What is Aromaticity: Definition and 11 Discussions

In chemistry, aromaticity is a property of cyclic (ring-shaped), planar (flat) structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to other geometric or connective arrangements with the same set of atoms. Aromatic rings are very stable and do not break apart easily. Organic compounds that are not aromatic are classified as aliphatic compounds—they might be cyclic, but only aromatic rings have enhanced stability.
Since the most common aromatic compounds are derivatives of benzene (an aromatic hydrocarbon common in petroleum and its distillates), the word aromatic occasionally refers informally to benzene derivatives, and so it was first defined. Nevertheless, many non-benzene aromatic compounds exist. In living organisms, for example, the most common aromatic rings are the double-ringed bases in RNA and DNA. An aromatic functional group or other substituent is called an aryl group.
The earliest use of the term aromatic was in an article by August Wilhelm Hofmann in 1855. Hofmann used the term for a class of benzene compounds, many of which have odors (aromas), unlike pure saturated hydrocarbons. Aromaticity as a chemical property bears no general relationship with the olfactory properties of such compounds (how they smell), although in 1855, before the structure of benzene or organic compounds was understood, chemists like Hofmann were beginning to understand that odiferous molecules from plants, such as terpenes, had chemical properties that we recognize today are similar to unsaturated petroleum hydrocarbons like benzene.
In terms of the electronic nature of the molecule, aromaticity describes a conjugated system often made of alternating single and double bonds in a ring. This configuration allows for the electrons in the molecule's pi system to be delocalized around the ring, increasing the molecule's stability. The molecule cannot be represented by one structure, but rather a resonance hybrid of different structures, such as with the two resonance structures of benzene. These molecules cannot be found in either one of these representations, with the longer single bonds in one location and the shorter double bond in another (see Theory below). Rather, the molecule exhibits bond lengths in between those of single and double bonds. This commonly seen model of aromatic rings, namely the idea that benzene was formed from a six-membered carbon ring with alternating single and double bonds (cyclohexatriene), was developed by August Kekulé (see History below). The model for benzene consists of two resonance forms, which corresponds to the double and single bonds superimposing to produce six one-and-a-half bonds. Benzene is a more stable molecule than would be expected without accounting for charge delocalization.

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  1. Eureka99

    Electronic transition in organic compounds

    Hello everybody :) I have a problem understanding the different types of pi -> pi * transition of organic compounds. I can't understand what's the difference between K-band, B- band, and E-bands, and I cannot find any explanation on the internet. Also I don't understand why benzene has two E...
  2. R

    Which of the following compounds is aromatic?

    1. Homework Statement : please refer to the picture attachedHomework Equations : [/B]A compound is aromatic if it is planar and it satisfies Huckles 4n+2 Π electron rule.The Attempt at a Solution : [/B] Option A is not correct...I don't know why...just speaking from intuition..probably its not...
  3. puddleduck

    Aromaticity of Conjugated Ring - Counting Pi Electrons

    My professor drew the following molecule on the board and asked us how many pi electrons this aromatic molecule has. Everyone in the class said 14, as there are 7 double bonds, with two pi electrons from each bond. He told us that there are only 10 pi electrons in this molecule, refused...
  4. Tony Stark

    Aromaticity of ortho substituted benzene

    Are ortho substituted benzene rings aromatic in nature as the planarity in such compounds breaks due to steric hindrance ?
  5. Y

    Is Diphenol Aromatic or Anti-Aromatic?

    Why is diphenol aromatic? Shouldn't it be anti aromatic as it has even pair of pie electrons?
  6. Titan97

    Partial Bond Fixation in Naphthalene

    From the resonance structures of Naphthalene, 1-2 Bond has more double Bond character than 2-3 Bond. In March's Advanced organic chemistry, its given that ozone preferentially reacts with 1-2 Bond. But the reaction is not given. Is this a normal ozonolysis reaction in which the 1-2 Bond is...
  7. Priyadarshini

    What is the IUPAC nomenclature for the given aromatic compound?

    Homework Statement Give the IUPAC nomenclature of the following compound (in the attachment) (sorry, i don't know why the picture has become upside down!) The Attempt at a Solution Shouldn't the answer be 1-phenyl-2,3-dibromopentane? The book says it is 2,3-dibromo-1-phenylpentane.
  8. L

    Aromaticity of 1-methyl-1H-pyridin-2-one?

    Hi everyone, I've recently had a little discussion at work concerning the concept of aromaticity applied to some molecules. To cut a long story short, while I see why 2-pyridone is considered aromatic: I don't understand why 1-methyl-2-pyridone is also considered aromatic: The two...
  9. S

    Organic Chemistry: Aromaticity

    Homework Statement Homework Equations Huckel's Rule: 4n +2 pi electrons The Attempt at a Solution I'm having problems with the last molecule. I said that the first molecule was indeed aromatic because it has 6 pi electrons and obeys the 4n + 2 rule. The second one is not aromatic because...
  10. A

    Is Hybridization Affecting Molecular Planarity and Aromaticity?

    I have two doubts in my course regarding resonance and aromaticity... (1) In various resonating structures, the hybridization of an atom may vary. Eg:- in aniline's nitrogen atom, it is sp3 as well as sp2 in different resonating structures. Since the actual molecule is a weighted mean of all...
  11. T

    Is Tropolone Aromatic Despite Its Unusual Structure?

    I understand the rules of aromaticity, but I do not understand why Tropolone is aromatic. The main reason is because I thought all the atoms in the ring have to have p-orbitals. How does the C connected the oxygen with a double bond have a p-orbital? Have to do with the resonance (that's my main...