Group
13.2TheCarbonyl
T97
II.2 Thecorbonylgroup
AIMS: Tonome ond drow structuresof simple aldehydesond
ketones.To exploin how intermoleculorinteroctionsof
the corbonylgroup offect the boiling point ond water
solubilityof oldehydesond ketones.
Focus
The polar carbonyl group
influences the physical
properties of aldehydes and
ketones.
Electron sharing in the carbon-oxygen double bond ofthe carbonyl group
is similar to that of the carbon-carbon double bond in alkenes. Compare
ethylene, H2C-CH2, with formaldehyde, H2C:O (Fig. 13.1).A carbon
atom has only four unshared electrons in its valence shell, whereas oxygen
has six. This means that carbon can make four covalent bonds, as in ethylene, but oxygen can make only two, as in the carbonyl group' The oxygen in
a carbonyl group has two unshared pairs of electrons, but the carbon in
ethylene has none.
In C:C bonds, the bonding electrons are shared equally. But oxygen is
much more electronegativethan carbon. The C:O bond is very polar
because oxygen draws the bonding electrons away from carbon. The oxygen in the carbonyl group carries a partial negative charge,and the carbon
carries a partial positive charge:
yE+ E1
t:o:
Hydrogen bonding cannot take place between molecules of aldehydes
or ketones because they lack O-H bonds. However, polar-polar interactions are possible:
b-
o
ll6a 6- b+7
\b+ 6c:o---c- -o:c
,/
\
,.\
\
Polar-oolar interaction
H
C:C
).=o:>,rnshared
u,
Ethylene
electron
palrs
Formaldehyde
Figurel5.l
modelsof ethyleneand
formulasand ball-and-stick
of the structural
Comparison
hastwo unTheorygenof the carbonylgroupof formaldehyde
formaldehyde.
sharedpairsof electrons.
598
CHAPTER
13 Aldehydesand Ketones
Bolling
points
Alcel'ryrles
andketoneswith lor,r, Aldehydes and ketones cannot form intermolecular hydrogen bonds
rnsLar"rnasses
areveryvolatileand becausetheylackhydroxyl (-OH) groups.Consequently,theyhave boiling
highlv flammable.
points lower than those of the corresponding alcohols. rne aiaenydes and
ketones can attract one another through polar-polar interactions of their
carbonyl groups, however, and their boiling points are higher than those of
the corresponding alkanes. Table 13.2 shows how the boiling points
increase in the sequence alkane, aldehyde, and alcohol for compounds
containing one and two carbons. Except for formaldehyde, which is an irritating, pungent gas,all aldehydesand ketones are either liquids or solids at
20 "C (Table 13.3).The vapors of aldehydesand ketones contribute to the
pleasant odors of many natural products, as described in A Closer Look:
Flavors and Fragrances.
Tablell,2 SomeCharacteristics
of One-andTwo-Carbon
Alkanes,Aldehydes,
andAlcohols
Compound
Formula
Boilingpoint("C)
Comments
One Carbon
methane
cH,
-161
no hydrogen bonding or
polar-polar interactions
polar-polar interactions
hydrogen bonding
formaldehyde
methanol
Two Carbons
ethane
acetaldehyde
ethanol
HCHO
cH3oH
-2r
czHo
-Bg
65
cH3cHo
cH3cH2oH
no hydrogenbonding or
polar-polar interactions
polar-polar interactions
hydrogen bonding
20
78
Tablel5.I PhysicalConstants
of SomeAldehydes
and Ketones
Compound
Aldehydes
formaldehyde
acetaldehyde
butyraldehyde
benzaldehyde
Ketones
acetone
methyl ethyl ketone
diethyl ketone
benzophenone
Melting
point('C)
Boiling
point ('C)
Solubility
in water
(9/100mt)
-92
-r23
-99
-26
-2r
20
76
L79
completely miscible
completely miscible
4
0.3
-95
-86
-42
56
80
101
306
completely miscible
25
5
insoluble
48
13.2 The CarbonylCroup
Flauors and Fragrances
All pleasant odors and tastes come from chemicals. Aldehydes are responsible for the delightful
odors of vanilla, cinnamon, and almonds. Other
odors such as that of camphor, which has been
used in medicine for thousands of years, are due
to ketones.
Perfumes are made by blending certain odoriferous substances.Fine perfumei may contain
more than 100ingredients. Perfumes are classified
by the dominant odor. Odors of lose, gardenia,lily
of the valley,and jasmine are the floral group. Aromas of clove, nutmeg, cinnamon, and carnation
provide the spicy blends. Cedar and sandalwood
make up the woody group. The Orientals group
combines the spicy and woody odors with the
sweetodors of balsamandvanilla and accentuates
the mixture with the odors of musk and civet. The
odors of aldehydesdominate the aldehydic group,
which have fruity character.
Natural products of vegetable and animal origin traditionally have been used as starting materials for perfume manufacture. Mixtures of essential oils (which give perfumes their odor or
essence)and waxes are obtained from these natural products by extraction into organic solvents.
Removalof the solvent givesa residue called a concrete.If the concrete is treated with ethanol, the
waxremains behind and the essentialoil dissolves
in the alcohol. Essentialoils are removed from citrus peel by pressing.Individual compounds used
in perfumesmaybe isolatedfrom the essentialoils
by distillation.
Some animal secretions contain odorous
compounds that improve the lasting quality of
perfumes. These substances and their compo-
Solubility
599
nents act as flxatives, preventing the more volatile
ingredients in the perfume from evaporating too
rapidly. Such materials are usually used as solutions in ethanol and give high-quality perfume its
strength, character, and tenacity. The traditional
animal products include castoror castoreumfrom
the beaver, civet from the civet cat (see figure),
ambergris frorn the sperm whale, and musk from
the musk deer.Today,becauseof ecologicalconcerns, chemists s1'nthesizethe compounds that
produce the musk odor. Musk, the most commonlyknorltrnmusky substance,has a penetrating,
persistent odor. Natural musk is produced by a
gland under the skin of the abdomen of the male
musk deer. The odorous substancein musk is a
ketone, muscone, which has the chemical name
3-methylcyclopentadecanone.The civet cat, a
native of Africa, southern Europe, andAsia, marks
its territory with another ketone, civetone. Civetone, like the musk of the deer, has a musky odor
that is excellent for use in perfumes.
The civetcat and musk deer producesecretionsthat are
used as fixativesin perfumes-they give the perfume a
lastingodor.
in water
Aldehydesand ketones can form hydrogen bonds with polar water molecules (Fig. 13.2).Formaldehyde,acetaldehyde,and acetone are soluble
in water in all proportions. As the length of the hydrocarbon chain
increases,water solubility decreases;when the carbon chain exceedsfive
400
CHAPTER
13 Aldehvdesand Ketones
ffi"%4*
wffiffi @,*f
re""ffi"
AldehYde
Figure15.2
Aldehydes
andketonescan
form hydrogenbondswith water.
W^
ffikV
W
W
Ketone
or six carbons, solubility of both aldehydesand ketones is very low. All aldehydes and ketones are soluble in nonpolar solvents.
PRACTICE
EXERCISE
I!.4
Arrange the following substances in order of increasing solubility in
water:
(a) acetone
(b) butanal
(c) pentanol
(d) benzophenone
(e) benzaldehyde
II.l Redoxreoctionsof orgoniccompounds
AIMS: To describethe processesof oxidotion ond reductionin
orgonic chemistryin termsof the lossor gain of oxygen,
hydrogen,or electrons.Torelote the energy contentof a
moleculeto its degreeof oxidotion or reduction,
Many reactions of organic
compounds involve oddation
or reduction.
Now that we are familiar with functional groups in organic chemistry we
can examine oxidation-reduction reactions of organic molecules. We are
interested in redox reactions of organic molecules becausethey are important in energy production in living organisms. Oxidation reactions are
energy-releasing,and the more reduced a carbon compound is, the more
energy the compound can release upon its complete oxidation. You may
recallthe principles of redoxreactionsfrom Section6.4: Oxidationreactions
involve a gain of oxygen,a loss of hydrogen, or a loss of electrons; reduction
reactionsinvolve a loss of oxygen,a gain of hydrogen, or a gain of electrons.
Oxidation and reduction reactions must be coupled; if a compound is oxidized in a reaction,some other compound in the reactionmust be reduced.
We will discuss electron transfers in redox reactions in living organisms in
Chapter 23. Our concern with redox reactions in this chapter will focus on
those reactions that involve oxygen and hydrogen. For example,methane, a