Is there any relation between resonance and smell of a hydrocarbon?
I wish I could convey the spirit of Professor Louis Fieser on the development of organic chemistry. I couldn't find it in his textbooks, but in class, it went something like this:
Coal distillation led to a large number of different unsaturated compounds, most of which had strong odors, so they were classified as &quot;aromatic&quot;. When more quantitative measurements (heats of combustion and heats of hydrogenation) were available, double bonds were investigated, and some compounds were found to be unusually stable. A single $C-C$ bond should have a certain energy, and so would a $C-H$ bond, and so also should a $C=C$ double bond, but some compounds gave surprising results.
Some molecules (like benzene) could be written with two positions for the double bonds - they could &quot;resonate&quot; between the two arrangements. Most of the compounds that had this sort of possibility were odorous (aromatic), so any compound with this sort of resonance possibility was called &quot;aromatic&quot;, even if it had no appreciable odor. And they all had some sort of extra stability (&quot;resonance energy&quot;) due to the double bond shifting.
Now while &quot;resonance energy&quot; ought to have some quantitative significance, it had some considerable variability, depending on what values you used to calculate the expected value for heat of hydrogenation or combustion. Another term popped into use: &quot;aromaticity&quot;, which was never quantitatively defined, but was really a measure of the surprise at the difference between the calculated value and the observed value of the heats.
But I expect that odor is not a result of extra stability, but more likely a result of some reactivity of the multiple double bond system. And volatility.