Molecular Formula as a Clue to Structure

As we have just seen, interpreting the fragmentation patterns in a mass spectrum in terms of a molecule s stmctural units makes mass spectrometry much more than just a tool for determining molecular weights. Nevertheless, even the molecular weight can provide more information than you might think. 

A relatively simple example is the nitrogen rule. A molecule with an odd number of nitrogens has an odd molecular weight a molecule with only C, H, and O or with an even number of nitrogens has an even molecular weight. 

You can t duplicate these molecular weights for C7H16 and C5H8O2 by using the atomic weights given in the periodic table. Those values are for the natural-abundance mixture of isotopes. The exact values are 12,00000 for 1,00783 for H, and 15,9949 for i 0, 

Other terms that mean the same thing as the index of hydrogen deficiency inciude elements of unsaturation, sites of unsaturation, and the sum of doubie bonds and rings. 

A second example concerns different compounds that have the same molecular weight, but different molecular formulas, such as heptane and cyclopropyl acetate. 

Once we have the molecular formula, it can provide information that limits the amount of trial-and-error structure writing we have to do. Consider, for example, heptane and its molecular formula of C7H16. We know immediately that the molecular formula belongs to an alkane because it corresponds to C H2 +2. 

What about a substance with the molecular formula C7H14 This compound cannot be an alkane but may be either a cycloalkane or an alkene, because both these classes of hydrocarbons correspond to the general molecular formula C H2 . Any time a ring or a double bond is present in an organic molecule, its molecular formula has two fewer hydrogen atoms than that of an alkane with the same number of carbons. 

The relationship between molecular formulas, multiple bonds, and rings is referred to as the index of hydrogen deficiency and can be expressed by the equation  

A molecule that has a molecular formula of C7H14 has an index of hydrogen deficiency of 1  

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Spectra by the Thousands 575 Infrared Spectra 576 Characteristic Absorption Frequencies Ultraviolet-Visible (UV-VIS) Spectroscopy Mass Spectrometry 584 Molecular Formula as a Clue to Structure 589 Summary 590 Problems 593... 

HjC CH3 25. A road-map problem. Hint There is "hidden" iiirormaiion. For example. if you just work out the molecular formula of the linal product, whose structure is shown, you obtain a useful clue. It is C(iHi SO . differing from unknown C by only two oxygens. So we can identify. structure C as the cyclic sulfide prior to oxidation (see structure in the margin). 

While Lavoisier had established a rational system for naming elements and compounds, Frankland developed the system that we use today for writing chemical formulas and for depicting the bonds between the atoms in molecules. As Frankland synthesized more and more isomers, compounds with the same formulas but different molecular structures, he found traditional formulas confusing they showed the types and numbers of elements but provided no clue as to how the atoms were arranged inside the molecule. To remedy the problem, Frankland depicted the atoms in functional groups and drew lines between them to indicate the bonds between the elements. 

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