Shaken, not stirred: a schools test for aldehydes and ketones

Article abstract of DOI:10.1177/1747519819886491

A schools test for aldehydes and ketones in water at room temperature using test tubes has been developed in this laboratory using either phenylhydrazine hydrochloride or phenylhydrazine hydrochloride with NaOAc ^. 3H₂O. The role of one equivalent of a strong or weak acid which catalyses the reaction is discussed.

Full text of DOI:10.1177/1747519819886491

886491CHL0010.1177/1747519819886491Journal of Chemical ResearchPlater
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Journal of Chemical Research
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Shaken, not stirred: a schools test
for aldehydes and ketones
https://doi.org/10.1177/1747519819886491
DOI: 10.1177/1747519819886491
journals.sagepub.com/home/chl
M John Plater
Abstract
A schools test for aldehydes and ketones in water at room temperature using test tubes has been developed in this
laboratory using either phenylhydrazine hydrochloride or phenylhydrazine hydrochloride with NaOAc . 3H2O. The role
of one equivalent of a strong or weak acid which catalyses the reaction is discussed.
Keywords
2,4-dinitrophenylhydrazine, acid catalysis, Brady’s reagent, hydrazone, phenylhydrazine hydrochloride
Date received: 3 July 2019; accepted: 14 October 2019
A comparison of the tests for glycolaldehyde using PhNHNH3Cl (left) or PhNHNH3Cl/NaOAc . 3H2O (right) in water
at 50°C for 5min.
test. Many publications involve hydrazone synthesis,8–12
Introduction
and it is part of popular textbook culture.13–18
The treatment of aldehydes or ketones with 2,4-dinitrophe-
However, this reagent is classed as an energetic sub-
nylhydrazine in either EtOH under reflux, in aqueous 2M
stance and is sold moistened with water to reduce its shock-
hydrochloric acid or in 5% cH2SO4/EtOH or MeOH at
sensitive nature.19–20 The dinitrated benzene ring and N–N
room temperature (RT) is known as Brady’s test.1–2 In 1926,
Brady reported the melting points of a series of crystalline
hydrazones and summarised previous studies in the field.1–5
Department of Chemistry, University of Aberdeen, Aberdeen, UK
The hydrazone precipitate has a characteristic melting point
Corresponding author:
M John Plater, Department of Chemistry, University of Aberdeen,
Meston Walk, Aberdeen AB24 3UE, UK.
for the aldehyde or ketone (Figure 1).6–7 This reagent is spe-
cial because it formed hydrazones in acidic water or metha-
nol which crystallized, hence its development as a school Email: m.j.plater@abdn.ac.uk

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Journal of Chemical Research 00(0)
bond are energetic moieties. A number of instances have
Figure 2 shows how phenylhydrazine is available to
occurred, in which bottles of dried reagent have been col- react from phenylhydrazine hydrochloride with mild acid
lected from schools for destruction. For this reason, we catalysis. The more vigorous reagent is produced from
.
have investigated the behaviour of other hydrazines to see treating phenylhydrazine hydrochloride with NaOAc
if they might be suitable as a test for aldehydes and ketones 3H2O13 because the equilibrium shown increases the con-
to be used in place of 2,4-dinitrophenylhydrazine. In these centration of phenylhydrazine. However, for both these
studies, indoles are not observed as these require treatment schemes in Figure 2, we showed that the small stoichiomet-
of the hydrazone formed with concentrated acids.21–22
ric quantity of acid, either hydrochloric acid or acetic acid,
is critical to producing an acceptable and rapid positive test
result. If phenylhydrazine hydrochloride in water was acid-
ified with 2–3 drops of conc. aqHCl, precipitate formation
was inhibited presumably because the hydrazine primary
amine is protonated. Also, if phenylhydrazine hydrochlo-
ride is neutralised with KOH in water, the test was not sat-
isfactorily fast enough with both aldehydes and ketones.
This illustrates that the one equivalent of acid present with
the reagent, aqHCl or aqHOAc, catalyses the hydrazone
formation. Methanol or ethanol was not satisfactory as sol-
vents to give precipitates under these conditions so only
water was used. The precipitates were presumed to be
hydrazones and were not characterised any further.
Discussion
Owing to the energetic structure of 2,4-dinitrophenylhydra-
zine and its withdrawal from some schools, we aimed to
introduce an alternative but related test for aldehydes and
ketones using known chemistry. Both phenylhydrazine and
phenylhydrazine hydrochloride are commercially available
and will give precipitates with aldehydes and ketones.
Here, we have carried out a series of tests in test tubes to
clarify the results that are obtained and make it suitable as a
test in schools. This takes into account the time required for
the test reaction to complete satisfactorily with a clear posi-
tive result, the quantity of test reagent required, the tem-
perature, the pH and general safety requirements. We have
not sought to isolate products and determine their melting
points because in schools, Brady’s reagent can be used as a
qualitative test for a carbonyl group of an aldehyde or
ketone. An analysis of melting points of hydrazones as a
means of identifying carbonyl containing compounds has
also been criticised.23 Phenylhydrazine has an m.p. near to
RT (18–21°C) and on a cold day is likely to be frozen in the
bottle. Warming is feasible, but it is quite a vigorous rea-
gent, and this would be an additional step so we chose to
work with phenylhydrazine hydrochloride. This is availa-
ble as a stable, white, fluffy solid and was easy to work
with. A small amount, about 50mg, was placed in a test
tube in a fume hood (fan off), and the test was done on an
open bench. Sample aliquots of 1mL were also measured
from a standard (0.5g of phenylhydrazine hydrochloride in
10-mL H2O). The tests were done in water using approxi-
mate amounts of reagents obtained with a spatula.
The results are shown in Table 1 and 2. Most of the
aldehydes reacted satisfactorily with phenylhydrazine
hydrochloride (Table 1, Entries 1–9, Test 1) to give pre-
cipitates within 2–3s. Methanal gave a white precipitate,
glyoxal gave an orange/brown precipitate and glycolalde-
hyde dimer gave an orange/yellow precipitate after heat-
ing. Photographs of Entries 1–3 and Entry 9, from vanillin,
are shown in the ESI (Supplemental Figure S1–S4).
Phenylhydrazine hydrochloride did not react with acetone
at RT or on heating (Table 2, Entry 10, Test 1). Other
ketones (Table 2 Entries 11–13) gave no precipitate at RT.
Ketones are less electrophilic than aldehydes, which slows
down the reaction. However, phenylhydrazine hydrochlo-
.
ride/NaOAc 3H2O reacts satisfactorily with both alde-
hydes and ketones at RT within a few seconds (Table 1 and
2, Test 2). The base will deprotonate the phenylhydrazine
hydrochloride, giving phenylhydrazine and acetic acid
which is a much weaker acid (Figure 2). The phenylhy-
drazine is more available to react compared to phenyl-
hydrazine hydrochloride which will dissociate less in
water, and acetic acid can catalyse the reaction. Methanal
gave a white precipitate, glyoxal gave a lemon yellow pre-
cipitate, glycolaldehyde dimer gave a green/grey haze on
heating and vanillin gave a white precipitate (Supplemental
Figures S1–S4 in the ESI). 3,5-Dinitrosalicylaldehyde gave
a brown precipitate (Entry 8). Glucose, fructose and other
reducing sugars can react in boiling water and is the basis
Figure 1. A classic test for an aldehyde or ketone with
2,4-dinitrophenylhydrazine in ethanol or water with acid catalysis.
Figure 2. Top: Phenylhydrazine hydrochloride is only weakly dissociated because hydrogen chloride is a strong mineral acid.
Bottom: Treatment with NaOAc . 3H2O liberates more phenylhydrazine because acetic acid is a weak acid.

Plater
3
Table 1. Test for aldehydes with a minimum quantity of 50mg of PhNHNH3Cl or PhNHNH3Cl/NaOAc . 3H2O in water (8mL) at
RT for 2–3s unless otherwise stated.
Entry
Name
Structure
Test 1
Test 2
PhNHNH3Cl (RT in water)
PhNHNH3Cl/NaOAc .
3H2O (RT in water)
1
2
3
4
5
6
7
Aqueous methanal solution
Glyoxal
White precipitate
White precipitate
Orange/brown precipitate
Lemon yellow precipitate
Glycolaldehyde dimer
(monomer shown)
Orange/yellow precipitate
(50°C for 5min)
Green/grey haze
(50°C for 5min)
Acetaldehyde
Isobutyraldehyde
Benzaldehyde
Salicylaldehyde
White precipitate
White precipitate
White precipitate
White precipitate
White precipitate
White precipitate
White precipitate
White precipitate
8
9
3,5-Dinitrosalicylaldehyde
Vanillin
Weak orange haze
Brown precipitate
White precipitate
White precipitate
(slower to form)
RT: room temperature.
Photographs of test tubes for Entries 1–3 and 9 are in the ESI (Supplemental Figures S1-S4).All tests were shaken manually in a test tube.
Table 2. Test for ketones with a minimum quantity of 50mg of PhNHNH3Cl/NaOAc . 3H2O in water (8mL) at RT for a few
seconds.
Entry
Name
Structure
Test 1
Test 2
PhNHNH3Cl (RT in water)
PhNHNH3Cl/NaOAc . 3H2O (RT in water)
10
Acetone
No precipitate
No precipitate
No precipitate
White precipitate
White precipitate
White precipitate
11
Butan-2-one
Acetophenone
12
13
Mesityl oxide
No precipitate
Haze
RT: room temperature.
All tests were shaken manually in a test tube.

4
Journal of Chemical Research 00(0)
of the osazone test for reducing sugars which was devel- Funding
oped by Emil Fischer.13,24–30
The author received no financial support for the research, author-
ship and/or publication of this article.
Conclusion
ORCID iD
A satisfactory school test for aldehydes and ketones in test
tubes has been developed using phenylhydrazine hydro-
chloride and NaOAc . 3H2O in water at RT, which gives a
precipitate in a few seconds with shaking. Phenylhydrazine
hydrochloride in water at RT will give a precipitate with
aldehydes in 2–3s but not ketones. The reagent is best
weighed out directly into test tubes in approximate quanti-
ties in a fume hood, but the test can be done on an open
bench. A 50-mg sample of phenylhydrazine hydrochloride
with or without NaOAc . 3H2O (50mg) is a suitable quan-
tity to be weighed out and mixed with five drops of the
carbonyl compound in water.
M John Plater
https://orcid.org/0000-0003-0204-9695
Supplemental material
Supplemental material for this article is available online.
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By wearing disposable gloves, solid phenylhydrazine
hydrochloride, a stable white fluffy solid, was trans-
ferred by a spatula from the reagent bottle into test tubes
in a fume hood. The fume hood was not switched on. The
test tubes can then be used on an open bench. Initially,
50-mg samples of phenylhydrazine hydrochloride were
weighed out with a balance, and it was verified that this
quantity was satisfactory for the test with aldehydes and
ketones. An estimated quantity from a spatula in a fume
hood, switched off, was used for simplicity as was the
.
quantity of NaOAc 3H2O (50 mg or an excess). We
assume this is likely to be the scenario in a school for
simplicity, speed and safety. We also used 1-mL aliquots
of reagent from a standard solution which had been kept
sealed for 2 months (0.5 g of phenylhydrazine hydro-
chloride in 10-mL H2O). Phenylhydrazine hydrochloride
.
or phenylhydrazine hydrochloride and NaOAc 3H2O
were mixed in water (8 mL) in a test tube at RT by shak-
ing or with upwards and downwards movements of a
spatula. Five drops of the aldehyde or ketone was added
and a precipitate formed within seconds with shaking.
The test tube was not stoppered with a bung but was
shaken from side to side with the top held in one hand
between thumb and forefinger. The test with glycolalde-
hyde was heated at 50 °C for 5 min in a water bath. See
Table 1 and 2 for the details and photographs of the test
results for Entries 1–3 and 9 are in the ESI (Supplemental
Figure S1–S4).
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with
respect to the research, authorship and/or publication of this
article.