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I know that cyclic planer molecules with $2n + 1$ $\pi$-electrons are aromatic. Also, the color of a compound is due to electronic transitions. But how is it related with aromaticity (smell)? Is there any such relation or is it the chemical property of any compound (matter)?

If it is chemical property of matter then what is its cause?

P.S. My question isn't about nomenclature, its about what is in benzene or any other aromatic compound that make it aromatic i.e. their smell? And what specifically is the roll of electrons in their smell?

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    $\begingroup$ I'm not certain I understand your question correctly; are you asking why aromatic compounds are named that way? $\endgroup$ – M.A.R. Oct 5 '15 at 19:07
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    $\begingroup$ @inɒzɘmɒЯ.A.M no, I think he’s asking if there’s a reason in the (electronic) structure of aromatic compounds that makes them smell. $\endgroup$ – Jan Oct 6 '15 at 18:55
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The smell of a compound as we perceive it usually depends on which receptors in the nose will bind the molecule (or not). (There are exceptions for molecules that actually react with these receptors, such as $\ce{HCl}$, but benzene and most aromatics are not one of these exceptions.)

These receptor, being proteins, are usually rather specific for certain shapes, not for bonding patterns. So chlorobenzene might fit into similar receptors as toluene which would lead to at least partly a similar smell, but that is it.

The definition of aromatic is, as Todd pointed out, one based on structure. Some aromatic compounds have similar structures, but there is no way that (say) furane would occupy the same or similar receptors as benzo(a)pyrene would. Therefore, their common feature aromaticity does not lead to a common odour.

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  • $\begingroup$ But what it has to do with electrons? As you said and from the answer of @Todd what i can understand is that its just a convention that is being followed and nothing related to electrons. Am I right at this??? $\endgroup$ – Chinmay Chandak Oct 6 '15 at 18:53
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    $\begingroup$ Yes. There is nothing in the electronic structure that would create a certain smell of any kind. $\endgroup$ – Jan Oct 6 '15 at 18:55
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I can address the part of your query pertaining to odor and nomenclature (more accurately, I can link the UC Davis ChemWiki page that does so and quote it here):

Benzene derived products are well known to be pleasantly fragrant. For this reason, organic compounds containing benzene rings were classified as being "aromatic" (sweet smelling) amongst scientists in the early 19th century when a relation was established between benzene derived compounds and sweet/spicy fragrances. There is a misconception amongst the scientific community, however, that all aromatics are sweet smelling and that all sweet smelling compounds would have a benzene ring in its structure. This is false, since non-aromatic compounds, such as camphor, extracted from the camphor laurel tree, release a strong, minty aroma, yet it lacks the benzene ring in its structure (See figure 1). On the other hand, benzene itself gives off a rather strong and unpleasant smell that would otherwise invalidate the definition of an aromatic (sweet-smelling) compound. Despite this inconsistency, however, the term aromatic continues to be used today in order to designate molecules with benzene-like rings in their structures. For a modern, chemical definition of aromaticity, refer to sections Aromaticity and Hückel's Rule.

You will find (in the linked section on Aromaticity) that presently:

The three general requirements for a compound to be aromatic are:

  1. The compound must be cyclic

  2. Each element within the ring must have a p-orbital that is perpendicular to the ring, hence the molecule is planar.

  3. The compound must follow Hückel's Rule (the ring has to contain 4n+2 p-orbital electrons).

Putting it all together: The historical naming convention for molecules that smelled good is responsible for the term aromatic. It turns out that many of the molecules that fall into the aromatic category happen to be rather stable/unreactive. These molecules happen to have certain physical characteristics which, taken together, are indicative of a molecule being classified as aromatic or not (based on present-day definitions not relating to odor/smell).

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The theory shape or chemical composition has anything to do with smell can be refuted based on three observations:

  1. Compounds with different shape can smell exactly the same. (creosol and eugenol)
  2. Compounds of different isotopes can smell different. (musk)
  3. Concentrations of compounds can change the odor completely. (phenol)

Since the advent of genetic technologies, a new theory is on the horizon is the underlying mechanism for smell is based on quantum oscillations. This theory is derived from information gathered from studying the genome which encodes the proteins responsible for the sense of smell.

Todd's answer is correct. The relationship to smell is a historical artifact from the time chemistry was in its infancy when chemical properties were described using the senses of taste and smell.

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