Chapter 15: Aldehyde and
Ketones
In this chapter, we discuss the hydrocarbon derivatives that contain the
element oxygen directly with a double bond. In last chapter, we discussed the functional
groups (alcohols phenols, and ethers) have the common feature of carbon-oxygen
single bonds. Carbon oxygen double bonds are also possible in hydrocarbon
derivatives. We will now consider the simplest types of compounds that contain
this structural feature: aldehydes and ketones.
15.1 The Carbonyl Group
Both
aldehydes and ketones contain a carbonyl functional group. A carbonyl group is
a n atom double-bonded to an oxygen atom. The structural representation for a
carbonyl group is
Carbon-oxygen and carbon-carbon double bonds differ in a major way. A
carbon oxygen double bond is polar,
and a carbon-carbon double bond is nonpolar. The electronegativity (Section
5.9) of oxygen (3.5) is much greater than that of carbon (2.5). Hence the carbon-oxygen
double bond is polarized, the oxygen atom acquiring a fraction of negative
charge and the carbon atom positive charge as shown in figure above.
15.2 Compounds Containing a Carbonyl Group
Aldehydes: Carbonyl is attached to at least one H atom
in aldehydes
Ketones: Carbonyl is directly attached with two carbon
atoms in ketones
Carboxylic acids: carbonyl carbon atom bonded to a hydroxyl group.
Esters: carbonyl carbon atom bonded to an oxygen atom
Amide: carbonyl carbon atom bonded to a nitrogen atom
Aldehydes Carbon
chain on one side of carbonyl and hydrogen on the other |
Ketones Carbon
chain on both sides of the carbonyl |
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Other Carbonyl Derivatives |
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15.3 The Aldehyde and Ketone Funcitonal Groups
Common
names of simple aldehydes and ketones
Aldehydes:
Formaldehyde: HCHO
Acetaldehyde:
CH3CHO
Propionaldehyde:
CH3CH2CHO
Butyraldehyde: CH3CH2CH2CHO
Valeraldehyde:
CH3CH2CH2CH2CHO
Ketones:
Acetone: CH3COCH3
Methyl
ethyl ketone
CH3CH2COCH3
Butyl propyl ketone
CH3CH2CH2CH2COCH2CH2CH3
15.4 Nomenclature for Aldehydes
Rule 1: Select as the parent carbon chain the longest carbon chain that
includes the carbon atom of the carbonyl group.
Rule 2: Name the parent chain by changing the “-e” ending of the
corresponding alkane name to “-one.”
Rule 3: Number the carbon chain such that the carbonyl carbon atom
receives the lowest possible number. The position of the carbonyl carbon atom
is noted by placing a number immediately before the name of the parent chain.
Rule 4: Determine the identity and location of any substituents, and
append this information to the front of the parent chain name.
Rule 5: Cyclic ketones are named by assigning the number 1 to the carbon
atom of the carbonyl group. The ring is then numbered to give the lowest
number(s) to the atom(s) bearing substituents.
Aldehydes: -al as suffix
Give
the condensed structures of following aldehydes:
Aldehydes: |
15.5 Nomenclature for Ketones
Ketones: -one as suffix
Aromatic ketones
Give
the condensed structures of following ketones:
Ketones:
propanal (propionaldehyde)
2-butanone (methylethylketone)
4,4-dimethyl-2-pentanone
cyclopentanone
4-hydroxy-2-cylohexenone
15.6 Isomerism for Aldehydes and Ketones
Constitutional isomers exist for aldehydes and
ketones
Isomers between aldehydes and ketones are called
functional group isomers
Two types of isomers:
•
Skeletal isomers: arrangements of atoms in space is
different.
•
Positional isomers: Position of the functional group
is different.
The keto
and enol forms of aldehydes and ketones
Aldehydes and ketones exsists in two different isomers also called tautomers.
Regular carbonyl form of the aldehyde or the ketone is called keto form
and the -OH containing form is called enol from.
In biological systems keto
form of aldehyde sugars (aldoses) are converted to ketone sugars (ketoses) via
enediol (enol) froms as shown below. Therefore D-fructose which is a ketone or
keto sugar (ketose) will give a positive test for Benedict's test because of
the ability of ketoses to get converted to aldoses (aldehydes).
15.7 Selected Common Aldehydes and Ketones
Methanal
or formaldehyde:
structure of fomaldehyde
Fomalin
40% water solution of formaldehyde has been widely used a preservative for
biological specimens. Formaldehyde is the simplest aldehyde. It is
manufactured by oxidizing methanol with air over a metal catalyst in a
temperature range of 400-650 degrees Celsius. The catalyst can be
copper, silver, or molybdeum alloy.
Ethanal or acetaldehyde:
Acetaldehyde
also called Ethanal (CH3CHO), an aldehyde used as a starting
material in the synthesis of acetic acid, n-butyl alcohol, ethyl
acetate, and other chemical compounds. It is manufactured by the oxidation of
ethyl alcohol and by the hydration of acetylene. Pure acetaldehyde is a
colourless, flammable liquid.
2-Propanone
or acetone:
Acetone also called
2-propanone, or dimethyl ketone (CH3COCH3),
organic solvent of industrial and chemical significance, the simplest and most
important of the aliphatic (fat-derived) ketones. Pure acetone is a colourless,
somewhat aromatic, flammable, mobile liquid that boils at 56 C. It is
also used as nail polish.
2-Butanone
or methyl ethyl ketone:
Important idustrial sovent.
Oil of
almonds or benzaldehyde:
Found in almond nuts.
Oil of
Cinnamon or cinnamaldehyde:
Found in cinnamon.
Berry
Flavoring a-Demascone:
Found in berry.
Oil of
vanilla beans or vanillin:
Found in vanilla beans.
Mushroom
flavoring or 2-octanone:
used as mushroom flavoring.
Oil of
lemongrass or citral:
Found in lemon grass.
15.8 Physical
Properties of Aldehydes and Ketones
15.9 Preparation of Aldehydes and Ketones
Preapration
of aldehydes:
Partial oxidation of primary alcohols with H2CrO4:
(primary
alcohol)
----
(aldehyde)
Preapration
of ketones:
Oxidation of secondary alcohols with KMnO4, H2CrO4
(Secondary
alcohol)
----
(ketone)
Tertiary
alcohols doesn't under go oxidation:
(Tertiary alcohols) ----
(no reaction)
15.10
Oxidation and Reduction of Aldehydes and Ketones
Aldehyde
are readily oxidized to corresponding carboxylic acids by number of
different oxidizing agents. The symbol [O] is used to indicate the
oxidation process.
Benedict's Test for aldehydes:
Aldehyde loses an e-
(aldehyde is oxidized to an acid) Cu2+ gains an e- (Cu2+
is reduced to Cu+)
Oxidizing Agents and Reducing
Agents in the Benedict's Test |
Oxidation = loss of an e- |
The aldehyde is a reducing
agent. |
The Cu2+ is an
oxidizing agent. |
The travel agent does not travel,
but the travel agent facilitates the travel of someone else. |
Glucose
is an aldehyde sugar which show positive test for the
Benedict's test. This test have been used in old days to detect excess bolld
sugsr in diabetic patients.
Tollen's
Test:
A basic Ag(NH3)2+
in aqueous ammonia reduces to metallic silver (mirror) with aldehyde
oxidized to carboxylic acid.
The commercial manufacture
of silver mirrors uses a similar process.
Reduction
of carbonyl compounds to alcohols:
Reduction aldehydes and ketones to primary sceondary alcohols Reduction Reactions:
Aldehyde and ketones are readily reduced to corresponding alcohols by number of
different reducing agents. The symbol [H] is used to indicate the reduction
process. One of the methods is hydrogenation
where H2 in the presence of a catalyst such as Pt rduces the
alcohol. The methods of reduction include:
Pt(H2)
platinum catalyzed hydrogenation
LiAlH4 lithium aluminum hydride
NaBH4 (sodium borohydride)
Reduction
of aldehydes:
(aldehyde)
---- (primary alcohol)
Reduction of
ketones:
(Secondary
alcohol)
----
(ketone)
15.11 Reaction of Aldehydes
and Ketones with Alcohols
Addition
reactions to carbonyl double bond in aldehydes and ketones:
Hemiacetal and hemiketal formation
Hemiacetals
Hemiketals
ketone
+ 1st alcohol --- hemiketal
Acetals, and Ketals.
In biological systems sugars which are complex aldehydes and ketones are found
in one of these forms: hemiacetals, and hemiketals or acetals and
ketals.
Cyanohydrin formation
Hydrate formation
Aldol
condensation reactions
Aldol condensation is a reaction in which aldehydes or ketones react to
form larger molecules. In biological systems this reaction is catalysed by an
enzyme named aldolase.
aldehyde
aldehyde
aldol
15.12 Formaldehyde-Based Polymers
Formaldehyde condense with phenols under acidic or
basic condition; polymerization gives a network of phenol rings
held together by methylene groups at the ortho and para positions.
This polymer called bakelite is a stiff, three dimensional
network with very little solubility in organic solvents and a high resistance
to electricity and heat. It is used in a wide variety of household
objects and electrical fixtures.
15.13 Sulfur-Containing Carbonyl Groups