2.6 PHYSICAL PROPERTIES OF ALKANES · 20–30 °C per carbon atom within a series is a general trend observed for many types of or- ... Figure 2.7 Boiling points of some unbranched

70 CHAPTER 2 • ALKANES

2.6 PHYSICAL PROPERTIES OF ALKANES

Each time we come to a new family of organic compounds, we’ll consider the trends in theirmelting points, boiling points, densities, and solubilities, collectively referred to as their phys-ical properties. The physical properties of an organic compound are important because theydetermine the conditions under which the compound is handled and used. For example, theform in which a drug is manufactured and dispensed is affected by its physical properties. Incommercial agriculture, ammonia (a gas at ordinary temperatures) and urea (a crystallinesolid) are both very important sources of nitrogen, but their physical properties dictate thatthey are handled and dispensed in very different ways.

Your goal should not be to memorize physical properties of individual compounds, butrather to learn to predict trends in how physical properties vary with structure.

A. Boiling Points

The boiling point is the temperature at which the vapor pressure of a substance equals atmos-pheric pressure (which is typically 760 mm Hg). Table 2.1 shows that there is a regular changein the boiling points of the unbranched alkanes with increasing number of carbons. This trendof boiling point within the series of unbranched alkanes is particularly apparent in a plot ofboiling point against carbon number (Fig. 2.7). The regular increase in boiling point of20–30 °C per carbon atom within a series is a general trend observed for many types of or-ganic compounds.

What is the reason for this increase? The key point for understanding this trend is that boil-ing points are a crude measure of the attractive forces among molecules—intermolecular at-tractions—in the liquid state. The greater are these intermolecular attractions, the more energy(heat, higher temperature) it takes to overcome them so that the molecules escape into the gasphase, in which such attractions do not exist. The greater are the intermolecular attractionswithin a liquid, the greater is the boiling point. Now, it is important to understand that thereare no covalent bonds between molecules, and furthermore, that intermolecular attractions

2.12 Represent each of the following compounds with a skeletal structure.(a)

(b) ethylcyclopentane

2.13 Name the following compounds.

(a) (b)

2.14 How many hydrogens are in an alkane of n carbons containing (a) two rings? (b) three rings?(c) m rings?

2.15 How many rings does an alkane have if its formula is (a) C8H10? (b) C7H12? Explain how youknow.

PROBLEMS

CH3

CH3CH2CH2CH CH

CH3

"" L C(CH3)3L

CH3

H3C

CH2CH3

02_BRCLoudon_pgs4-4.qxd 11/26/08 8:36 AM Page 70

2.6 PHYSICAL PROPERTIES OF ALKANES 71

have nothing to do with the strengths of the covalent bonds within the molecules themselves.What, then, is the origin of these intermolecular attractions?

In Chapter 1, we learned that electrons in bonds are not confined between the nuclei butrather reside in bonding molecular orbitals that surround the nuclei. We can think of the totalelectron distribution as an “electron cloud.” Electron clouds are rather “squishy” and can un-dergo distortions. Such distortions occur rapidly and at random, and when they occur, they re-sult in the temporary formation of regions of local positive and negative charge; that is, thesedistortions cause a temporary dipole moment within the molecule (Fig. 2.8, p. 72). When asecond molecule is located nearby, its electron cloud distorts to form a complementary dipole,called an induced dipole. The positive charge in one molecule is attracted to the negativecharge in the other. The attraction between temporary dipoles, called a van der Waals attrac-tion or a dispersion interaction, is the cohesive interaction that must be overcome to vapor-ize a liquid. Alkanes do not have significant permanent dipole moments. The dipoles dis-cussed here are temporary, and the presence of a temporary dipole in one molecule induces atemporary dipole in another. We might say, “Nearness makes the molecules grow fonder.”

Now we are ready to understand why larger molecules have higher boiling points. Van derWaals attractions increase with the surface areas of the interacting electron clouds. That is, thelarger the interacting surfaces, the greater the magnitude of the induced dipoles. A larger mole-cule has a greater surface area of electron clouds and therefore greater van der Waals interactionswith other molecules. It follows, then, that large molecules have higher boiling points.

The shape of a molecule is also important in determining its boiling point. For example, acomparison of the boiling point of the highly branched alkane neopentane (9.4 °C) and its un-branched isomer pentane (36.1 °C) is particularly striking. Neopentane has four methylgroups disposed in a tetrahedral arrangement about a central carbon. As the following space-filling models show, the molecule almost resembles a compact ball, and could fit readily

0 2 4 6 8 10 12number of carbon atoms

-200

-150

-100

-50

50

100

150

200

250

0

boili

ng p

oint

, °C

Figure 2.7 Boiling points of some unbranched alkanes plotted against number of carbon atoms. Notice thesteady increase with the size of the alkane, which is in the range of 20–30 °C per carbon atom.



within a sphere. On the other hand, pentane is rather extended, is ellipsoidal in shape, andwould not fit within the same sphere.

The more a molecule approaches spherical proportions, the less surface area it presents toother molecules, because a sphere is the three-dimensional object with the minimum surface-to-volume ratio. Because neopentane has less surface area at which van der Waals interactionswith other neopentane molecules can occur, it has fewer cohesive interactions than pentane,and thus, a lower boiling point.

neopentane:compact, nearly spherical

pentane:extended, ellipsoidal

d–

t1: molecules moving at random approach each other

d+

t2: the electron cloud of one molecule distorts randomly to form a temporary dipole

t3: the dipole in one molecule induces a complementary dipole in the othert4: temporary dipoles dissipate

weak attractions developbetween opposite charges

(van der Waals attractions)

d–

d–d+

d+

Figure 2.8 A stop-frame cartoon showing the origin of van der Waals attraction.The frames are labeled t1, t2, andso on, for successive points in time. The time scale is about 10_10 s. The colors represent electrostatic potentialmaps (EPMs). The green color of the isolated molecules (t1 and t4) shows the absence of a permanent dipole mo-ment. As the molecules approach (t1), the electron cloud of one molecule undergoes a random distortion (t2) thatproduces a temporary dipole, indicated by the red and blue colors. This dipole induces a complementary chargeseparation (induced dipole) in the second molecule (t3), and attractions between the two dipoles result.Throughrandom fluctuations of the electron clouds (t4), the temporary dipoles vanish. Averaged over time, this phenome-non results in a small net attraction.This is the van der Waals attraction.



In summary, by analysis of the boiling points of alkanes, we have learned two generaltrends in the variation of boiling point with structure:

1. Boiling points increase with increasing molecular weight within a homologous series—typically 20–30 °C per carbon atom. This increase is due to the greater van der Waals at-tractions between larger molecules.

2. Boiling points tend to be lower for highly branched molecules that approach spherical pro-portions because they have less molecular surface available for van der Waals attractions.

B. Melting Points

The melting point of a substance is the temperature above which it is transformed sponta-neously and completely from the solid to the liquid state. The melting point is an especiallyimportant physical property in organic chemistry because it is used both to identify organiccompounds and to assess their purity. Melting points are usually depressed, or lowered, by im-purities. Moreover, the melting range (the range of temperature over which a substance melts),usually quite narrow for a pure substance, is substantially broadened by impurities. The melt-ing point largely reflects the stabilizing intermolecular interactions between molecules in thecrystal as well as the molecular symmetry, which determines the number of indistinguishableways in which the molecule fits into the crystal. The higher the melting point, the more stableis the crystal structure relative to the liquid state. Although most alkanes are liquids or gasesat room temperature and have relatively low melting points, their melting points neverthelessillustrate trends that are observed in the melting points of other types of organic compounds.

One such trend is that melting points tend to increase with the number of carbons (Fig. 2.9).Another trend is that the melting points of unbranched alkanes with an even number of carbonatoms lie on a separate, higher curve from those of the alkanes with an odd number of carbons.This reflects the more effective packing of the even-carbon alkanes in the crystalline solidstate. In other words, the odd-carbon alkane molecules do not “fit together” as well in thecrystal as the even-carbon alkanes. Similar alternation of melting points is observed in otherseries of compounds, such as the cycloalkanes in Table 2.3.

-2000 2 4 6 8

number of carbons10 12

-180

-160

-140

-120

-100

-80

-60

-40

-20

0

mel

ting

poi

nt, °

C

odd carbons

even carbons

Figure 2.9 A plot of melting points of the unbranched alkanes against number of carbon atoms.Notice the gen-eral increase of melting point with molecular size. Also notice that the alkanes with an even number of carbons(red) lie on a different curve from the alkanes with an odd number of carbons (blue). This trend is observed in anumber of different types of organic compounds.



Branched-chain hydrocarbons tend to have lower melting points than linear ones becausethe branching interferes with regular packing in the crystal. When a branched molecule has asubstantial symmetry, however, its melting point is typically relatively high because of theease with which symmetrical molecules fit together within the crystal. For example, the melt-ing point of the very symmetrical molecule neopentane, -16.8 °C, is considerably higher thanthat of the less symmetrical pentane, -129.8 °C. (See models on p. 72.) Compare also themelting points of the compact and symmetrical molecule cyclohexane, 6.6 °C, and the ex-tended and less symmetrical hexane, -95.3 °C.

In summary, melting points show the following general trends:

1. Melting points tend to increase with increasing molecular mass within a series.2. Many highly symmetrical molecules have unusually high melting points.3. A sawtooth pattern of melting point behavior (see Fig. 2.9) is observed within many ho-

mologous series.

C. Other Physical Properties

Among the other significant physical properties of organic compounds are dipole moments,solubilities, and densities. A molecule’s dipole moment (Sec. 1.2D) determines its polarity,which, in turn, affects its physical properties. Because carbon and hydrogen differ little in theirelectronegativities, alkanes have negligible dipole moments and are therefore nonpolar mole-cules. We can see this graphically by comparing the EPMs of ethane, with a dipole moment ofzero, and fluoromethane, a polar molecule with a dipole moment of 1.82 D.

Solubilities are important in determining which solvents can be used to form solutions;most reactions are carried out in solution. Water solubility is particularly important for several

EPM of ethane EPM of fluoromethane (H3C—F)

2.16 Match each of the following isomers with the correct boiling points and melting points. Ex-plain your choices.

Compounds: 2,2,3,3-tetramethylbutane and octane

Boiling points: 106.5 °C, 125.7 °C

Melting points: -56.8 °C, +100.7 °C

2.17 Which compound has (a) the greater boiling point? (b) the greater melting point? Explain.(Hint: What is the geometry of benzene?)

PROBLEMS

HH

HH

HH

CH3H

HH

HH

benzene toluene



reasons. For one thing, water is the solvent in biological systems. For this reason, water solu-bility is a crucial factor in the activity of drugs and other biologically important compounds.There has also been an increasing interest in the use of water as a solvent for large-scale chem-ical processes as part of an effort to control environmental pollution by organic solvents. Thewater solubility of the compounds to be used in a water-based chemical process is crucial.(We’ll deal in greater depth with the important question of solubility and solvents in Chapter8.) The alkanes are, for all practical purposes, insoluble in water—thus the saying, “Oil andwater don’t mix.” (Alkanes are a major constituent of crude oil.)

The density of a compound is another property, like boiling point or melting point, that de-termines how the compound is handled. For example, whether a water-insoluble compound ismore or less dense than water determines whether it will appear as a lower or upper layer whenmixed with water. Alkanes have considerably lower densities than water. For this reason, amixture of an alkane and water will separate into two distinct layers with the less dense alkanelayer on top. An oil slick is an example of this behavior (Fig. 2.10).

2.18 Gasoline consists mostly of alkanes. Explain why water is not usually very effective in extin-guishing a gasoline fire.

2.19 (a) Into a separatory funnel is poured 50 mL of CH3CH2Br (bromoethane), a water-insolublecompound with a density of 1.460 g/mL, and 50 mL of water. The funnel is stoppered andthe mixture is shaken vigorously. After standing, two layers separate. Which substance isin which layer? Explain.

(b) Into the same funnel is poured carefully 50 mL of hexane (density = 0.660 g/mL) so thatthe other two layers are not disturbed. The hexane forms a third layer. The funnel is stop-pered and the mixture is shaken vigorously. After standing, two layers separate. Whichcompound(s) are in which layer? Explain.

PROBLEMS

Figure 2.10 The lower density of hydrocarbons and their insolubility in water allows an oil spill in flood watersto be contained by plastic tubes at a Texas refinery in the aftermath of Hurricane Rita in 2005.


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2.6 PHYSICAL PROPERTIES OF ALKANES · 20–30 °C per carbon atom within a series is a general trend observed for many types of or- ... Figure 2.7 Boiling points of some unbranched