Physical properties of alkanes

Alkanes are nonpolar molecules, hence van der Waals forces are responsible for attractions between the molecules. Increasing the number of carbon atoms causes an increase in the strength of the van der Waals forces. The first four members of the alkanes are gases, those with 5-17 carbons are liquids, and the rest are solids. 

The boiling points, melting points and densities of alkanes increase directly with the increase in the number of carbon atoms. 

Their densities are smaller than Ig/ml and they are generally colorless, tasteless and odorless. 

Increasing of the number of branches in isomers decreases the boiling point. For example, n-pentane boils at 36°C, whereas isopentane boils at 28°C. 

They are insoluble in water, but soluble in organic solvents such as benzene and carbon tetrachloride. 

Alkanes are nonpolar, so they dissolve in nonpolar or weakly polar organic solvents. Alkanes are said to be hydrophobic ( water hating ) because they do not dissolve in water. Alkanes are good lubricants and preservatives fa- metals because they keep water from reaching the metal surface and causing corrosion. 

Densities of the n-alkanes are listed in Table 3-2 (p. 82). Alkanes have densities around 0.7g/mL, compared with a density of 1. Og/mL fa water. Because alkanes are less dense than water and insoluble in water, a mixture of an alkane (such as gasoline or oil) and water quickly separates into two phases, with the alkane on top. 

Oil floats on water. Note how the oil slick (from the leaking Exxon Valdez) spreads across the top of the water. Oil recovery booms, containing nonpolar fibers, are used to soak up and contain the spilled oil. Note how most of the oil slick ends at the oil recovery booms. 

Alkane boiling points. The boiling points of the unbranched alkanes (blue) are compared with those of some branched alkanes (green). Because of their smaller surface areas, branched alkanes have lower boiling points than unbranched alkanes. 

List each set of compounds in order of increasing boiling point 

The mutual insolubility of nonpolar oil and very polar water leads to the common expression, Oil and water don t mix.  

Alkanes contain only nonpolar C-C and C-H bonds, and as a result they exhibit only weak van der Waals forces. Table 4.2 summarizes how these intermolecular forces affect the physical properties of alkanes. 

Because nonpolar alkanes are not water soluble, crude petroleum spilled into the sea from a ruptured oil tanker creates an insoluble oil sUck on the surface. The insoluble hydrocarbon oil poses a special threat to birds whose feathers are coated with natural nonpolar oils for insulation. Because these hydrophobic oils dissolve in the crude petroleum, birds lose their layer of natural protection and many die. 

Problem 4.18 Rank the following products of petroleum refining in order of increasing boiling point diesel fuel, kerosene, and gasoline. 

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Solubility in Water A familiar physical property of alkanes is contained m the adage oil and water don t mix Alkanes—indeed all hydrocarbons—are virtually insoluble m water In order for a hydrocarbon to dissolve m water the framework of hydrogen bonds between water molecules would become more ordered m the region around each mole cule of the dissolved hydrocarbon This increase m order which corresponds to a decrease m entropy signals a process that can be favorable only if it is reasonably... 

J 6 Describe general trends in the physical properties of alkanes (Section 18.3). 

Initially, we will be concerned with the physical properties of alkanes and how these properties can be correlated by the important concept of homology. This will be followed by a brief survey of the occurrence and uses of hydrocarbons, with special reference to the petroleum industry. Chemical reactions of alkanes then will be discussed, with special emphasis on combustion and substitution reactions. These reactions are employed to illustrate how we can predict and use energy changes — particularly AH, the heat evolved or absorbed by a reacting system, which often can be estimated from bond energies. Then we consider some of the problems involved in predicting reaction rates in the context of a specific reaction, the chlorination of methane. The example is complex, but it has the virtue that we are able to break the overall reaction into quite simple steps. 

Comparison of Physical Properties of Alkanes, Alkenes, and Alkynes... 

Summary Rules for Naming Alkanes 94 3-4 Physical Properties of Alkanes 95 3-5 Uses and Sources of Alkanes 97 3-6 Reactions of Alkanes 99 3-7 Structure and Conformations of Alkanes 100 3-8 Conformations of Butane 104 3-9 Conformations of Higher Alkanes 106 3-10 Cycloalkanes 107 3-11 Cis-trans Isomerism in Cycloalkanes 109 3-12 Stabilities of Cycloalkanes Ring Strain 109 3-13 Cyclohexane Conformations 113... 

Molecular connectivity indices were first proposed by Randic in 1975 (16) as a means of estimating physical properties of alkanes. This formalism was quickly extended to other types of molecules (17) and, since then, a wide range of indices has been proposed, as reviewed by Hall and Kier (18) and Randic (19). The indices are derived from a graph theoret-... 

Edward, J.T. (1982b). The Relation of Physical Properties of Alkanes to Connectivity Indices A Molecular Explanation. Can.J.Chem., 60,480-485. 

Estrada, E. (1996). Spectral Moments of the Edge Adjacency Matrix of Molecular Graphs. 1. Definition and Applications to the Prediction of Physical Properties of Alkanes. J.Chem.Inf... 

Physical properties of alkanes What types of physical properties do nonpolar compounds have A comparison of two molecular substances—one... 

Estrada, E. (1996) Spectral moments of the edge adjacency matrixofmolecular graphs. 1. Definition and applications to the prediction of physical properties of alkanes./. Chem. Inf. Comput. Sci., 36, 844-849. 

Ivanciuc, O. (2001b) Design of topological indices. Part 18. Modehng the physical properties of alkanes with molecular graph descriptors derived from the Hosoya operator. Rev. Roum. Chim., 46, 129-141. 

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