The structures of organic compounds

An element is a substance that cannot be divided further by chemical methods—it is the basic substance upon which chemical compounds are built. The Periodic Table classes all known elements in a systematic manner based on the increasing number of electrons and protons (which are equal), starting with hydrogen (number 1 as it has 1 electron and 1 proton). 

Atoms are the smallest particle within elements. They are made up of protons and neutrons (in the nucleus) and electrons (in orbits around the nucleus). Each orbit represents an energy level and these give the atom stability. Electrons in the outer orbit, or valence shell, control how the atom bonds. When atoms are linked together by chemical bonds they form molecules. 

To achieve chemical stability, an atom must fill its outer electron shell, and it does this by losing, gaining or sharing electrons. These are known as valence electrons and the valence is specific for each element. 

A bond is a pair of electrons shared by the two atoms it holds together. There are many types of chemical bonds including hydrogen, ionic and covalent bonds. In organic chemistry (based on the element carbon) we deal mainly with covalent bonds, which may occur as single, double or triple bonds. 

Covalent bonds have a shared pair of electrons between two atoms—they neither gain nor lose electrons, as ionic bonds do. They occur in elements towards the centre of the Periodic Table, the most significant element being carbon. Covalent bonds are stronger than hydrogen or ionic bonds and don t form solutions with water. They may be polar or non-polar depending on the relationship between the electric charges emitted by the respective atoms. 

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Braude, Ultra-Violet Light Absorption and the Structure of Organic Compounds Annual Reports, 42, 105 (1945). ... 

Before the advent of NMR spectroscopy infrared (IR) spectroscopy was the mstrumen tal method most often applied to determine the structure of organic compounds Although NMR spectroscopy m general tells us more about the structure of an unknown com pound IR still retains an important place m the chemist s inventory of spectroscopic methods because of its usefulness m identifying the presence of certain functional groups within a molecule... 

Mass spectrometry can be used for gas analysis, for the analysis of petroleum products, and in examining semiconductors for impurities. It is also a very useful tool for establishing the structure of organic compounds. 

For even more convenience in representing the structures of organic compounds, particularly in printed material, line formulas are used, so-called because they are printed on one line. In line formulas, each carbon atom is written on a line adjacent to the symbols for the other elements to which it is bonded. Line formulas show the general sequence in which the carbon atoms are attached, but in order to interpret them properly, the permitted total bond orders of all the respective atoms must be kept in mind. Again referring to the compounds (a) to (e) described above, the line formulas are as follows ... 

A type of spectroscopy used in chemical analysis and the determination of the structure of organic compounds and polymers. 

C nuclei has become a powerful tool for elucidation of the structures of organic compounds. A large number of textbooks, monographs, articles, and data collections (1-15) deal with this method, and individual reviews have been devoted to practically all classes of natural products and related organic compounds (lb-24). 

This series of volumes, established by Victor Gold in 1963, aims to bring before a wide readership among the chemical community substantial, authoritative and considered reviews of areas of chemistry in which quantitative methods are used in the study of the structures of organic compounds and their relation to physical and chemical properties. 

The 5 major spectroscopic methods (MS, UV, IR, H NMR and NMR) have become established as the principal tools for the determination of the structures of organic compounds, because between them they detect a wide variety of structural elements. 

DETERMINiNG THE STRUCTURE OF ORGANiC COMPOUNDS FROM SPECTRA... 

Chapter 8 Determining the Structure of Organic Compounds from Spectra... 

Infrared (IR) spectroscopy was the first modern spectroscopic method which became available to chemists for use in the identification of the structure of organic compounds. Not only is IR spectroscopy useful in determining which functional groups are present in a molecule, but also with more careful analysis of the spectrum, additional structural details can be obtained. For example, it is possible to determine whether an alkene is cis or trans. With the advent of nuclear magnetic resonance (NMR) spectroscopy, IR spectroscopy became used to a lesser extent in structural identification. This is because NMR spectra typically are more easily interpreted than are IR spectra. However, there was a renewed interest in IR spectroscopy in the late 1970s for the identification of highly unstable molecules. Concurrent with this renewed interest were advances in computational chemistry which allowed, for the first time, the actual computation of IR spectra of a molecular system with reasonable accuracy. This chapter describes how the confluence of a new experimental technique with that of improved computational methods led to a major advance in the structural identification of highly unstable molecules and reactive intermediates. 

III. Influence of the Structure of Organic Compounds on Their Electrical... 

It is now known that dark- and photoconductivity is connected with the structure of organic compounds 10>. The conductivity of organic dyes and other organic compounds, like that of inorganic semiconductors, is attributable to electronic charge carriers, i.e. electrons and positive holes. The dark conductivity [Pg.87]

The structure of organic compounds is discussed in the first five chapters of Part 1. This section provides a necessary background for understanding mechanisms and is also important in its own right. The discussion begins with chemical bonding and includes a chapter on stereochemistry. There follow two chapters on reaction... 

Mass spectrometers provide a wealth of information concerning the structure of organic compounds, their elemental composition and compound types in complex mixtures. A detailed interpretation of the mass spectrum frequently allows the positions of the functional groups to be determined. Moreover, mass spectrometry is used to investigate reaction mechanisms, kinetics, and is also used in tracer work. 

The role of functional groups in directing molecular fragmentation, and the value of mass spectrometry in elucidation of the structure of organic compounds was clearly demonstrated by Biemann [28], Beynon [29], and McLafferty [30]. 

Postcolumn UV irradiation to destroy the structure of organic compounds leaving the chromatographic column Oxidizing organic matter contained in the sample Hybrid photocatalysis/membrane treatment of water UV digestion of the sample... 

This exercise helps you revise the key features of the families of hydrocarbons and develops your understanding of the structures of organic compounds. 

The structure of organic compounds of heavy group 14 elements in the condensed state, for example R4M, depends not only on intramolecular R R (R = Aik, Ar, H or other nucleophobic substituent) non-bonded interactions, but also on intermolecular R R and R M interactions. If the M atom is greatly shielded by bulky R substituents, the intra-and inter-molecular R R interactions prevail. If the M atom is weakly shielded by R substituents, the possibility of intermolecular R M non-bonded interactions as well as M M (if R = H) interactions become more important. This favours the association of R4M molecules70. As the atomic number of M increases, the shielding of the central M atom in isostructural molecules R4M diminishes. This is the reason why intra-molecular non-bonded interactions are more typical for M = Ge, and inter-molecular interactions are more typical for M = Sn and Pb. 

N. A. Domnin, in Theoretical Problems Concerning the Structure of Organic Compounds, p. 107. Leningrad Univ. Press, 1960. 

Chapters 3 and 4 (familiarity with which is assumed) provide us with powerful techniques and methods to elucidate the structures of organic compounds especially when combined with information derived from IR and mass spectrometry. These NMR methods are collectively referred to as one-dimensional techniques. To extend our capabilities, we turn once more to NMR. We will use four compounds as examples ipsenol (see Chapter 3), caryophyllene oxide (a sesquiterpene epoxide), lactose (a j3-linked disaccharide), and a small peptide (valine-glycine-serine-glutamate, VGSE). The structures of these compounds are shown in Figure 5.1. 

Fig. 12.47. Corrosion inhibition by n-alkyl-triethyl- and -trimethylammonium bromides in 1 W-H2S04 at 20 °C. (Reprinted from W. P. Singh, Relationships between the Structure of organic Compounds and Corrosion Inhibition, in Green Inhibitor Consortium, Texas A M University, 1997.)...

How did organic chemists identify the structures of organic compounds before the advent of spectroscopy Basically, the structure had to be consistent with all the facts known about a compound. Often, these facts included the results of a number of chemical reactions. Let s examine the case of the substitution products that occur on reaction of benzene with bromine. 

Q Combine the chemical shifts, integrals, and spin-spin splitting patterns in NMR spectra with information from infrared and mass spectra to determine the structures of organic compounds. Problems 13-47 and 48... 

Braude EA (1955) Ultraviolet light absorption and the structure of organic compounds Ann Rep Prog Chem, Chem Soc Lond 17 105-130 Brauns FE (1952) The chemistry of lignin Academic Press New York, 217-230 Brauns FE, Brauns DA (1960) The chemistry of lignin, Suppl Vol Academic Press, New York, 199-219... 

There are two main problems to consider. One is the formulation of quantitative relationships using physicochemical parameters and regression analysis. As such equations are derived, the problem of organization of the mass of data must be solved. The most suitable, relatively inexpensive method for dealing with the structures of organic compounds via computers is the Wiswesser Line Notation method. This notation and the proper computer program can be of great help in comparative pharmacodynamics. 

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