The Carbonyl Compounds  Aldehydes and Ketones
Aldehydes and Ketones both have the general formula, CnH2nO and both contain the carbonyl group,
Aldehydes have an alkyl (or aryl) group and a hydrogen atom attached to the carbonyl carbon, whereas
ketones have two alkyl (or aryl) groups attached to the carbonyl carbon atom.
Methanal, the first aldehyde, has all of its atoms lying in the same plane. All other aldehydes and ketones
have the three atoms around the carbonyl carbon lying in one plane.
ALDEHYDES - at least one hydrogen atom attached to the carbonyl
carbon atom
KETONES - 2 carbon atoms attached
Some examples of aldehydes and ketones are shown below : -
Nomenclature
Aldehydes • look for the longest chain of C atoms containing the carbonyl group
equivalent alkane name and add ‘AL’
• remove ‘E’ from the
• substituents are numbered based on the C with the O being number 1.
Ketones • look for the longest chain of C atoms containing the carbonyl group
equivalent alkane name and add ‘ONE’.
• remove ‘E’ from the
• if necessary, the position of the C=O is given (lower number counting from one end)
• substituents are numbered based on the number allocated to the C in the C=O
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Distinguishing between Aldehydes and Ketones
Most of the reactions of Aldehydes and Ketones are similar and are due to the carbonyl functional group.
They differ in their ability to be oxidised and this is the reaction that is used to distinguish them.
Aldehydes are easily oxidised to carboxylic acids but ketones are difficult to oxidise. This is because
aldehydes have a hydrogen atom attached to the carbonyl carbon atom but ketones do not. The oxidising
agent used to distinguish them must be mild because powerful oxidising agents can oxidise ketones by
rupturing the carbon – carbon bond. Mild oxidising agents will not affect ketones but will affect
aldehydes.
Tollens reagent is a solution of the complex ion [Ag(NH3)2] + , and is made by adding aqueous ammonia to a
solution of silver nitrate. A brown precipitate of silver oxide appears which redissolves in excess
ammonia. (Complex ions are often quite soluble)
2 Ag +
+
2 NH4 OH
Ag2 O
Ag2 O
+
4 NH4 OH
2 [Ag(NH3)2] +
+
H2 O
+
+ 2 OH
2 NH4+

+
3 H 2O
The Ag+ in the [Ag(NH3)2] + complex ion is very easily reduced to silver metal and so when a few drops of an
aldehyde are added to a few cm3 of tollens reagent in a perfectly clean test tube and carefully warmed,
metallic silver is deposited on the walls of the test tube as a mirror. Ketones have no effect.
Ag+
+
e
Ag
A safety aspect of this particular reaction is that the product mixture must be disposed of as soon after
the reaction as possible because Tollens Reagent deteriorates rapidly forming an explosive complex.
Fehling’s Solution or Benedict’s Solution is a solution containing a deep blue complex copper (II) ion. In
this complex state, it is very easy to reduce the Cu 2+ ion to Cu+ that is produced in the form of red
insoluble copper (I) oxide (Cu2O).
CH3CHO
+
2 Cu2+ + 4 OH
CH3COOH
+
Cu2O + 2 H2O
Warming a mixture of a few cm3 of Fehling’s solution or Benedict’s solution, to which has been added a few
drops of an aldehyde, will result in the formation of a red or a green precipitate of copper (I) oxide.
Ketones fail to react.
RCHO
CH3CHO(l) +
+
[O]
[O]
RCOOH
CH3COOH(l)
Distinguishing between Primary, Secondary and Tertiary alcohols
Separate samples are warmed with a few drops of acidified potassium dichromate (VI). The primary and
secondary alcohols react showing a colour change from orange to green. The tertiary alcohol remains
unoxidised and there is no colour change. Therefore the tertiary alcohol can now be identified.
The primary alcohol is oxidised to an ALDEHYDE, whilst the secondary alcohol is oxidised to a KETONE.
The products of these two reactions should now be separated from their mixtures by distillation. A few
drops of tollen’s reagent or benedict’s solution added to the distillates and warmed should distinguish
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between the primary and the secondary alcohols. The distillate from the primary alcohol will show a
positive result (either a silver mirror or a red ppt) when warmed with tollen’s or benedict’s showing that an
ALDEHYDE had been formed as the alcohol oxidation product, whereas the secondary alcohol distillate will
show no result, showing that a KETONE was formed as the alcohol oxidation product.
The Nature of the C=O bond
The double bond between the carbon and the oxygen atoms consists of a sigma bond and a bond. The
difference between the bond in the alkene and in carbonyl compounds like aldehydes and ketones is that
the differences in electronegativities between the carbon and oxygen atoms makes the carbonyl double
bond polar. The oxygen atom attracts electrons in the bond towards itself as shown in the diagram
below.
Addition reactions across the double bond are a feature of the chemistry of aldehydes and ketones, but,
unlike the case with alkenes where the addition is characterised by electrophilic addition, carbonyl
compounds react by nucleophilic attack.
1.
Reduction of Carbonyl Compounds
a) Hydrogenation
This is exactly the same reaction as hydrogenation of alkenes. Both the carbon – carbon and the carbon –
oxygen double bonds are saturated in the same way by this process. The aldehyde or ketone is vaporised
and mixed with the correct amount of hydrogen and passed over a nickel catalyst at 200o C. (or platinum
at R.T.)
CH3 CO CH2 CH3
+
H2
Butanone
CH3 CH(OH) CH2 CH3
butan – 2 – ol ( a SECONDARY alcohol)
CH3 CH2 CH2 CHO
+
H2
Butanal
CH3 CH2 CH2 CH2 OH
butan – 1 – ol (a PRIMARY alcohol)
CH2 = CH CH2 CHO +
2 H2
CH3 CH2 CH2 CH2 OH
But – 3 – enal
ALDEHYDES are reduced to PRIMARY alcohols
whereas
KETONES
are reduced to
SECONDARY alcohols.
b) by reaction with sodium tetrahydridoborate (III) in methanol
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Nucleophilic Addition
NaBH4 is a reagent that adds hydrogen across the double bond of a carbonyl group by nucleophilic attack
on the carbonyl carbon atom. The mixture is then acidified and the intermediate formed is then converted
to the alcohol. This is the reverse of the oxidation reaction where the alcohol is oxidised by acidified
potassium dichromate to the aldehyde or the ketone.
Since the electron rich cloud around the double bond of an alkene would repel nucleophiles, this reagent is
specific to the double bond of the carbonyl group and leaves the alkene double bond untouched.
The mechanism involves the initial attack on the carbonyl carbon by the nucleophile, H, from the NaBH4.
Sodium tetrahydrido borate (III) is in aqueous or alcoholic solution. The water provides the proton to
attach onto the carbonyl oxygen. Overall, the reaction is
NUCLEOPHILIC ADDITION.
Aldehydes form primary alcohols, whilst ketones form secondary alcohols. A similar result can be obtained
if the aldehyde or ketone is first vaporised and then mixed with the correct volume of hydrogen and
passed over a heated nickel catalyst at 200 0C.
NaBH4 will NOT attack the carboncarbon double bond in an alkene because this bond is not polar.
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2. Another Nucleophilic Addition to Carbonyl compounds
The carbon – oxygen double bond is unsaturated and so is able to undergo addition reactions. The  electron cloud is unevenly distributed over the bond and is more concentrated over the oxygen atom
because of its electronegativity whereas it is evenly distributed over the carbon – carbon double bond in
alkenes. A feature of the reactivity of the alkene double bond is, therefore, electrophilic addition.
The nature of the carbon – oxygen double bond means that the initial attack on the carbonyl carbon atom is
nucleophilic attack on the  + carbon atom.
HCN, which is usually generated by adding dilute H2SO4 to KCN at R.T., will produce an hydroxynitrile.
2 – hydroxypropanenitrile
2 – hydroxypropanenitrile has a chiral or asymmetric carbon atom and therefore has optically active
isomers. In fact, all aldehydes (except methanal) and all ketones produce hydroxynitriles that have optical
activity. The product formed in these reactions is not optically active, but is a racemate or a racemic
mixture because attack by the cyanide ion occurs on either side of the carbonyl carbon with equal
probability (since the carbonyl group is planar).
From above
From below
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The hydroxynitrile compounds can then be converted into hydroxycarboxylic acids by refluxing them with
concentrated hydrochloric acid. (see haloalkanes)
H+
CH3 CH(OH)CN
CH3 CH(OH)COOH
2 – hydroxypropanoic acid
or lactic acid
Lactic acid found as an energy store in muscle tissue is the d – enantiomer and is optically active. Lactic
acid made in the laboratory from ethanal is optically inactive.
Identifying a Carbonyl compound – a Nucleophilic Addition – Elimination
Reaction
• reacts with carbonyl compounds (aldehydes and ketones)
• used as a simple test for aldehydes and ketones
• makes orange crystalline derivatives - 2,4dinitrophenylhydrazones
• derivatives have sharp, well-defined melting points
• also used to characterise (identify) carbonyl compounds.
A carbonyl compound will form yellow or orange crystals with BRADY’S
2,4-dinitrophenylhydrazine and is, itself, an orange solution in ethanol.
REAGENT
which is
The first initial reaction is an addition reaction of the 2,4-dinitrophenylhydrazine to the carbonyl group.
This is then followed by an elimination reaction where a molecule of water splits off from the adjacent ‘C’
and ‘N’ atoms leaving a C=N double bond. The product is a dinitrophenylhydrazone.
NO2
H
CH3−C
NO2
H
addition
:NH2 NH2
NO2
CH3−C NH NH2
O
OH
elimination
− H2O
NO2
H
CH3−C
N NH2
NO2
+
ethanal 2,4 – dinitrophenylhydrazone
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H2O
NO2
This reaction is used to tell if the compound is an aldehyde or a ketone, but it cannot be used to
distinguish between the two. For this, a further test such as Benedict’s Test, or Tollen’s Reagent, or
acidified Potassium Dichromate (VI) is required. (see p 2).
The production of orange or yellow crystals with 2, 4 – dinitrophenylhydrazine shows that the compound
being test is EITHER and aldehyde or a ketone. The aldehyde or ketone 2, 4 – dinitrophenylhydrazone
product can then be used to actually identify the exact aldehyde or ketone it has been derived from.
Every aldehyde or ketone 2, 4 – dinitrophenylhydrazone product has its own specific melting point and,
because these compounds can be very easily recrystallised from ethanol and obtained in a high state of
purity, their melting points can be determined very accurately and can be used to identify the original
carbonyl compound with a high degree of certainty.
The products can also be used to determine which isomer is obtained. For Example, the three isomeric
chlorophenylmethanals below have very similar van der Waal’s forces between their molecules and
therefore very similar boiling points. The 2, 4, dinitrophenylhydrazone products have quite widely
differing melting points and so the three compounds can be readily recognised.
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