Physical Properties of Alkanes and Cycloalkanes

Modem petroleum refining involves more than distillation, however, and includes two major additional operations  

Petroleum is not the only place where alkanes occur naturally. Solid n-alkanes, especially those with relatively long chains, have a waxy constituency and coat the outer surface of many living things where they help prevent the loss of water. Pentacosane [CH3(CH2 23CH3] is present in the waxy outer layer of most insects. Hentriacontane [CH3(CH2)29CH3] is a component of beeswax (see Problem 2.8) as well as the wax that coats the leaves of tobacco, peach trees, pea plants, and numerous others. The C23, C25, C27, C29, and C31 n-alkanes have been identified in the surface coating of the eggs of honeybee queens. 

Cyclopentane and cyclohexane are present in petroleum, but as a mle, unsubstituted cycloalkanes rarely occur naturally. A significant exception is a group of more than 200 hydrocarbons called hopanes, related to the parent having the carbon skeleton shown. 

Hopanes were first found in petroleum and geological sediments, later as components of certain bacterial cell membranes. Although present in small amounts, hopanes are so widespread that they rank among the most abundant natural products on Earth. 

Boiling Point. As we have seen earlier in this chapter, methane, ethane, propane, and butane are gases at room temperature. The unbranched alkanes pentane (C5H12) through heptadecane (CnHse) are liquids, whereas higher homologs are solids. As shown in 

If we examine the unbranched alkanes in Table 4.3, we notice that each alkane differs from the preceding alkane by one —CH2— group. Butane, for example, is CH3(CH2)2CH3 and pentane is CH3(CH2)3CH3. A series of compounds like this, where each manber differs 

At room temperature (25°C) and 1 atm pressure the first four members of the homologous series of unbranched alkanes are gases (Fig. 4.3), the C5—C-17 unbranched alkanes (pentane to heptadecane) are liquids, and the unbranched alkanes with 18 and more carbon atoms are solids. 

X-Ray diffraction studies, which provide information about molecular structure, have revealed the reason for this apparent anomaly. Alkane chains with an even number of carbon atoms pack more closely in the crystalline state. As a result, attractive forces between individual chains are greater and melting points are higher. 

The effect of chain branching on the melting points of alkanes is more difficult to predict. Generally, however, branching that produces highly symmetrical structures results in 

Cycloalkanes also have much higher melting points than their open-chain counterparts (Table 4.4). 

A substance exists as a liquid rather than a gas because attractive forces between molecules (intermolecular attractive forces) are greater in the liquid than in the gas phase. Attractive forces between neutral species (atoms or molecules, but not ions) are referred to as van der Waals forces and may be of three types  

These forces are electrical in nature, and in order to vaporize a substance, enough energy must be added to overcome them. Most alkanes have no measurable dipole moment, and therefore the only van der Waals force to be considered is the induced-dipole/induced-dipole attractive force. 

It might seem that two nearby molecules A and B of the same nonpolar substance would be unaffected by each other. 

In fact, the electric fields of both A and B are dynamic and fluctuate in a complementary way that results in a temporary dipole moment and a weak attraction between them. 

Bolling points of unbranched alkanes and their 2-methyl-branched isomers. (Temperatures in this text are expressed in degrees Celsius, °C. The SI unit of temperature is the kelvin, K. To convert degrees Celsius to kelvins add 273.15.) 

A substance exists as a liquid rather than a gas because attractive forces between 

Appendix 1 lists selected physical properties for representative alkanes as well as members of other families of organic compounds. 

Van der Waals forces involving induced dipoles are often called London forces, or dispersion forces. 

Aggregated molecules (solution) held together by opposite charges 

FIGURE 2.53 The stabilization of molecules through van der Waals forces. 

The more extended a molecule is, the stronger its induced dipole can be. More compact, more spherical molecules have smaller induced dipoles and therefore lower boiling points. A classic example is the difference between pentane and neopentane (Fig. 2.54). 

Highly symmetrical neopentane has a nearly spherical cloud of electrons (bp 9.5 C mp-16.5 C) 

The more extended molecule pentane has a much greater surface area and has greater intermolecular interactions (bp 36.1 °C mp-130 °C) 

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Physical Properties of Alkanes and Cycloalkanes 3.49 Highest boiling point heptane... 

Conformations of Alkanes and Cycloalkanes HOW TO Draw Alternative Chair Conformations of Cyclohexane 2- Cis,Trans Isomerism in Cycloalkanes and Bicycloalkanes HOW TO Convert Planar Cyclohexanes to Chair Cyclohexanes 2-7 Physical Properties of Alkanes and Cycloalkanes 2-8 Reactions of Alkanes... 

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