Question:
So as we all know synthetic chemicals are often used to replace difficult to find or expensive natural ingredients in perfumes. My question is how is compound X found to be similar to rose? Is it by trial and error, or are they trying to match chemicals with similar structures (for example both share an alcohol group in one location)?
Thanks...

Answer:
There's a lot about this question to be found in books by and about Luca Turin (The Secret of Scent and The Emperor of Scent, respectively).
One of the bases of Turin's critique of theories that tie scent to molecular shape is that these theories have very poor predictive power, so that the design of new fragrance molecules requires extensive trial and error. Turin claims, furthermore, that his theory -- which ties scent to the vibrational frequencies of a molecule's constituent atoms -- has better predictive power. (See the front page of http://www.flexitral.com/ for an instance of this claim.)
I don't know how accurate these claims are about the relatively predictive success of the two theories, and I don't know if it's appropriate to use the predictive success of either theory as a way of deciding in favor of one theory or the other.
I guess the roundabout answer to your question is, they are trying to match chemicals to known structures, but this matching process, as it is usually performed, seems to require a degree of trial and error.

Answer:

There are three types of odorants:
(1) Molecules that don't exist in nature and are manufactured in the laboratory by chemical means
(2) Odorant elements of essential oils that are synthesized from natural sources
(3) The same odorant elements of essential oils that are synthesized from non-natural sources, such as coal tar derivatives
In the first case, one never knows how a specific molecule will smell until is it synthesized (unless Turin is correct; in which case, one can predict to a certain extent how it will smell beforehand; although, there are a lot of variables, and it's not as simple as it sounds as far I understand with my limited understanding).
In both the second and third cases, perfumers pretty much know how these odorants smell because they have a chance to smell them as synthetically created isolates or nature derived isolates. Even though these isolates smell a certain way by themselves, when they occur naturally in essential oils their scent is the total synergistic effect of their specific odor modified and enhanced by many other odorants. Rose oil has over 300 different odorants that make up its odor profile, but there are about four major odorants (rose alcohols) in terms of volume: geraniol, phenyl ethyl alcohol, citronellol, and nerol. It is the other approxmately 296 odorant constituents of rose oil, even in minute amounts, however, that give a rose oil its complex, inimitable nature specific rosiness. Different kinds of roses also have different proportions of these various major and minor odorants. These minor odorants are more difficult to isolate and to synthesize.
It's one of the cruel ironies of Nature that the easiest odorants to isolate are those that give us the general sense of the odor of a particular thing, say a rose, but all the other myriad odorants that are more difficult to isolate, and in some cases even identify, are what give a rose, for instance, its specific odor that only inimitably exists in toto in Nature.
scentemental


Answer:
Thanks for the replies guys. I guess I'm going to pick up Turin's book.