Properties of Alkanes

Alkanes are sometimes referred to as paraffins, a word derived from the Latin parum affinis, meaning little affinity. This term aptly describes their behavior, for alkanes show little chemical affinity for other substances and are chemically inert to most laboratory reagents. They are also relatively inert biologically and are not often 

Reaction with oxygen occurs during combustion in an engine or furnace when the alkane is used as a fuel. Carbon dioxide and water are formed as products, and a large amount of heat is released. For example, methane (natural gas) reacts with oxygen according to the equation 

Another effect seen in alkanes is that increased branching lowers an alkane s boiling point. Thus, pentane has no branches and boils at 36.1 °C, isopentane (2-methylbutane) has one branch and boils at 27.85 °C, and neopentane (2,2-dimethylpropane) has two branches and boils at 9.5 °C. Similarly, octane 

Up to now, we ve viewed molecules primarily in a two-dimensional way and have given little thought to any consequences that might arise from the spatial arrangement of atoms in molecules. Now it s time to add a third dimension to our study. Stereochemistry is the branch of chemistry concerned with the three-dimensional aspects of molecules. We ll see on many occasions in future chapters that the exact three-dimensional stmcture of a molecule is often cm-cial to determining its properties and biological behavior. 

The different arrangements of atoms that result from bond rotation are called conformations, and molecules that have different arrangements are called conformational isomers, or conformers. Unlike constitutional isomers, however, different conformers often can t be isolated because they interconvert too rapidly. 

Some common uses of haloalkanes include solvents and anesthetics. For many years, carbon tetrachloride was widely used in dry cleaners, and in home spot removers to take oils and grease out of clothes. However, its use was discontinued when carbon tetrachloride was found to be toxic to the liver, where it can cause cancer. Today, dry cleaners use other halogenated compounds such as dichloromethane, 1,1,1-trichloroethane, and l,l,2-trichloro-l,2,2-trifluoroethane. 

One of the most widely used general anesthetics is halothane (2-bromo-2-chloro-l,l,l-trifluoroethane), also called fluothane. It has a pleasant odor, is nonexplosive, has few side effects, undergoes few reactions within the body, and is eliminated quickly fiom the hody. 

Anesthetics, such as halothane, decrease the sensation of pain. 

Many types of alkanes are the components of fuels that power our cars and oil that heats our homes. You may have used a mixture of hydrocarbons such as mineral oil as a laxative or petrolatum to soften your skin. The differences in uses of many of the alkanes result from their physical properties, including solubility and density. 

The first four alkanes— methane, ethane, propane, and butane—are gases at room temperature and are widely used as heating fuels. 

Some trends in physical properties can be noted. As the number of carbon atoms in the compounds increases, so does their molecular weight. As molecular weight increases, melting and boiling points increase. 

At 1 atm pressure, methane, ethane, propane, and butane have boiling points below 20 C, so they are gases under normal conditions. The next heavier alkanes boil at temperatures above 20 C, so they are liquids under normal conditions. By the time the carbon chains have twenty or more atoms, the melting point is above 20 C, so those compounds are fairly solid under normal conditions. 

The most important uses of alkanes are as fuels, lubricating oils, paraffin wax, and asphalt. All alkanes (actually, all organic compounds) are combustible, but the Ughter ones are notably the substances found in natmal gas and petroleum. 

The thickness of a liquid, known as its viscosity, or resistance to flow, also increases with a higher molecular weight. The lighter liquids are suitable to be used in gasoline or jet fuels. 

As the substances become more viscous, they are more likely to be used as lubricating oils. The weight of motor oil is an indication of viscosity. A lOW oil has a low viscosity, and is more suitable for use in cold weather because it flows more easily. A 40W oil has a high viscosity and is more suitable for driving in hot weather. 

At room temperature, the first four members of the alkane family are gases. The straight chain alkanes from pentane and up are liquids, and octadecane (18 carbons) and up in the alkane family are solids. As the number of carbons increases, the boiling point increases. Branched alkanes have lesser boiling points than their unbranched or less branched isomeric counterparts. The reason for this is that the unbranched molecules have more intermolecular interactions than the branched ones. 

Problem 3.11 Give tUPAC names for the following compounds  

Problem 3.12 Draw structures corresponding to the following IUPAC names  

Problem 3.13 s Name the eight 5-carbon alkyl groups you drew in Problem 3.7. 

Problem 3.14 Give the IUPAC name for the following hydrocarbon, and convert the drawing into a skeletal structure. 

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Figure 7-1 shows the groups that are obtained for alkanes, and the corresponding notation of these groups as introduced by Benson [Ij. Table 7-2 contains the group contributions to important thermochemical properties of alkanes. Results obtained with these increments and more extensive tables can be obtained from Refs. [1] and [2]. 

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). 

Dodd, L. R. and Theodorou, D. N. Atomistic Monte Carlo Simulation and Continuum Mean Field Theory of the Structure and Equation of State Properties of Alkane and Polymer Melts. Vol 116,pp, 249-282,... 

Thermodynamic properties of alkane chains 13 Thermodynamic properties of ring compounds 15 Enthalpy and entropy effects on ring closure 21... 

Vroon, Z.A.E.P., Keizer, K., Gilde, M.J., Verweij, H., and Burggraaf, A.J. (1995) Transport properties of alkanes through ceramic thin zeolite MEI membranes. 

Nomenclature of Alkanes — Physical and Chemical Properties of Alkanes... 

Garbalena, M., and W. C. Herndon, Optimum Graph-Theoretical Models for Enthalpic Properties of Alkanes. J. Chem. Inf. Comput. Sci., 1992 32, 37-42. 

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... 

There is considerable similarity in the spectroscopic properties of alkanes and cycloalkanes. We mentioned previously the main features of their infrared... 

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