{"id":2379,"date":"2018-06-19T20:28:11","date_gmt":"2018-06-19T20:28:11","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/chapter\/alkanes-2\/"},"modified":"2020-06-23T02:59:09","modified_gmt":"2020-06-23T02:59:09","slug":"3-3-properties-of-alkanes","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-potsdam-organicchemistry\/chapter\/3-3-properties-of-alkanes\/","title":{"raw":"3.3. Properties of alkanes","rendered":"3.3. Properties of alkanes"},"content":{"raw":"
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\r\n\r\nAlkanes are organic compounds that consist entirely of single-bonded carbon and hydrogen atoms and lack any other functional groups.\u00a0Alkanes have the general formula $$C_nH_{2n+2}$$ and can be subdivided into the following\u00a0three groups: the linear straight-chain alkanes<\/strong>, branched alkanes<\/strong>, and cycloalkanes<\/strong>. Alkanes are also saturated hydrocarbons<\/em>.\u00a0Alkanes are the simplest and least reactive\u00a0hydrocarbon\u00a0species containing only carbons and hydrogens. They are commercially very important, being the principal constituent of gasoline and lubricating oils and are extensively employed in organic chemistry; though the role of pure alkanes (such as hexanes) is relegated mostly to serving as solvents. The distinguishing feature of an alkane, making it distinct from other compounds that also exclusively contain carbon and hydrogen, is its lack of\u00a0unsaturation. That is to say, it contains no double or triple bonds, which are highly reactive in organic chemistry. Though not totally devoid of reactivity, their lack of reactivity under most laboratory conditions makes them a relatively uninteresting, though very important component of organic chemistry. As you will learn about later, the energy confined within the carbon-carbon bond and the carbon-hydrogen bond is quite high and their rapid oxidation produces a large amount of heat, typically in the form of fire.\r\n\r\nMore on properties of alkanes<\/span><\/a>\r\n
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Isomerism<\/h3>\r\nAlkanes with four or more carbon atoms can have more than one arrangement of atoms, so they can form structural isomers. The carbon atoms can form a single unbranched chain, or the primary chain of carbon atoms can have one or more shorter chains that form branches. For example, butane (C4<\/sub>H10<\/sub>) has two possible structures. Normal<\/em> butane (usually called n<\/em>-butane or simply butane<\/em>) is CH3<\/sub>CH2<\/sub>CH2<\/sub>CH3<\/sub>, in which the carbon atoms form a single unbranched chain. In contrast, the condensed structural formula for the isomer 2-methylpropane<\/em> (sometimes called isobutane<\/em>) is (CH3<\/sub>)2<\/sub>CHCH3<\/sub>, in which the primary chain of three carbon atoms has a one-carbon chain branching at the central carbon. Three-dimensional representations of both structures are as follows:\r\n
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One of the major components of gasoline is commonly called isooctane; its structure is as follows:<\/p>\r\n\r\n

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The compound has a chain of five carbon atoms, so it is a derivative of pentane. There are two methyl group branches at one carbon atom and one methyl group at another. Using the lowest possible numbers for the branches gives 2,2,4-trimethylpentane for the systematic name of this compound.<\/p>\r\n\r\n<\/div>\r\n

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Uses for alkanes<\/h3>\r\nThe simplest alkane is methane (CH4<\/sub>), a colorless, odorless gas that is the major component of natural gas. Other simple alkanes are used as fuels and solvents.\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
Table<\/strong>: The First 10 Straight-Chain Alkanes<\/em><\/caption>\r\n
Name<\/th>\r\nNumber of Carbon Atoms<\/th>\r\nMolecular Formula<\/th>\r\nCondensed Structural Formula<\/th>\r\nBoiling Point (\u00b0C)<\/th>\r\nUses<\/th>\r\n<\/tr>\r\n<\/thead>\r\n
methane<\/td>\r\n1<\/td>\r\nCH4<\/sub><\/td>\r\nCH4<\/sub><\/td>\r\n\u2212162<\/td>\r\nnatural gas constituent<\/td>\r\n<\/tr>\r\n
ethane<\/td>\r\n2<\/td>\r\nC2<\/sub>H6<\/sub><\/td>\r\nCH3<\/sub>CH3<\/sub><\/td>\r\n\u221289<\/td>\r\nnatural gas constituent<\/td>\r\n<\/tr>\r\n
propane<\/td>\r\n3<\/td>\r\nC3<\/sub>H8<\/sub><\/td>\r\nCH3<\/sub>CH2<\/sub>CH3<\/sub><\/td>\r\n\u221242<\/td>\r\nbottled gas<\/td>\r\n<\/tr>\r\n
butane<\/td>\r\n4<\/td>\r\nC4<\/sub>H10<\/sub><\/td>\r\nCH3<\/sub>CH2<\/sub>CH2<\/sub>CH3<\/sub> or CH3<\/sub>(CH2<\/sub>)2<\/sub>CH3<\/sub><\/td>\r\n0<\/td>\r\nlighters, bottled gas<\/td>\r\n<\/tr>\r\n
pentane<\/td>\r\n5<\/td>\r\nC5<\/sub>H12<\/sub><\/td>\r\nCH3<\/sub>(CH2<\/sub>)3<\/sub>CH3<\/sub><\/td>\r\n36<\/td>\r\nsolvent, gasoline<\/td>\r\n<\/tr>\r\n
hexane<\/td>\r\n6<\/td>\r\nC6<\/sub>H14<\/sub><\/td>\r\nCH3<\/sub>(CH2<\/sub>)4<\/sub>CH3<\/sub><\/td>\r\n69<\/td>\r\nsolvent, gasoline<\/td>\r\n<\/tr>\r\n
heptane<\/td>\r\n7<\/td>\r\nC7<\/sub>H16<\/sub><\/td>\r\nCH3<\/sub>(CH2<\/sub>)5<\/sub>CH3<\/sub><\/td>\r\n98<\/td>\r\nsolvent, gasoline<\/td>\r\n<\/tr>\r\n
octane<\/td>\r\n8<\/td>\r\nC8<\/sub>H18<\/sub><\/td>\r\nCH3<\/sub>(CH2<\/sub>)6<\/sub>CH3<\/sub><\/td>\r\n126<\/td>\r\ngasoline<\/td>\r\n<\/tr>\r\n
nonane<\/td>\r\n9<\/td>\r\nC9<\/sub>H20<\/sub><\/td>\r\nCH3<\/sub>(CH2<\/sub>)7<\/sub>CH3<\/sub><\/td>\r\n151<\/td>\r\ngasoline<\/td>\r\n<\/tr>\r\n
decane<\/td>\r\n10<\/td>\r\nC10<\/sub>H22<\/sub><\/td>\r\nCH3<\/sub>(CH2<\/sub>)8<\/sub>CH3<\/sub><\/td>\r\n174<\/td>\r\nkerosene<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n

Physical properties<\/h3>\r\n
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Boiling points<\/h4>\r\nThe boiling points shown are\u00a0 for the \"straight chain\" isomers\u00a0in which\u00a0there are more than one (Figure 1). Notice that the first four alkanes are gases at room temperature, and solids do not start to appear until about C17<\/sub>H36<\/sub>.\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"363\"]\"bptsalkanes.gif\" Figure 1:<\/strong> Normal boiling points of first four alkanes<\/em>[\/caption]\r\n\r\nThe temperatures\u00a0cannot be more precise than those given in this\u00a0chart\u00a0because each isomer has a different melting and boiling point. By the time you get 17 carbons into an alkane, there are unbelievable numbers of isomers!\u00a0 Cycloalkanes have boiling points\u00a0that are approximately 10 - 20 o<\/sup>C higher than the corresponding straight chain alkane.\r\n\r\nThere electronegativity <\/a>difference between carbon and hydrogen (2.1 vs. 1.9) is small; therefore,\u00a0there is only a slight bond polarity, meaning that the only attractions between one molecule and its neighbors will be Van der Waals dispersion forces. These\u00a0 forces will be very small for a molecule like methane but will increase as the size of the molecules increase. Therefore, the boiling points of the alkanes increase with the molecular size.\r\n\r\nRegarding isomers, the more branched the chain, the lower the boiling point tends to be. Van der Waals dispersion forces are smaller for shorter molecules and only operate over very short distances between one molecule and its neighbors. It is more difficult for short, bulky molecules (with substantial amounts of branching) to lie close together (compact)\u00a0compared with\u00a0long, thin molecules.\r\n
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Example<\/h3>\r\nThe boiling points of the three isomers of C5<\/sub>H12<\/sub> are as follows:\r\n