Carbonyl Compounds

CARBONYL COMPOUNDS
Carbonyl group
Some common classes of carbonyl group

Naming Aldehydes
Replace -e with -al.

The aldehyde carbon is number 1.
For dialdehydes the location numbers for
both carbonyls are omitted, the ending –
dial is added to the end of the parent chain
name.
Naming Ketones
Replace -e with -one. Indicate the position

of the carbonyl with a number.
The carbonyl carbon MUST BE the lowest
number.
For cyclic ketones the carbonyl carbon is
assigned the number 1.
For diketones both carbonyls require a
location number. The ending -dione added
to the end of the parent chain.
Naming Ketones
Replace -e with -one. Indicate the position

of the carbonyl with a number.
The carbonyl carbon MUST BE the lowest
number.
For cyclic ketones the carbonyl carbon is
assigned the number 1.
For diketones both carbonyls require a
location number. The ending -dione added
to the end of the parent chain.
When and aldehyde or ketone is present in a molecule
which also contains an alcohol, in the case of alcohols
the OH is named as a hydroxy substituent.
Example:
pentanal 3-methylbutanal
3-methyl-2-butanone 3-hexanone
Practice !
butanal
2-hydroxycyclopentanone
2,3-pentanedione 1,3-cyclohexanedione
4-hydoxy-3-methyl-2-butanone 3-methyl-2-hepten-4-one
3-oxobutanal
3-bromocyclohexanecarbaldehyde
butanedial 2-methyl-3-hexenal
Naming Aldehydes and Ketones in the Same Molecule
As with many molecules with two or more functional

groups, one is given priority while the other is named as
a substituent. Because aldehydes have a higher priority
than ketones, molecules which contain both functional
groups are named as aldehydes and the ketone is
named as an "oxo" substituent.
More Practice!
c
a
b d
e g
f
Preparation of aldehydes and ketones
They can be prepared from the oxidation of primary and
secondary alcohols
1 Oxidation of a primary alcohol
The oxidising agent used is a solution of potassium

dichromate(VI), (K2Cr2O7) acidified with dilute sulfuric
acid. To make the aldehyde, the primary alcohol is heated
gently with acidified dichromate solution. The reaction
mixture turns green as the orange dichromate ions, Cr2O7
2–
(aq), are reduced to green Cr3+(aq) ions.
The formed aldehyde has
a lower boiling point than
the alcohol and can
therefore be distilled off
as soon as it forms.
If the aldehyde formed is not distilled off,

further refluxing with excess oxidising agent will oxidise
the aldehyde to a carboxylic acid
2 Oxidation of a secondary alcohol
The oxidising agent used is a solution of potassium

dichromate(VI), acidified with dilute sulfuric acid.
To produce a ketone, this oxidising agent must be heated
with a secondary alcohol. The ketone formed cannot be
further oxidised, even if we reflux the reaction mixture and
add excess oxidising agent. Therefore we do not need to
distil out the ketone product immediately.
Reduction of aldehydes and ketones
Chemical reduction of an aldehyde or ketone produces an

alcohol.
The reducing agent used is usually an aqueous alkaline

solution of sodium tetrahydridoborate, NaBH4, or lithium
tetrahydridoaluminate, LiAlH4, in dry ether.
The reduction reaction is carried out by either:
■■ warming the aldehyde or ketone with an aqueous alkaline
solution of NaBH4
■■ adding LiAlH4 dissolved in a dry ether, such as diethyl
ether, at room temperature. The organic solvent has to be dry
because lithium tetrahydridoaluminate is a more powerful
reducing agent than sodium tetrahydridoborate and reacts
vigorously in water.
Nucleophilic addition with HCN
Aldehydes and ketones both undergo addition reactions with
hydrogen cyanide, HCN.
In this case, addition of HCN takes place across the C=O bond.
The attack is by a nucleophile, not an electrophile.
Example:
nucleophilic addition of propanal with HCN. The HCN is
generated in situ (in the reaction vessel) by the reaction of
sodium cyanide, NaCN, and dilute sulfuric acid.
The nitrile CN group can then be easily:
■■ hydrolysed to a carboxylic acid
■■ reduced to an amine.
The hydrolysis can be carried out by refluxing with dilute

hydrochloric acid:
CN + H+ + H2O  COOH + NH4+
The reduction of the nitrile group to an amine can be carried
out using sodium and ethanol:
CN + 4[H]  CH2NH2
Mechanism of nucleophilic addition
As the name of the reaction suggests, nucleophilic addition
involves the addition of a nucleophile to a molecule.
The nucleophile can be a negatively charged ion such as

cyanide (CN- ) hydride (H-), hydrogen sulfite ion (SO3H-)
or it can be a neutral molecule such as water, alcohol,
ammonia, hydrazine (NH2-NH2)
Neutral nucleophiles where nitrogen or oxygen is the

nucleophilic center are relatively weak nucleophiles, and an
acid catalyst is usually required.
Nucleophilic addition to a carbonyl group
Mechanism of nucleophilic addition
First step
The carbonyl group, C=O, is polarised due to the high
electronegativity of the oxygen atom. The electrons in the
C=O bond are drawn nearer to the O atom, giving it a partial
negative charge O and leaving the C atom with a partial
positive charge. This makes the C atom open to attack by a
nucleophile, such as the cyanide ion, CN–.
Second step
The negatively charged intermediate formed in the first

step in the mechanism is highly reactive and quickly
reacts with an H+ ion (from HCN, from dilute acid or from
water present in the reaction mixture).
PRACTICE
a. Name the organic product that would be formed in
the nucleophilic addition of HCN to:
(i) ethanal
(ii) propanone.
b Use diagrams and curly arrows to describe the

mechanism of the reaction in part a .
MORE PRACTICE
Outline the mechanisms and products of the following
reactions:
MORE PRACTICE
Testing for aldehydes and ketone
1. Testing with 2,4-DNPH
 The presence of a carbonyl group in an aldehyde or ketone

can be easily tested for by adding a solution of 2,4-
dinitrophenylhydrazine (often abbreviated to 2,4-DNPH).
 If an aldehyde or ketone is present, a deep-orange
precipitate is formed.
2,4-dinitrophenylhydrazine
 The reaction of an aldehyde or ketone with 2,4-
dinitrophenylhydrazine is an example of a condensation
reaction.
2. Testing with Tollens’ reagent
 Tollens’ reagent is an aqueous solution of silver nitrate in
excess ammonia solution, sometimes called ammoniacal
silver nitrate solution.
 The silver ions, Ag+ , in the solution act as a mild
oxidising agent. When warmed, the Ag+ ions will oxidise
an aldehyde to form a carboxylate ion. The Ag+ ions
themselves are reduced to silver atoms. The silver atoms
form a ‘mirror’ on the inside of the tube, giving a positive
test for an aldehyde
The ‘before’ and ‘after’
observations when Tollens’
reagent is warmed with an
aldehyde, such as ethanal.
 There will be no change observed when a ketone is warmed

with Tollens’ reagent as no redox reaction takes place. It
remains a colourless mixture in the test tube
3. Testing with Fehling’s solution
 Fehling’s solution is an alkaline solution containing

copper(II) ions.
 When warmed with an aldehyde, the Cu2+ ions act as an
oxidising agent. The aldehyde is oxidised to a carboxylate
ion while the Cu2+ ions are reduced to Cu+ ions.
 The clear blue Fehling’s solution turns an opaque red colour
as a precipitate of copper(I) oxide
 Ketones are not oxidised, so the Fehling’s solution remains
blue when warmed.
The ‘before’ and ‘after’ observations
when Fehling’s solution is warmed
with an aldehyde, such as ethanal.
 Ketones are not oxidised, so the Fehling’s solution remains

blue when warmed.
Test for the presence of a CH3CO group in aldehydes and
ketones.
 Step 1
the carbonyl compound is halogenated – the three hydrogen
atoms in the CH3 group are replaced by iodine atoms
 Step 2
the intermediate is hydrolysed to form the yellow precipitate
of tri-iodomethane, CHI3
Tri-iodomethane (iodoform)
The tri-iodomethane test can also be

used to identify the presence of a
secondary alcohol.
Infrared spectroscopy
In infrared spectroscopy a sample being analysed is irradiated
(exposed to radiation) with electromagnetic waves in the
infrared region of the electromagnetic spectrum.
We cannot be too specific when quoting the characteristic

absorption frequency of a bond. Therefore, we can only
identify the presence (or absence) of different functional
groups from the absorbance pattern on an infrared spectrum.
broad
free alcohol: when the alcohol is dissolved in a

solvent.
The infrared spectrum
of ethanol, CH3CH2OH.
O-H
The infrared spectrum of

ethanoic acid, CH3COOH.
O-H
C=O
C=O C-O
The infra-red spectrum of ethyl ethanoate, CH3COOCH2CH3

PRACTICE
Which infra-red spectrum is most likely to be produced by

ethanal? Give three reasons for your choice.

Carbonyl Compounds

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CARBONYL COMPOUNDS

Some common classes of carbonyl group

Replace -e with -al.

Replace -e with -one. Indicate the position

Replace -e with -one. Indicate the position

As with many molecules with two or more functional

1 Oxidation of a primary alcohol

The oxidising agent used is a solution of potassium

If the aldehyde formed is not distilled off,

The oxidising agent used is a solution of potassium

Chemical reduction of an aldehyde or ketone produces an

The reducing agent used is usually an aqueous alkaline

The hydrolysis can be carried out by refluxing with dilute

The nucleophile can be a negatively charged ion such as

Neutral nucleophiles where nitrogen or oxygen is the

The negatively charged intermediate formed in the first

b Use diagrams and curly arrows to describe the

 The presence of a carbonyl group in an aldehyde or ketone

 There will be no change observed when a ketone is warmed

 Fehling’s solution is an alkaline solution containing

 Ketones are not oxidised, so the Fehling’s solution remains

The tri-iodomethane test can also be

We cannot be too specific when quoting the characteristic

free alcohol: when the alcohol is dissolved in a

The infrared spectrum of

The infra-red spectrum of ethyl ethanoate, CH3COOCH2CH3

Which infra-red spectrum is most likely to be produced by

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