Experiment:
Aldehyde and Ketone Reactions
Aldehydes and ketones are an important class of compounds in organic chemistry. They both
have carbonyl groups. The aldehyde carbonyl is bonded to an R (or Ar) group and a hydrogen,
while the ketone carbonyl is bonded to two R (or Ar) groups.
O
O
C
C
R
H
aldehyde
R
R
ketone
The chemistry of these compounds is highly dependent on the polar carbonyl group. Many of
the reactions of aldehydes and ketones are addition reactions of nucleophiles to the carbonyl
group. The products of some of these addition reactions are solids, easily formed and purified,
whose melting point can be used to help identify the aldehydes and ketones. These products are
called derivatives.
You will study aldehydes and ketones from four perspectives. You will study several tests used
to distinguish aldehydes from ketones and certain types of ketones from other ketones. You will
also form derivatives of some aldehydes and ketones as means of identifying them. You will
make physical measurements of an unknown aldehyde and ketone to aid in its identification.
Finally, you will run an IR and an NMR spectrum to help you identify the unknown. With the
help of these classification tests, derivatives, physical and spectral data, you will determine the
identity of an unknown aldehyde or ketone, while learning some interesting chemistry along the
way.
 Classification Reactions
1. Tollen’s Tests
This test is an oxidation test. Its usefulness comes from the fact that aldehydes can be
oxidized by Tollen's reagent, Ag(NH3)2+1, while ketones cannot.
O
O
+ 2Ag(NH3)2OH + OHR
H
Tollen's
Reagent
R
-
O
oxidized
product
+ 2Ag + 4NH3 + 2H2O
reduced
silver
mirror
When the Tollen's reagent oxidizes the aldehyde, the silver in the reagent is reduced to pure
silver. When done correctly with a sparkling clean test tube, the reduced silver will form a
silver coating, the famous "silver mirror," on the insides of the test tube. Ketones, on the
other hand, are oxidized only with great difficulty, therefore should not react with a mild
oxidizing agent such as the Tollen’s reagent. This test, therefore, was once used to
distinguish between aldehydes and ketones. It was, in fact, a particularly useful test before
the development of NMR, which offers an easy and unambiguous way to distinguish between
aldehydes and ketones.
2. Iodoform Test
Methyl ketones and acetaldehyde have methyl groups directly attached to the carbonyl group.
These compounds react with an alkaline iodine solution to form iodoform and the carboxylic
acid salt.
O
OH-
O
+ I2 (excess)
R
CH3
R
O
O
-
+ OH
R
CI3
CI3
CI3- +
R
O
OH
CHI3
+
Iodoform
(yellow ppt)
R
-
O
A positive test is the formation of a yellow precipitate of iodoform.
Note: Alcohols which can be oxidized to methyl ketones or acetaldehyde will also give a
positive result in this test. This is important information if you are testing your general
unknown, which might be an alcohol.
The key to this test is to add sufficient I2 solution to the aldehyde or ketone to maintain a
light yellow color or until a yellow precipitate of iodoform appears. Remember, any
aldehyde or ketone with an alpha hydrogen, that is a hydrogen on a carbon adjacent to the
carbonyl group, will react with I2 (thus causing the brownish-yellow to disappear) but only
compounds with the methyl directly bonded to the carbonyl carbon will have the three alpha
H’s needed to produce a yellow precipitate of iodoform, CHI3.
 Formation of Derivatives
The term “derivative” can have several different meanings to an organic chemist. You will learn
one meaning later in the semester when you study derivatives of carboxylic acids, compounds
formed by replacing the –OH of a carboxylic acid with other groups. The process of forming
such derivatives may be simple or it may be complex. However, in this experiment, the term
“derivative” has a different meaning. This other meaning refers to solid products that can be
quickly made and easily purified for most compounds in a class of organic compounds, using
the same reactants and procedures. Since these derivatives are solids, their melting points can be
used to identify the compounds from which they were made. The CRC Handbook for
Identification of Organic Compounds lists these derivatives, when known, for many of these
compounds.
For aldehydes and ketones 2,4-dinitrophenylhydrazones are one very common type of derivative
used for identification. You are required to make, purify and characterize by melting point a 2,4dinitrophenylhydrazone derivative to aid in the identification of your unknown. You will also be
making derivatives of some known compounds to get familiar with the procedures and to allow
you to visually characterize the derivatives. These known derivatives, however, do not have to
be purified or tested for melting point.
Note: A comment about the derivatives is in order here. The procedures you are given for
making the derivatives are general procedures to be used for all aldehydes and ketones. But all
aldehydes and ketones are not the same, and often they will behave somewhat differently in the
procedure. One solid may not dissolve as well as another. One derivative may require far more
solvent to recrystallize it than another. Or a precipitate may form immediately on mixing the
reagents while another may take ten minutes to form. Don't assume that the derivatives will all
behave the same.
Let's look at the chemistry of the derivatives that you will make. The following reaction shows
the formation of 2,4-dinitrophenylhydrazone derivatives, which are orange solids.
O
C
H+
+ H2N NH
C
+
H2O
NO2
NO2
2,4-dinitrophenylhydrazine
NO2
NNH
NO2
2,4-dinitrophenylhydrazone
The crystals of the 2,4-dinitrophenylhydrazones vary in color, with the more conjugated ones a
darker orange than the non-conjugated ones. You will be preparing these derivatives with two
knowns (acetophenone, a conjugated ketone, and acetone, a non-conjugated one) to see the
differences in color. This color alone may help you with your unknown determination.
Recrystallization of the 2,4-dinitrophenylhydrazone derivatives is sometimes difficult. Do the
best you can, and if you are having trouble, ask your instructor for help.
 Pre-lab Preparation
Before coming to lab, you need to complete the following:
1.
Read the experiment carefully and be familiar with the procedures. Put together a plan for
completing this experiment over a two-week period (unless otherwise announced in class).
2.
Review the procedures for determining melting points, boiling points, refractive indices, and
densities, all of which you will have to determine for your unknown. (Appendix 1 and 2)
3.
Write in your notebook a definition of derivatives and give a short explanation of how they
are used in helping to identify compounds.
4.
Write general equations for the following test reactions of aldehydes and ketones you will
be doing in this lab.
a. aldehyde + Tollen's reagent →
ketone + Tollen's reagent →
b. ketone + I2 + OH-1 →
(For this equation, write which types of aldehydes or ketones will react.)
c. carbonyl group + 2,4-dinitrophenylhydrazine →
Experimental Procedure
! Safety Considerations
! The Tollen's reagent can explode if it sits around after it is made. Once you have made the
reagent use it promptly. Don't make it up before you need it, and don't store it overnight in
your locker for the following week. Make it, use it, and throw it in the waste jar containing
aqueous sodium chloride solution when you are finished with it. Likewise, throw out your
completed Tollen's test solutions when you are finished.
! Semicarbazide hydrochloride is carcinogenic. Use gloves when working with this compound.
If you get any on your skin, wash it thoroughly with soap and hot water.
! Use boiling stones; never plug sand baths directly into the wall outlet – use Variac controllers.
! Keep unknown liquids off of skin and avoid breathing the vapors.
! Dispose of waste organics in the waste container provided, not in the sink.
As with the hydrocarbons lab, this set of experiments requires you to be very organized. It will
take you the full lab periods to finish the known reactions and to run tests to enable you to
identify your unknown.
Once again, this is a lab where organization and careful recording of observations play a big role.
Take a minute before each part of the experiment to plan your strategy. The experiments do not
need to be carried out in the order listed below. As there are only a few bottles of each reagent,
you will need to spread out and start at different points in the experiment to avoid long lines.
As you are doing the knowns and observing how they react, you should do your unknown with it
so you can compare the behavior of your unknown to that of the knowns. Remember, just
because your unknown gives a result like one of the knowns doesn't mean that it is that particular
compound.
Note: Don’t forget to test your general unknown at the same time if, based upon the solubility
tests, it could possibly be an aldehyde or a ketone.
1. Chemical Tests
Obtain a vial of aldehyde/ketone unknown and record the number in your notebook. Run the
following chemical tests on the known samples indicated below and on your unknown at the
same time.
 Tollen’s Test
To a thoroughly clean, well scrubbed test tube, place 2 mL of a 5% solution of silver
nitrate. Add one drop of 10% sodium hydroxide solution. To this precipitate add dropwise
with continuous shaking a 2% aqueous ammonia solution until the silver oxide just
dissolves. To this Tollen’s reagent add a few drops of the aldehyde to be studied and mix
well. Allow the solution to sit five minutes at room temperature. If no silver forms, warm
the mixture gently in a 50C hot water bath. Aldehydes will leave a silver mirror on the
sides of the tube. Ketones do not react. Carry out the test only with formalin (37%
solution of formaldehyde and water) so that you can see the reduction of silver ion to form
the silver mirror. Dispose of all waste and unused reagent in the bottle provided.
Note: There are some things to watch out for in this test. The reagent must be made
carefully. If too much or too little aqueous ammonia is added to the precipitate of silver
oxide, the reagent will not be as sensitive as it should be. Also, to get a good silver mirror,
you must have a very clean test tube. Sometimes the silver will not mirror, and the silver
will appear as a black precipitate of finely divided silver. This is a common problem with
water-insoluble aldehydes where a black precipitate may indicate the presence of an
aldehyde. Also, avoid overheating. The Tollen’s reagent alone, with no aldehyde present,
will result in a silver mirror if heated too much. The only heat that a Tollen's test needs is a
little warming in a water bath. Don't leave the test reaction in the boiling water bath for a
long period of time. This may result in your getting a false positive test for an aldehyde.
Keep in mind that the NMR spectrum will allow a definitive determination of whether or
not your unknown is an aldehyde or ketone.
 Iodoform Test
To a test tube containing 3 mL of water, add 3 drops of the substance to be tested. Add 10
drops of the I2/KI solution followed by just enough 10% NaOH to produce a light yellow
color. Mix the reagents thoroughly with “thrumping”. A yellow precipitate may form
immediately. If no precipitate forms, heat the test tube in a water bath to 60°C. As you do
this, you will notice that the yellow color will disappear. Continue to add more I2/KI
solution to maintain a yellow color. If a yellow precipitate forms at any time, this is a
positive result in the iodoform test. If no precipitate forms after a minute or two at 60°C
with sufficient reagent to exhibit pale yellow coloring, then this is a negative result for the
iodoform test. Carry out this test with acetone, diethylketone, and your unknown.
Discard all waste in the bottle provided.
Note: I2 is only slightly soluble in water, but it is quite soluble in aqueous KI. Therefore
it is necessary to use an I2/KI solution even though we only really require the I2 for this
reaction.
2. Derivatives
You are required to make and purify a derivative to characterize your unknown sample. Have
your crystals in a labeled vial to turn in to your instructor with your lab report.
Note: The procedures given below are general ones designed for all aldehydes and ketones.
But different aldehydes and ketones react differently. Some react quickly, others react more
slowly. Some have derivatives which crystallize well, while others have low melting or liquid
derivatives which crystallize poorly or not at all. Some derivatives require different amounts of
solvent for recrystallization. Your instructor can help you with difficulties that arise because of
these differences.
Note: It is important that you realize that the acetone that you use to clean your glassware is a
ketone. Be sure that any glassware you use in making a derivative has been rinsed of any
acetone which may have been used to clean it.
 2,4-Dinitrophenylhydrazones
Dissolve 0.5 g of the unknown aldehyde or ketone in 20 mL of 95% ethanol. To this is added 15
mL of 2,4-dinitrophenylhydrazine solution already prepared for you. Crystallization may occur
immediately but should occur at least within 10-15 minutes. Do not put into an ice bath unless
crystals do not form after 15 minutes. Filter the crystals and recrystallize them from ethanol. If
your derivative won’t dissolve in a reasonable amount of ethanol, filter the undissolved
dinitrophenylhydrazone solid from the hot ethanol with suction and wash it with ice cold ethanol.
It is safe to assume that any impurities that would be present will be soluble in the hot ethanol
and thus removed. Carry out this test with your unknown only. Dispose of all wastes in the
waste container in the hood.
Note: The color of the derivative may give some clue as to the structure of the unknown.
Aldehydes or ketones with carbonyls not conjugated with other double bonds or aromatic rings
usually give dinitrophenyhydrazones which are yellow to orange-yellow in color. Conjugated
carbonyls give derivatives which are more orange or orange-red in color.
3. Physical tests
 Boiling point: Measure the boiling point of your unknown. If you do not recall the
procedure, review Appendix 1.
 Refractive index (nD): Using the Abbe Refractometer, measure the refractive index of
the unknown. If you do not recall the procedure, review Appendix 2 and ask your
instructor to review the necessary steps with you. This will avoid errors in reading or
damage to the instrument.
 Note: Because the densities of compounds within a given class of compounds do not
usually vary significantly, this physical property will not be useful in distinguishing
between different aldehydes and ketones. As a result, you will not measure the density
of your unknown.
3. Spectroscopy
 IR Spectrum: Run an IR spectrum of your unknown. If you do not recall the
necessary procedures, see Appendix 3 for background information and then check
with your instructor before running the IR.
 NMR Spectrum: Prepare an NMR sample using your unknown and following the
instructions in Appendix 3. With your instructor’s assistance, you will run an NMR
spectrum. Again, if you do not recall the necessary procedures, see Appendix 3 for
review. Students are not permitted to run an NMR spectrum without direct supervision.
 Post-Lab and Report Requirements
1. Data and Discussion Requirements: See Appendix 5 for detailed instructions on the
requirements for the unknown reports. You will be following these requirements for
all unknown reports, so review this Appendix carefully to be certain that you include all
of the required tables and discussion sections.
2. Do you think you could have accurately identified you unknown without qualitative
testing and derivative formation? Explain by discussion which information was most
useful.