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The original definition, (see Historical Outline ) which came from the perception that these compounds were always related to life processes is no longer valid as over the years a very large number of chemical compounds have been artificially produced which, do not necessarily relate to life processes, but due to their similarity in characteristics to the previously known organic compounds are classed as organic compounds. Those compounds that are related to life processes are dealt with in the branch of organic chemistry which is called Biochemistry. However: organic compounds are all compounds containing carbon. Inorganic chemistry deals, apart from elemental carbon, only with simple carbon compounds, with molecular structures which do not contain carbon to carbon connections (its oxides, acids, salts, carbides, and minerals) This does not mean that single-carbon organic compunds do not exist (viz. methane and its simple derivatives) The nexus is thus Carbon , which is almost unique among the Elements of the Periodic Table in that its atoms combining directly with one another can form long Molecular chains and rings which may also include Nitrogen , Oxygen , Halogens , Phosphorus , Sulfur , and a variety of other Atom ic species. The straight chains and rings can again combine with one another to make their structure very complex. Because of their unique properties, multi-carbon compounds exhibit extremely large variety and the range of application of organic compounds is enormous. They form the basis or are important constituents of many industries (paints, plastics, food, explosives, drugs, petrochemicals, and many others) and of course (apart from a small exception) they form the basis of all life processes. The different shapes and chemical reactivities of the Substituent s provide an astonishing variety of functions, like those of Enzyme Catalyst s in biochemical reactions of live systems. The autopropagating nature of these is what life is all about. Because of the special properties of carbon, it is likely that life on other Star systems will be found to be carbon-based, in spite of speculations about the possibility of substituting Silicon , which lies just below carbon in the periodic table. Trends in organic chemistry include Chiral Synthesis , Green Chemistry , Microwave Chemistry and Microwave Spectroscopy which has identified dozens of organic molecules in Interstellar Space Another subject area is the group of substances called Fullerene s but one could argue that these should be included with inorganic chemistry, because they are allotrope modifications of pure carbon such as graphite or diamond, though their structures are complex. HISTORIC HIGHLIGHTS ]] Towards the beginning of the nineteenth century when modern chemistry has really begun the chemists generally thought that the compounds that came from living organisms were too complicated in structure, and could only be made by life itself through a 'vital force' or 'vitalism'. They named these compounds 'organic' and in general ignored them. Organic chemistry has really started when someone demonstrated that these compounds could be treated in similar ways to inorganic compounds and finally to manufacture them by other means than by a 'life process'. Around 1816 Michel Chevreuil started to a study of soaps made from various fats and alkali. He separated the different acids that, in combination with the alkali produced the soap, and recognised that these were all individual compounds, thereby demonstrating that it was possible to make a chemical change, producing new compounds without life processes in fats which came from an organic source. The real event that has completely destroyed the myth of 'vitalism' occurred, however, when in 1828 Friedrich Woehler first manufactured urea (carbamide), a constituent of the liquid waste matter urine and in his report stated that he made it entirely without the assistance of a kidney. He obtained it after trying unsuccessfully to make ammonium cyanate by reacting cyanides with ammonium hydroxide. When he used cyanogen instead of the salts he produced something else, which he analysed and recognised to be urea. Making urea by evaporating an Aqueous Solution of Ammonium Cyanate NH4OCN, is now called the Wöhler Synthesis . Progress was slow at the beginning. A great step was when in 1856 William Henry Perkin, whilst trying to manufacture quinine, again accidentally came to manufacture the organic dye now called Perkin's mauve. Another step was the laboratory preparation of DDT by Othmer Zeidler in 1874, but the insecticide properties of this compond were not discovered till much later. The history of organic chemistry continues with the discovery of petroleum and its separation into fractions according to boiling ranges and dissecting these further by fine fractionation, and by type separation using, for instance, solvent extraction or refrigeration. The chemical conversion of different compound types or individual compounds by various chemical processes created the petroleum chemistry leading to the birth of the petrochemical industry, which successfully linked to the manufacture of artificial rubbers and plastics. The pharmaceutical industry began in the last decade of the XIX century when acetyl-salicilic acid (aspirin) manufacture started in Germany by Bayer. Biochemistry, the chemistry of living organisms, their structure and their interactions in vitro and inside live systems, has only started in the XX century after advances in chemical techniques and instruments, which was also linked to advances in medicine and resulted in accelerating the development of the pharmaceutical industry. A notable advance was the publication in 1928 of the discovery of penicillin by Alexander Fleming, who noticed that blue-green mould (penicillium notatum) had destroyed some of his bacteria when it had got near to it in his fridge. So this again, was a lucky accident. It took some really devoted work by a brilliant team in Oxford to isolate the active ingredient, and to succeed in developing a manufacturing process to mass produce it in the early 1940's The publication in 1952 of the discovery of the chemical structure of DNA by Francis Crick and James Watson at Cambridge, its relative RNA, and their constituent nucleotides, and the demonstration of how these molecules, by the way they react in the organism, can be regarded as the 'propagators of life'. A great help for those who wish to study biochemistry with the aid of Internet is the site of 'The Medical Biochemistry Page' created by Michael W King. CHARACTERISTICS OF ORGANIC SUBSTANCES Organic compounds are generally Covalently Bonded . This allows for unique structures such as long carbon chains and rings. The reason carbon is excellent at forming unique structures and that there are so many carbon compounds is that carbon atoms form very stable covalent bonds with one another ( Catenation ). In contrast to inorganic materials, organic compounds typically melt, boil, sublimate, or decompose below 300°C. Neutral organic compounds tend to be less Soluble in Water compared to many inorganic Salts , with the exception of certain compounds such as ionic organic compounds and low Molecular Weight Alcohols and Carboxylic Acids where Hydrogen Bonding occurs. Organic compounds tend rather to dissolve in organic Solvent s which are either pure substances like Ether or Ethyl Alcohol , or mixtures, such as the parffinic solvents such as the various petroleum ethers and white spirits, or the range of pure or mixed aromatic solvents obtained from petroleum or tar fractions by physical separation or by chemical conversion. Solubility in the different solvents depends upon the solvent type and on the Functional Groups if present. Solutions are studied by the science of Physical Chemistry. Like inorganic salts, organic compounds may also form Crystal s. Unique property of carbon in Organic Compound s is that its valency does not always have to be taken up by atoms of other elements, and when it is not a condition termed unsaturation results. In such cases we talk about carbon carbon Double Bonds or Triple Bonds . Double bonds alternating with single in a chain are called Conjugated double bonds. An Aromatic structure is a special case in which the conjugated chain is a closed ring (see Molecular Structure). CATEGORIES OF ORGANIC SUBSTANCES Because so very many compounds exist, a clear, unambiguous naming system is necessary. . In chemistry, structure is quite synonymous with function, and so the structural categories double as categories of property or activity. According to the chain type the two main categories are Open Chain s without any double or triple bonds, olefins Alkene s with double bonds, which can be mono-olefins with a single double bond, di-olefins with two, or poly-olefins with more. The third group with a triple bond is named after the name of the shortest member of the homologue series as the acetylenes Alkyne s. Another classification specifies the difference in complexity. According to this the compound can be straight chain or branched chain compound. Because of the bonding angle of carbon, mentionned earlier, the most stable configuration of the cyclic compounds contain six carbon atoms, but rings with five carbon atoms are also frequent. The cyclic compounds may be homocyclic, containing only carbon in the closed chain itself, or heterocyclic, with atoms of other elements in the chain of the molecule. Cyclo-paraffins do not contain double bonds, whilst cyclo-olefins do. What is different in Aromatic hydrocarbons is that they contain conjugated (alternating)double bonds. The simplest example of this is Benzene . The structure of this was formulated by Kekulé . For more about the organic structure see Resonance Structure and Delocalization pages. The hetero atom of the heterocyclic molecules is most often nitrogen. The basic constituents of live systems are compounds with such heterocyclics. Simple examples of Heterocyclic compounds are Pyrrole and Indole . Examples of Functional Group -based categories are Alcohol s, Aldehyde s, Ketone s, Amide s, Amine s, Carboxylic Acid s, Ether s and Ester s. Organic compounds containing bonds of carbon to nitrogen, oxygen and the halogens are considered part of regular organic chemistry. Those with carbon connected to other elements are often treated separately such as in Organosulfur Chemistry , Organophosphorus Chemistry and Organosilicon Chemistry . Polymers Polymers consist of long chains of repeating segments of smaller molecular units, the Monomer s. When the segments are all the same the molecule is called a ''homopolymer''; when the segments vary in chemical structure the molecule is called a ''heteropolymer''. Common Synthetic organic polymers include Polyethylene , Polypropylene , Nylon , Polyester s, and Polymethyl Methacrylate . There are also a few common inorganic polymers such as the Silicone s. Biomolecules Biomolecular Chemistry is a major category within organic chemistry. Many complex multi-functional group molecules are important in living organisms. Some are long-chain Biopolymers . The main classes are Carbohydrate s, Amino Acid s and Protein s, Polysaccharide s, Lipid s, and Nucleic Acid s. MOLECULAR STRUCTURE OF AN ORGANIC COMPOUND Organic Compounds are generally made from carbon atoms, hydrogen atoms, and Functional Groups . The Valence of carbon is 4, and hydrogen is 1, functional groups are generally 1. Many, but not all structures can be envisioned by the simple valence rule that there will be one bond for each valence number. The knowledge of the Chemical Formula for an organic compound is not sufficient information because many Isomer s can exist. Organic compounds often exist as mixtures. Because many organic compound have relatively low boiling points and/or dissolve easily in organic solvents there exist many methods for separating mixtures into pure constituents that are specific to organic chemistry such as Distillation , Crystallization and Chromatography techniques. Currently, there exist several methods for deducing the structure an organic compound. In general usage are (in alphabetical order):
Additional methods are provided by Analytical Chemistry . ORGANIC REACTIONS Organic Reaction s are Chemical Reaction s involving Organic Compound s. While pure Hydrocarbon s undergo certain limited classes of reactions, many more reactions which organic compounds undergo are largely determined by Functional Group s. The general theory of these reactions involves careful analysis of such properties as the Electron Affinity of key atoms, and Bond Strength s. These issues can determine the relative stability of short-lived Reactive Intermediate s, which usually directly determine the path of the reaction. A common reaction is generically written here as an example: :R-F + X-Y → R-Y + X-F where F is some Functional Group such as the Hydroxyl or -OH group . It is presumed that Functional Group F is bonded to one of the carbon atoms in R. R is often one of the hydrocarbon categories mentioned previously. The example above is a Substitution Reaction , since Y is substituted for F. There are many important aspects of a specific reaction. Whether it will occur spontaneously or not is determined by the Gibbs Free Energy change of the reaction. The heat that is either produced or needed by the reaction is found from the total Enthalpy change. Other concerns include whether side reactions occur from the same reaction conditions. Any side reactions which occur typically produce undesired compounds which may be anywhere from very easy or very difficult to separate from the desired compound. SEE ALSO REFERENCES
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