Chemistry_Grade_10-12 (1).pdf

In other words scientists believed that organic

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compounds. In other words, scientists believed that organic compounds would not follow the normal physical and chemical laws that applied to other inorganic compounds because the very ’force of life’ made them different. This idea of a mystical ’vital force’ in organic compounds was weakened when scientists began to manufacture organic compounds in the laboratory from non-living materials. One of the first to do this was Friedrich Wohler in 1828, who successfully prepared urea, an organic compound in the urine of animals which, until that point, had only been found in animals. A few years later a student of Wohler’s, Hermann Kolbe , made the organic compound acetic acid from inorganic compounds. By this stage it was acknowledged that organic compounds are governed by exactly the same laws that apply to inorganic compounds. The properties of organic compounds are not due to a ’vital force’ but to the unique properties of the carbon atom itself. Organic compounds are very important in daily life. They make up a big part of our own bodies, they are in the food we eat and in the clothes we wear. Organic compounds are also used to make products such as medicines, plastics, washing powders, dyes, along with a list of other items. 9.2 Sources of carbon The main source of the carbon in organic compounds is carbon dioxide in the air. Plants use sunlight to convert carbon dioxide into organic compounds through the process of photosyn- thesis . Plants are therefore able to make their own organic compounds through photosynthesis, while animals feed on plants or plant products so that they gain the organic compounds that they need to survive. 151
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9.3 CHAPTER 9. ORGANIC MOLECULES - GRADE 12 Another important source of carbon is fossil fuels such as coal, petroleum and natural gas. This is because fossil fuels are themselves formed from the decaying remains of dead organisms (refer to chapter 21 for more information on fossil fuels). 9.3 Unique properties of carbon Carbon has a number of unique properties which influence how it behaves and how it bonds with other atoms: Carbon has four valence electrons which means that each carbon atom can form bonds with four other atoms. Because of this, long chain structures can form. These chains can either be unbranched (figure 9.1) or branched (figure 9.2). Because of the number of bonds that carbon can form with other atoms, organic compounds can be very complex. C C C C Figure 9.1: An unbranched carbon chain C C C C C C C Figure 9.2: A branched carbon chain Because of its position on the Periodic Table, most of the bonds that carbon forms with other atoms are covalent . Think for example of a C-C bond. The difference in electroneg- ativity between the two atoms is zero, so this is a pure covalent bond. In the case of a C-H bond, the difference in electronegativity between carbon (2.5) and hydrogen (2.1) is so small that C-H bonds are almost purely covalent. The result of this is that most organic compounds are non-polar. This affects some of the properties of organic compounds.
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