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8.3 EFFECT OF MOLECULAR POLARITY AND HYDROGEN BONDING ON PHYSICAL PROPERTIES 333

8.3EFFECT OF MOLECULAR POLARITY AND HYDROGEN BONDING ON PHYSICAL PROPERTIES

A. Boiling Points of Ethers and Alkyl Halides

Most alkyl halides, alcohols, and ethers are polar molecules; that is, they have permanent di-pole moments (Sec. 1.2D). The following examples are typical.

H3CLF

1.82 Ddipole moment

H3CLCl

1.94 D

methyl fluoride methyl chlorideH3CLOH

1.7 D

methanolH3CLOLCH3

1.31 D

dimethyl ether

0.08 D

L LCH3H3C CH2propane

8.9 Using the data in Table 8.1, estimate the carbon–selenium bond length in H3CLSeLCH3.

8.10 From the data in Fig. 8.1, tell which bonds have the greater amount of p character (Sec. 1.9B):CLO bonds or CLS bonds. Explain.

PROBLEMS

109°1.426 Å

0.96 Å

HH3C

O

96°1.82 Å1.335 Å

HH3C

S

99°

1.803 Å

CH3H3C

S

111.4°

1.413 ÅCH3H3C

O

21

21

11

21

Figure 8.1 Bond lengths and bond angles in a simple alcohol, thiol, ether, and sulfide. Bond angles at sulfur aresmaller than those at oxygen, and bonds to sulfur are longer than the corresponding bonds to oxygen.

Bond Lengths (in Angstroms) in Some Methyl Derivatives

H3 CLCH3 H3 CLNH2 H3 CLOH H3 CLF1.536 1.474 1.426 1.391

H3 CLSH H3 CLCl1.82 1.781

H3 CLBr1.939

H3 CL I2.129

TABLE 8.1

Increasing electronegativity

Increasin

g atom

ic radiu

s

08_BRCLoudon_pgs5-1.qxd 12/8/08 11:05 AM Page 333

334 CHAPTER 8 • INTRODUCTION TO ALKYL HALIDES, ALCOHOLS, ETHERS, THIOLS, AND SULFIDES

The EPMs of dimethyl ether and propane, two molecules of about the same size and shape,show the greater polarity of the ether:

The polarity of a compound affects its boiling point. When the boiling points of two mole-cules with the same shape and molecular mass are compared, the more polar molecule typi-cally has the higher boiling point.

What is the reason for this effect? A higher boiling point results from greater attractionsbetween molecules in the liquid state (Sec. 2.6A). Polar molecules are attracted to each otherbecause they can align in such a way that the negative end of one dipole is attracted to the pos-itive end of another.

Although just two molecules are shown here, interactions like this can occur among manymolecules at the same time. Molecules in the liquid state are in constant motion, so their rela-tive positions are changing constantly; however, on the average, this attraction exists andraises the boiling point of a polar compound.

When a polar molecule contains a hydrocarbon portion of even moderate size, its polarityhas little effect on its physical properties; it is sufficiently alkanelike that its properties resem-ble those of an alkane.

boiling point

H3CLOLCH2CH2CH2CH2CH3

99 °C

H3CLCH2LCH2CH2CH2CH2CH3

98 °C

EPMs of two polar moleculesaligned for attraction

d– d+

d–d+

dipole momentboiling point

dipole momentboiling point

O

1.7 D66 °C

1.31 D-23.7 °C

%%H3C CH3

O

0 D49.3 °C

0.08 D-42.1 °C

H3C% %

CH3

CH2

dimethyl ether propane

tetrahydrofuran (THF) cyclopentane

EPM of propaneEPM of dimethyl ether



From the preceding discussion, you might expect that an alkyl halide should have a higherboiling point than an alkane of the same molecular mass. However, this is not so. Alkyl chlo-rides have about the same boiling points as alkanes of the same molecular mass, and alkyl bro-mides and iodides have lower boiling points than the alkanes of about the same molecularmass.

The key to understanding these trends is to realize that although the molecules compared ineach row have similar molecular masses, they have very different molecular sizes and shapes.From their relatively high densities, it is apparent that alkyl halide molecules have largemasses within relatively small volumes. Thus, for a given molecular mass, alkyl halide mol-ecules have smaller volumes than alkane molecules. Recall that the attractive forces betweenmolecules—van der Waals forces, or dispersion forces—are greater for larger molecules (Sec.2.6A). Larger intermolecular attractions translate into higher boiling points. The greater mol-ecular volumes of alkanes, then, should cause them to have higher boiling points than alkylhalides. The polarity of alkyl halides, in contrast, has the opposite effect on boiling points: ifpolarity were the only effect, alkanes would have lower boiling points than alkyl halides.Thus, the effects of molecular volumes and polarity oppose each other. They nearly cancel inthe case of alkyl chlorides, which have about the same boiling points as alkanes of about thesame molecular mass. However, alkane molecules are so much larger than alkyl bromide andalkyl iodide molecules of the same molecular mass that the volume effect dominates, and alka-nes have higher boiling points.

B. Boiling Points of Alcohols

The boiling points of alcohols, especially alcohols of lower molecular mass, are unusuallyhigh when compared with those of other organic compounds. For example, ethanol has amuch higher boiling point than other organic compounds of about the same shape and molec-ular mass.

8.11 The measured dipole moment of cyclopentane is 0 D. Yet the dipole moment of cyclopentanecalculated from molecular orbital theory is 0.38 D—small, but definitely not zero. Assumingthe theory is reliable, how do you account for the discrepancy between calculated and mea-sured dipole moments? (Hint: The measurement is made on a sample of cyclopentane, butthe calculation is performed on a single molecule.)

8.12 The boiling points of the 1,2-dichloroethylene stereoisomers are 47.4 °C and 60.3 °C. Givethe structure of the stereoisomer with the higher boiling point. Explain.

PROBLEMS

CH3CH2CH2CH2Cl CH3CH2CH2CH2CH2CH3

molecular mass 92.6 86.2boiling point 78.4 °C 68.7 °Cdensity 0.886 g mL_1 0.660 g mL_1

CH3CH2Br CH3CH2CH2CH2CH2CH2CH3

molecular mass 109 100.2boiling point 38.4 °C 98.4 °Cdensity 1.46 g mL_1 0.684 g mL_1

CH3I CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3

molecular mass 142 142boiling point 42.5 °C 174 °Cdensity 2.28 g mL_1 0.73 g mL_1

08_BRCLoudon_pgs5-1.qxd 12/8/08 11:05 AM 35


The contrast between ethanol and the last two compounds is particularly striking: All havesimilar dipole moments, and yet the boiling point of ethanol is much higher. The fact thatsomething is unusual about the boiling points of alcohols is also apparent from a comparisonof the boiling points of ethanol, methanol, and the simplest “alcohol,” water.

Generally, each additional LCH2L group results in a 20–30 °C increase in the boiling pointsof successive compounds in a homologous series (Sec. 2.6A). Yet the difference in the boilingpoints of methanol and ethanol is only 13 °C; and water, although the “alcohol” of lowest mol-ecular mass, has the highest boiling point of the three compounds. This unusual trend is due tovery important intermolecular interaction called hydrogen bonding.

C. Hydrogen Bonding

Hydrogen bonding is an attraction that results from the association of a hydrogen on oneatom with an unshared electron pair on another. Hydrogen bonding can occur within the samemolecule, or it can occur between molecules. For example, in the case of the simple alcohols,hydrogen bonding is a weak association of the OLH proton of one molecule with the oxygenof another.

Formation of a hydrogen bond requires two partners: the hydrogen-bond donor and the hy-drogen-bond acceptor. The hydrogen-bond donor is the atom to which the hydrogen is fullybonded, and the hydrogen-bond acceptor is the atom bearing the unshared pair to which thehydrogen is partially bonded.

In a classical Lewis sense, a proton can only share two electrons. Thus, a hydrogen bond is dif-ficult to describe with conventional Lewis structures. Consequently, hydrogen bonds are oftendepicted as dashed lines. The hydrogen bond results from the combination of two factors: first,

$$H

CH3

CH3

H

hydrogen bonddonor

O11 3 3L O

hydrogen bondacceptor

%

$ $$R R

H

HL O

O H hydrogen bond length

HL O covalent bond length

hydrogen bond

O331.8–1.9 Å

0.96 Å

..

..

CH3CH2LOH H3CLOH HLOH

boiling pointethanol78 °C

methanol65 °C

water100 °C

boiling pointdipole moment

LOHCH3CH2

ethanol78 °C1.7 D

LFCH3CH2

ethyl fluoride-38 °C1.8 D

CH3CH2CH3

propane-42 °C0 D

H3CLOLCH3

dimethyl ether-24 °C1.3 D



a weak covalent interaction between a hydrogen on the donor atom and unshared electronpairs on the acceptor atom; and second, an electrostatic attraction between oppositely chargedends of two dipoles. Opinions differ as to the relative importance of these two factors.

The hydrogen bond between two molecules resembles the same two molecules poised toundergo a Brønsted acid–base reaction:

(8.2)

The hydrogen-bond donor is analogous to the Brønsted acid in Eq. 8.2, and the acceptor isanalogous to the Brønsted base. In fact, it is not a bad analogy to think of the hydrogen bondas an acid–base reaction that has just started! In an acid–base reaction, the proton is fullytransferred from the acid to the base; in a hydrogen bond, the proton remains covalently boundto the donor, but it interacts weakly with the acceptor.

The best hydrogen-bond donor atoms in neutral molecules are oxygens, nitrogens, andhalogens. In addition, as might be expected from the similarity between hydrogen-bondinteractions and Brønsted acid–base reactions, all strong Brønsted acids are also goodhydrogen-bond donors. The best hydrogen-bond acceptors in neutral molecules are the elec-tronegative first-row atoms oxygen, nitrogen, and fluorine. All strong Brønsted bases are alsogood hydrogen-bond acceptors.

Sometimes an atom can act as both a donor and an acceptor of hydrogen bonds. For exam-ple, because the oxygen atoms in water or alcohols can act as both donors and acceptors, someof the molecules in liquid water and alcohols exist in hydrogen-bonded chains. The hydrogenbonds in these chains are not static, but rather are rapidly breaking and re-forming.

hydrogen bonds

$RH

33O $RH

33O $RH

33O% % %

3L $) )22O 2OH

H

R R

3L L22O _ |R $)2OH

H

R

Brønstedacid

Brønstedbase

Brønsted acid–base reaction:

$HR

H O1L 3 $

R

$ 11O

hydrogen bonddonor

hydrogen bondacceptor

Hydrogen bonding:

$R$R$H

33 H O11LO

weak covalent interaction

$R$R$H

33 H O11LO

electrostatic attraction ofopposite charges

d–d–

d+

d+



In contrast, the oxygen atom of an ether is a hydrogen-bond acceptor, but it is not a donor be-cause it has no hydrogen to donate. Finally, some atoms are donors but not acceptors. The am-monium ion, |NH4, is a good hydrogen-bond donor; but, because the nitrogen has no un-shared electron pair, it is not a hydrogen-bond acceptor.

Hydrogen bonding accounts for the unusually high boiling points of alcohols. In the liquidstate, hydrogen bonding is a force of attraction that holds molecules together. In the gas phase,hydrogen bonding is much less important (because molecules are farther apart than in a liquidor solid) and, at low pressures, it does not exist. To vaporize a hydrogen-bonded liquid, then,the hydrogen bonds between molecules must be broken, and breaking hydrogen bonds re-quires energy. This energy is manifested as an unusually high boiling point for hydrogen-bonded compounds such as alcohols.

Hydrogen bonding is also important in other ways. You’ll see in Sec. 8.4B how it can affectthe solubility of organic compounds. It is also a very important phenomenon in biology. Hydro-gen bonds have critical roles in maintaining the structures of proteins and nucleic acids. With-out hydrogen bonds, life as we know it would not exist.

In summary, the tendency of molecules to associate noncovalently in the liquid state in-creases their boiling points. The most important forces involved in these intermolecular as-sociations are

1. hydrogen bonding: hydrogen-bonded molecules have greater boiling points;2. attractive interactions between permanent dipoles: molecules with permanent dipole

moments have higher boiling points;3. attractive van der Waals forces, which are influenced by

a. molecular size: larger molecules have greater boiling points; andb. molecular shape: more extended, less spherical molecules have greater boiling points.

An understanding of these factors will allow you to predict trends in boiling points within agroup of compounds, as illustrated in Study Problem 8.4.

Study Problem 8.4Arrange the following compounds in order of increasing boiling point: 1-hexanol, 1-butanol, tert-butyl alcohol, pentane.

Solution 1-Butanol and pentane have almost the same molecular mass and about the same sizeand shape. However, because 1-butanol is a polar molecule that can both donate and accepthydrogen bonds, it has a considerably higher boiling point than pentane. Because 1-hexanol, alsoa primary alcohol, is a larger molecule than 1-butanol, its boiling point is the highest of the three.So far, the order of increasing boiling points is: pentane < 1-butanol < 1-hexanol. Tert-butyl al-cohol has about the same molecular mass as pentane, but the alcohol has a higher boiling pointbecause of its polarity and hydrogen bonding. However, a tert-butyl alcohol molecule is morebranched and more nearly spherical than the isomeric 1-butanol molecule; thus, the boiling pointof tert-butyl alcohol should be lower than that of 1-butanol. Therefore, the correct order of boilingpoints is: pentane < tert-butyl alcohol < 1-butanol < 1-hexanol. (The respective boiling points in°C are 36, 82, 118, and 157.)

PROBLEMS8.13 Within each set, arrange the compounds in order of increasing boiling point.

(a) 4-ethylheptane, 2-bromopropane, 4-ethyloctane(b) 1-butanol, 1-pentene, chloromethane


8.4 SOLVENTS IN ORGANIC CHEMISTRY 339

8.14 Label each of the following molecules as a hydrogen-bond acceptor, donor, or both. Indicatethe hydrogen that is donated or the atom that serves as the hydrogen-bond acceptor.(a) (b) (c)

(d) (e) (f) H3CLCH2L|NH3

8.4 SOLVENTS IN ORGANIC CHEMISTRY

A solvent is a liquid used to dissolve a compound. Solvents have tremendous practical impor-tance. They affect the acidities and basicities of solutes. In some cases, the choice of a solventcan have dramatic effects on reaction rates and even on the outcome of a reaction. Understand-ing effects like these requires a classification of solvent types, to which Section 8.4A is devoted.

The rational choice of a solvent requires an understanding of solubility—that is, how wella given compound dissolves in a particular solvent. Section 8.4B discusses the principles thatwill allow you to make general predictions about the solubilities of both covalent organic com-pounds and ions in different solvents. The effects of solvents on chemical reactions are closelytied to the principles of solubility.

Solubility is also important in biology. For example, the solubilities of drugs determine theforms in which they are marketed and used, and such important characteristics as whether theyare absorbed from the gut and whether they pass from the bloodstream into the brain. Some ofthese ideas are explored in Section 8.5.

Because certain alcohols, alkyl halides, and ethers are among the most important organicsolvents, this is a good point our survey of organic chemistry to study solvent properties.

A. Classification of Solvents

There are three broad solvent categories, and they are not mutually exclusive; that is, a solventcan be in more than one category.

1. A solvent can be protic or aprotic.2. A solvent can be polar or apolar.3. A solvent can be a donor or a nondonor.

A protic solvent consists of molecules that can act as hydrogen-bond donors. Water,alcohols, and carboxylic acids are examples of protic solvents. Solvents that cannot act as hy-drogen-bond donors are called aprotic solvents. Ether, methylene chloride, and hexane areexamples of aprotic solvents.

A polar solvent has a high dielectric constant; an apolar solvent has a low dielectric con-stant. The dielectric constant is defined by the electrostatic law, which gives the interaction en-ergy E between two ions with respective charges q1 and q2 separated by a distance r:

E = k (8.3)

In this equation, k is a proportionality constant and e is the dielectric constant of the solvent inwhich the two ions are imbedded. This equation shows that when the dielectric constant e islarge, the magnitude of E, the energy of interaction between the ions, is small. This means that

q1q2!er

cLOH11LCLNHLCH3H3CSO3 3 1

LCLCH3H3CSO3 3L 311FHL 311BrH


effect of molecular polarity and hydrogen bonding … › media › loudon › ... · hydrogen...

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