CHAPTER ONE
ALDEHYDES and KETONES
1.1 Introduction-The Carbonyl Bond (C=O)
The geometry of the carbonyl is determined by the sp 2–hybridized carbon. The molecule
is planar around the trigonal sp2 carbon.
C O C O
a carbonyl bond polar
The C==O bond is composed of a sigma (σ) bond and a pi (π) bond. The oxygen atom
has two pairs of unshared electrons. Because of the significant difference in
electronegativity between carbon and oxygen, the carbonyl group is polarized in such a
way that oxygen is slightly negative (δ-) and the carbon slightly positive (δ+). The
carbonyl group is more polar than the C-O bond of the alcohol or ether molecule. The pi
electrons in the carbonyl are drawn more easily to the electronegative oxygen than the
sigma electrons in the C-O bond. Keeping polarity in mind will often be helpful in
understanding the chemical reactivity of carbonyl containing compounds.
The name aldehyde is derived from alcohol dehydrogenated and may be represented as
follows –CHO or in structural form as below.
O
C
H
Aldehydes have the general formula R-CHO and the simplest, methanal, is prepared by
passing methanol over heated copper metal.
O
CH3OH + Cu metal / high temp. H2 +H C
H
alcohol: methanol aldehyde: methanal
Aldehydes are very reactive compounds because they contain an aldehyde group or more
generally the carbonyl group -C==O. An aldehyde has a hydrogen atom attached to the
carbonyl group while a ketone has no hydrogen attached but two organic groups, R,
which could be alkyl or aryl. Much of the chemistry of aldehydes and ketones involves
addition reaction to the pi bond. Because of the presence of a hydrogen atom on the
carbonyl group of aldehydes, they are easily oxidised while ketones are oxidised with
difficulty. Aldehydes are more reactive towards nucleophilic addition.
1
, O O
C O unshared electrons C
C
R H R R
carbonyl bond aldehyde ketone
1.2 Physical Properties
Aldehydes and ketones have higher boiling points than non-polar compounds of similar
molecular weight. This is because they are polar and undergo intermolecular dipole-
dipole attractions. To a limited extent aldehydes and ketones can solvate ions. An
example is sodium iodide (NaI), which is soluble in acetone (CH3COCH3).
CH3 O OH
CH3CHCH3 CH3CCH3 CH3CHCH3
bp 12 0 bp 56 0 bp 82 0
O H H
O
O H O R C O
C R H
R C R H O R C
R
H
H H
O
Because of unshared electrons on the oxygen, a carbonyl compound can form hydrogen
bonds (but not with another carbonyl compound unless it has an acidic hydrogen
available for hydrogen bonding). Aldehydes and ketones of low molecular weight, those
with four or fewer carbons, can form hydrogen bonds with water and alcohols and are
soluble.
1.3 Aldehydes
Methanal (formaldehyde): This is a colourless gas with a sharp penetrating odor (i.e.
irritating to mucus membrane). It is readily soluble in water. When concentrated
methanal solution is evaporated it forms a solid polymer known as paraformaldehyde (an
acetal).
2
,A solution of 37% methanal and 7-15% methanol in water is known as formalin.
Methanal is conveniently shipped or stored as formalin or as a solid polymer
(paraformaldehyde) or trimer - trioxane (mp 620C).
Formalin acts as a disinfectant (a germicide).
Is used in embalming fluid.
Is used as a preservative of various tissues i.e. hardens tissues.
Formaldehydes gas and the polymer paraformaldehyde are used extensively as
insecticides, fumigating agents and antiseptics. Large quantities of formaldehyde are used
in the manufacture of synthetic (lightweight) resins (e.g. bakelite foam) and the syntheses
of other organic compounds.
HCHO + H2O heat
(formalin)
O heat CH2OCH2OCH2OCH2O
H2 heat C
paraformaldehyde
C H H
O O evaporate
methanal (formaldehyde)
H2C CH2
O
trioxane
Ethanal bp 200 (acetaldehyde) is also stored and shipped in a cyclic trimer paraldehyde
or as a tetramer metaldehyde.
It is a colourless liquid and has a characteristic sharp odor
Produced by oxidation of ethanol or hydration of ethane.
Ethanal is used in the production of ethanoic acid (acetic acid, vinegar), ethyl
acetate, synthetic rubber, etc.
Paraldehyde, (mp 125 o C; polymer of ethanol) an acetal (cyclic ether), is more
stable than ethanol and serves as a source of the latter compound when heated.
When three molecules of ethanal is heated with a trace of acid paraldehyde is
formed.
Paraldehyde–used to depress the central nervous system and to desensitize the
gums of the mouth against heat or cold.
It is an effective hypnotic or sleep producer but has been replaced by other drugs
since it has an irritating odor and an unpleasant taste.
CH3
H3C O CH3 H
C O H
C C O heat
H H heat O C
O O CH3C CH3
C H H3C C O
H3C H ethanal H O C
acid
H CH3
0
paraldehyde mp 125 C 0
metaldehyde mp 246 C
1.4 Nomenclature of Aldehydes
3
, 1) Pick the longest carbon chain containing the aldehyde or ketone group.
2) Number the chain so that the terminal aldehyde is always in position 1.
3) Locate and number the branched alkyl groups on the main chain.
4) For aldehydes change the alkane name by changing the “e” to “al”
Structure IUPAC common name.
a) HCHO methanal formaldehyde
b) CH3CHO ethanal acetaldehyde
c) CH3CH2CHO propanal propionadehyde
d) CH3CH2CH2CHO butanal buytraldehyde
e) CH3CH2CH(CH3)CHO 2–methylbutanal
f) CH3CH2CH(CH2CH2CH2CH3)CH(CH3) CHO 3-ethyl-2-methylhexanal
(NOT 2-methyl-3-ethylhexanal!)
g) C6H5CHO benzaldehyde
h) ClC6H5CHO 4-chlorobenzaldehyde
i) HOC6H5CHO 2-hydroxybenzaldehyde.
j) OHCCH2CH(CH3)CH2C(C2H5)(CH3)CH2CH2CH3 5-ethyl-3,5-dimethyloctanal
k) OHCCH2CH2CH2CH2CHO hexanedial (the final e is retained in dial names)
F
CHO H3C CHO CHO
CH3 H3C
benzaldehyde 2 , 4 - dimethylbenzaldehyde 3 - fluoro - 5 - methylbenzaldehyde
Note: Commas and hyphens should be used properly just like in naming of other
organic compounds.
If two or more substitutes present: e.g. 2 methyl groups–dimethyl- or 3 ethyl groups-
triethyl etc.
In alphabetizing, a prefix denoting the number of times a substituent is found (di, tr.) is
disregarded, but m for methyl.
With 2 or more branches attached to a parent chain, more prefixes are added to the parent
name. Use alphabetical order – ethyl, methyl, propyl etc.
Nomenclature Priority
4
ALDEHYDES and KETONES
1.1 Introduction-The Carbonyl Bond (C=O)
The geometry of the carbonyl is determined by the sp 2–hybridized carbon. The molecule
is planar around the trigonal sp2 carbon.
C O C O
a carbonyl bond polar
The C==O bond is composed of a sigma (σ) bond and a pi (π) bond. The oxygen atom
has two pairs of unshared electrons. Because of the significant difference in
electronegativity between carbon and oxygen, the carbonyl group is polarized in such a
way that oxygen is slightly negative (δ-) and the carbon slightly positive (δ+). The
carbonyl group is more polar than the C-O bond of the alcohol or ether molecule. The pi
electrons in the carbonyl are drawn more easily to the electronegative oxygen than the
sigma electrons in the C-O bond. Keeping polarity in mind will often be helpful in
understanding the chemical reactivity of carbonyl containing compounds.
The name aldehyde is derived from alcohol dehydrogenated and may be represented as
follows –CHO or in structural form as below.
O
C
H
Aldehydes have the general formula R-CHO and the simplest, methanal, is prepared by
passing methanol over heated copper metal.
O
CH3OH + Cu metal / high temp. H2 +H C
H
alcohol: methanol aldehyde: methanal
Aldehydes are very reactive compounds because they contain an aldehyde group or more
generally the carbonyl group -C==O. An aldehyde has a hydrogen atom attached to the
carbonyl group while a ketone has no hydrogen attached but two organic groups, R,
which could be alkyl or aryl. Much of the chemistry of aldehydes and ketones involves
addition reaction to the pi bond. Because of the presence of a hydrogen atom on the
carbonyl group of aldehydes, they are easily oxidised while ketones are oxidised with
difficulty. Aldehydes are more reactive towards nucleophilic addition.
1
, O O
C O unshared electrons C
C
R H R R
carbonyl bond aldehyde ketone
1.2 Physical Properties
Aldehydes and ketones have higher boiling points than non-polar compounds of similar
molecular weight. This is because they are polar and undergo intermolecular dipole-
dipole attractions. To a limited extent aldehydes and ketones can solvate ions. An
example is sodium iodide (NaI), which is soluble in acetone (CH3COCH3).
CH3 O OH
CH3CHCH3 CH3CCH3 CH3CHCH3
bp 12 0 bp 56 0 bp 82 0
O H H
O
O H O R C O
C R H
R C R H O R C
R
H
H H
O
Because of unshared electrons on the oxygen, a carbonyl compound can form hydrogen
bonds (but not with another carbonyl compound unless it has an acidic hydrogen
available for hydrogen bonding). Aldehydes and ketones of low molecular weight, those
with four or fewer carbons, can form hydrogen bonds with water and alcohols and are
soluble.
1.3 Aldehydes
Methanal (formaldehyde): This is a colourless gas with a sharp penetrating odor (i.e.
irritating to mucus membrane). It is readily soluble in water. When concentrated
methanal solution is evaporated it forms a solid polymer known as paraformaldehyde (an
acetal).
2
,A solution of 37% methanal and 7-15% methanol in water is known as formalin.
Methanal is conveniently shipped or stored as formalin or as a solid polymer
(paraformaldehyde) or trimer - trioxane (mp 620C).
Formalin acts as a disinfectant (a germicide).
Is used in embalming fluid.
Is used as a preservative of various tissues i.e. hardens tissues.
Formaldehydes gas and the polymer paraformaldehyde are used extensively as
insecticides, fumigating agents and antiseptics. Large quantities of formaldehyde are used
in the manufacture of synthetic (lightweight) resins (e.g. bakelite foam) and the syntheses
of other organic compounds.
HCHO + H2O heat
(formalin)
O heat CH2OCH2OCH2OCH2O
H2 heat C
paraformaldehyde
C H H
O O evaporate
methanal (formaldehyde)
H2C CH2
O
trioxane
Ethanal bp 200 (acetaldehyde) is also stored and shipped in a cyclic trimer paraldehyde
or as a tetramer metaldehyde.
It is a colourless liquid and has a characteristic sharp odor
Produced by oxidation of ethanol or hydration of ethane.
Ethanal is used in the production of ethanoic acid (acetic acid, vinegar), ethyl
acetate, synthetic rubber, etc.
Paraldehyde, (mp 125 o C; polymer of ethanol) an acetal (cyclic ether), is more
stable than ethanol and serves as a source of the latter compound when heated.
When three molecules of ethanal is heated with a trace of acid paraldehyde is
formed.
Paraldehyde–used to depress the central nervous system and to desensitize the
gums of the mouth against heat or cold.
It is an effective hypnotic or sleep producer but has been replaced by other drugs
since it has an irritating odor and an unpleasant taste.
CH3
H3C O CH3 H
C O H
C C O heat
H H heat O C
O O CH3C CH3
C H H3C C O
H3C H ethanal H O C
acid
H CH3
0
paraldehyde mp 125 C 0
metaldehyde mp 246 C
1.4 Nomenclature of Aldehydes
3
, 1) Pick the longest carbon chain containing the aldehyde or ketone group.
2) Number the chain so that the terminal aldehyde is always in position 1.
3) Locate and number the branched alkyl groups on the main chain.
4) For aldehydes change the alkane name by changing the “e” to “al”
Structure IUPAC common name.
a) HCHO methanal formaldehyde
b) CH3CHO ethanal acetaldehyde
c) CH3CH2CHO propanal propionadehyde
d) CH3CH2CH2CHO butanal buytraldehyde
e) CH3CH2CH(CH3)CHO 2–methylbutanal
f) CH3CH2CH(CH2CH2CH2CH3)CH(CH3) CHO 3-ethyl-2-methylhexanal
(NOT 2-methyl-3-ethylhexanal!)
g) C6H5CHO benzaldehyde
h) ClC6H5CHO 4-chlorobenzaldehyde
i) HOC6H5CHO 2-hydroxybenzaldehyde.
j) OHCCH2CH(CH3)CH2C(C2H5)(CH3)CH2CH2CH3 5-ethyl-3,5-dimethyloctanal
k) OHCCH2CH2CH2CH2CHO hexanedial (the final e is retained in dial names)
F
CHO H3C CHO CHO
CH3 H3C
benzaldehyde 2 , 4 - dimethylbenzaldehyde 3 - fluoro - 5 - methylbenzaldehyde
Note: Commas and hyphens should be used properly just like in naming of other
organic compounds.
If two or more substitutes present: e.g. 2 methyl groups–dimethyl- or 3 ethyl groups-
triethyl etc.
In alphabetizing, a prefix denoting the number of times a substituent is found (di, tr.) is
disregarded, but m for methyl.
With 2 or more branches attached to a parent chain, more prefixes are added to the parent
name. Use alphabetical order – ethyl, methyl, propyl etc.
Nomenclature Priority
4
