The Chemistry of Nature – There are endless thousands of chemical substances in nature, divided into many categories or classifications such as hormones, alkaloids, vitamins, neurotransmitters, enzymes, proteins, nucleic acids, and so on. One group of chemicals, the one I wish to discuss here, follows a kind of rule of manufacture that has come to be called the isoprene rule . Many substances we eat and or clean with are examples of chemicals from this group, which include turpentine we use to clean our paint brushes and beta-carotene found in carrots! Chemicals that can be made from derivatives of isoprene following the stipulations of this rule are known as terpenoids .
Accompanying this article, but not in the text itself, I have provided illustrations representative of the molecular structure of the various chemicals, so that you can get an idea of what I am talking about. I do not show the individual atoms of carbon or hydrogen, but I use the ” bond line notation ” standard to much of advanced organic chemistry, which is a kind of stick notation.
Thus, to use the simplest example, ethane, C2H6 would be, a bit more accurately,
H3C-CH3 or simplifying the drawing by removing the hydrogens,
C-C or even more simplified, the stick notation,
where the left side of the stick or line segment represents one carbon atom, and the right side one carbon atom. The hydrogen atoms, as I mention above, are not represented at all, but are understood to be there, just the same. Carbon here is assumed to have a valence of four. Thus, for our purposes, no carbon has more or less than 4 connections to other atoms. In the above instance, both carbons are attached to each other, and, if you add three bonds for each carbon to its respective hydrogens, you have a total of four bonds to each carbon.
So a short, straight line segment pictures ethane. What about the next compound of this series (known as alkanes)? Showing it as a V-shaped line segment, it is called propane, C3H8 or,
Or, in other words, a carbon on each end of the V and a carbon at the point of the V. Now the molecule drawn out may not appear to have a V-shape at all, but looks like it exists in a straight line of three carbons in a row with attached hydrogens. In reality, the three carbons do not lie along a straight line, but somewhat resemble a V on the average (the molecule is not static, but it moves about, bending, twisting, vibrating, and moving in three dimensions).
The Structure of Isoprene and the Statement of the Rule
Isoprene, although consisting only of carbon and hydrogen, is not limited to using “single bonds.” See Figure 1 for an illustration of isoprene, C5H8. The double lines represent double bonds, which is to say the two carbons involved have two connections, rather than one, to each other, and so a hydrogen is missing on each of the carbons having the double bonds. If there had been only single bonds, the chemical would have been C5H12. Two double bonds, though, involve a total of four carbons, each of which loses a hydrogen, and so the twelve hydrogens are reduced in number to eight.
Now, isoprene is said to have a “head” end and a “tail” end. The isoprene rule involves how there is a class of chemicals made by polymerizing, or putting together, more than one isoprene unit, in a configuration of “head-to-tail.” So the tail of the first molecule joins to the head of the second molecule, forming one larger species. As the simplest example, see Figure 2, which represents myrcene. Myrcene used as an intermediate in the manufacture of flavor and fragrance chemicals.
One cleaning agent found in industry is limonene. Figure 3 gives an illustration of that chemical structure. It, like myrcene, involves only two isoprene units, but it uses them in a fashion that it generates a ring-structure. I have provided a URL at the end of this article that has a shockwave flash animation of the two isoprene units joining to become the one limonene unit.
Another chemical that is well known, and is of major importance is the species responsible for a carrot’s orange color, beta-carotene. I do not provide a structure for that, but one is readily found on Wikipedia. Break this chemical in half, and you get retinal, a form of Vitamin A, which is considered essential to good eyesight. Can you see why it is said that carrots are good for the eyes?
This article touches on a very small selection of terpenoids, and represents the briefest of introductions. There are untold numbers more of these fascinating compounds. Some examples not discussed are pinene, geraniol, menthol, and camphor. Many scientists particularly enjoy pursuing the chemistry of naturally-occurring substances. Perhaps you, too, one day, may desire to study the chemistry of the terpenoids, as well as the vast number of other substances that exist in nature all around you. If so, I can guarantee you will never be bored!