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Experiment 1 Recrystallization & Melting Point Determination
Chemistry (Skyline College )
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Experiment 1: Recrystallization & Melting Point Determination
Sophia Olvera
February 2, 2020
Section: AA
Lab Partner: Katrine Taran
purpose
To purify benzoic acid, sulfanilamide by the technique of recrystallization in order to determine
purity of crystals using the final melting point. In addition, the best solvent for recrystallizing an
unknown solid will be determined and identified using the melting point of the compound and
comparing to known melting point values of compounds.
theory
The most commonly used technique for purifying a solid organic compound is recrystallization.
The basic form for recrystallization occurs when the solid compound is dissolved in a minimal
amount of hot solvent, and later allowed to cool. While cooling the solid will begin to precipitate
out of solution newly formed crystals (this is recrystallization in action.) These crystals will only
form from the main compound present in the mixture, leaving any impurities in the solution.
This occurs as the impurities are excluded from the crystal lattice, allowing for a pure solid
crystal to form. Complete purification is often possible by simply filtering new crystals out from
the solvent, resulting in the separation of the newly formed compound from its impurities. It is
important to note that there are often times when impurities may not be soluble in hot solvent. If
this occurs, it is imperative to filter out in a hot solution almost immediately in order to remove
insoluble material (this process is known as ‘hot-filtration’), and then left to cool to allow the
pure compound to recrystallize.
Selecting the appropriate solvent is critical to successfully performing the recrystallization of a
pure compound. The proper recrystallization solvent should 1) dissolve a moderate quantity of a
given compound near its boiling point, allowing it to be freely soluble, BUT only slightly soluble
near room temperature (0°C), 2) not react with the given compound, 3) dissolves impurities
readily at lower temperatures, or not dissolve them at all, 4) be readily removed from the newly
purified product.
Therefore, it is critical to examine all possible potential solvents and check the solubility
of a compound at high and low temperatures when developing a method of
recrystallization. This is normally done with a small amount of the given compound.
Once a suitable compound has been selected, the recrystallization of the remaining
compound may begin.
Following recrystallization, the purity of the newly formed crystal must be done independently.
For solid compounds, melting point determination is often used. Melting points of pure
substances are constant and are not altered even when a different method of preparation is used.
This method can also be used to identify a compound. Melting points are generally a temperature
range over which samples melt. Pure samples will melt with ± 1°C of said range, while impure
samples will melt over a wider range. This occurs because impurities tend to lower the melting
point of a substance. Determination of identity of a substance may also be done via mixed
melting points; the unknown sample is mixed with another compound, suspected to be the same.
If they are the same, the melting point will remain the same and have a narrow temperature
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range. If different, the melting point will decrease and broaden as the mixing of the two would
simply taint each substance by adding impurities to each.
Name /
Formula
Benzonic
Acid
C7H6O2 (s)
table of reagents
Density
Amount
(g/mL)
Molecular
Weight
(g/mol)
122.123
1.27
Physical
Constants
0.5g
Hazards
severe irritant
mp, 122.4 °C
Methanol
C7H6O2 (l)
32.04
0.791
Toulene
C6H5CH3 (l)
19.141
Flourene
C13H10 (s)
166.22
3mL
bp, 64.7 °C
0.87
3mL
bp, 110.6 °C
1.2
0.1g
flammable, irritant,
solvent
flammable, toxic,
suspected
carcinogen, irritant
solvent
irritant
mp, 114.8 °C
procedure
data & observations
part a: purification of benzoic acid by
recrystallization
1. obtain .5 grams of “impure” benzoic acid
and place in 50-mL Erlenmeyer flask with a
boiling stone
2. heat ~50mL distilled water in 150 mL beaker
3. add ~10mL heated water to “impure”
benzoic acid and place mixture onto hot plate
a. record total amount of water used
4. continuously add small increments of heated
water to benzoic acid until substance has
completely dissolved, stirring constantly and
keeping solution hot
5. once completely dissolved out an additional 1
mL hot water to keep substance dissolved
a. Record total amount of water used
6. Remove flask from hot plate, cool mixture to
room temperature and observe any changes.
o obtained 0.512g of impure
benzoic acid
o 10mL total amount water used
o 2.5mL water added, resulting in a
total amount of 12.5mL water
used
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clamp in air and place the inverted beaker
over the flask. once at room temperature
transfer flask to ice water bath and let sit for
five minutes (minimum)
a. scratch sides if crystallization does
not occur, add seed crystals, ice chips
7. Filter crystals using Hirsch funnel/ Büchner
funnel
a. place filter paper or surface, wet with
solvent, and apply suction to make
sure seal occurs. break vacuum, add
crystals and liquid, apply vacuum.
repeat until crystals are clean and
filtrate comes through clear.
i.
wash is either an ice-cold
portion of main solvent in
small amounts or a “worse”
solvent.
8. let sample air dry, weigh and record the
amount benzoic acid recovered.
9. calculate percent recovery.
10. measure melting point of both impure and
pure benzoic acid.
o Crystallization occurred by itself,
no need for ice chips or seed
crystals
o amount of benzoic acid
recovered: 0.454g
o percent recovery: 90.8%
o impure benzoic acid melting
point range: 114-118°C
o pure benzoic acid melting point
range: 118-120°C
part d: determining the best solvent to use and
identification of ~ unknown ~
1. obtain and unknown and record #
2. place 0.10 g of your fluorene (weighed out to
0.01 g) in test tube
3. add 3 mL of a solvent, top tube with stoppers,
shake strongly
a. if all solid dissolved at room
temperature, solid is soluble! do not
use this solvent as recrystallization
solvent. make a note of all
observations
b. if none (or very little) dissolves,
unstopped tube and heat with boiling
stone. shake, heat, shake, may need to
heat to gently boil, simmer! if
dissolving doesn’t occur do not use
this as recrystallization solvent.
c. if dissolved when heated and not and
o obtained 0.117g of fluorene

toluene: all solid was dissolved
instantly when agitated at room
temperature. solid was soluble in
this solvent. was not used as
recrystallization solvent.

methanol: did not dissolve at
room temperature, however, did
dissolve when heated, and not at
room temperature.
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4.
5.
6.
7.
room temperature?
i.
place tube once filled into ice
water bath and cool it to ~ 5
degrees C
ii.
if crystals are present this is
your recrystallization solvent!
iii.
no crystals? agitate solution in
tube with glass rod. rub inside/
stir rod— still no crystals? this
is not your solvent
once decided, recrystallize the “pure”
flourene
collect obtained crystals using
Buchner/Hirsch funnel and dry
once dry, conduct melt analysis on “pure”
solid to determine melting point range
explain results from tests for best
recrystallization solvent in terms of polarity/
solubility predictions



once the tube was in an ice bath
and cooled, crystals appeared.
methanol was used as the
recrystallization solvent
melt analysis on “pure” fluorene
melting point range 105-107°C.
table of results
Name / Formula
Impure Benzoic
Acid C7H6O2
Pure Benzoic Acid
C7H602
Flourene C13H10
Mass of Product
% yield
0.512g
Experimental
Melting/Boing Point
114-118°C
0.454g, 90.8%
118-120°C
0.117g,
105-109°C
discussion
1. Discuss how your percent recovery informs you of the success (or failure) of the
recrystallization and if the value might be expected from a typical recrystallization
experiment. If there are any sources of error or doubt in your values from your observation
notes, discuss them here.
Percent recovery informs you of the success of failure of the recrystallization by
The amount of benzoic acid recovered was 0.454 g, resulting in a percent recovery of 90.8%.
This value is indicative of a successful recrystallization. The percent yield value should not reach
100% as that would indicate too much of the impurity remaining within the newly crystallized
benzoic acid.
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2. Use the comparison of your melting point data of the benzoic acid crystals to the impure
benzoic acid to discuss the success of your recrystallization. Also compare your melting
point data of benzoic acid to literature values to determine if your value matches or not. If
not, discuss the sources or possible reasons of the discrepancy.
The melting point range of impure benzoic acid was recorded as 114-118°C
3. For Part D, explain the chemical reasons (in terms of polarity and solubility predictions)
to the observed differences between the tested solvents. then discuss the success (or failure)
of the recrystallization procedure using the % recovery and the melting point.
In terms of solubility, it is important to keep in mind that “like dissolves like.” This statement
simply reminds us that polar solvents will dissolve polar solids while nonpolar solvents will
dissolve nonpolar solids. Being polar, Fluorene will only successfully dissolve with the help of
another polar solvent. With this in mind, it is simple to choose which solvent would help in order
to continue a successful recrystallization process. Due to its polar OH bond, Methanol will
dissolve Fluorene properly in comparison to Toluene, which is known to be a nonpolar solvent.
post-lab questions
1. List the most important criteria for selecting a recrystallization solvent.
In order to select the proper recrystallization solvent, the compound must be insoluble in the
solvent at room temperature, soluble in the boiling solvent, and the solvent’s boiling point must
be lower than that of the compound’s melting point. Lastly, it is crucial to be able to recover an
abundant quantity of crystals from the cooled solvent.
2. When is it necessary to use a solvent-pair recrystallization?
It is possible to come across an insoluble solid requiring the use of a solvent pair or a mix of
multiple solvents, in order to continue with the recrystallization process.
3. Why should the recrystallization solvent have a fairly low boiling point? Why should the
boiling point of the solvent be lower than the melting point of the compound being
recrystallized?
Recrystallization solvents must have a fairly low boiling point so that the solute vapors are not
lost in the process of boiling off excess water. A lower boiling point allows for a smoother
purification process.
4. Why should the recrystallization mixture be cooled in an ice bath prior to filtering?
In order to make sure that all of the dissolved solid has crystallized, the recrystallization mixed
must be cooled in an ice bath. Solubility decreases as the temperature decreases in temperature.
This process increases the percent yield of the recrystallized solid.
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5. What effect would each of the following operations have on the success of the
crystallization of benzoic acid? Explain your answers.
a) The hot solution containing the dissolved benzoic acid is immediately placed in an ice
bath.
The crystallization will be less successful because the product will be less pure. There will be a
rapid formation of solid material with inclusion of impurities rather than a true crystallization.
Slow cooling encourages pure crystal formation.
b) After crystallization has taken place, the cold solution is vacuum filtered and product
crystals are collected on a Buchner funnel, then the crystals are washed with hot water.
Product will be lost. The crystals would redissolve in the hot hexanes instead of remaining on the
filter paper on the Büchner funnel, causing loss of product.
c) After isolation of the benzoic acid crystals on a Büchner funnel, they are washed with
diethyl ether.
The crystals may redissolve in the new solvent, or even react with it. It might be okay to rinse
with a different solvent, such as diethyl ether, if you know that the compound is totally insoluble
in the new solvent.
6. A student crystallizes 5 g of a solid and isolates 3.5 g as the first crop. He/she then isolates
a second crop of 1.2 g solid from the filtrate.
a) What is the percent recovery in the first crop?
o Percent recovery is an amount recovered divided by the total possible amount recovered
Amount recovered = 3.5g
Total amount recoverable = 5.0g
% recovery = 3.5/5.0 = .70 = 70.%
b) What is the total percent recovery?
o Amount recovered = (3.5+1.2)g = 4.7g Total amount Recoverable = 5.0g
% recovery = 4.7/5 = .94 = 94%
7. Sometimes crystallization does not occur, even after cooling the solution in the ice bath.
Describe the probable cause of this problem and explain what steps might be taken to
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induce crystallization.
If crystals do not form upon slow cooling of the solution to room temperature, there are a variety
of procedures you can perform in order to stimulate growth. First, the solution should be cooled
in an ice bath. Slow cooling of the solution leads to slow formation of crystals and the slower
crystals form, the purer they are. The rate of crystallization slows as temperature decreases so
cooling with an ice bath should only be used until crystals begin to form; after they do, the
solution should be allowed to warm to room temperature so crystal formation occurs more
slowly. If no crystals form even after the solution has been cooled in an ice bath, take a fire
polished stirring rod and scratch the glass of your glass. The small pieces of glass that are
etched off of the beaker serve as nuclei for crystal formation. If crystals still do not form, take a
small amount of your solution and spread it on a watch glass. After the solvent evaporates, the
crystals that are left behind can serve as seeds for further crystallization. Both these methods
will include nucleation and may even cause very rapid crystallization, which can lead to the
formation of impure crystals.
8. A compound has a solubility in ethanol of 4.24 g/100 mL at 78°C and of 0.86 g/100 mL at
0°C.
a. What volume of hot ethanol will be necessary to dissolve 50 mg of the compound?
(Assume no impurities.)
1.18 mL
b. How much of the compound will remain dissolved in the solvent after recrystallization is
complete? (Assume no loss of crystals due to faulty technique!)
10.15mg
c. What is the maximum percent recovery that could be attained?
79.7%
9. The solubility of acetanilide in hot and in cold water is given in the table below. What is
the maximum percent recovery if 5.0 g of acetanilide is recrystallized from 100 mL of
water?
Assuming we chill the 100 mL down to 0°C, the 100 mL will hold onto 0.53 grams of acetanilide.
5 grams - 0.53 grams = 4.47 grams recovered
(4.47 grams/5.00 grams) x 100 = 89.4% yield,
therefore the percent recovery would be 89.4%.
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10.The CRC lists the melting point for a compound as 182-183°C. You observe a melting
point for this same compound isolated in your experiment as 177-181°C.What can you
conclude about the compound isolated in your experiment?
When isolating a compound for recrystallization, it is common to not fully purify the compound
which will later lead to a lower melting point of the compound than recorded as a literature
value. In terms of recrystallization, if not properly purified, impurities will find themselves
getting stuck in between the crystals of the compound which comes between the atoms of the
compound which later results in a lower melting point range for the compound.
11.How could the identity of a compound be established by melting point determination?
12.The goal of this problem is to find an appropriate solvent to crystallize a given
compound. Rather than doing this experimentally, you will try to predict which one of
three given solvents is the best. For each compound, one of the solvents has the desired
solubility characteristics to be a good solvent for crystallization. In a second solvent, the
compound will be highly soluble, even at room temperature. The compound will be
relatively insoluble in the third solvent, even at the boiling point of the solvent. After
making your predictions, you will check them by looking up the appropriate information in
The Merck Index.
For example, consider naphthalene, which has the following structure
Consider the three solvents ether, water, and toluene. (Look up their
structures if you are unsure. Remember that ether is also called
diethyl ether.) Based on your knowledge of polarity and solubility
behavior, make your predictions. It should be clear that naphthalene
is insoluble in water because naphthalene is a hydrocarbon that is
nonpolar and water is very polar. Both toluene and ether are
relatively nonpolar, so naphthalene is most likely soluble in both of
them. One would expect naphthalene to be more soluble in toluene
because both naphthalene and toluene are hydrocarbons. In
addition, they both contain benzene rings, which means that their
structures are very similar. Therefore, according to the solubility
rule "Like dissolves like," one would predict that naphthalene is very
soluble in toluene. Perhaps it is too soluble in toluene to be a good
crystallizing solvent. If so, then ether would be the best solvent for
crystallizing naphthalene.
For each of the following sets of compounds (the solid is listed first,
followed by the three solvents), use your understanding of polarity
and solubility to predict:
a. The best solvent for crystallization
b. The solvent in which the compound is too soluble
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c. The solvent in which the compound is not sufficiently
soluble
1.
2.
3.
4.
Phenanthrene; toluene, 95% ethyl alcohol, water
Cholesterol; ether, 95% ethyl alcohol, water
Acetaminophen; toluene, 95% ethyl alcohol, water
Urea; hexane, 95% ethyl alcohol, water
The best solvent for crystallization is the one that does not dissolve the substance completely at
room temperature, but will at higher temperature. “Like dissolves like”, a polar compound will
dissolve in polar solvent like water, a nonpolar compound will dissolve in a nonpolar solvent like
toluene.
Phenanthrene is highly nonpolar, it’s highly soluble in toluene and poorly soluble in
water. Phenanthrene can be crystallised in ethyl alcohol, ethyl alcohol is polar and
nonpolar at the same time.
Cholesterol is mostly nonpolar, but is very slightly polar. It is soluble in ether, poorly
soluble in water, and crystallisable in ethyl alcohol.
Acetaminophen is moderately polar. Acetaminophen is soluble in ethanol, poorly soluble
in toluene, and crystallisable in water.
Urea is highly polar. Urea is soluble in water, and poorly soluble in hexane. It is
crystallisable in ethanol.
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