0.8 Molecular structures  (Page 2/5)

Observation 1: compounds with identical molecular formulas

In simple molecules, such as CH ₄ , it is fairly clear that the molecule consists of a single carbon atom surrounded by and bonded to four hydrogen atoms. There really is only one way that the molecule could be structured. But even in molecules only slightly larger than this, we discover some interesting observations.

Let’s consider the molecular formula C ₂ H ₆ O. Experimentally, we discover that there are two different compounds with the molecular formula C ₂ H ₆ O, and these two compounds have very different physical and chemical properties. One of the compounds is a liquid at room temperature with a boiling point of 78 ˚C. The other is a gas with a boiling point of -25 ˚C. This might seem surprising in a couple of ways. First, how could two compounds have the same molecular formula? We might have expected that a specific combination of the elements would produce a specific compound, but this is clearly not true. And second, even if we do imagine more than one possible compound, we would most probably have guessed that they would be similar in their properties since they contain the same elements in the same proportions. How can two compounds with the same molecular formula have so very different properties?

To understand these observations, we can use our model for molecular structures. From our earlier work, it is clear that the two carbon atoms and the oxygen atom must be bonded together, surrounded by hydrogen atoms. But in what order are they bonded? One option would be C-C-O, and a second would be C-O-C. The first of these would give us the molecular structure:

The second would give us:

By all of our criteria for the Lewis model of molecular structures, both of these look right. Each carbon atom and the oxygen atom in each structure show the proper valence and a complete valence shell. There are no unpaired electrons in either structure. These two molecular structures must be consistent with our experimental observation that there are two compounds with this molecular formula. Each of the compounds must correspond to one of the different molecular structures.

Which one is which? Determining which molecular structure corresponds to which compound is an interesting experimental problem. We’ll explore this further in the next section. For now, we’ll assume that it is experimentally possible to determine which is which. In this case, the first molecular structure above, containing the C-O-H arrangement, is the liquid at room temperature and is called ethanol or ethyl alcohol. The second molecular structure, containing the C-O-C arrangement, is a gas at room temperature and is called dimethyl ether.

Compounds with the same molecular formula are called “isomers” of one another. In the example above, dimethyl ether is an isomer of ethanol and vice versa. It is very common to observe isomers for any given molecular formula. Some of these are simple to see. Let’s consider the molecular formula C ₂ F ₄ Cl ₂ . Each halogen atom generally has a valence of 1 with seven valence electrons. This suggests that the two carbon atoms should be bonded together, as in ethane, with the six halogen atoms arranged three per carbon. But there is more than one way to do this: