Formula, molecular

A molecular formula shows the molecular composition of a substance. It provides the actual number of atoms of each element in a molecule of the compound. The formula of sarin, for example, is C4H,oF02P This formula shows that one sarin molecule contains 4 carbon atoms, 10 hydrogen atoms, 1 fluorine atom, 2 oxygen atoms, and 1 phosphorus atom. The formulas for CWAs and some of the most hazardous TICs are included in this chapter s tables. 

By identifying the functional groups present in a molecule, a molecular formula provides insight into numerous properties. These include the molecule s water and lipid solubility, the presence of fracture points for gas chromatography (GC) determinations, sources of potential markers such as chromophores, an indication as to the molecule s UV absorbance, whether derivatization is likely to be required when quantifying residues of the compound, and the form of ionization such as protonated ions or adduct ions when using electrospray ionization. The molecular formulas of the antimicrobial agents described in this chapter are shown in Tables 1.2-1.15. 

This formula indicates the actual number of atoms of each constituent element which is present in the molecule in question. It is derived from a knowledge of the empirical formula and the relative molecular mass (RMM). For example, ethyne and benzene each have the same empirical formula, namely CH. How-ever, they have different RMMs, namely 26 and 78 respectively. Accordingly, the molecular formula for each compound is C2H2 and C6H6 respectively. Like the empirical formula, 

In indexes, the compounds are first listed in order of increasing carbon number and, for molecules with the same number of carbons, in increasing hydrogen number. The remaining elements are then listed in alphabetical order. The symbols D and T are used for the isotopes of hydrogen, i. e. deuterium and tritium respectively. Other non-standard isotopes are indicated by a preceding 

All structural notations attempt to give some indication of the real-life arrangement of the atoms within the molecule in question. The notations fall into two categories two-dimensional and three-dimensional. The former type portrays the molecule as though it were flat, and leaves unresolved any explicit information about the three-dimensional structure. This type only indicates the connectivities of the component atoms to each other. In contrast, the latter type sets out to give as much information as is possible on a flat sheet of paper of the spatial configuration of the constituent parts of the molecule in question. 

This structural notation is very cumbersome. It is used only when it is necessary to illustrate the origin of each electron in a covalent bond. A dot is used to indicate an electron originating from one type of atom, while a cross is used for an electron originating from a different type of atom. It is used only for very simple molecules. 

This notation can be used to represent the electronic configuration of a molecule that contains more than two different elements, but this can easily become confusing. Often the electrons that originate from the third element would be represented by a small square, or such like. If that element occurred only in an isolated part of the molecule, then it might be represented by either a dot or a cross, whichever was the more convenient. 

With the growing availability of mass spectrometers of sufficiently high range and resolving power for most organic unknowns, mass spectrometry has become the most useful tool for determining molecular formulas. It is discussed at length before the other methods. 

Mass spectra have long been used (a) as fingerprints for organic compounds (previous section) and (b) for the determination of accurate molecular weights (well within one mass unit). More recently, they have been exploited (c) for the determination of exact molecular formulas. The emphasis in this section is on applications (b) and (c). 

The chief difficulty with mass spectrometric molecular weights is that a small percentage are, for several different reasons, off by one or more mass units, thus introducing an element of doubt in molecular weights obtained in this way. To appreciate the techniques used to minimize these sources of error, it is necessary to know something of the changes occurring in a sample between its introduction into a mass spectrometer and the appearance of peaks from it in the spectrum. 

The coincidence that organic compounds containing the usual elements have even-numbered masses except when an odd number of nitrogens is present is often of considerable aid in the selection of the molecular ion peak. 

Even when these two tests have been applied, the possibility exists that the molecular ion is so unstable that it does not appear in the spectrum. This is especially true for tertiary alcohols. In these cases, it is sometimes possible to determine the exact molecular weight by studying the fragmentation pattern. Tertiary alcohols, for example, usually give strong peaks at P — 18 (loss of water from the molecular ion). 

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Simmons-Smith reagent Named after the duPont chemists who discovered that diiodo-mechane would react with an active zinc-copper couple in ether to give a reagent with molecular formula ICHiZnl. The reagent adds stereospecifically cis- to alkenes to give cyclopropanes in high yields. 

Separation of families by merely increasing the resolution evidently can not be used when the two chemical families have the same molecular formula. This is particularly true for naphthenes and olefins of the formula, C H2 , which also happen to have very similar fragmentation patterns. Resolution of these two molecular types is one of the problems not yet solved by mass spectrometry, despite the efforts of numerous laboratories motivated by the refiner s major interest in being able to make the distinction. Olefins are in fact abundantly present in the products from conversion processes. 

The molecular formula of ozone was determined by comparing its rate of diffusion with that of a known gas. The geometric structure... 

Three crystalline compounds, one violet, one pale green, and one deep green in colour, all have the molecular formula CrClj. 6H2O. When equal masses of the three compounds are separately treated with an excess of aqueous silver nitrate at room temperature, the masses of white precipitate produced are in the ratio 3 2 1. Suggest an explanation for these results. 

Factual databases may provide the electronic version of printed catalogs on chemical compoimds. The catalogs of different suppliers of chemicals serve to identify chemical compounds with their appropriate synonyms, molecular formulas, molecular weight, structure diagrams, and - of course - the price. Sometimes the data are linked to other databases that contain additional information. Structure and substructure search possibihties have now been included in most of the databases of chemical suppliers. 

Thus, if the user wants to look for literature including requested chemicals or reactions, it is possible to query the database by the first option Chemical Substance or Reaction , The compound can be entered as a query in three different ways drawing the chemical structure in a molecule editor (Chemical Structure) searching by names or identification number, such as the CAS Number (Structure Identifier) and searching by molecular formula (Figure 5-12). 

As already mentioned (Section 5.3), the stored structure information in this type of database makes it possible to search for chemical structures in several ways. One method is to draw a structure (via a molecule editor) and to perform either a precise structure search (full structure search) or a search containing part of the input structure (substructure search) (see Sections 6.2-6.4). The databases also allow the searching of chemical names and molecular formulas (see Section 6.1). The search results are in most cases displayed in a graphical manner. 

The problem of perception complete structures is related to the problem of their representation, for which the basic requirements are to represent as much as possible the functionality of the structure, to be unique, and to allow the restoration of the structure. Various approaches have been devised to this end. They comprise the use of molecular formulas, molecular weights, trade and/or trivial names, various line notations, registry numbers, constitutional diagrams 2D representations), atom coordinates (2D or 3D representations), topological indices, hash codes, and others (see Chapter 2). 

Empirical molecular formulas and molecular weights usual identify a whole class of compounds (chemical isomers) rather than a single structure. Further-... 

Furthermore, the prediction of and NMR spectra is of great importance in systems for automatic structure elucidation. In many such systems, aU isomers with a given molecular formula are automatically produced by a structure generator, and are then ranked according to the similarity of the spectrum predicted for each isomer to the experimental spectrum. 

Robust implementations can currently propose correct structures from spectroscopic data, especially when the molecular formula and C NMR spectrum are available, or from 2D NMR spectra. 

The analyses which follow are arranged in the order in which they would be applied to a newly discovered substance, the estimation of the elements present and molecular weight deter-minations(f.e., determination of empirical and molecular formulae respectively) coming first, then the estimation of particular groups in the molecule, and finally the estimation of special classes of organic compounds. It should be noted, however, that this systematic order differs considerably from the order of experimental difficulty of the individual analyses. Consequently many of the later macro-analyses, such as the estimation of hydroxyl groups, acetyl groups, urea, etc. may well be undertaken by elementary students, while the earlier analyses, such as estimation of elements present in the molecule, should be reserved for more senior students. 

Amylene is a general name for the ethylenic hydrocarbons of the molecular formula CjHio. Two of these hydrocarbons are the main products of the dehydration of the appropriate amyl alcohols ... 

Carbohydrates may be divided into monosaccharides, disaccharides and polysaccharides. The monosaccharides under certain conditions react as polyhydroxy-aldehydes or polyhydroxy-ketones two important representatives are glucose CjHjjO (an aldose) and fructose (laevulose) CgHuO, (a ketose). Upon hydrolysis di- and polysaccharides 3deld ultimately monosaccharides. Common disaccharides are sucrose, lactose and maltose (all of molecular formula C,2H2. 0,), whilst starch, dextrin and cellulose, (CjHjoOj), in which n > 4, are typical polysaccharides. 

SMILES (simplified molecular-input line-entry specification) a way of specifying a molecular formula and connectivity, but not the three-dimensional geometry... 

The irradiation of tetra-/-butylcyclopentadienone with 254 nm light at 77 K produced a tricyclopentanone which, upon extended irradiation, lost carbon monoxide. Tetra-f-butyltetrahedrane was formed. This derivative of the second fundamental hydrocarbon of molecular formula (CH), namely tetrahedrane, is stable at room temperature and could be isolated after chromatography on silica gel in crystalline form (G. Maier, 1978). 

The most intriguing hydrocarbon of this molecular formula is named buUvalene, which is found in the mixture of products of the reaction given above. G. SchrOder (1963, 1964, 1967) synthesized it by a thermal dimerization presumably via diradicais of cyciooctatetraene and the photolytical cleavage of a benzene molecule from this dimer. The carbon-carbon bonds of buUvalene fluctuate extremely fast by thermal Cope rearrangements. 101/3 = 1,209,6(X) different combinations of the carbon atoms are possible. 

What particularly seemed to excite Wohler and his mentor Berzelius about this experiment had very little to do with vitalism Berzelius was interested m cases m which two clearly different materials had the same elemental composition and he invented the term isomerism to define it The fact that an inorganic compound (ammonium cyanate) of molecular formula CH4N2O could be transformed into an organic compound (urea) of the same molecular formula had an important bearing on the concept of isomerism... 

The molecular formula and the connectivity are determined experimentally and are included among the information given in the statement of the problem... 

Methyl nitrite has the molecular formula CH3NO2 All hydrogens are bonded to carbon and the order of atomic connections is CONO... 

Table 1 4 summarizes the procedure we have developed for writing Lewis structures Notice that the process depends on knowing not only the molecular formula but also the order m which the atoms are attached to one another This order of attachment is called the constitution, or connectivity, of the molecule and is determined by experiment Only rarely is it possible to deduce the constitution of a molecule from its molecular formula... 

In the introduction we noted that both Berzelius and Wohler were fascinated by the fact that two different compounds with different properties ammonium cyanate and urea pos sessed exactly the same molecular formula CH4N2O Berzelius had studied examples of similar phenomena earlier and invented the word isomer to describe different compounds that have the same molecular formula... 

We can illustrate isomensm by referring to two different compounds mtromethane and methyl nitrite both of which have the molecular formula CH3NO2 Nitromethane... 

Write structural formulas for al avTng the given molecular formula... 

Table 14m this section sets forth the procedure to be followed m writ mg Lewis structures for organic molecules It begins with experimentally determined information the molecular formula and the constitution (order m which the atoms are connected)... 

Different compounds that have the same molecular formula are called isomers If they are different because their atoms are connected m a dif ferent order they are called constitutional isomers... 

Formamide (/eft) and formaldoxime (right) are constitutional isomers both have the same molecular formula (CH3NO) but the atoms are con nected m a different order... 

Wnte structural formulas for all the constitutional isomers of molecular formula C3HgO that contain... 

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 ... 

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