Chemistry of Smells--Structure of Aldehydes and Other Compounds?

Question

I am wondering how and/or why specific molecular structure affects the way odor is processed in the human body.

Specifically, is the structure of aldehydes unique from the other odiferous compounds (esters, ketones, phenols, and any other groups of compounds), which in turn causes the body to process the compound differently than the other compounds?

Is there a group of compounds that are very close to aldehydes? If so, what are common sources of that compound found in the everyday world?

Why would a person be physically irritated by aldehyde-based odors, but not others?

I am a lay person, without chemistry knowledge, so I am not well-versed in terminology and explinations are helpful.

Both great answers. Thanks for the time and effort in writing. I chose first one because links were especially helpful for me at this stage.

Answer 1

Brief mechanism of olfactory system:

You smell something.
The molecules from what you smell bind to a receptor in your nose.
The receptors then transmit a signal to your brain where the odor is processed.

Each odor binds to a specific odor receptor. Humans have about 400 different types of odor receptors (that actually work). Each receptor binds to a certain range of odors.

Organic Chemistry:

Functional group:

From Wikipedia: In organic chemistry, functional groups (or moieties) are specific groups of atoms within molecules, that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction(s) regardless of the size of the molecule it is a part of.

All the compounds you listed above are functional groups:
In the formulas, the symbols R and R' usually denote an attached hydrogen, or a hydrocarbon (formula CnHn) side chain of any length, but may sometimes refer to any group of atoms.

(the dashed lines in parenthesis are just for spacing purposes)
(---------------)O
(----------------)||
Aldehyde: RC-H


(---------------)O
Ketone:(---) ||
(-----------)R'-C-R

Phenol: OH
(----------) |
(----------)Benzene Ring (6 carbon ring with alternating double bonds)

Ester: O
(-------) ||
(----)R-C-OR'

structurally I would say ketones are the closest things to aldehydes as an aldehyde is a special form of ketone (the double bond O is located at the end of the chain)

As for exactly why a person is irritated by aldehyde based odors and not any others I don't know. My guess is it would have to do with the receptors. Maybe the signal it sends to the brain is stronger in that individual.

Check out this page:
http://en.wikipedia.org/wiki/Aroma_compound

Answer 2

I have always found it absolutely facinating how the structure relates to the odor of various chemicals.

For example, you can smell the difference in length of a hydrocarbon chain! Pentane smells different than hexane than heptane etc!

You can smell the different length alcohols; methanol has no odor but ethanol, propanol, isopropanol, butanol all have characteristic odors and can be told apart.

You can smell the difference that the presence of oxygen makes! pentane (CH3CH2CH2CH2CH3 smells way different that ethyl ether CH3CH2OCH2CH3; the only difference is there is an oxygen in the center of ethyl ether which causes the molecule to have a bent configuration; so are we smelling "oxygen", or "bent-ness"? The molecule tetrahydrofuran is very similar to ethyl ether, but it is cyclic and thus even more "bent" than ethyl ether and smells way different.

You can smell differing levels of halogenation in a compound. Methylene chloride (CH2Cl2) chloroform (CHCl3) and carbon tetrachloride (CCl4) all smell different!

You can smell the difference from single vs multiple bonds Ethane CH3CH3 has no odor but ethylene CH2CH2 and acetylene HCCH both have strong odors.

A lot of aldehydes tend to have strong, unpleasant odors, benzene smells pretty bad, however, benzaldehyde smells pleasant! There are things that smell worse than aldehydes; if you want to smell something really nasty, try butyric acid, thiols (like alcohols but a sulfur instead of the oxygen), or compounds with carbon-carbon double bonds and thiol groups, organic selenium compounds, or amino silane compounds.

As to why people preceive certian odors as unpleasant it might have been a self-protective mechanism. Like when you find leftovers in the back in the of the fridge of questionable vintage; the first instinct is to sniff; if it smells like death, you leave it for someone else.

It is believed that the shape and probably the polarity of different molecule are responsible for the precieved odors.

Answer 3

if i remember correctly from my organic chem class, virtually all scents come from aldehydes. dont quote me on that though.