Molecular formulas determination

The resolution of most mass spectrometers (the ability to separate ions of similar m/z values - see Section 3.3 above) in routine use is sufficient to allow the separation of the ions containing the individual isotopes if low-molecular-weight compounds (<1000 Da) are being studied. This is illustrated in Figure 4.13 which shows the molecular-ion region of a compound having the molecular formula C35H48N8O11S determined with a mass spectrometer resolution of 1500. The masses of the isotopes present in this molecule are shown in Table 4.2. 

Now the molecular formula is determined using the steps outlined above ... 

Consider the mass spectra below of two isomers (A and B) of molecular formula C10H14. Determine their structures and explain the major spectral features for each. 

Important as the molecular formula is. it does not describe fully the properties, or even in some cases the identity, of chemical compounds. For example, there are two compounds that have the molecular formula CjFLO. They are different in all their properties, both chemical and physical. This difference is due to a difference in the manner in which the atoms are connected in the molecules of the two substances. These differences can be shown only by the use of structural formulas, such as those shown in Fig. I, in which the valence bonds between the atom are shown. These structural formulas are determined circumstantially, lhat is. by the chemical reactions into which the compounds enter. (However, (heir arrangements have been confirmed In many cases by a direci instrumental means such us speclrometric methods, x-ray studies, etc.) These reactions differ markedly for ethyl alcohol and methyl ether. Such compounds which have the same molecular formula but differ due to the arrangements or positions of their atoms are called isomers, and the type just cited, in which the difference is in the grouping of the atoms, are called functional isomers. These, and many other lypes of isomers, are treated in the entry on Isomerism. 

Elemental analysis can provide only an empirical formula. To determine the molecular formula, it is also necessary to know the substance s molecular mass. In the present problem, the molecular mass of naphthalene is 128.2 amu, or twice the empirical formula mass of C5H4 (64.1 amu). Thus, the molecular formula of naphthalene is C(2x5)H(2x4)/ or Ci0H8. 

We said in the previous section that elemental analysis gives only an empirical formula. To determine a compound s molecular formula, it s also necessary to know the compound s molecular mass. How is molecular mass determined ... 

Given a simple molecular formula, draw the electron-dot formula and determine whether each atom in the structure carries a formal charge. 

A unique compound with a 7,7-dimethylated cularine skeleton bearing oxygen substituents at positions 1, 2, 3, and 9 was isolated from G. ouregou and named gouregine (132) (38, 41). Its molecular formula was determined as C2oH 9N05... 

A compound with the molecular formula C7H13NO2, determined by mass spectrometry, was isola-ted from the plant Petiveria alliacea (Phytolaccaceae). What is its structure given the set of NMR results 52 ... 

Taking into account the results reported here on the various fractions, Figure 3 shows structures consistent with the data. Molecular formulae were determined from the elemental analyses of the fractions. Other functionalities and linking units could also satisfy the data however, those shown were considered most likely based on the results and steric and stability considerations. The structures in Figure 3 contain only major features such as the nature of the aromatic clusters, as well as the approximate numbers of aromatic and naphthenic rings per cluster. Functional groups and side chains are indicated only in a qualitative manner. 

Calculation of the Empirical Formula Molecular formulas can be determined by a two-step process. The first step is the determination of an empirical formula, simply the relative ratios of the elements present. Suppose, for example, that an unknown compound was found by quantitative elemental analysis to contain 40.0% carbon and 6.67% hydrogen. The remainder of the weight (53.3%) is assumed to be oxygen. To convert these numbers to an empirical formula, we can follow a simple procedure. 

Chemists can use a mass spectrometer to determine the molar mass of a compound. They can use the molar mass, along with the molar mass of a known empirical formula, to determine the compound s molecular formula. For example, the empirical formula CH has a molar mass of 13 g/mol. We know, however, that acetylene, C2H2, and benzene, C6H6, both have the empirical formula CH. Suppose it is determined, through mass spectrometry,... 

It is apparent that the compounds C2H2 and CeHe have different molecular masses. That of C2H2 is 26 amu that of CeHe is 78 amu. It straightforward to determine the molecular mass from the molecular formula, but how can the molecular formula be determined from the empirical formula and the molecular mass The following steps are used, with benzene having a molecular mass of 78 amu and an empirical formula of CH serving as an example. 

The molecular formula is determined from the empirical formula and the experimentally determined molar mass. 

Determine the integer that Experimental molar mass relates the empirical and Mass of empirical formula molecular formulas. ... 

Fractionation of the aqueous MeOH-soluble extract guided by antibacterial activity against Rhodospirillum salexigens, which has adhering properties and forms a microbial biofilm [28], led to the isolation of a novel bromotyrosine derivative, zamamistatin (17) [29]. In its ESIMS spectrum, 17 showed 1 4 6 4 1 quintet ion peaks at m/z 697, 699, 701, 703 and 705, indicative of the presence of four bromine atoms. The molecular formula was determined to be Ci8Hi8Br4N204 by HRESIMS. The observation of only 9 carbons by NMR and the specific rotation of +248° suggested that 17 was an optically active dimmer with a symmetrical structure. Not many resonances in the H and NMR spectra of 17 in contrast to its molecular formula made the structure determination of 17 difficult. 

The molecular formula is some whole-number multiple of the empirical formula. To determine the molecular formula, you must know the approximate molar mass of the compound under study. From Avogadro s hypothesis, the ratio of molar masses of two gaseous compounds is the same as the ratio of their densities, provided that those densities are measured at the same temperature and pressure. (This is true because a given volume contains the same number of molecules of the two gases.) The density of the welding gas from Example 2.4 is 1.06 g at 25°C... 

A chemist dissolves 1.406 g pure platinum (Pt) in an excess of a mixture of hydrochloric and nitric acids and then, after a series of subsequent steps involving several other chemicals, isolates a compound of molecular formula Pt2CioHi8N2S20g. Determine the maximum possible yield of this compound. 

Percent composition data yield only simplest formulas. To determine the molecular formula for a molecular compound, both its simplest formula and its molecular weight must be known. Some methods for experimental determination of molecular weights are introduced in Chapters 12 and 14. 

Iran mix, Arab mix, and Dubai short residues (550°C+) obtained from Madras Refineries Ltd., Madras were used as the feed stocks. Their Physico-chemical characteristics, compositional analysis, elemental compositions and average molecular formulae as determined by standard ASTM/BIS/IP methods are given in Table 1. 

Write a complete structural formula and determine the I.U.P.A.C. name for each of the carboxylic acids of molecular formula C4Hg02. 

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