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We all know the very famous esterification reaction, where a carboxylic acid and an alcohol react to give an ester and water. But why do they actually smell? What is the reason behind its aromatic character?

I initially thought that it must be due to the conjugation between the two oxygens present. I do know that aromaticity is decided by the conjugated lone pairs present in the compound where the compound must follow Hückels rule. But it's only valid for a cyclic and a planar compound.

Why do esters have that fruity smell? What is the chemical explanation for a smelly non planar compound?

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    $\begingroup$ Don't confuse the term aromaticity when used to describe electron configuration and the term aromatic when used to describe smell. Smell came first as a simple way of characterising chemicals. When structures of the chemicals started to be understood the term became associated with a particular subset of compounds like benzene and used to describe their electronic configurations. There are a lot more compounds with interesting smells than compounds with aromaticity in their electronic configuration. $\endgroup$ – matt_black Oct 29 '16 at 17:46
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    $\begingroup$ @matt_black The question is not the difference between the two. The basic question is that why does the $ -COO- $ group make anything smell at all. $\endgroup$ – Rohinb97 Oct 29 '16 at 17:52
  • $\begingroup$ Oh I got it. I've asked for its "aromatic" character there. It's not to be confused with aromaticity, of course. $\endgroup$ – Rohinb97 Oct 29 '16 at 17:54
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    $\begingroup$ Why things smell at all is a big subject about how the sense of smell works. It has nothing to do with electronic aromaticity as you seemed to assume in the question. $\endgroup$ – matt_black Oct 29 '16 at 17:55
  • $\begingroup$ I did not assume that. Like I said in the previous comment I meant "aromatic" in the smelly way. $\endgroup$ – Rohinb97 Oct 29 '16 at 18:00
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The term aromaticity originated with the discovery of unusually stable hydrocarbons that also happened to have strong smells. Many hydrocarbons smell, but not all are aromatic.

Nowadays, a compound being classified as "aromatic" has little to nothing to do with its smell and everything to do with its electron configuration. There are also non-aromatic compounds and anti-aromatic compounds. If we try to relate these to smell as well, what would an anti-aromatic compound smell like? Bad? But isn't that a little too subjective for science? Doesn't benzene smell "bad" too? Then why is it aromatic (i.e. unusually resistant to chemical change?) Why not anti-aromatic?

Esters smell partly because they exhibit weak intermolecular forces. This allows ester molecules to enter the gas phase and reach your nose. Esters don't exhibit intermolecular hydrogen bonding, unlike alcohols, for example. These are no strongly positively polarized hydrogens in esters to participate in hydrogen bonding. Consider for example ethyl butyrate, which smells like pineapples.

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Most of the molecule resembles a plain aliphatic hydrocarbon! And we know these only exhibit weak van der Waals intermolecular forces. No wonder esters smell (good and bad) - they're volatile and reach our noses easily!

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  • $\begingroup$ Thanks for the answer. I always thought that compounds were classified because they smell. But it's actually because they were smelly that's why they were classified...nice. $\endgroup$ – Rohinb97 Dec 26 '14 at 10:01
  • $\begingroup$ This is not an answer to the question. This is not because molecules are volatile than they smell ! $\endgroup$ – Hexacoordinate-C Oct 29 '16 at 13:49
  • $\begingroup$ @9-BBN you can always give your own answer by writing one below $\endgroup$ – Rohinb97 Oct 29 '16 at 17:50
  • $\begingroup$ @Rohinb97 I won't because I have that kind of knowlegde. I found your answer great but it is more an answer about why can we smell some molecules and not some others. And as I said this is not because molecules are volatiles than they smell. In my town they add a smell in the gas to be able to smell it if there is a problem. $\endgroup$ – Hexacoordinate-C Oct 30 '16 at 13:06
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Dissenter is correct about aromatic compounds and that the basic requirement for a compound to be detected by smell is for it to be volatile enough to reach the nose. This is why things like dry salt don't really smell like anything. Once they reach the nose, things become more complex.

Why many esters smell fruity while most thiols smell rather unpleasant is a rather interesting question. Just as most sugars taste sweet, many molecules that have structural similarities also smell similar. Humans have about 350 different olfactory receptors. These are G protein-coupled receptors that change conformation when they bind a compound and activate a signal pathway that is eventually sent to the brain for processing. However, since we only have 350 different receptors and there are far more than 350 odoriferous compounds, there is not a one-to-one mapping of receptors to compounds—there isn't an "acetone receptor". Instead, olfactory receptors have partial and overlapping selectivity. Each receptor type responds to more than one compound, thiols with a certain general shape, for instance, and different receptor types can respond to some of the same compounds. A given compound then has a more or less unique pattern of receptor activation. All this information gets sent to the brain and is processed into the perception of smell. Compounds that have similar structural properties activate many of the same receptors, so they smell similar.

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