46. The formation of solid derivatives of aldehydes and ketones

174
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46. The formation of solid derivatives of
aldehydes and ketones using 2,4dinitrophenylhydrazine (Brady’ s Test)
Topic
Aldehydes and ketones.
Level
Post-16.
Timing
15 min.
Description
In this experiment various aldehydes and ketones are mixed with
2,4-dinitrophenylhydrazine solution in a well-plate to form solid derivatives. Two
alcohols are also included to show that they do not react.
Apparatus (per group)
▼
One plastic well-plate (24 well size, eg Sigma ref: M 9655).
Chemicals (per group)
Solutions contained in plastic pipettes, see p. 2
▼
Ethanol
▼
Propanone
▼
p-Methoxybenzaldehyde (or other aromatic aldehyde or ketone)
▼
Methanol
▼
Ethanal
▼
Solution of 2,4-dinitrophenylhydrazine (prepared as described below) which
requires 24 hours to dissolve completely.
Preparing a solution of dinitrophenylhydrazine
1.
Weigh out 0.5 g of 2,4-dinitrophenylhydrazine and dissolve in 10 cm3 of
concentrated sulphuric acid in a 100 cm3 beaker. (Leave overnight to allow the
compound to dissolve completely.)
2.
Add this solution carefully to a solution of 15 cm3 of ethanol and 5 cm3 of
deionised water. This gives the yellow-brown stock solution.
The solution can be used for about 10 days but it gradually deteriorates as a
precipitate forms.
This procedure is based on that described in L. Harwood and R. Moody,
Experimental organic chemistry – principles and practice, p 242. Oxford: Blackwell
Scientific, 1989.
Microscale Chemistry
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Observations
Compound
Observation
Ethanal
Immediate yellow precipitate
Propanone
Yellow, crystalline precipitate
forms slowly (after 1-2 minutes)
p-Methoxybenzaldehyde
(or other aromatic
aldehyde or ketone)
Immediate red precipitate
Methanol
No change
Ethanol
No change
The reaction between propanone and dinitrophenylhydrazine
O2N
H3C
C
O +NH2 — NH
NO2
H3C
O2N
H3C
C
N — NH
NO2 + H2O
H3C
Safety
Students must wear eye protection.
It is the responsibility of the teacher to carry out a risk assessment.
82
Microscale Chemistry
THE ROYAL
SOCIETY OF
CHEMISTRY
46. The formation of solid derivatives
of aldehydes and ketones using
2,4-dinitrophenylhydrazine (Brady’ s
Test)
In this experiment you will be adding various liquid aldehydes and ketones to
2,4-dinitrophenylhydrazine to form solid derivatives. To show that the reaction does
not occur with alcohols you will also do the test with methanol and ethanol.
Instructions
1.
In a well-plate, add 10 drops of 2,4-dinitrophenylhydrazine solution to each of
the wells A1–A5 (see diagram).
2.
Carefully add three drops of ethanal to well A1 (ethanal is very volatile!).
3.
Repeat adding three drops of the other liquids to wells A2–A5.
4.
Observe any changes over the next few minutes.
1
2
3
4
5
6
A
B
C
D
Questions
1.
Can you write equations for any reactions occurring?
2.
Why do you think that these reactions serve such a useful purpose in
identifying aldehydes and ketones?
3.
Aldehydes and ketones will also form derivatives with hydrazine itself. What is
the purpose of using 2,4-dinitrophenylhydrazine in this experiment instead of
hydrazine?