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Chapter 16
Aldehydes and
Ketones
Structure
The functional group of an aldehyde is a
carbonyl group bonded to a hydrogen
atom
 The functional group of a ketone is a
carbonyl group bonded to two carbons

O
O
O
HCH
CH3 CH
CH3 CCH3
Meth anal
Ethanal
Prop anone
(Formaldehyde) (Acetaldeh yd e) (Aceton e)
Nomenclature

IUPAC names for aldehydes



to name an aldehyde, change the suffix -e of the
parent alkane to -al
the aldehyde carbon must be carbon-1
for unsaturated aldehydes, indicate the presence of
a carbon-carbon double bond and an aldehyde by
changing the ending of the parent alkane from -ane
to -enal; show the location of the carbon-carbon
double bond by the number of its first carbon
Nomenclature
O
5
6
3
4
Hexanal

3
1
2
O
O
H
4
2
3
1
2
H
3-Methylbutan al
1
H
2-Prop enal
(Acrolein)
the IUPAC system retains common names
for some aldehydes, including these three
O
CHO
CHO
H
OCH3
t rans-3-Phenyl-2-prop enal
(Cinn amald ehyd e; in
oil of cin namon)
Ben zaldehyde
(in almond s)
OH
Van illin
(from van illa
bean s)
Nomenclature

IUPAC names for ketones




parent = longest chain that contains the carbonyl
indicate with parent alkane as -one
carbonyl carbon gets the smaller number
the IUPAC retains the common name acetone for 2propanone
O
O
1
Acetone
O
2
3
4
5
1
6
5-Meth yl-3-h exanone
2
2-Methylcycloh exanone
Nomenclature

To name an aldehyde or ketone that also
contains an -OH or -NH2 group



give the carbonyl carbon the lower number
indicate an -OH substituent by hydroxy-, and an -NH2
substituent by aminohydroxy and amino substituents are numbered and
alphabetized along with other substituents
O
OH O
5
4
3
1
H
3-Hydroxy-4-meth ylp entanal
6
4
3
2
1
NH2
3-Amino-4-ethyl-2-h exanone
Nomenclature

Common names


derived from the common name of the
corresponding carboxylic acid; drop the word "acid"
and change the suffix -ic or -oic to -aldehyde
name each alkyl or aryl group bonded to the
carbonyl carbon as a separate word, followed by
the word "ketone”;
O
O
O
CH3 CH
CH3 COH
Acetaldehyde Acetic acid
O
Methyl ethyl ketone Ethyl isopropyl ketone
Physical Properties

A C=O bond is polar, with oxygen
bearing a partial negative charge and
carbon bearing a partial positive charge
Physical Properties
in liquid aldehydes and ketones, the
intermolecular attractions are polar
 no hydrogen bonding is possible between
aldehyde or ketone molecules
 aldehydes and ketones have lower boiling
points than alcohols and carboxylic acids,
compounds in which there is hydrogen
bonding between molecules

Physical Properties
Name
diethyl ether
pentane
butanal
2-b utanone
1-b utanol
propan oic acid


Molecular
Stru ctural Formu la Weight (amu)
CH3 CH2 OCH2 CH3
74
CH3 CH2 CH2 CH2 CH3
72
CH3 CH2 CH2 CHO
72
72
CH3 CH2 COCH3
74
CH3 CH2 CH2 CH2 OH
72
CH3 CH2 COOH
bp
(°C)
34
36
76
80
117
141
formaldehyde, acetaldehyde, and acetone are
infinitely soluble in water
aldehydes and ketones become less soluble in
water as the hydrocarbon portion of the molecule
increases in size,
Oxidation

Aldehydes are oxidized to carboxylic acids by
a variety of oxidizing agents, including
potassium dichromate
O
O
H
Hexan al

K2 Cr2 O7
H2 SO4
OH
Hexan oic acid
liquid aldehydes are sensitive to oxidation by O2
O
C
H
Benzald ehyde
O
C
+ O2
OH
Benzoic acid
Oxidation

Ketones resist oxidation by most oxidizing
agents, including potassium dichromate and
molecular oxygen
Reduction

The carbonyl group of an aldehyde or ketone
can be reduced to an -CHOH group by
hydrogen in the presence of a metal catalyst
O
H + H2
transition
metal catalyst
Pentanal
O + H2
Cyclopen tan on e
tran sition
metal catalyst
OH
1-Pentanol
OH
Cyclopen tanol
Reduction

The most common laboratory reagent for the
reduction of an aldehyde or ketone is sodium
borohydride, NaBH4



hydrogen in the form of hydride ion, H:in a reduction by sodium borohydride, hydride ion
adds to the partially positive carbonyl carbon which
leaves a negative charge on the carbonyl oxygen
reaction of this intermediate with aqueous acid
gives the alcohol
Reduction
C O
H C O-
NaBH4
O-
H3 O+
: :
:
:
H: +
: :
:
-
H C O-H
Hydride
ion
O

O-H
H3 O+
H
H
does not affect a carbon-carbon double bond
O
C
H
1 . NaBH4
CH2 OH
2 . H2 O
Cin namaldehyde
Cinnamyl alcoh ol
Reduction

In biological systems, the agent for the
reduction of aldehydes and ketones is NADH
(Section 26.3)


this reducing agent also delivers a hydride ion
reduction of pyruvate, the end product of glycolysis,
by NADH gives lactate
O
CH3 -C-COO
Pyruvate
NADH
OCH3 -C-COO
H
+
H3 O
O-H
CH3 -C-COO
H
Lactate
Addition of Alcohols

Addition of a molecule of alcohol to the
carbonyl group of an aldehyde or ketone forms
a hemiacetal (a half-acetal)


the functional group of a hemiacetal is a carbon
bonded to one -OH group and one -OR group
in forming a hemiacetal, H of the alcohol adds to
the carbonyl oxygen and OR adds to the carbonyl
carbon
O
H
C + O-CH2 CH3
H
Benzaldehyde Ethanol
O-H
C OCH2 CH3
H
A hemiacetal
Addition of Alcohols


hemiacetals are generally unstable and are only
minor components of an equilibrium mixture
If a five- or six-membered ring can form, the
compound exists almost entirely in a cyclic
hemiacetal form
O
5
4
3
2
1
H
O-H
4-Hyd roxypentanal
redraw to
show the -OH
an d -CHO clos e
to each oth er
3
2
1
4
5
O
H
C
H
O
H
O-H
O
A cyclic hemiacetal
Addition of Alcohols

A hemiacetal can react further with an
alcohol to form an acetal plus water
this reaction is acid catalyzed
 the functional group of an acetal is a carbon
bonded to two -OR groups

O-H
H
C OCH2 CH3 + OCH2 CH3
H
A hemiacetal
Eth anol
(from benzaldeh yd e)
H+
OCH2 CH3
C OCH2 CH3 + H2 O
H
An acetal
Addition of Alcohols
O-H
OCH2 CH3
H
+ OCH2 CH3
A hemiacetal
(from cyclohexan one)
+
H
Ethanol
OCH2 CH3
OCH2 CH3
+ H2 O
An acetal
all steps are reversible
 Le Chatelier's principle

to drive it to the right, we either use a large
excess of alcohol or remove water from the
equilibrium mixture
 to drive it to the left, we use a large excess of
water

Keto-Enol Tautomerism

A carbon atom adjacent to a carbonyl
group is called an a-carbon, and a
hydrogen atom bonded to it is called an
a-hydrogen
a-hydrogens
O
CH3 -C-CH2 -CH3
a-carbons
Keto-Enol Tautomerism

A carbonyl compound that has a hydrogen on
an a-carbon is in equilibrium with a
constitutional isomer called an enol
O
CH3 -C-CH3
Acetone
(k eto form)

OH
CH3 -C=CH2
Aceton e
(enol form)
in a keto-enol equilibrium, the keto form
generally predominates
Keto-Enol Tautomerism

example: draw structural formulas for the
two enol forms for each ketone
O
(a)
(b)
O
Keto-Enol Tautomerism
example: draw structural formulas for the
two enol forms for each ketone
 solution:

O
OH
OH
(a)
(b )
O
OH
OH
Aldehydes and Ketones
End
Chapter 17