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8 Focus on Health & Medicine: Ethers as Anesthetics

8 Focus on Health & Medicine: Ethers as Anesthetics

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ALKYL HALIDES



PROBLEM 14.20



437



Classify each alkyl halide as 1°, 2°, or 3°.

CH3



a.



CH3CH2CH2CH2CH2



b.



Br



CH3



F



c.



CH3



C



CHCH3



CH3 Cl



PROBLEM 14.21



Draw the structure of an alkyl halide that fits each description: (a) a 2° alkyl bromide of

molecular formula C4H9Br; (b) a 3° alkyl chloride of molecular formula C4H9Cl; (c) a 1° alkyl

fluoride of molecular formula C3H7F.



14.9A



PHYSICAL PROPERTIES



Alkyl halides contain a polar C X bond, but since they have all of their hydrogens bonded to

carbon, they are incapable of intermolecular hydrogen bonding. Alkyl halides with one halogen

are polar molecules, therefore, because they contain a net dipole. As a result, they have higher

melting points and boiling points than alkanes with the same number of carbons.

polar bond

C



X



H



H



H



C Cl

δ+ δ−



net dipole



X = F, Cl, Br, I



The size of the alkyl groups and the halogen also affects the physical properties of an alkyl halide.

• The boiling point and melting point of an alkyl halide increase with the size of the alkyl

group because of increased surface area.

• The boiling point and melting point of an alkyl halide increase with the size of the halogen.



Thus, CH3CH2CH2Cl has a higher boiling point than CH3CH2Cl because it has one more carbon,

giving it a larger surface area. CH3CH2CH2Br has a higher boiling point than CH3CH2CH2Cl

because Br is further down the column of the periodic table, making it larger than Cl.

CH3CH2Cl



CH3CH2CH2Cl



CH3CH2CH2Br



chloroethane

bp = 12 °C



1-chloropropane

bp = 47 °C



1-bromopropane

bp = 71 °C



Increasing boiling point

Increasing size of the alkyl group

Increasing size of the halogen



Since alkyl halides are incapable of hydrogen bonding, they are insoluble in water regardless of

size.



PROBLEM 14.22



Rank the compounds in each group in order of increasing boiling point.

a. CH3CH2CH2I, CH3CH2CH2Cl, CH3CH2CH2F

b. CH3(CH2)4CH3, CH3(CH2)5Br, CH3(CH2)5Cl



14.9B



NOMENCLATURE



In the IUPAC system, an alkyl halide is named as an alkane with a halogen substituent—that

is, as a halo alkane. To name a halogen substituent, change the -ine ending of the name of the

halogen to the suffix -o (e.g., chlorine → chloro).



smi26573_ch14.indd 437



12/9/08 4:41:25 PM



438



ORGANIC COMPOUNDS THAT CONTAIN OXYGEN, HALOGEN, OR SULFUR



HOW TO

EXAMPLE



Name an Alkyl Halide Using the IUPAC System

Give the IUPAC name of the following alkyl halide.

CH3



Cl



CH3CH2CHCH2CH2CHCH3



Step [1]



Find the parent carbon chain containing the halogen.

CH3



Cl



CH3CH2CHCH2CH2CHCH3

7 C’s in the longest chain

7 C’s



Step [2]



• Name the parent chain as an alkane, with the halogen as a

substituent bonded to the longest chain.



heptane



Apply all other rules of nomenclature.

a. Number the chain.

CH3



b. Name and number the substituents.

Cl



methyl at C5



chloro at C2

CH3



CH3CH2CHCH2CH2CHCH3



Cl



CH3CH2CHCH2CH2CHCH3

7



6



5 4



3



2 1



• Begin at the end nearest the first

substituent, either alkyl or halogen.



7



6



5 4



3



2 1



c. Alphabetize: c for chloro, then m for methyl.

ANSWER: 2-chloro-5-methylheptane



Common names for alkyl halides are used only for simple alkyl halides. To assign a common

name, name the carbon atoms as an alkyl group. Then name the halogen by changing the -ine

ending of the halogen name to the suffix -ide; for example, bromine → bromide.

CH3CH2



Cl



chlorine



chloride



ethyl group

Common name: ethyl chloride



PROBLEM 14.23



Give the IUPAC name for each compound.

CH3



a.



CH3CHCH2CH2CH2CH3



b.



(CH3)2CHCHCH2CH3



Br



PROBLEM 14.24



c.

Br



Cl



Give the structure corresponding to each name.

a. 3-chloro-2-methylhexane

b. 4-ethyl-5-iodo-2,2-dimethyloctane



c. 1,1,3-tribromocyclohexane

d. propyl chloride



14.9C INTERESTING ALKYL HALIDES

Many simple alkyl halides make excellent solvents because they are not flammable and they

dissolve a wide variety of organic compounds. Two compounds in this category include CHCl3

(chloroform or trichloromethane) and CCl4 (carbon tetrachloride or tetrachloromethane). Large

quantities of these solvents are produced industrially each year, but like many chlorinated organic

compounds, both chloroform and carbon tetrachloride are toxic if inhaled or ingested. Two other

simple alkyl halides are given in Figure 14.5.



smi26573_ch14.indd 438



12/9/08 4:41:25 PM



ALKYL HALIDES



439







FIGURE 14.5 Two Simple Alkyl Halides

• Chloromethane (CH3Cl) is produced by giant kelp and algae and is also

found in emissions from volcanoes such as Hawaii’s Kilauea. Almost all of

the atmospheric chloromethane results from these natural sources.



CH3Cl



• Dichloromethane (or methylene chloride, CH2Cl2) is an important

solvent, once used to decaffeinate coffee. Coffee is now decaffeinated

using liquid CO2 due to concerns over the possible ill effects of trace

amounts of residual CH2Cl2 in the coffee. Subsequent studies on rats

have shown, however, that no cancers occurred when animals ingested

the equivalent of over 100,000 cups of decaffeinated coffee per day.



CH2Cl2



14.9D

The 1995 Nobel Prize in Chemistry

was awarded to Professors

Mario Molina, Paul Crutzen, and

F. Sherwood Rowland for their work

in elucidating the interaction of ozone

with CFCs. What began as a very

fundamental research project turned

out to have important implications in

the practical world.



FOCUS ON THE ENVIRONMENT

ALKYL HALIDES AND THE OZONE LAYER



While the beneficial effects of many organic halides are undisputed, certain synthetic chlorinated organics, such as the chlorofluorocarbons, have caused lasting harm to the environment.

Chlorofluorocarbons (CFCs) are simple halogen-containing compounds having the general

molecular structure CFxCl4 – x. CFCs are manufactured under the trade name Freons. Two

examples are CFCl3 (Freon 11) and CF2Cl2 (Freon 12).

CFCs are inert and nontoxic, and they have been used as refrigerants, solvents, and aerosol propellants. Because CFCs have low boiling points and are water insoluble, they readily escape into

the upper atmosphere. There, CFCs are decomposed by sunlight. This process forms highly reactive intermediates that have been shown to destroy the ozone layer (Figure 14.6).





FIGURE 14.6 CFCs and Ozone Destruction



mesosphere



ENVIRONMENTAL NOTE

stratosphere



protective natura

l

ozone layer



• CFCs are decomposed by sunlight.

• The reactive intermediates convert

ozone (O3) to oxygen (O2).



troposphere



O3



CFCs



O2



CFCs used at ground level

escape to the upper atmosphere.



Mount Everest



Propane and butane are now used

as propellants in spray cans in place

of CFCs.



smi26573_ch14.indd 439



5



10



15



20



de

ltitu



25



30



35



)



(km



a



12/9/08 4:41:26 PM



440



ORGANIC COMPOUNDS THAT CONTAIN OXYGEN, HALOGEN, OR SULFUR



ENVIRONMENTAL NOTE



The ozone (O3) in the upper atmosphere is vital to life: it acts as a shield, protecting the earth’s

surface from destructive ultraviolet radiation. A decrease in ozone concentration in this protective layer will have some immediate consequences, including an increase in the incidence of skin

cancer and eye cataracts. Other long-term effects include a reduced immune response, interference with photosynthesis in plants, and harmful effects on the growth of plankton, the mainstay

of the ocean food chain.

These findings led to a ban on the use of CFCs in aerosol propellants in the United States in 1978

and to the phasing out of their use in refrigeration systems. While it is now easy to second-guess

the extensive use of CFCs, it is also easy to see why they were used so widely. CFCs made

refrigeration available to the general public. Would you call your refrigerator a comfort or a

necessity?



O3 destruction is most severe in

the region of the South Pole, where

a large ozone hole (shown in purple)

is visible with satellite imaging.



PROBLEM 14.25



There are two newer alternatives to CFCs. Hydrochlorofluorocarbons (HCFCs) such as

CF3CHCl2 contain the elements of H, Cl, and F bonded to carbon, and hydrofluorocarbons

(HFCs) such as FCH2CF3 contain the elements of H and F bonded to carbon. These compounds

have many properties in common with CFCs, but they are decomposed more readily before they

reach the upper atmosphere, so they are less destructive to the O3 layer.

Classify each compound as a CFC, HCFC, or HFC: (a) CF3Cl; (b) CHFCl2; (c) CH2F2.



14.10 ORGANIC COMPOUNDS THAT CONTAIN SULFUR

Thiols are organic compounds that contain a sulfhydryl group (SH group) bonded to a

tetrahedral carbon. Since sulfur is directly below oxygen in the periodic table, thiols can be

considered sulfur analogues of alcohols. Because the sulfur atom is surrounded by two atoms and

two lone pairs, thiols have a bent shape around sulfur.

Thiol



C



S



CH3CH2



H



sulfhydryl

group



H



S



=



ethanethiol

bent shape around S



Thiols differ from alcohols in one important way. They contain no O H bonds, so they are incapable of intermolecular hydrogen bonding. This gives thiols lower boiling points and melting

points compared to alcohols having the same size and shape.

Hydrogen bonding can occur.

stronger intermolecular forces

higher boiling point



CH3CH2



OH



ethanol

bp 78 °C



CH3CH2



SH



ethanethiol

bp 35 °C



Hydrogen bonding is impossible.

weaker intermolecular forces

lower boiling point



To name a thiol in the IUPAC system:

• Name the parent hydrocarbon as an alkane and add the suffix -thiol.

• Number the carbon chain to give the SH group the lower number.

CH3

CH3



SH



CH3CH2CH2



SH



3

methanethiol



smi26573_ch14.indd 440



1-propanethiol



Begin numbering at this C.



CH3CHCH2CH2



SH



1



3-methyl-1-butanethiol



12/9/08 4:41:30 PM



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