COMPOUNDS CONTAINING A CARBONYL GROUP
ALDEHYDES & KETONES
Aldehydes and ketones both contain a carbonyl group. The carbon-oxygen bond in
the carbonyl group is polar, because oxygen is more electronegative than carbon.
The less electronegative carbon atom forms the positive end of the dipole.
-
+
 
C=O
1. Nucleophilic Addition
The electron-deficient carbon atom is susceptible to attack by nucleophiles. In
general, substitution occurs at sp3 hybridised carbon atoms, and addition takes
place at sp2 hybridised carbon atoms. Since the carbonyl carbon atom is sp2
hybridised, aldehydes and ketones can undergo NUCLEOPHILIC ADDITION.
a) Reaction with hydrogen cyanide, HCN
Aldehydes and ketones react with hydrogen cyanide to form the corresponding
hydroxynitrile. For example, when ethanal reacts with HCN, 2-hydroxypropanenitrile
is formed, which contains an asymmetric carbon atom.
CH3CHO + HCN
CH3C*H(OH)CN
The nucleophile is the cyanide ion, CN-, which attacks the + carbon of the carbonyl
group. This forms an oxyanion, which subsequently accepts a proton.
H3C
-
H3C
C
H
O
:CN
-
O:
H
+
H3C
OH
C
C
CN
H
CN
H
2-hydroxypropanenitrile
The carbonyl group is planar, and therefore the attack occurs with equal probability
from above or below this plane. Therefore, both enantiomers are formed in equal
amounts, giving a racemate.
-
:CN
CN
H
C*
OH
CH3
H
CH3
C
O
-
:CN
OH
H
C*
CN
CH3
The reaction of aldehydes and ketones with HCN is useful in synthesis, because it
increases the length of the carbon chain by one carbon atom.
TOPIC 13.5: COMPOUNDS CONTAINING A CARBONYL GROUP 1
b) Reaction with sodium tetrahydridoborate(III), NaBH4
Aldehydes and ketones are reduced by sodium tetrahydridoborate(III) to the
corresponding alcohol. Aldehydes are reduced to primary alcohols and ketones to
secondary alcohols.
For example, when ethanal reacts with NaBH4, ethanol is formed.
CH3CHO + 2[H]
CH3CH2OH
When propanone reacts with NaBH4, propan-2-ol is formed.
CH3COCH3 + 2[H]
CH3CH(OH)CH3
Sodium tetrahydridoborate(III) acts as a source of the hydride ion H -. This is a
nucleophile and attacks the + carbon of the carbonyl group. This forms an oxyanion,
which subsequently accepts a proton from water or an acid.
H3C
C
H
-
H3C
O
:H
-
O:
H
+
H3C
C
C
H
OH
H
H
ethanol
H
NaBH4 is used in aqueous ethanol. A similar reduction takes place with the more
powerful reducing agent, lithium tetrahydridoaluminate(III), LiAlH4. This requires
scrupulously dry conditions and is usually used in tetrahydrofuran which has been
dried with sodium metal.
This reduction is specific and will not reduce any alkene double bonds which are
present in the same molecule as the aldehyde or ketone. (see CHM3)
Catalytic Hydrogenation
Reduction of aldehydes and ketones to alcohols can also be brought about by
catalytic hydrogenation. The aldehyde or ketone is treated with hydrogen gas in the
presence of a finely-divided catalyst , such as Ni or Pt.
CH3CHO + H2
CH3CH2OH
This reduction is not specific and will reduce any alkene double bonds which are
present in a molecule as well as an aldehyde or ketone.
2. Oxidation
(see CHM3)
Aldehydes are readily oxidised by acidified potassium dichromate(VI) to the
corresponding carboxylic acid. The colour change is from orange (Cr 2O72-) to green
(Cr3+).
CH3CHO + [O]
CH3COOH
Ketones are not oxidised under these conditions.
Since aldehydes are readily oxidised, they behave as reducing agents, whereas
ketones do not. This difference allows aldehydes and ketones to be distinguished.
TOPIC 13.5: COMPOUNDS CONTAINING A CARBONYL GROUP 2
Tollen’s Reagent
Tollen’s reagent is made by adding ammonia solution dropwise to a solution of silver
nitrate until the initial brown precipitate of Ag2O just re-dissolves to give a clear
colourless solution. This solution contains the ion, [Ag(NH3)2]+. When heated with
aldehydes, the complex ion is reduced to silver, which is deposited as a mirror on the
walls of the test tube.
RCHO + 2[Ag(NH3)2]+ + 3OH-
RCOO- + 2Ag + 4NH3 + 2H2O
Fehling’s Solution
Fehling’s solution is made by adding an alkaline solution of potassium sodium
tartrate to copper(II) sulphate solution. This forms a deep blue solution containing a
copper(II)-tartrate complex ion. When aliphatic aldehydes are heated with Fehling’s
solution, the complex ion is reduced to copper(I) oxide, which appears as a brick-red
precipitate.
RCHO + 2Cu2+ + 5OH-
RCOO- + Cu2O + 3H2O
CARBOXYLIC ACIDS
Carboxylic acids contain the carboxyl group COOH.
C=O
O-H
A carboxyl group can only occur at the end of a carbon chain. The presence of a
carboxyl group is indicated in the name of a compound by the suffix -oic acid.
e.g. CH3CH2COOH propanoic acid
Preparation
Carboxylic acids can be prepared:
 by the oxidation of aldehydes or primary alcohols
RCH2OH + [O]
RCHO + H2O
RHCO + [O]
RCOOH

by the hydrolysis of nitriles
RCN + 2H2O
RCOOH + NH3
Acidity
Carboxylic acids are weak acids and dissociate only to a small extent in water.
RCOOH
RCOO- + H+
In general terms, any structural feature which helps to delocalise the charge on the
anion formed from the acid will stabilise it. This has the effect of moving the above
equilibrium further to the r.h.s. and increasing the strength of the acid. Carboxylic
acids are stronger acids than alcohols, because a mesomeric effect in the
carboxylate anion delocalises the charge.
..
O-
O
C
O
..
O
or
C
O
TOPIC 13.5: COMPOUNDS CONTAINING A CARBONYL GROUP 3
C O
Carboxylic acids, like most acids, will react with metal carbonates (or
hydrogencarbonates) to form carbon dioxide and with reactive metals, such as
magnesium, to form hydrogen.
2CH3COOH + Na2CO3
2CH3COONa + H2O + CO2
2CH3COOH + Mg
(CH3COO)2Mg + H2
Solubility and Boiling Point
Low Mr carboxylic acids are very soluble in water, because the carboxyl group can
form hydrogen bonds with water. However, if the length of the carbon chain is
increased, the solubility gradually decreases.
The presence of hydrogen bonding between carboxylic acid molecules is responsible
for their relatively high boiling points.
ESTERS
Ester contain the functional group COOR, where R represents an alkyl group. They
are named as alkyl derivatives of a carboxylic acid. For example:
CH3CH2COOH
propanoic acid
(carboxylic acid)
CH3CH2COONa
sodium propanoate
(salt)
CH3CH2COOCH2CH3
ethyl propanoate
(ester)
Esters are formed by the reaction of a carboxylic acid with an alcohol in the presence
of a strong acid catalyst, such as concentrated sulphuric acid. For example:
CH3COOH + CH3CH2OH
CH3COOCH2CH3 + H2O
The reaction is reversible. It is driven to completion by using a small excess of the
carboxylic acid and by distilling out the water from the reaction mixture as it is
formed.
Uses of Esters
1. As solvents
Esters do not have any free OH groups and therefore they are unable to form
hydrogen bonds, either with other ester molecules or with water. This means that:
 the boiling points of esters are lower than the boiling points of carboxylic acids
of similar Mr
 esters are almost insoluble in water
Ester molecules are, however, polar. They are useful solvents for polar organic
compounds and, because of their fairly low boiling points, can easily be removed.
This leads to their use in printing inks, lacquers and glues.
For example, butyl ethanoate (see practical) is used as a solvent for nitrocellulose.
Ethyl ethanoate is used as a solvent in Evostik.
2. As plasticisers
The intermolecular forces in thermosoftening plastics are weak, and this allows the
plastic to be easily melted and re-shaped. Flexibility in plastics, however, requires the
individual polymer chains to be able to move past each other easily.
TOPIC 13.5: COMPOUNDS CONTAINING A CARBONYL GROUP 4
This movement occurs easily with, for example, poly(ethene), whose molecules are a
long smooth sausage shape. However, the more irregular shape of
poly(chloroethene) molecules, caused by the presence of a large chlorine atom on
every second carbon atom, makes the molecules lock together. The result is that
poly(chloroethene) or p.v.c. is a rigid plastic. Incorporating a plasticiser into this
plastic holds the polymer chains further apart, allowing them to move past each other
more easily. The plastic, therefore, becomes more flexible. The plastic can contain
up to 50% by mass of plasticiser. The esters of benzene-1,4-dicarboxylic acid are
widely used as plasticisers for p.v.c. For example:
CH2CH3
COOCH2CH(CH2)3CH3
COOCH2CH(CH2)3CH3
CH2CH3
The plasticisers have fairly high boiling points and are not particularly volatile, but
over time they may evaporate from the plastic, causing it to become stiffer and more
brittle.
3. As Food Flavourings
Many aliphatic esters have fruity smells and are used as artificial fruit flavours.
Natural flavours are caused by a complex mixture of compounds, and therefore the
artificial substitutes are rarely more than an approximation of the real thing. Two
examples of esters used as flavourings are:
 pentyl ethanoate (pear drops)
 octyl ethanoate (orange)
Hydrolysis of Esters
1. Making soap
When an ester is hydrolysed, it forms an alcohol and a carboxylic acid. The reaction
can be brought about by heating the ester with either dilute aqueous hydrochloric
acid (acid-catalysed hydrolysis) or dilute aqueous sodium hydroxide (alkali-catalysed
hydrolysis or saponification). The acid-catalysed route is reversible, the alkalicatalysed route is not and therefore tends to be preferred.
CH3COOCH2CH3 + NaOH
CH3CH2OH + CH3COONa
Natural oils and fats are esters of propane-1,2,3-triol (glycerol) and long chain
carboxylic acids (fatty acids), which typically have 12 to 20 carbon atoms. Commonly
occurring fatty acids are:
octadecanoic acid
CH3(CH2)16COOH
which is saturated and is found in tallow and other animal fats
octadec-9-enoic acid
CH3(CH2)7CH=CH(CH2)7COOH
which is unsaturated and is found in plant oils, such as olive oil
When fats and oils are heated with sodium hydroxide solution, they are hydrolysed to
form glycerol and a mixture of the sodium salts of the component fatty acids. These
salts are soaps. (Saponification means ‘soap making’)
TOPIC 13.5: COMPOUNDS CONTAINING A CARBONYL GROUP 5
CH2O.CO.R
CHO.CO.R
CH2OH
+ 3NaOH
CH2O.CO.R
CHOH
+ 3RCOONa
CH2OH
Glycerol is a viscous, colourless liquid with a slightly sweet taste. It has three O-H
bonds and can, therefore, hydrogen bond easily to other molecules, such as water.
Its ability to attract water is put to use in the manufacture of cosmetics and glues. It is
used in food and is added to wine to adjust its sweetness and viscosity.
2. Making biodiesel
Biodiesel is a fuel made from vegetable oils but can also be made from animal fats. It
is made by heating the oils (triesters) with methanol and sodium hydroxide (catalyst).
The products are long chain methylesters of fatty acids (biodiesel) and propane1,2,3-triol.
CH2O.CO.R
CHO.CO.R
CH2O.CO.R
CH2OH
+ 3CH3OH
CHOH
CH2OH
TOPIC 13.5: COMPOUNDS CONTAINING A CARBONYL GROUP 6
+ 3RCOOCH3