Molecular formulas of organic compounds

Molecular formulas of organic compounds are customarily presented in the fashion C2H5Br02 The number of carbon and hydrogen atoms are presented first followed by the other atoms in alphabetical order Give the molecular formulas corresponding to each of the compounds in the preceding problem Are any of them isomers ... 

PeDegrin, V. (1983) Molecular formulas of organic compounds. The nitrogen rule and degree of unsaturation. J. Chem. Educ., 60, 626-633. 

Therefore in order to calculate the molecular formula of a compound it is necessary to know its molar mass. Molar masses can be determined by a variety of physical measurements, including back titrations (for weak acids and bases) and weighing gases. Mass spectrometry is frequently used to determine the molar masses of molecular substances (Chapter 2). Automated instruments for determining the empirical and molecular formulas of organic compounds are available. 

To find the molecular formula of a compound, we need one more piece of information—its molar mass. Then all we have to do is to calculate how many empirical formula units are needed to account for the molar mass. One of the best ways of determining the molar mass of an organic compound is by mass spectrometry. We saw this technique applied to atoms in Section B. It can be applied to molecules, too and, although there are important changes of detail, the technique is essentially the same. 

A technique called infrared (IR) spectroscopy is valuable in the study of organic compounds. This technique allows researchers to determine the kinds of bonds and functional groups that are present in a molecule. Using a more advanced analysis, researchers are even able to determine other groups and bonds that are nearby. This information, paired with the molecular formula of a compound, helps researchers puzzle out the precise structure of an unknown molecule. 

Traditionally, the molecular formula of a compound was derived from elemental analysis and its molecular weight which was determined independently. The concept of the degree of unsaturation of an organic compound derives simply from the tetravalency of carbon. For a non-cyclic hydrocarbon i.e. an alkane) the number of hydrogen atoms must be twice the number of carbon atoms plus two, any deficiency in the number of hydrogens must be due to the presence of unsaturation, i.e. double bonds, triple bonds or rings in the structure. 

In Section F, we remarked that one technique used in modern chemical laboratories or the agencies that carry out contract work on behalf of other chemists is combustion analysis. This technique—which has been used since the earliest days of chemistry—is used to establish the empirical formulas of organic compounds and, in combination with mass spectrometry, their molecular formulas. It is used both for newly synthesized compounds and to identify active compounds in natural products. We are now in a position to understand the basis of the technique, for it makes use of the concept of limiting reactant. 

An organic compound was prepared containing at least one and no more than two sulfur atoms per molecule. The compound had C and H, no N, but O could have been present. The molecular mass of the predominant nuclidic species was 110.020 u, as determined by mass spectrometry. What is the molecular formula of the compound ... 

An organic compound whose molar mass is 60 g/mol contains 40% carbon, 53.3% oxygen and 6.7% hydrogen. What is the molecular formula of this compound ... 

Le Bel published his stereochemical ideas two months later, in November 1874, under the title, The relations that exist between the atomic formulas of organic compounds and the rotatory power of their solutions" (14). An English translation is presented in Le Bel (15). Le Bel approached the problem from a different direction from van t Hoff. His hypothesis was based on neither the tetrahedral model of the carbon atom nor the concept of fixed valences between the atoms. He proceeded purely from symmetry arguments he spoke of the asymmetry, not of individual atoms, but of the entire molecule, so that his views would nowadays be classed under the heading of molecular asymmetry. Only once does he mention the tetrahedral carbon atom, which he regarded as not a general principle but a special case. Today, substituted allenes, spiranes, and biphenyls are but a few examples of asymmetric molecules that do not contain any asymmetric carbons, thus confirming Le Bel s views on molecular asymmetry. The reason for the different approaches by van t Hoff and le Bel is easy to understand, van t Hoff came from the camp of structural chemists, and he... 

An organic compound was found by combustion analysis to contain 38.7% carbon, 9.7% hydrogen, and the remainder oxygen. In order to determine its molecular formula a l.OOg sample was added to 10.00g of water, and the freezing point of the solution was found to be -2.94 °C. What is the molecular formula of the compound ... 

The empirical formula of an organic compound is C3H502. The molecular mass of the compound is 146.1 amu. What is the molecular formula of the compound ... 

Today, a number of different instrumental techniques are used to identify organic compounds. These techniques can be performed quickly on small amounts of a compound and can provide much more information about the compound s structure than simple chemical tests can provide. We have already discussed one such technique ultraviolet/visible (UVA/is) spectroscopy, which provides information about organic compounds with conjugated double bonds. In this chapter, we will look at two more instrumental techniques mass spectrometry and infrared (IR) spectroscopy. Mass spectrometry allows us to determine the molecular mass and the molecular formula of a compound, as well as certain structural features of the compound. Infrared spectroscopy allows us to determine the kinds of functional groups a compound has. In the next chapter, we will look at nuclear magnetic resonance (NMR) spectroscopy, which provides information about the carbon-hydrogen framework of a compound. Of these instrumental techniques, mass spectrometry is the only one that does not involve electromagnetic radiation. Thus, it is called spectrometry, whereas the others are called spectroscopy. 

Before attempting to deduce the structure of an unknown organic substance from an examination of its spectra, we can simplify the problem somewhat by examining the molecular formula of the substance. The purpose of this chapter is to describe how the molecular formula of a compound is determined and how structural information may be obtained from that formula. The early portion of this chapter reviews the classical analytical methods of determining a molecular formula. Many of these methods are still in routine use today, but the use of mass spectrometry has become a common alternative. A brief introduction to the use of the mass spectrometer is given at the end of this chapter (Section 1.6) and a more complete discussion may be found in Chapter 8. 

Recognize the molecular formulas of organic and inorganic compounds. (Section 1.1)... 

Landolt made a careful study of the formula proposed by J. H. Gladstone and J. Dale for the molecular refractivity of organic compounds. He investigated the time-reaction between iodic and sulphurous acids leading to the liberation of iodine, collaborated with Richard Bornstein, professor of physics in the Agricultural Institute, Berlin, in compiling the valuable Physikalisch-chemische Tabellen (1883 and later eds.), and did much work on the practical applications of optical activity and polarimetry. ... 

In the following section, we present an exploratory investigation into the use of HR MS data to determine the correct molecular formula of measured compounds. Ten organic compounds chosen with masses ranging between 132 and 1218 Da are listed in Table 8.20 together with the PubChem [218] Compound Identifier (CID), molecular formula ji and nominal mass nip. Compounds were obtained fi om Sigma-Aldrich (St. Louis, MO, USA). All compound solutions were prepared in MeOH/Hj 0 (1 1). 

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