CHEM 322: ANALYSIS OF DOUBLE UNKNOWNS
You will have to pass a quiz (≥80%) that covers this document + supplemental handouts before you can start this experiment.
OVERVIEW
In this experiment, you will isolate two unknown compounds from each other, and purify and identify each one.
The mixture must be separated before either substance can be identified. We can modify the single unknown scheme to accomplish
this. Once isolated, purified, and characterized using physical and chemical info, the identity of each component of the double
unknown can be confirmed by making and analyzing a solid derivative with control comparisons (like in the single unknown analysis).
The mixture you receive will contain two* of the following different classes of organic compounds (A+B, A+C, or B+C):
A. basic compound (amines, pKa ranging from ~5 - 10)
B. acidic compound (phenols, pKa ~10 or carboxylic acids, pKa less than 5)
C. neutral compound (*exception: if you get an ester, it will be the only compound present)
If you receive combination A+C or B+C, this mixture will be completely soluble in diethyl ether (referred to as “ether” from now on).
If you receive combination A + B, a reaction will have taken place, and you will see a large amount of wet, messy-looking solid. The
solid is the salt created by the acid-base reaction. Salts are NOT soluble in ether. The wetness is caused by an excess of either the acid
or the base. The acid and the base will each become ether-soluble at different stages of the separation scheme.
HOW THIS SEPARATION SCHEME WORKS
Acidic and basic organic compounds can be separated from neutral organic compounds and from each other by taking
advantage of acid/base character, differences in pKa values, and solubility differences between their uncharged and ionic forms. This is
done by shaking a solution of the mixture in ether with either aqueous HCl or aqueous NaOH (both of these are mainly water). A
reaction may occur, in which a component may become either protonated or deprotonated, and each component of the mixture then has
a “choice” of staying in the ether (the organic phase) or migrating to the water provided by the dilute acid or base (the aqueous phase).
An acid (H-A) will dissociate into H+ and A- when it is in a solution with a pH that is at or above its pKa. At this pH, there
isn’t a lot of free H+ in solution, so adding more H+ to the mix is energetically favorable. H-A will not dissociate if it is in a solution
with a pH that is below its pKa. At this pH, there is a lot of free H+ already, so the acid may hang onto its proton. If the acid has a
conjugate base (A-) that is stable, the acidic proton can dissociate even if there is already lots of free H+ in solution. Stability of the
conjugate base increases as acid strength increases. So, you’ll find a stronger acid in its dissociated form (H+ + A-) in more pH
environments than you would find a weaker acid (or stronger base) in its dissociated form.
Isolation of Acids
If a mixture of compounds in ether is shaken with aqueous
NaOH (creating a basic, high-pH environment and adding an
aqueous layer to the organic ether), phenols and carboxylic
acids get deprotonated, become anions (Ar-O- or RCOO-), and
put more H+ into solution. Anions are NOT ether-soluble but
usually dissolve in water due to their charge. So, shaking the
mixture in ether with aqueous NaOH will pull acidic
compounds into the aqueous phase and leave all other
compounds (amines and neutrals) in the ether.
Ordinary phenols: pKa ≈ 10; Acidic phenols have lower values
Isolation of Bases
If a mixture of compounds in ether is shaken with aqueous HCl
(creating a low-pH environment and adding an aqueous layer
to the organic ether), amines get protonated and become
cations (RNH3+ or RR’NH2+). When in acidic solution,
amines prefer their H-A form, which is cationic in this case.
Cations are NOT ether-soluble but usually dissolve in water
due to their charge. So, shaking with aqueous HCl will pull
amines into the aqueous phase and leave all other compounds
(acids and neutrals) in the ether layer.
Aliphatic amines: pKa ≈ 10
Carboxylic acids: pKa ≤ 5
Aromatic amines: pKa ≈ 5
Ordinary phenols can be separated from carboxylic acids if the
pH of an aqueous solution containing both is adjusted to ~7.
At this pH, phenols will not be deprotonated (due to their
higher pKa values), remaining neutral and ether-soluble, while
carboxylic acids will be deprotonated (due to their lower pKa
values), becoming anionic and insoluble in ether.
EXPERIMENTAL SECTION
A. TEST YOUR MIXTURE OF COMPOUNDS FOR WATER SOLUBILITY
1.
2.
3.
Put about 1 mL of water in a tiny test tube.
To this gently add about 4 drops (or a small crystal) of well-mixed double unknown (enough of the unknown to actually if a
reaction is occurring, while still conserving your unknown stock).
Swirl gently as you watch for the following observations:
 Can you see any Schlieren lines forming? What does this indicate?
 Is there any significant loss in volume of the sample you added? If so at least one of your substances is water-soluble.
 Does the mixture get warm? If so, at least one is water reactive.
 If you have a salt formed by reaction of an acid with a base, most of the solid will be water-soluble even though its
individual components may not be. To avoid loss, treat it as if at least one component is in fact water-soluble.
If you fail to detect water solubility before you start, you will lose at least one of your compounds. If you note that one (or both) of the
components is (are) water soluble you MUST shake all separations vigorously (with appropriate venting) to force the water soluble
compound from the water into the ether layer. If you fail to do this, you will lose your compound in the water layer, and you will have
to start the separation scheme over again with your mixed unknown.
B. SEPARATE YOUR COMPOUNDS INTO DIFFERENT ETHER LAYERS
1.
Follow the Double Unknown Separation Scheme. This scheme produces up to four separate ether solutions (defined A, B, C,
D). Your unknowns may be in any of them, but if you are alert to evidence of reaction (or lack of it), you may be able to focus
on specific ones.
Each time after you shake ("extract") an ether layer with an aqueous solution, perform steps 2-4:
2.
3.
4.
Physically separate the ether and aqueous layers and save both in labeled containers (E-flasks for ether layers, beakers for
aqueous layers).
Test the pH of the lower (aqueous) layer with pH paper. If you just extracted with HCl, the most recent aqueous layer formed
must have pH < 2; if with NaOH, the most recent aqueous layer formed much have pH > 12. If its pH is not correct,
save/combine it in a beaker with other lower layers from this stage of the separation. Extract the ether (upper) layer again with
fresh aqueous HCl or NaOH, repeating steps 1 & 2 as many times as necessary until the newest lower layer has the desired
pH. At this point, combine all lower layers from extraction of the current ether in a single beaker. Separately save ether and
combined aqueous layers, each labeled as on next page.
[Skip this step if none of your original mixture was water soluble] Extract combined aqueous solutions from step (2) twice
with fresh ether, 10 mL each time. Combine these ether layers with the ether saved from final step (2).
There are two important reasons for following the procedure above:

First, if you are not careful about lower-layer pH, you will not cleanly separate compounds from each other. This will create huge
problems when you try to make derivatives. The reasoning behind testing the aqueous layer for pH is shown in this example:
o Imagine you have an amine. Shaking with aqueous HCl will protonate all of the amine molecules you have to bring them all
into the aqueous layer. H+ ions are used up as they protonate amine molecules, so the pH will reflect the current [H +]
concentration, and if pH is still high, H+ ions are still be used up, and thus it can be reasoned that there are still amine
molecules left to be collected. Once pH < 2, there is an excess of H+ ions signifying no more are being used to protonate
amine molecules, and so it is reasoned that you have recovered ALL the amine possible into the aqueous layer.

Second, if a neutral compound is water-soluble (e.g., 2-propanol), then a large portion of it will be in the lower, aqueous layer
instead of the ether. You must recover it via the above "back extractions". Back-extractions provide the neutral molecules more
opportunities to come back into the ether layer, even if they “like” both organic and aqueous layers. This is the only way to avoid
loss of a water-soluble neutral compound. (Note that ionized organics will stay in the aqueous layer.)
C. ISOLATE AND PURIFY EACH COMPOUND FROM THE ETHER SOLVENT
1.
2.
3.
4.
5.
Each ether solution (except “ether Z”) that you save must be dried with CaCl2 for at least 10-15 minutes.
If you are not sure whether a particular ether layer (A, B, C, or D) contains a compound, put a few drops of that solution on a
watch glass and evaporate the ether. A residue is a clue that something with a fairly high boiling point is present in that ether.
Distill "suspicious" ethers first. Drive off the ether into a small round flask. Any compound dissolved in that ether will remain
as a residue in the boiling flask. Watch the vapor temp: ether boils at 36 C. If the vapor temp rises to about 50, you better
stop because there may be a low-boiling compound present. A residue of less than about one mL usually indicates that nothing
was in that ether. You should have an idea of the class to which each belongs based on which ether layer it comes out in.
If you’re left with a solid, recrystallize it from a suitable solvent and dry it at least 24 hours.
If a liquid: Distill through dry glassware and collect boiling range now if you have enough to boil over to the receiving flask.
If you don’t have enough volume, use the large boiling flask test tubes at the back of the lab (we can show you how). Since
the compound came from an ether solution, its vapor temperature is likely lower than its “real” boiling range.
See the board for instructions on how to handle and dispose of spent chemicals and waste.
D. CHARACTERIZE EACH COMPOUND TO CONFIRM ITS IDENTITY
1.
2.
3.
Obtain each compound’s melting or boiling range.
Perform appropriate characterization tests for both of your pure compounds as you did for the single unknown. (If an amine,
do a Hinsberg test.) You should have a good idea of the class to which each belongs based on where it came out in the
separation scheme, so use that as guidance about where to begin.
Finally, make a suitable solid derivative of each compound and of an authentic control sample for each. See the “Procedures
for Synthesis of Solid Derivatives” and “Useful Procedural Goodies” handouts to complete this.
REPORT Write in the standard format which has been established in this course, but with the modifications indicated below. There
is a grading rubric provided on the website for further guidance.
1.
Introduction: State the purpose of the work and observations about the initial appearance, odor, and amount of mixed
unknown. Include the unknown number.
2. Experimental Methods and Results: Combine these into 1 fluid section. Include water solubility results for the double
unknown. Write two major sections, one for each compound. In each section, gather the info pertinent to that compound.:
a. Give the experimental evidence that allowed you to establish the unknown’s class and likely identity:
i. describe the chemical and physical treatments that allowed you to cleanly isolate each compound
(Specifically, DO NOT mention “ether B” or others – instead, tell what aqueous medium the substance was
(or was not) soluble in, and how you recovered it if soluble.
ii. justify choice of specific functional group chemical tests or acid-based tests used
iii. provide a balanced chemical reaction for each test that led to a reaction occurring
iv. briefly describe how each test was performed
v. explain what its results imply/indicate
vi. include the melting or boiling range of each compound (with the published melting or boiling range of the
authentic compound(s) closest to your data)
b. Give the experimental evidence provided by your solid derivative analysis to confirm the identity:
i. justify your choice of a specific solid derivative option
ii. briefly describe the complete procedure you used to prepare the solid derivative (including purification
method; don't forget a bibliographic reference to source – whether web site, handout, or lab text),
iii. provide a balanced chemical reaction for the solid derivative reaction
iv. report and analyze/compare 4 melting ranges: experimental melting ranges for the derivative of your
compound, the derivative of the authentic compound, and of the 50/50 mix; and the published mp of the
authentic derivative.
c. If you used IR or NMR spectroscopy to clinch your identification, provide an interpretation and comparison (with
citation for source) of the most important features of each spectrum submitted (as you would normally do for IR).
3. Discussion: need not be long unless there are parts of your work that need elaboration. Items may include:
a. Any major sources of error
b. Topics considered/explained when deliberating between two potential identities
c. Further explanation of chemical concepts utilized in this experiment
Submit your report electronically via email as a Word document by the start of your lab period during the week of April 11-15, 2016.
Submit your notebook pages in lab.
This report is worth 40 points.
DOUBLE UNKNOWN SEPARATION SCHEME (instructions refer to the layer under which they are printed)
omit steps marked (*1) if no watersoluble components are present
1) dissolve all of it in 30 mL ether
2) shake w/ 15 mL portions of 3 M HCl until pH of final lower layer
<2;cool if warm. Save and label both layers.
aqueous Z (pH must be < 2) Note a ; Note b
Note b
ether Z
(*1) extract 2X with 10 mL fresh ether; combine
these ethers with ether Z
2) add 10 M NaOH slowly & mix until pH >12
3) cool to room temp!
4) shake twice with 15 mL portions of ether (save)
shake with 15 mL portions of 3 M NaOH
until pH of final lower layer > 12– save both
ether A
aqueous A (pH must be >12; discard if so)
(dry this)
=====================================
ether B
aqueous B ( pH must be > 12) Note c
(dry this)
(*1) extract 2X with 10 mL of fresh ether; combine these ethers with ether B
2) add conc HCl slowly to pH 7 Note d. If you overshoot and the pH suddenly drops
from >12 to < 4, you have no phenols and can skip #3 below (there will be
no ether C). Otherwise, cool if warm, then…
3) shake twice with 15 mL portions of ether (save both layers; combine ethers as "C")
ether C
aqueous C (pH ~7 or possibly lower)
(dry this)
Note e
1) add conc HCl to pH < 2
2) if solid: filter, rinse with a little water, & save solid. Otherwise,
cool, then shake twice with 15 mL of ether (save)
solid carboxylic acid
or ether D (dry this)
Note a
Note b
Note c
Note d
Note e
aqueous D (discard if certain that there is no organic acid,
or if you recovered a solid acid)
If the first treatment with HCl does not produce heat and its aqueous phase pH is <2, it's likely that no amine is present. Label and save this
aqueous phase, but do not do any of the steps on the leg below unless you fail to find 2 components in the remaining legs of the scheme.
If an amine is suspected, extract ether Z once more with 15 mL of 3M HCl. Combine the new aqueous layer with aqueous Z.
If a solid forms during NaOH treatment, it may be a relatively insoluble anion of a phenol. Sometimes addition of more water will bring this
into solution. In any case, it will not be soluble in ether. Keep any solid with the aqueous phase.
• If you have an ordinary phenol, beginning about pH 11 you will see persistent cloudiness appear in the aqueous phase that you are
acidifying, and the pH will change downward slowly. Acidic phenols have anions that are intensely yellow, and they don't become waterinsoluble (ether-soluble) until pH 7 or below. Ask instructor for advice if in doubt how to proceed. [If a phenol falls out as a solid upon
adding acid, do not dissolve it in ether – instead, filter it, saving the solid. Extract the aqueous filtrate with ether to remove as much remaining
phenol (which may be colored) as possible. You can save this ether as a backup sample of your phenol. Then proceed with the rest of the
scheme.]
• Poorly soluble large acids start to precipitate if the local concentration of HCl is high (as it is when drops are added) or if the pH is <6.
These acids are white, are obviously crystalline, and do not have a phenol odor. Don’t confuse them with phenols.
If pH plummets to below 2 after pH ~7, you probably don't have an acid. In this case, keep the pH<2 aq. layer but don't bother extracting it
with ether unless the instructor asks you to do so. If it takes a lot of HCl to get from pH ~7 to pH 2 you probably do have an acid. If this
happens but there is no solid, extract with ether, making ether D. Solid carboxylic acids are white and precipitate from water solution if pH is
below 5; just filter these to recover the acid (no ether D needed).
ether
contents
A
amines
B
alcohols, aldehydes, ketones, esters, alkenes, aliphatic/aromatic hydrocarbons and halides, nitriles
C
ordinary phenols (will probably be recovered as liquid if m.p. < 60° C)
D
acids (solid or liquid) and strongly acidic phenols