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| Nomenclature of Hydrocarbons
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| Hydrocarbons are molecules consisting of two elements: hydrogen and carbon. Hydrocarbons are the simplest component of organic chemistry, the branch of chemistry based on living organisms. Nomenclature deals with the rules for naming items. In this case, it refers to the names of chemical compounds.
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| The fundamental rule is knowing the prefixes denoting the number of carbon atoms in a molecule. These prefixes, commonly used in science, come from the Greek language. The prefixes for one through ten are: mon(o) for 1, di for 2, tri for 3, tetr(a) for 4, pent(a) for 5, hex(a) for 6, hept(a) for 7, oct(a) for 8, non(a) for 9 and dec(a) for 10. The vowels written in the parentheses are used for words starting with consonants. If the word starts with a vowel, that letter is deleted. In the case of carbon chains, the first four prefixes are "meth" for 1, "eth" for 2, "prop" for 3 and "but" for 4. An archaic prefix for one carbon atom is "form", as in formic acid or formaldehyde. An archaic prefix for two carbon atoms is "acet", as in acetic acid, acetone or acetylene.
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| Alkanes have a chain of carbon atoms all joined by single bonds. The remaining bonds are then filled with hydrogen atoms. The ending "ane" denotes an alkane and the pattern of the chemical formula is C2H2n+2, where n refers to the number of carbon atoms. By applying this rule, methane is CH4, propane is C3H8 and octane is C8H18. Alkanes are colorless, odorless, nonpolar molecules with relatively low densities. Many are commonly used as fuel, producing water and carbon dioxide when they undergo complete combustion.
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| Unlike alkanes, alkenes are unsaturated, meaning they do not consist of only single bonds. This makes them more reactive. Basic alkenes have one double bond between two carbon atoms. This results in the molecules having two fewer hydrogen atoms, so the pattern of the chemical formula is C2H2. Ethene is C2H4 and propene is C3H6. Isomers are possible when there are more than three carbon atoms. Isomers are molecules made up of the same ("iso") parts ("mers"), but they are arranged differently. A numerical prefix denotes the position of the double bond. For example, in 1-pentene, the double bond is between the first and second carbon atoms. In 2-pentene, the double bond is between the second and third carbon atoms. Both have the general formula of C5H10. The nomenclature rule requires that one starts counting from the end closer to the double bond. These therefore are the only two isomers of pentene, which is the generic name that does not specify the isomer.
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| Alkynes are a third group of hydrocarbons. Like alkenes, alkynes are unsaturated, but they have a triple bond between two neighboring carbon atoms. Other than having the ending of "yne", these molecules follow the nomenclature rules that have already been noted. The pattern of the chemical formulas is CnH2n-2. Ethyne is C2H2, propyne is C3H4, and 3-hexyne is C6H10, with the triple bond between the third and fourth carbon atoms.
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| Hydrocarbons with more than three carbon atoms can also have isomers that depend of the structure of the molecules. The molecules may be either linear or branched. Linear molecules have the carbon atoms connected end to end, as in the prevvious examples. Branched molecules have carbon atoms joined to interior points in the chain. There are two isomers of butane that are named as n-butane and 2-methylpropane. In the first, the prefix of "n" denotes that the molecule is linear (or "normal"). In the second, there is a one carbon branch ("methyl") that is joined to the second carbon atom of a three-carbon chain, noted by the numerical address and the name propane, denoting a chain of three carbon atoms. [The ending "-yl" denotes a branch.} As a further example, 3,3-dimethyl-1-butyne has the general formula of C6H10 and is an isomer of hexyne. It has a basic four-carbon chain with a triple bond between the first and second carbon atoms and two one-carbon chains of the third carbon atom. In an unsaturated hyd rocarbon molecule with branches, the nomenclature rule requires that the counting starts from the end closer to the multiply-bonded carbon atoms.
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| Another factor comes into play in alkenes when the double bond is within the carbon chain. The double bond creates a three-dimensional property in the molecule. If both parts of the bond extend in the same direction (that is, both "up" or "down"), it is referred to as a "cis" molecule. If they project in opposite direction (that is, one "up" and one "down"), it is referred to as a "trans" molecule. For example, cis-2-butene is an asymmetrical molecule while trans-2-butene is symmetrical, meaning it cannot be divided to have two different halves. Both have the formula C4H8.
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| Another group that fits in this topic are cyclic hydrocarbons. These molecules have a closed ring, as opposed to linear or branched chains. Since there is a closed ring, there are two less hydrogen atoms in the molecule. Examples include cyclohexane, which is C6H12, cyclohexene, which is C6H10, and cyclooctyne, which is C8H12. Cyclic hydrocarbons can also have branches, such as methyl cyclopentane, which is C6H12.
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