Checking Molecular Formulae

When working out the molecular formula of a neutral organic compound, it is useful to remember that there must be an even number of odd-valent atoms (e.g., H, halogens, N, P). Thus, the formula C27H45O is obviously incorrect unless it is a radical. 

A more sophisticated check on the accuracy of a formula is to calculate the number of rings/ double bonds in the compound from the formula. You can then count the number of rings and double bonds and compare it with the results of the calculation. 

If H = number of univalent atoms (H, halogen), N = number of trivalent atoms (N, P), and C = number of tetravalent atoms, then 

Organic Chemist s Desk Reference, Second Edition 

The following rapid method can be nsed to check the hydrogen connt in a structure without using a computer, for example  

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Reality Check Note that although molecular formulas give the composition of the molecule, they reveal nothing about the way the atoms fit together. In that sense they are less useful than structural formulas. 

It follows that the molecular formula of the compound is (C2H5) x 2 = C4H10 (1). A quick check confirms that the molar mass is correct molar mass of C4H10 in g mol-1 = (12 x 4) + (1 x 10) = 58 g mol-1 9 (a) Start with the full equation ... 

If you completed the Concept Check activity on page 12, you drew a possible structure for benzene. For many years, scientists could not determine the structure of benzene. From its molecular formula, CeHe, scientists reasoned that it should contain two double bonds and one triple bond, or even two triple bonds. Benzene, however, does not undergo the same reactions as other compounds with double or triple bonds. 

Constitutional isomers may differ in ways other than the branching of their carbon chain. They may differ in the position of a double bond or other group or by having a ring or some other feature. Notice how the following constitutional isomers all differ by the ways in which atoms are bonded to other atoms. (Check the number of hydrogens bonded to each carbon.) These compounds are not isomers of the pentanes just shown, however, because these have a different molecular formula (C5H10). 

When you need a structure for a particular molecular formula, it helps to find the number of elements of unsaturation. Calculate the maximum number of hydrogen atoms from the saturated formula, C H(2n+2)> and see how many are missing. The number of elements of unsaturation is simply half the number of missing hydrogens. This simple calculation allows you to consider possible structures quickly, without always having to check for the correct molecular formula. 

Mercury(I) chloride (and all other Hg(I) salts) are diamagnetic. Explain how this can be tme. You may want to check the molecular formulas of these compounds. 

Methods of characterization aim to determine the products of a reaction. The level of detail expected depends on the circumstances, and determines the range of methods required. If the aim has been to make a known compound, one needs to check its identity and purity. Fingerprinting techniques measure a spectrum or some other property and compare it with results published for known compounds and available in literature databases. Such techniques may also show whether impurities are present, but it is often desirable to check the purity of the compound independently, for example by elemental analysis. However, if the compound prepared is a new one, more thorough investigation is appropriate. The stoichiometric formula may be found by elemental analysis, and the full molecular formula in principle by mass spectrometry (MS). MS combined with other spectroscopic techniques, especially infrared (IR) and nuclear magnetic... 

The chapters are organized for easy reference and incorporate tables by which information on specific compounds can be readily traced. Table entries on individual compounds are listed in order of their molecular formula. We suggest that a molecular formula check is the surest way of ascertaining if a particular compound is listed in the tabulation. Also, a molecular formula provides a convenient search term for locating specific information in Chemical Abstracts. It is clearly impossible to include the entries for polyfunctional compounds in each appropriate table so that in the case, for example, of a chloroamino compound the tabulation of both chloro- and amino compounds must be consulted. 

Check The calculated molecular formula has the same ratio of moles of elements (3 6 3) as the empirical formula (1 2 1) and corresponds to the given molar mass ... 

O Reading Check Write the molecular formula for an alkane that has 13 carbon atoms in its molecular structure. 

Notice that the NMR shows a total of five peaks, corresponding exactly to the number of carbons in the molecular formula This is a nice check on our calculation of the formula via the Rule of Thirteen (five earbon atoms). The peak at 174 ppm corresponds to the ester C=0 carbon. The peak at 60 ppm is a deshielded earbon atom eaused by a neighboring single-bonded oxygen atom. The rest of the earbon atoms are relatively shielded. These three peaks eorrespond to the remaining part of the carbon chain in the ester. 

We wrote a program to remove in-house idiosyncrasies from the WLN file and to reformat it for CROSSBOW. Programs from this suite were then used to remove outdated conventions, check for what errors could be detected algorithmically and generate molecular formulae. This was achieved and the file, then of some 13,000 FPL Compounds, was subsequently maintained by CROSSBOW. 

Check Note that in determining the molecular formula from the empirical formula, we need only know the approximate molar mass of the componnd. The reason is that the true molar mass is an integral multiple (IX, 2X, 3X,. . . ) of the empirical molar mass. Therefore, the ratio (molar mass/empirical molar mass) will always be close to an integer. 

An isomer of butane, for example, has the same molecular formula as the straight-chained compound shown in Figure 14-2, C4H10, but a different bonding pattern. This isomer is mostly referred to by the common name isobutane and is what I call a branched hydrocarbon. Check out Figure 14-3 to see it shown in a variety of ways. 

The data were subjected to very rigorous consistency checks to ensure only the inclusion of coherent spectrum-structure pairs, before further use The spectrum file needed to contain the molecular formulas and names of compounds as well as peak... 

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