Question

Our present world could not exist without hydrocarbons. Nearly everything we touch today is either comprised of or coated with hydrocarbon products. In the practice of clinical toxicology, initial efforts usually entail precisely determining the specific xenobiotics that might be involved in a specific exposure, followed by defining the type and extent of the exposure. In this regard, hydrocarbon exposures are always clinically challenging. Despite significant chemical diversity, most classification schemes group hydrocarbons by specific uses or applications, rather than by chemical structure or physiologic properties. Most hydrocarbons in everyday use, such as gasoline, charcoal starter, and lamp oil, are actually mixtures of chemicals obtained from a common distillation fraction. The chemical diversity within a mixture makes it challenging to try to assess individual contribution to toxicity. As a result, generalities are often needed to describe the behavior of these complex mixtures.

Q1 Theory: Why does carbon form such a large diversity of compounds?

Answer 1

Carbon is the only element that can form so many different compounds because each carbon atom can form four chemical bonds to other atoms, and because the carbon atom is just the right, small size to fit in comfortably as parts of very large the atomic molecules.

Carbon's ability to form long carbon-to-carbon chains is the first of five reasons that there can be so many different carbon compounds; a molecule that differs by even one atom is, of course, a molecule of a different compound. The second reason for carbon's astounding compound-forming ability is that carbon atoms can bind to each other not only in straight chains, but in complex branchings, like the branches of a tree. They can even join "head-to-tail" to make rings of carbon atoms. There is practically no limit to the number or complexity of the branches or the number of rings that can be attached to them, and hence no limit to the number of different molecules that can be formed.

The third reason is that carbon atoms can share not only a single electron with another atom to form a single bond, but it can also share two or three electrons, forming a double or triple bond. This makes for a huge number of possible bond combinations at different places, making a huge number of different possible molecules. And a molecule that differs by even one atom or one bond position is a molecule of a different compound.

The fourth reason is that the same collection of atoms and bonds, but in a different geometrical arrangement within the molecule, makes a molecule with a different shape and hence different properties. These different molecules are called isomers.

The fifth reason is that all of the electrons that are not being used to bond carbon atoms together into chains and rings can be used to form bonds with atoms of several other elements. The most common other element is hydrogen, which makes the family of compounds known as hydrocarbons. But nitrogen, oxygen, phosphorus, sulfur, halogens, and several other kinds of atoms can also be attached as part of an organic molecule. There is a huge number of ways in which they can be attached to the carbon-atom branches, and each variation makes a molecule of a different compound. It's just as if moving a Christmas tree ornament from one branch to another created a completely different tree.