STAGE 2 CHEMISTRY
Organic and Biological Chemistry
READING
Unit 4
The Essentials book pg 258-260 Q4.15, 4.16, 4.17
Aldehydes and Ketones:
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draw structural formulae for the alcohol given the structural formula of the aldehyde or
ketone from which it was produced
describe the optimum conditions for the oxidation of alcohols,
draw the structural formula of the oxidation product of an aldehyde in acidic conditions,
draw the structural formula of the oxidation product of an aldehyde in alkaline conditions,
describe how to distinguish between an aldehyde and a ketone using acidified dichromate
solution or Tollen’s reagent.
The Syllabus statement says:
Key Ideas
Intended Student Learning
Aldehydes and ketones are produced by the
oxidation of the corresponding primary and
secondary alcohols respectively. Aldehydes are
readily oxidised, and so must be distilled off from the
reaction mixture as they are formed.
Draw, given the structural formula of the
aldehyde or ketone, the structural formula of
the alcohol from which it could be produced by
oxidation, and describe the necessary reaction
conditions.
Aldehydes can be oxidised to form carboxylic acids
or, in alkaline solutions, carboxylate ions.
Draw the structural formula of the oxidation
product of a given aldehyde in either acidic or
alkaline conditions.
Ketones cannot readily be oxidised. This difference
in properties can be used to distinguish aldehydes
from ketones.
Describe how acidified dichromate solution and
Tollen’s reagent (ammoniacal silver nitrate
solution) can be used to distinguish between
aldehydes and ketones.
The key ideas relate to:
Key Idea 1
Aldehydes and ketones are produced by the oxidation of the corresponding primary and secondary
alcohols respectively.
Key Idea 2
When preparing aldehydes, the aldehyde is readily oxidised and must be distilled off from the
reaction mixture as it forms.
Key Idea 3
Aldehydes can be oxidised to form carboxylic acids or, in alkaline conditions, carboxylate ions.
Key Idea 4
Ketones cannot be readily oxidised. This difference in properties can be used to distinguish
aldehydes from ketones.
Page 1
Preparation
Common laboratory apparatus for organic chemistry practicals involve the use of quick-fit apparatus. The apparatus
has ground glass joints that enable it to be connected easily, in a number of variations, to perform a range of
functions.
You are required to be able to select apparatus and undertake the preparation of organic substances using
controlled oxidation, reflux, distillation and liquid-liquid extraction.
Aldehydes
The oxidation of primary alcohols can be used to prepare aldehydes. However, the oxidation must be carefully
controlled because aldehydes are easily oxidised to carboxylic acids.
Aldehydes are prepared using distillation apparatus, with a separating funnel used to provide controlled oxidation of
the alcohol. The oxidising agent, dichromate ions, is added drop by drop into a heated primary alcohol and acid
mixture. The alcohol is oxidised to the aldehyde which immediately boils off before it is further oxidised to the
carboxylic acid.
The aldehyde distils off first because it will have a lower boiling point than the alcohol.
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Ketones
Ketones are prepared by the oxidation of
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Ketones are not oxidised any further, so do not have to be distilled off as they form.
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Page 2
Oxidation of aldehydes: Some distinguishing chemical tests
When aldehydes are heated with an acidified solution of dichromate ions, they undergo oxidation to carboxylic acids.
RCHO
RCOOH
Cr2O72-
Cr3+
Aldehydes also undergo oxidation when heated with ammoniacal silver nitrate solution – commonly called Tollen’s
reagent. The oxidising agent is the silver diamine ion which reduced to metallic silver. Under certain conditions, the
silver can be made to deposit as a silver mirror on the inside surface of the reaction vessel.
The formation of a silver mirror is a positive test for the presence of an aldehyde group in an organic compound.
With Tollen’s reagent, the oxidation product is a carboxylate ion, RCOO -, not a carboxylic acid, because the oxidation
is performed under alkaline conditions.
Tollen’s reagent is a weak oxidising agent and will not oxidise an alcohol functional group in an organic molecule.
For example, how could we distinguish between samples of
propan-1-ol
propanal
propanone
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Page 3
SUPPORTING QUESTIONS – aldehydes and ketones
1
State whether the following compounds are aldehydes, ketones, both or neither.
i
CH3CH2CHO
______________________
ii
CH3CH2COOCH2CH3
______________________
iii
CH3CH2COOH
______________________
O
iv
C
H
______________________
C
CH2
O
v
HCCH2CH2CO
O
O
______________________
2
Name the first three compounds in Question 1.
3
i
Name the alcohol that would be oxidised by acidified dichromate solution to prepare butanal.
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ii
Name the other product that could be formed if the oxidation reaction was allowed to continue.
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iii
State the reaction conditions that would be used in the preparation of butanal that would prevent the
formation of the compound named in part ii.
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iv
Draw the structural formula and name the alcohol that would be oxidised by acidified dichromate
solution to prepare butanone.
v
Explain why the conditions that were necessary for the preparation of butanal would not be needed for
the preparation of butanone.
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Page 4
vi
4
Write an annotated equation for the preparation of butanal using acidified dichromate ions.
An organic compound, C3H6O, was mixed with ammoniacal silver nitrate solution in a test tube.
A silver mirror was formed on the inside wall of the test tube.
i
State the common name for ammoniacal silver nitrate solution.
____________________________________________
ii
Write the structural formula of the organic compound C3H6O and give its systematic name.
iii
Name and write the structural formula of the product formed from C 3H6O in the reaction.
iv
Ammoniacal silver nitrate solution contains silver ions Ag+, which are converted to silver.
Write the half-equation for this reaction.
Page 5
SUPPORTING QUESTIONS – aldehydes and ketones - SOLUTIONS
1
2
3
i
aldehyde
ii
neither
iii
neither
iv
both
v
aldehyde
i
propanal
ii
ethyl propanoate
iii
propanoic acid
i
butan-1-ol
ii
butanoic acid
iii
controlled oxidation by drop-wise addition of acidified dichromate ions and distillation of the
aldehyde as it forms.
iv
CH3
CH
CH2
CH3
OH
v
Once the butanone forms it is not further oxidised, so no other product id formed.
vi
CH3CH2CH2CH2OH
butan – 1 - ol
CH3CH2CH2CHO
H+/Cr2O72
butanal
-
4
i
Tollens reagent
ii
CH3CH2CHO
iii
CH3CH2COO-
iv
Ag+
+
e-
propanoate ion
Ag
Page 6