Structural Formulas

If we accept hydrogen and bromine as being univalent and carbon as tetravalent, we can write

as a structural formula for C2H5BrC2H5Br.22 However, we also might have written

There is a serious problem as to whether these formulas represent the same or different compounds. All that was known in the early days was that every purified sample of C2H5Br, no matter how prepared, had a boiling point of 38 ^oC and density of 1.460 gml⁻¹. Furthermore, all looked the same, all smelled the same, and all underwent the same chemical reactions. There was no evidence that C2H5Br  was a mixture or that more than one compound of this formula could be prepared. One might conclude, therefore, that all of the structural formulas above represent a single substance even though they superficially, at least, look different. Indeed, because H−Br and Br−H  are two different ways of writing a formula for the same substance, we suspect that the same is true for

There are, though, two of these structures that could be different from one another, namely

In the first of these, CH3− is located opposite the Br−  and the H−'s on the carbon with the Br also are opposite one another. In the second formula, CH3−  and Br− are located next to each other as are the H−'s on the same carbon. We therefore have a problem as to whether these two different formulas also represent different compounds.