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Ch_322a_4.010 - large relative to going from an even to the...

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Physical Properties of Alkanes and Cycloalkanes Homologous Series The unbranched alkanes (CH 4 , CH 3 CH 3 , CH 3 CH 2 CH 3 , CH 3 CH 2 CH 2 CH 3 , etc.) form a regular series where each member differs from the next in order by -CH 2 -. Such a regular series is called a homologous series . Boiling Points and Melting Points of the Unbranched Alkanes The boiling points of the unbranched alkanes increase more or less regularly with increasing size reflecting the increasing van der waals attractions. pentane hexane heptane octane bp ( o C) 36 69 98 126
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The Importance of Packing Forces in the Solid State The melting points of the unbranched alkanes increase with increasing size but with some notable features. Propane (mp = -188 o C) melts lower than methane (mp = -182 o C) or ethane (mp = -183 o C). Also, the incremental increase in melting point in going from an odd numbered alkane to the next even numbered alkane is
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Unformatted text preview: large, relative to going from an even to the next odd numbered alkane. pentane hexane heptane octane mp ( o C)-130 -95 -91 -57 ! = 35 ! = 4 ! = 34 It has been suggested that these melting point anomalies are due to variable packing forces in the solid state . Methane and ethane have higher melting points than propane because they are more compact and pack better in the solid state. Alkane chains with an even number of carbons pack more closely in the solid state. The resulting stronger attractive forces lead to higher melting points. Density and Solubility All alkanes and cycloalkanes have densities less than the density of water (1.00 g/mL at 4 o C), and are insoluble in water . Since they are nonpolar materials, they dissolve well in other nonpolar or low polarity organic solvents....
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