organicnomenclature - CH 310M/318M Dr. Brian M. Bocknack...

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CH 310M/318M Dr. Brian M. Bocknack Organic Nomenclature – The Basics There are two compounds having the molecular formula C 4 H 10 , which differ only in the way the atoms are connected to one another. Molecules which have the same molecular formula, but different connectivities among the constituent atoms, are called structural isomers . The structural isomers having the formula C 4 H 10 are shown below. Note that the prefix “iso” means “same”, so isobutane received its name because it has the same formula as butane. CH 3 CH 2 CH 2 CH 3 CH 3 CH CH 3 Butane CH 3 Isobutane The situation become slightly more complicated when the number of carbon atoms is increased to 5. There are 3 different structural isomers having the molecular formula C 5 H 12 : pentane, isopentane, and neopentane (“neo” means “new”). CH 3 CH 2 CH 2 CH 3 CH 3 CH CH 3 Pentane CH 3 Isopentane CH 2 CH 3 C CH 3 CH 3 CH 2 CH 3 Neopentane As the number of carbon atoms increases, the number of possible structural isomers increases geometrically. For example, 5 different compounds have the molecular formula C 6 H 14 . These are different compounds, and thus require different names. What prefixes should be used to distinguish one “hexane” from another? CH 3 CH 2 CH 2 CH 3 CH 3 CH CH 3 Hexane CH 3 CH 2 CH 3 C CH 3 CH 3 CH 2 CH 3 CH 2 CH 2 CH 3 CH CH 3 CH 3 CH 2 CH 2 CH 2 CH 3 CH 3 CH 3 CH 3 CH CH It only gets worse. There are 75 possible structural isomers having the formula C 10 H 22 , 366,319 possible structural isomers having the formula C 20 H 42 , and 4,111,846,763 possible structural isomers having the formula C 30 H 62 ! Since isomers are different compounds, they need to have different names . Furthermore, the name assigned to a given compound must be unambiguous, so that chemists all over the world draw the same structure when they see the name. As you might expect, a series of rules have been devised to make this entire process highly systematic. The rules for naming organic compounds were developed by the International Union of Pure and Applied Chemistry (IUPAC). The basic IUPAC rules for naming simple branched alkanes are outlined below. These rules form the basis for the nomenclature of all organic molecules, so it is important to become familiar with them! As we progress through the course, additions and modifications to the IUPAC rules will be introduced as necessary to accommodate the nomenclature of the various functional groups we will encounter. The rules are listed below in order of decreasing priority
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organicnomenclature - CH 310M/318M Dr. Brian M. Bocknack...

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