Download 6 cH3cH2-o-cH3 .r.-o<o

t76
Alcohols,and Ethers
t2 Halocarbons,
CHAPTER
form the strong bases called hydroxides, reactiue metals react uigorously
with alcohols to form strongbasescalledakoxtdes:
2H-OH+
2Na -
+ 2Na+ + H2
2H-OHydroxide
+ 2Na .----- 2R-O-
2R-OH
+ 2Na+ + H2
Alkoxide
Becauseof their weak acidic character, aqueous solutions of alcohols are
essentiallyneutral. Phenols are much stronger acids than alcohols, but they
are still veryweak acids. Phenols produce slightly acidic solutions.
I2.8
EXERCISE
PRACTICE
for
the reaction of sodium with each of the following:
Write the equation
(b) phenol.
(a) methanol
12.7Ethers
AIMS: Tonome ond draw stracturesof ethers.To illustratethe
synthesisof on etherfrom a holocorbonond on olkoxide
ion.
Focus
Ethers are disubstituted derivatives of water.
Ethers are cornpoundsin which both hydrogensof water are replacedby car'
The R
bon chains or rings. The general formula for ethers is R-O-R.
stands for any alkyl or aryl group.
6
HH
Watermolecule
o
RR
Ethermolecule
The alkyl or aryl groups joined by the ether linkage are named in alphabetical order and are followed by the word,ether For example:
.r.-o<o
cH3cH2-o-cH3
Ethylmethyl ether
Methylphenyl ether
Ethylmethyl ether and methylphenyl ether are asymmetrical ethers becausethe R groups attached to the oxygen are different.When both R
groups are the sarne,the ether is symmetrical. Syrnmetrical ethers are
named by using the prefix di-.For example:
cH3cH2-o-cH2cH3
O"O
Diethyl ether
Diphenyl ether
(phenyl ether)
I2.7 Ethers
,77
Many modern anestheticscontain halogen and ether functional groups,as
describedinA CloserLook Halocarbonand EtherAnesthetics.
Halocarbon and EtherAnesthetics
Enflurane is a stable liquid that is somewhat
less volatile than halothane. Enflurane provides
rapid anesthesia and rapid recovery for the
patient. Although enflurane is broken down in
the liver to produce fluoride ions, elevatedfluoride levels in the blood are not considereda
problem.
Isoflurane has physical, pharmacologic, and
clinical properties that are similar to those of
halothane and enflurane. This anesthetic is a
more potent muscle relaxant than halothane,
and induction of anesthesiais relativelyrapid.
General anesthesiais usually induced with
the administration of an intravenous anesthetic,
regardless of the inhalant anesthetic subsequently used for anesthesiamaintenance. The
most commonly used induction agentis the barbiturate thiopental, also known as Pentothal (see
Sec. 15.10).In many instances,low concentrations of halothane, enflurane, or isoflurane are
used in conjunction with nitrous oxide (N2O).
Anesthetics have alleviated a great deal of pain
and suffering during surgery.Zo cal anesthetics
make one part of the body insensitive to pain but
leave the patient conscious. Generalanesthetics
act on the brain to produce unconsciousnessand
insensitivity to pain. Many general anesthetics
are halocarbons or ethers or contain both kinds
of functional groups in their molecular structures.
Diethyl ether (CzH5-O-C2H5), the first
general anesthetic, was introduced into surgery
in 1846byWilliam Morton, a Boston dentist (see
figure). InIB47, chloroform (CHCI3)was introduced as a general anesthetic.Both diethyl ether
and chloroform can cause undesirable side
effects. Today, neither compound is used as an
anestheticin the WesternHemisphere.
lVhy are compounds such as diethyl ether
and chloroform generalanesthetics?The potency
of an anestheticis related to its solubility in fatty
tissue. One theory for the action of generalanesthetics is that they dissolve in the fatlike membranes of nerve cells of the brain (neurons).This
changes the properties of the membranes. As a
consequence, the activity of the neurons is
depressed,leading to anesthesia.
The fat-solubility theory carries over into
modern anesthetics.Today'sanestheticsinclude
relatively nonpolar fluorine-containing organic
compounds such as halothane (Fluothane,
CF3CHBTCI). Enflurane (Ethrane, Efrane,
CHFCICF2-O-CHF2) and isoflurane (Forane,
CF3CHCI-O-CHF2) are also used. All these
compounds are inhalant anesthetics.Thesecompounds are nonflammable and relativelysafe, and
the patient recoversrapidly from their effects.
Halothane is nonexplosive.The start of anesthesia is rapid, but slower than for anesthetics of
greater solubility in membranes such as enflurane
Thefirstuseof etheras an anestheticin 1846is
depictedin this painting.
and isoflurane.
t78
Alcohols,
and Ethers
l2 Halocarbons,
CHAPTER
Just as alcohols can be prepared by using hydroxide ions to displace
halogenfrom a halocarbon,etherscan be preparedby using alkoxideions'
For example:
CH.
------ CH3CH2-O-CH3
+ CH3CH2O
EthVlmethVl
Sodium ethoxide
Iodomethane
+
i:it;;;r
The ether linkage is often found in rings. Rings that contain elements
other than carbon are called heterocyclic rings or heterocycles.Here are
some common oxygen-containing heterocycles:
na
\o/
Furan
\n/
P1'ran
Tetrahydrofuran
\o/
Tetrahydropyran
The fundamental ring structures of furan and pgan are found in many natural sugars.Compounds containing an oxygenatom in a three-membered
ring are called epoxides.Epoxyethaneis the simplest example.
Ho
\616/
,/\
H
H
H
Epoxyethane
(ethylene oxide)
Other epoxidesare used to make cements and adhesives.Withthe exception
of the epoxides,the ether linkage is very resistantto chemical modification.
Sincethey arethree-memberedrings,epoxiderings arehighly strained.
Therefore,epoxidesare much more reactive than other ethers' For example,
ro rHECnsrlx Potnr:A mistakenconviction
Folr.ow-up
You may recall from the Case in Point that a mother
was accused in 1989 of murdering her oldest son by
ethylene glycol poisoning. She was convlcted and
sentenced to life in prison. The case might have
ended there, except for a quirk of fate and the efforts
of Dr. William S1yand Dr, James Shoemaker of the St.
Louis University School of Medicine. The quirk of
fate was that while the accused mother was in custody ln 1990, she gave birth to a second son, who
soon began to exhibit the same symptoms as his late
older brother. It was impossible that she had polsoned her youngest son. The St. Louis University scientlsts, who had foilowed the case on television, rec-
ognized that a rare inherited disease, methylmalonic
acidemia, has symptoms very similar to those of ethylene glycol poisoning. Contrary to findings reported
in.previous blood tests, new tests undertaken by Dr.
Shoemaker revealed no evidence of ethylene glycol
in the blood of either child. Dr. Piero Rinaldo of the
Yale University School of Medicine verifled from
blood samples that bolh sons had been born with
methylmalonic acidemia. In September 1991, the
mother's conviction was reversed and all charges
were dismissed. Thanks to science and caring scientists, she has resumed her Lifewith her husband and
-^-^i-i--
rurrt4[lttrv
^^n
JUrl.
12.8Physical
Properties ,79
the epoxide ring of epoxyethaneis easily opened. In aqueous solution containing a trace of strong acid, the product is ethylene glycol:
A ,"
"..c_c
Hl
t"
OH
+ Ll-(lH
+
HCI
ll
OH
H-C-C_H
tt
HH
Although very useful as antifreeze,ethylene glycol is toxic and can be fatal if
ingested, a fact that led to the accusation of murder described in the Casein
Point early in this chapter.
12.8Physicolproperties
AIM: To relate differencesin boiling point ond solubility to the
molecular structuresof hydrocarbons,holocorbons,
alcohols,and ethers.
The physical properties of
organic molecules depend on
their molecular structure.
Thus far we have seen the aliphatic and aromatic hydrocarbons,halocarbons, alcohols, and ethers. Except for an occasional comment, not much
has been said about their physical properties. This is not an oversight. Discussion of the physical properties of all four classesof compounds at one
time will help us understand why the properties are what they are.
Boillngpoints
Hydrocarbons and halocarbons of low molar mass tend to be gasesor lowboiling liquids. Hydrocarbon molecules such as the alkanes are nonpolar.
The electron pair in a carbon-hydrogenor carbon-carbon bond is about
equally shared by the nuclei of the elements involved. The carbon-halogen
bond is only slightly polar. You,may recall from Section 8.1 that attractions
between molecules, because of hydrogen bonding, require that the molecules contain hydrogen attached to very electronegativeatoms such as oxygen.There is no hydrogen bonding in hydrocarbons and halocarbons. Consequently, the forces that hold hydrocarbon or halocarbon molecules
together in the liquid state are very weak. Table 11.1 shows that all alkanes
containing fewer than five carbons are gasesat room temperature.
Boiling points of closely related organic compounds usually increaseas
molar mass increases.The data in Table 12.4show this principle. Remember that a pure liquid boils when enough heat energy has been supplied to
let molecules in the liquid escape.The sum of weak forces holding heary
nonpolar molecules together in a liquid is greater than the sum of weak
forces holding light nonpolar molecules together.
Like water, alcohols are capable of intermolecular hydrogen bonding.
Alcohols therefore boil at higher temperaturcs than alkanes and halocarbons containing comparablenumbers of atoms (Table12.5).
Ethers usually have lower boiling points than alcohols of comparable
molar mass, but they have higher boiling points than comparable hydrocarbons and halocarbons.
580
l2 Halocarbons,
Alcohols,and Ethers
CHAPTER
Compared
Table12.4MolarMassesand BoilingPointsof the Chloromethanes
with Thoseof Methane
Molecular
structure
Molar
mass(g)
Name
methane
chloromethane
(methyl chloride)
dichloromethane
(methylene chloride)
trichloromethane
(chloroform)
tetrachloromethane
(carbon tetrachloride)
CHn
cH3cl
cH2cl2
cHC13
CCL
16
50.5
Boiling
point("C)
- 161
-24
85.0
40
129.5
61
154.0
74
Alkyl andAryl Chlorides
TableI2.5 BoilingPointsof Alcoholsand €omparable
Boiling
point("C)
Alcohol
CH3OH
CH3CH2OH
CH3CH2CH2OH
o""
o""
Solubility
Hydrophobic
hudor(Greek): water
phobos(Greek): fear
65
7B
97
t62
t82
Alkylchloride
CH3CI
CH3CH2CI
CH3CH2CH2CI
O"
G"
Boiling
point("C)
-64
13
47
143
t32
in water
The hydrocarbon parts of chains and rings of organic molecules are
hydrophobic ("water-hating")-repelled by water Oil and water dont mix.
If we mix two nonpolar liquids, however, they form a solution. A good rule
of thumb is that "like dissolveslike."
With the principle that "like dissolves like" in mind, how would we
expect alcohols to behave with respect to their solubilities in water? Since
alcohols are derivatives of water, we might expect them to have similar
properties. And to a point, this is correct. Alcohols of up to four carbons are
soluble in water in all proportions. The solubility of alcohols with four or
more carbons in the chain is usually much less.For example, the solubility
of l-butanol is only 7.9 gll00 mL of water.The reasonis that alcoholsconsist of two parts: the carbon chain and the hydroxyl group. These parts are
in opposition to each other. The carbon chain is nonpolar and hydrophobic, but the hydroxyl group forms hydrogen bonds with water. Groupssuch
12.9SulfurCompounds I8l
Hydrophilic
hudor (Greek): water
philos (Greek): loving
as -OH that interact strongly with water, usually W hydrogen bonding, are
called hydrophilic ("water-Iouing") gtoups. Alcohols with short carbon
chains are soluble in water. Those with longer carbon chains will not dissolve. And some alcohols whose carbon chains are not too long are only
slightly soluble.
Ethers are more soluble in water than hydrocarbons and halocarbons
but less soluble than alcohols of approximately the same molar mass.The
reason is that the oxygens in ethers are hydrogen-bond acceptors, but
ethers have no hydroxyl hydrogens to donate in hydrogen bonding. This
lower solubility compared with alcohols is overcome in molecules with
more than one ether linkage. Dioxane, a cyclic compound with two ether
linkages, is soluble in water in all proportions; diethyl ether, with the same
number of carbons but only one ether linkage, is not.
O:
:O
CH3CH2.-O-CH2CH3
Dioxane
(soluble in water
in all proportions)
Diethyl ether
(solubility in water:
B 9/100 mL)
PRACTICE
EXERCISE
I2.9
Name and classitrbyfunctional groups the following compounds, and
identi{i the one that is most polar.
(a) CHsOCHg
(c) CH3CHTOH
&) CH3CHzCI
(d) cH3cH2cH3
PRACTICE
EXERCISE
I2.I O
Arrange the compounds in Practice Exercise12.9in order of increasing
polarity then comment on their relative boiling points and water solubilities.
12.9Sulfurcompounds
AIM: Toidentify nome, structures,
ond usesof somecommon
thiols, thioethers,ond disulfides.
Just as alcohols and ethers are organic derivatives of water, thioalcohols
and thioethers are organic derivatives of hydrogen sulfide, H2S.
Thiols and thioethers are sulfur
analogues of alcohols and
ethers.
Thioalcohols
and thioethers
Thioalcohols, compounds wi,th the general formula R-S-fi,
are also
calledthiols. Thiols are also calledtnercaptans-a name coined becauseof
their readyability to "captttrre"or react with the element mercury.The odor of
582
Alcoholt and Ethers
CHAPTER
l2 Halocarbons,
the fluid that skunks eject to protect themselvesfrom predators comes partiallv from two thiols.
H"c
'\
Thiol
theian {Greek): sulfur
The odor frorn a skunk can be
removedfrom a contaminated
object by treatment with a solution of the follollring composition:
1 quart 3YoIJ2A2,I /4 cup baking
soda,and I teaspconliquid soap.
After treatment, the object should
be rinsed with tap water.
H
C:C.
,/\
,/
H
9H.
|
CH3-CHCH2CHz-SH
CH2-SH
3-Methyl- I -butanethiol
rrans-2-Butenethiol
Sulfur compounds are well knornm among chemists as the most foul
smelling of all organic compounds. Sometimes,however, even an odor can
be put to good use. Natural gasis odorless,but the gas piped to a gasrange
has a characteristic smell. The gas company has added a trace of
methanethiol or ethanethiol to the natural gas so that leaks can be
detected.
cH3-sH
cH3cH2-sH
Methanethiol
fmethyl mercaptan)
Ethanethiol
(ethyl mercaptan)
In very low concentrations,the odors of some thiols and sulfides are even
desirablein cooking. The pungent smell of onions comes from propanethiol.
CH3CH2CH2-SH
Propanethiol
(propyl mercaptan)
Thioethers, compounds with the generalformula R-S-R, are also
called sulfides.The characteristic aroma and flavor of garlic comes from
divinyl sulfide.
\,/\./
C : C ---,,/
H-S-H
-..-
C:C
Divinyl sulfide
Disulfrdes
Sulfur atoms can form relatively stable bonds with other sulfur atoms. The
sulfur-sulfur bond is not as stable as the carbon-carbon bond, and sulfur
does not form long chains. But disulfides, organic compounds of the general
are found in many proteins, especially
structural formula R-S-S-R,
those of hair, hooves,and nails (seeSec. 18.6).A third compound in the
defensefluid of skunks is a disulfide:
Htc\
C:C
Hl
./H
ta"r-r-r-cHs
12.10 Polyfunctional
Compounds
585
Disulfides are prepared by the mild oxidation of thiols. Thiols permitted
to stand in air spontaneously oxidize to disulfides:
2CH3S-H i.'
Methanethiol
Crt S-SCH.
Methyl disr'lfids
Once formed, disulfides can be further oxidized to sulfonic acids bv
hydrogen peroxide:
CH3S-SCH3
Methyl disulfide
Hz{)2,
2C1I3SO3H
Methyl sulfonic acid
Disulfides are easily reduced to thiols. Although many reducing agents
are suitable, hydrogen works well:
CH3S-SCI{3 -En' ZCU.-SH
Methyl disulflde
Methanethiol
I2.l 0 Polyfunctionolcompounds
AIM: Torecognizethe functionolgroups of a given
polyfunctionol molecule.
Focus
Many organic molecules
contain more than one
functional group.
exA fig'i
';=-':'':;'
So far our introduciion to functionatgroup chemistry has been focused on
molecules with only one kind of functional group. Polyfunctional organic
compounds contain two or more functional groups. Tetrahydrocannabinol
is an example of a polyfunctional molecule, as shor,rrnin the following
example.
idtiigi4t+,Ett*i,t,":'';,:';,:;'t:,',
: : ' , , ' , ' ; ; ; , , , " " , , ' ,""',,'', " , , , " 1 1 -
IdentiSrthe functionalgroupsin the tetrahydrocannabinol
molecule.
c}{2cH2cH2cH2cH3
CH.
Tetrahydrocannabinol
SOLUTION
Inspect the molecule to find the functional groups, such as multiple carbon-carbon bonds, hydroxyl groups, ether linkages, and so forth. The
ta4
Alcohols,and Ethers
l2 Halocarbons,
CHAPTER
tetrahydrocannabinol molecule contains three functional groups: a cyclic
ether linkage, a phenolic hydroxyl group, and a carbon-carbon double
bond. These functional groups are in color in the following structure.
CH2CH2CH2CHzCH3
CHe
Tetrahydrocannabinol is the active ingredient of the Cannabis satiua
(marijuana) plant. The effects of marijuana use on health and society are
still being debated. One interesting finding is that smoking marijuana
reduces the pressure of the optic fluid and may therefore be helpful in
relieving the symptoms of glaucoma, a serious eye disease.
Another polyfunctional compound is hexachlorophene, which is both
an aromatic halocarbon and aphenol:
HO
CI
\--J
."r\
/-\
/
cr,Tct
CI
Hexachlorophene
Hexachlorophene is an antiseptic. Until recently, it was an important ingredient in germicides and soaps used in hospitals. Its use was curbed, however,when it was found that the babies of female hospital workers who frequently washed their hands with hexachlorophene soap had a higher
incidence of birth defects than babies born to women in the general population.
I2.I I
PRACTICE
EXERCISE
The structure that follows is a urushiol, a family of compounds that are
the irritants in poison ivy. It is a polyfunctional compound with the following structure. How many funciional groups can you identify?
HO
OH
(CHz)zCH:CHzCH : CH(CH2) CH3