Carey - Organic Chemistry - chapt04

Carey - Organic Chemistry - chapt04

(Parte 1 de 7)

CHAPTER 4 ALCOHOLS AND ALKYL HALIDES

Our first three chapters established some fundamental principles concerning the structureof organic molecules. In this chapter we begin our discussion of organic chemical reactionsby directing attention to alcoholsand alkyl halides.These two rank among the most useful classes of organic compounds because they often serve as starting materials for the preparation of numerous other families.

Two reactions that lead to alkyl halides will be described in this chapter. Both illustrate functional group transformations. In the first, the hydroxyl group of an alcohol is replaced by halogen on treatment with a hydrogen halide.

In the second, reaction with chlorine or bromine causes one of the hydrogen substituents of an alkane to be replaced by halogen.

Both reactions are classified as substitutions,a term that describes the relationship between reactants and products—one functional group replaces another. In this chapter we go beyond the relationship of reactants and products and consider the mechanismof each reaction. Amechanismattempts to show howstarting materials are converted into products during a chemical reaction.

While developing these themes of reaction and mechanism, we will also use alcohols and alkyl halides as vehicles to extend the principles of IUPAC nomenclature, con-

R±HAlkane

X2Halogen R±XAlkyl halide

H±X Hydrogen halide

R±OHAlcohol

H±XHydrogen halide R±XAlkyl halide

H±OH Water

126 BackForwardMain MenuTOCStudy Guide TOCStudent OLCMHHE Website tinue to develop concepts of structure and bonding, and see how structure affects properties. Areview of acids and basesconstitutes an important part of this chapter in which a qualitative approach to proton-transfer equilibria will be developed that will be used throughout the remainder of the text.

4.1IUPAC NOMENCLATURE OF ALKYL HALIDES

The IUPAC rules permit alkyl halides to be named in two different ways, called functional class nomenclature and substitutivenomenclature. In functional class nomenclaturethe alkyl group and the halide (fluoride, chloride, bromide,or iodide) are designated as separate words. The alkyl group is named on the basis of its longest continuous chain beginning at the carbon to which the halogen is attached.

Substitutive nomenclatureof alkyl halides treats the halogen as a halo-(fluoro-, chloro-, bromo-,or iodo-) substituenton an alkane chain. The carbon chain is numbered in the direction that gives the substituted carbon the lower locant.

When the carbon chain bears both a halogen and an alkyl substituent, the two substituents are considered of equal rank, and the chain is numbered so as to give the lower number to the substituent nearer the end of the chain.

PROBLEM 4.1Write structural formulas, and give the functional class and substitutive names of all the isomeric alkyl chlorides that have the molecular formula

Substitutive names are preferred, but functional class names are sometimes more convenient or more familiar and are frequently encountered in organic chemistry.

4.2IUPAC NOMENCLATURE OF ALCOHOLS

Functional class names of alcohols are derived by naming the alkyl group that bears the hydroxyl substituent (±OH) and then adding alcoholas a separate word. The chain is always numbered beginning at the carbon to which the hydroxyl group is attached.

Substitutive names of alcohols are developed by identifying the longest continuous chain that bears the hydroxyl group and replacing the -eending of the

5-Chloro-2-methylheptane

CH3CHCH2CH2CHCH2CH3

CH3W Cl W

2-Chloro-5-methylheptane

CH3CHCH2CH2CHCH2CH3

ClW CH3

1-Fluoropentane 2-Bromopentane

CH3CHCH2CH2CH3

Br W

3-Iodopentane

CH3CH2CHCH2CH3

CH3CH2CH2CH2CH2ClPentyl chloride CH3FMethyl fluoride

CH3CH2CHCH2CH2CH3

Br W

1-Ethylbutyl bromide

I Cyclohexyl iodide

4.1IUPAC Nomenclature of Alkyl Halides127

The IUPAC rules permit certain common alkyl group names to be used. These include n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, and neopentyl (Section 2.10).

Prior to the 1993 version of the IUPAC rules, the term “radicofunctional” was used instead of “functional class.”

BackForwardMain MenuTOCStudy Guide TOCStudent OLCMHHE Website corresponding alkane by the suffix -ol.The position of the hydroxyl group is indicated by number, choosing the sequence that assigns the lower locant to the carbon that bears the hydroxyl group.

Hydroxyl groups take precedence over (“outrank”) alkyl groups and halogen substituents in determining the direction in which a carbon chain is numbered.

PROBLEM 4.2Write structural formulas, and give the functional class and substitutive names of all the isomeric alcohols that have the molecular formula

4.3CLASSES OF ALCOHOLS AND ALKYL HALIDES

Alcohols and alkyl halides are classified as primary, secondary, or tertiary according to the classification of the carbon that bears the functional group (Section 2.10). Thus, pri- mary alcoholsand primary alkyl halidesare compounds of the type RCH2G (where G is the functional group), secondary alcoholsand secondary alkyl halidesare compounds of the type R2CHG, and tertiary alcoholsand tertiary alkyl halidesare compounds of the type R3CG.

Many of the properties of alcohols and alkyl halides are affected by whether their functional groups are attached to primary, secondary, or tertiary carbons. We will see a number of cases in which a functional group attached to a primary carbon is more reactive than one attached to a secondary or tertiary carbon, as well as other cases in which the reverse is true.

6-Methyl-3-heptanol (not 2-methyl-5-heptanol)

CH3CHCH2CH2CHCH2CH3

CH3W OH W

3-Fluoro-1-propanol

CH3 trans-2-Methylcyclopentanol

CH3CH2OH

Ethyl alcoholEthanol 1-Methylpentyl alcohol 2-Hexanol

CH3CHCH2CH2CH2CH3

1,1-Dimethylbutyl alcohol 2-Methyl-2-pentanol

CH3CCH2CH2CH3 OH

Functional class name: Substitutive name:

128CHAPTER FOURAlcohols and Alkyl Halides

CH3CCH2OH

2,2-Dimethyl-1-propanol (a primary alcohol)

CH3CCH2CH2CH3

2-Chloro-2-methylpentane (a tertiary alkyl halide)

CH3CH2CHCH3

Br W

2-Bromobutane (a secondary alkyl halide)

CH3

1-Methylcyclohexanol (a tertiary alcohol)

Several alcohols are commonplace substances, well known by common names that reflect their origin (wood alcohol, grain alcohol) or use (rubbing alcohol). Wood alcohol is methanol

(methyl alcohol, CH3OH), grain alcohol is ethanol

(ethyl alcohol, CH3CH2OH), and rubbing alcohol is

2-propanol [isopropyl alco-

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4.4BONDING IN ALCOHOLS AND ALKYL HALIDES

The carbon that bears the functional group is sp3-hybridized in alcohols and alkyl halides. Figure 4.1 illustrates bonding in methanol. The bond angles at carbon are approximately tetrahedral, as is the C±O±H angle. Asimilar orbital hybridization model applies to alkyl halides, with the halogen substituent connected to sp3-hybridized carbon by a bond. Carbon–halogen bond distances in alkyl halides increase in the order C±F (140 pm) C±Cl (179 pm) C±Br (197 pm) C±I (216 pm).

PROBLEM 4.4Bromine is less electronegative than chlorine, yet methyl bromide and methyl chloride have very similar dipole moments. Why?

Figure 4.2 shows the distribution of electron density in methanol and chloromethane. Both are similar in that the sites of highest electrostatic potential (red) are near the electronegative atoms—oxygen and chlorine. The polarization of the bonds

CH3 Cl

O H3C H

H Lone-pair orbitals bondσ

FIGURE 4.1 Orbital hybridization model of bonding in methanol. (a) The orbitals used in bonding are the 1s orbitals of hydrogen and sp3- hybridized orbitals of carbon and oxygen. (b) The bond angles at carbon and oxygen are close to tetrahedral, and the carbon–oxygen bond is about 10 pm shorter than a carbon–carbon single bond.

Methanol (CH3OH)Chloromethane (CH3Cl)

FIGURE 4.2 Electrostatic potential maps of methanol and chloromethane. The most positively charged regions are blue, the most negatively charged ones red. The electrostatic potential is most negative near oxygen in methanol and near chlorine in chloromethane.

BackForwardMain MenuTOCStudy Guide TOCStudent OLCMHHE Website to oxygen and chlorine, as well as their unshared electron pairs, contribute to the concentration of negative charge on these atoms.

Relatively simple notions of attractive forces between opposite charges are sufficient to account for many of the properties of chemical substances. You will find it helpful to keep the polarity of carbon–oxygen and carbon–halogen bonds in mind as we develop the properties of alcohols and alkyl halides in later sections.

4.5PHYSICAL PROPERTIES OF ALCOHOLS AND ALKYL HALIDES: INTERMOLECULAR FORCES

Boiling Point.When describing the effect of alkane structure on boiling point in Section 2.14, we pointed out that the forces of attraction between neutral molecules are of three types listed here. The first two of these involve induced dipoles and are often referred to as dispersion forces,or London forces.

1. Induced-dipole/induced-dipole forces 2. Dipole/induced-dipole forces 3. Dipole–dipole forces

Induced-dipole/induced-dipole forcesare the only intermolecular attractive forces available to nonpolar molecules such as alkanes. In addition to these forces, polar molecules engage in dipole–dipole and dipole/induced-dipole attractions. The dipole–dipole attractive forceis easiest to visualize and is illustrated in Figure 4.3. Two molecules of a polar substance experience a mutual attraction between the positively polarized region of one molecule and the negatively polarized region of the other. As its name implies, the dipole/induced-dipole forcecombines features of both the induced-dipole/induceddipole and dipole–dipole attractive forces. Apolar region of one molecule alters the electron distribution in a nonpolar region of another in a direction that produces an attractive force between them.

Because so many factors contribute to the net intermolecular attractive force, it is not always possible to predict which of two compounds will have the higher boiling point. We can, however, use the boiling point behavior of selected molecules to inform us of the relative importance of various intermolecular forces and the structural features that influence them.

Consider three compounds similar in size and shape: the alkane propane, the alcohol ethanol, and the alkyl halide fluoroethane.

Both polar compounds, ethanol and fluoroethane, have higher boiling points than the nonpolar propane. We attribute this to a combination of dipole/induced-dipole and dipole–dipole attractive forces that stabilize the liquid states of ethanol and fluoroethane, but that are absent in propane.

The most striking aspect of the data, however, is the much higher boiling point of ethanol compared with both propane and fluoroethane. This suggests that the attractive forces in ethanol must be unusually strong. Figure 4.4 shows that this force results from a dipole–dipole attraction between the positively polarized proton of the OH group of one ethanol molecule and the negatively polarized oxygen of another. The term hydrogen bondingis used to describe dipole–dipole attractive forces of this type. The

CH3CH2OH

CH3CH2CH3

130CHAPTER FOURAlcohols and Alkyl Halides

FIGURE 4.3 A dipole–dipole attractive force. Two molecules of a polar substance are oriented so that the positively polarized region of one and the negatively polarized region of the other attract each other.

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proton involved must be bonded to an electronegative element, usually oxygen or nitrogen. Protons in C±H bonds do not participate in hydrogen bonding. Thus fluoroethane, even though it is a polar molecule and engages in dipole–dipole attractions, does not form hydrogen bonds and, therefore, has a lower boiling point than ethanol.

Hydrogen bonding can be expected in molecules that have ±OH or ±NH groups.

Individual hydrogen bonds are about 10–50 times weaker than typical covalent bonds, but their effects can be significant. More than other dipole–dipole attractive forces, intermolecular hydrogen bonds are strong enough to impose a relatively high degree of structural order on systems in which they are possible. As will be seen in Chapter 27, the three-dimensional structures adopted by proteins and nucleic acids, the organic molecules of life, are dictated by patterns of hydrogen bonds.

PROBLEM 4.5The constitutional isomer of ethanol, dimethyl ether (CH3OCH3), is a gas at room temperature. Suggest an explanation for this observation.

Table 4.1 lists the boiling points of some representative alkyl halides and alcohols.

When comparing the boiling points of related compounds as a function of the alkyl group,we find that the boiling point increases with the number of carbon atoms, as it does with alkanes.

TABLE 4.1Boiling Points of Some Alkyl Halides and Alcohols

Name of alkyl group

Methyl Ethyl Propyl Pentyl Hexyl

Formula

Functional group X and boiling point, C (1 atm) X F

FIGURE 4.4 Hydrogen bonding in ethanol involves the oxygen of one molecule and the proton of an ±OH group of another. Hydrogen bonding is much stronger than most other types of dipole–dipole attractive forces.

Hydrogen bonds between ±OH groups are stronger than those between ±NH groups, as a comparison of the boiling points of water

(H2O, 100°C) and ammonia (NH3, 33°C) demonstrates.

For a discussion concerning the boiling point behavior of alkyl halides, see the January 1988 issue of the Journal of Chemical Education, p. 62–64.

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With respect to the halogenin a group of alkyl halides, the boiling point increases as one descends the periodic table; alkyl fluorides have the lowest boiling points, alkyl iodides the highest. This trend matches the order of increasing polarizabilityof the halogens. Polarizabilityis the ease with which the electron distribution around an atom is distorted by a nearby electric field and is a significant factor in determining the strength of induced-dipole/induced-dipole and dipole/induced-dipole attractions. Forces that depend on induced dipoles are strongest when the halogen is a highly polarizable iodine, and weakest when the halogen is a nonpolarizable fluorine.

The boiling points of the chlorinated derivatives of methane increase with the number of chlorine atoms because of an increase in the induced-dipole/induced-dipole attractive forces.

Fluorine is unique among the halogens in that increasing the number of fluorines does not produce higher and higher boiling points.

Thus, although the difluoride CH3CHF2boils at a higher temperature than CH3CH2F, the trifluoride CH3CF3boils at a lower temperature than either of them. Even more striking is the observation that the hexafluoride CF3CF3is the lowest boiling of any of the fluo- rinated derivatives of ethane. The boiling point of CF3CF3is, in fact, only 11°higher than that of ethane itself. The reason for this behavior has to do with the very low polar- izability of fluorine and a decrease in induced-dipole/induced-dipole forces that accompanies the incorporation of fluorine substituents into a molecule. Their weak intermolecular attractive forces give fluorinated hydrocarbons (fluorocarbons)certain desirable physical properties such as that found in the “no stick” Tefloncoating of frying pans.

(Parte 1 de 7)

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