smell or aroma of coffee and naphthalene?

Coffee and naphthalene have structures that are referred to as aromatic structures but aromaticity has nothing to do with the smell or aroma of the compounds.


Then what is aromaticity?

You might have heard the story of unity is strength. A single stick or a log of wood can be easily broken or cut. But a bundle of sticks cannot be that easily cut or broken. The arrangement gives extra strength to each stick. When alone -weaker, but together -stronger 

This is applicable in chemical molecules also, The unsaturated double bond or triple bond may indicate a weak spot of the molecule. But when these multiple bonds occur in conjugation (alternative with a single bond) the molecule gets extra stability and special properties. One such arrangement of pi-bonded electrons is called Aromaticity.


Aromaticity is the property of cyclic compounds which possess delocalized electrons in the π orbitals and a planar geometry resulting in enhanced stability of the molecule.

The chemical meaning of the word ‘aromatic’ signifies certain kinds of chemical structures.

TABLE OF CONTENT

Aromaticity

Aromaticity may be defined as a characteristic feature of conjugated cycloalkenes whose primary function is to enhance the stability of the corresponding organic compound. The aromaticity of the compound is mainly because of the delocalizing ability of electrons present in pi orbitals.



Aromaticity is one of the crucial concepts in organic chemistry because most of the organic compounds we come across are aromatic. Organic compounds exhibiting delocalization of pi electrons are aromatic and are referred to as aromatic compounds. On the other hand, organic compounds which do not exhibit delocalization of pi electrons are referred to as non-aromatic or aliphatic compounds. Compared to aliphatic compounds, aromatic compounds exhibit more excellent stability.

Evidence of aromaticity - Heat of hydrogenation

One of the efficient ways to prove the existence of the concept called aromaticity is by comparing the heat of hydrogenation of similar compounds. Comparing the heat of hydrogenation enables us to identify the relative amount of resonance energies in a given molecule.

Let us understand the evidence of aromaticity a little better by taking the example of three-ring structures - benzene, cyclohexene, and 1,3-cyclohexadiene.


  • The number of double bonds present in benzene, 1,3-cyclohexadiene, and cyclohexene is 3, 2, and 1, respectively.
  • We usually expect the hydrogenation value to increase proportionately with an increase in the number of double bonds present in the ring structure. However, experimental evidence is not as we generally expect. The heat of hydrogenation of cyclohexene containing one double bond is 28.6kcal mol-1
  • The heat of hydrogenation values of 1,3-cyclohexadiene and benzene having two and three double bonds should be 57.2 and 85.8kcal mol-1 respectively. But the experimental heat of hydrogenation of 1,3cyclohexadiene is 54.9kcal mol-1, lower than expected. The heat of hydrogenation (ΔH ) of benzene was expected to be greater than 54.9 kcal mol-1 because it has three double bonds. But it is much less at 49.3 kcal mol-1
  • This decrease in the heat of hydrogenation in the benzene molecule is because of its aromaticity. In addition, benzene has 6 electrons in pi orbitals capable of undergoing delocalization. The presence of 6 delocalizing electrons makes the compound relatively more stable, thereby causing a decrease in the ΔH value.

Classification of Compounds

Cyclic compounds are classified into three categories:

  1. Aromatic compounds
  2. Anti aromatic compounds
  3. Non-aromatic compounds

Rules of aromaticity

Most of the organic compounds that we see in our daily lives are aromatic. Let us now understand what it takes for a compound to exhibit aromaticity.


The four rules mentioned below are the features that determine the aromaticity of the given compound.

Rule 1: The organic compound must have a cyclic structure. Acyclic, i.e., linear and branched organic compounds, do not exhibit aromaticity.

Rule 2: The cyclic ring must be planar. Every element of the aromatic ring should contain a p-orbital perpendicular to the ring. 

Rule 3: The organic compound must obey Huckel's rule. Huckel's rule states that the compound having (4n+2) pi electrons exhibit aromaticity, whereas the others do not.

The total number of pi electrons present in the given ring structure must satisfy Huckle's formula 4n+2, where n is any natural number (0, 1, 2, 3, 4, and so on).

Rule 4: All the pi-electrons must be in conjugation (in alternate carbon with a single bond in between).

Examples

Now that we have studied the four rules of aromaticity let us try to apply the same.

1. Benzene

Consider the example of benzene. However, first, let us see whether benzene satisfies all the four rules of aromaticity or not.

Rule 1: Yes, benzene has a cyclic structure.

Rule 2: Yes, benzene is a planar molecule. All carbon atoms are sp2 hybridized and have a hexagon-like planar shape.

Rule 3: Let us see whether benzene satisfies Huckel's rule of aromaticity or not. The total number of pi electrons present in benzene is 6.

If n = 1,

4n+2 → 4(1) + 2 = 4+2 = 6.

The total number of pi electrons in benzene equals the value obtained by substituting '1' in Huckel's rule. Hence, benzene is said to satisfy Huckel's rule.

Rule 4: Yes, All, the six pi electrons are involved in conjugated delocalization. The delocalized electrons lie above and below the plane of benzene. It contains alternate single and double bonds. It is thus concluded that benzene is an aromatic compound.

Anti Aromatic Compounds

Conditions for anti-aromaticity

  • It has to be a cyclic/ring compound. Generally, all the atoms of the cycle or the ring should be sp2 hybridized. As sp2 hybridization has a planar structure, if all the atoms of the ring are sp2 hybridized, then the whole molecule can be planar.
  •  It should be a conjugated system.
  • It should have 4n pi electrons, where, n = 1, 2, 3, 4 .....

Examples:

Cyclobutadiene is an anti-aromatic compound.



Non Aromatic Compounds

Compounds which are neither aromatic nor antiaromatic are called non-aromatic compounds. If any structure does not follow even a single rule of either aromatic or antiaromatic, then it is considered a non-aromatic compound.

Example: 

The given organic structure is not a cyclic compound, so it does not fulfill the criteria of being aromatic or antiaromatic so it is a non-aromatic compound.

2. (10) Annulene

Let us consider another example of (10) annulene, which is otherwise called Cyclodecapentaene.

Rule 1: Yes, (10) annulene is a cyclic structure.

Rule 2: When observed experimentally, no two adjacent double bonds were found to exist on the same plane. Hence, Cyclodecapentaene is said to deviate from being a planar molecule.

Rule 3: The total number of pi electrons present in Cyclodecapentaene is 10. Let us check whether this molecule satisfies Huckel's rule or not.

If n = 2,

4n+1 → 4(2) + 2 = 8+2 = 10.

The total number of pi electrons present in (10) annulene equals the value obtained by substituting '2' in Huckel's rule. Hence, Cyclodecapentaene is said to satisfy Huckel's rule.

Rule 4: (10) Annulene is not a flat structure.


Although (10) annulene satisfies Huckel's rule, it is not obeying the rule of planarity. Hence, the compound is considered non-aromatic. From this example, we can conclude that the compound must satisfy all four rules to be considered an aromatic compound.

Stability Order

Aromatic > Non-aromatic > Anti-aromatic

Anti-aromatic compounds are the least stable because more electrons are present in the anti-bonding molecular orbitals, which leads to an increase in energy and a decrease in the stability of the molecule. Whereas in aromatic compounds, more electrons are present in the bonding molecular orbitals, which leads to a decrease in energy and an increase in the stability of the molecule.

Quasi-Aromatic Compounds

The aromatic species whose charge contributes to the aromaticity of the compound are known as quasi-aromatic compounds.

Quasi-aromatic molecules are not aromatic in their neutral form but are aromatic in their charged form.

Example:

c It is aromatic in its dipolar ionic form.


Properties of Quasi-Aromatic Compounds

The properties of Quasi-aromatic compounds are listed below.

  • Crystalline in nature
  • Highly stable
  • High dipole moment
  • High Boiling Point and Melting Point
  • Soluble in Polar Solvents

Benzenoid aromatic system

Aromatic compounds with two or more fused rings, having at least one benzene ring are called benzenoid aromatic systems.

If only part (ring) of the system is aromatic, then the whole system can be considered aromatic.

Examples: Naphthalene, anthracene, and phenanthrene.


Non-benzenoid aromatic system

  • Aromatic compounds with two or more fused rings without any benzene ring are known as non-benzenoid aromatic systems.
  • Azulenes are fused non-benzenoid aromatic compounds.
  •  These compounds are found in anti-blemishing cosmetic creams.
  • Azulenes are polar compounds but look like non-polar compounds.


Benzyne

  • Benzyne is similar to benzene.
  •  An additional weak bond is formed by two sp2 hybridized orbitals (in the plane of the ring).
  • It is strained, but the conjugation is maintained.
  • It is aromatic in nature.


Practice Problems

Q. 1. Find out the number of compounds that are stable in ionic form from the following:


Solution: A dipolar species is created for all the given compounds A, B, C, and D as follows:


A: The ring is cyclic, planar, and fully conjugated. Since it has two π electrons, according to

Hückle’s rule is that it is aromatic.

B: Here also the ring is cyclic, planar, and fully conjugated. Since it has four π electrons, therefore it is anti-aromatic.

C: The ring is cyclic, planar, and fully conjugated. Since it has six π electrons, according to Hückle’s rule, it is aromatic.

D: If a negative charge is given to a 5-membered ring and the positive charge is given to a 3-membered ring, it makes both the rings aromatic.

Compounds A, C, and D will be stable in their ionic form.

Hence, the number of compounds stable in their ionic form is 3.

Q. 2. Azulene looks like a nonpolar compound. However, on analysis, it is observed to be polar. Why?


Solution: 

Both the seven and the five-membered rings of azulene exhibit aromatic character in their ionic form.

So, it is more stable in ionic form.

Hence, it is polar in nature and has a high dipole moment. It is a quasi-aromatic compound.

Q 3. Find if the given compound is aromatic, anti-aromatic, or non-aromatic.


Solution: The given organic molecule i.e. cyclopropene is cyclic, non-planar (as one of the carbons is sp3 hybridized) and the system is not conjugated. So, it does not fulfill the criteria of aromatic or antiaromatic compounds.

Hence, cyclopropene is a non-aromatic compound.

Q 4. Consider the following reactions and find out the stability order of the following compounds.


a. 1 > 2 > 3
b. 2 > 3 > 1
c. 2 > 1 > 3
d. 3 > 2 > 1

Solution:

NaNH2 (amide) is a strong base which abstracts H+ ions. In the given systems, amide abstracts a proton from the sp3 hybridized carbon.

  • The removal of a proton from the sp3 hybridized carbon from the given compound (cyclopentadiene) results in the formation of cyclopentadienyl anion. It is a cyclic, planar and conjugated system. So, it fulfills the criteria of aromatic or antiaromatic compounds. Cyclopentadienyl anion has two pi bonds and a negative charge i.e. 6 pi electrons. It follows the 4n + 2 rule i.e. 4 × 1 + 2 = 6 pi electrons. Hence, the organic compound 1 is an aromatic system.

  • The removal of one proton from the sp3 hybridized carbon from a given compound (1,3-Cyclohexadiene) results in the formation of an anion, but the structure is still non-planar. So, it does not fulfill the criteria of being aromatic or antiaromatic. Hence, the organic compound 2 is a non-aromatic system.



  • The removal of a proton from the sp3 hybridized carbon from a given compound (Cyclopropene) results in the formation of cyclopropenyl anion. It is a cyclic, planar and conjugated system. So, it fulfills the criteria of aromatic or anti-aromatic compounds. Cyclopropenyl anion has one pi bond and a negative charge i.e. 4 pi electrons. It follows 4n i.e. 4 × 1 = 4 pi electrons rule. Hence, the organic compound 3 is an anti-aromatic system.

  • We know that the order of stabilities is: aromatic > non-aromatic > anti-aromatic. Thus, the correct order of stabilities of the given organic compounds is 1 > 2 > 3.

Hence, option A is correct.

Frequently Asked Questions - FAQ

Q 1. Why is tropylium a non-aromatic compound?
Answer: 

The organic structure (Tropylium/Cycloheptatriene) is cyclic, non-planar (as one of the carbons is sp3 hybridized) and the system is not conjugated. So, it does not fulfill the criteria of aromatic or antiaromatic compounds.

Hence, tropylium/cycloheptatriene is a non-aromatic system.

Q 2. Why is [8]-annulene a non-aromatic system?
Answer: [8]-annulene is a tub-shaped molecule i.e. it is non-planar and non-aromatic.


Q 3. Comment on the aromaticity of styrene? 
Answer: Styrene is ethenyl Benzene, Benzene is aromatic in the complete compound. Hence, Styrene is Aromatic.


Q 4. Is planarity a necessary condition for aromaticity?
Answer: Yes, if a planar molecule has cyclic conjugation and follows Huckel’s rule is said to be aromatic.

Related Topics

Benzene

Hyperconjugation

Inductive effect

Organic Compounds

Free Radical

Carbocation

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