131n - lab4 - recrystallization erika.doc

8/17/2019 131N - Lab4 - Recrystallization Erika.doc

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Lab 4: Recrystallization

Objectives:

- Purify an impure sample of an antibiotic.

- Practice the crystallization technique.

Introduction:

The purpose of this experiment is to introduce the technique of crystallization, a

very common procedure used to purify crude solids in the organic laboratory. Thegeneral technique involves dissolving the material to be crystallized in a hot solvent and

cooling the solution slowly. The dissolved material has a decreased solubility at lower

temperatures and will separate from the solution as it is cooled. This phenomenon iscalled crystallization if the crystal growth is relatively slow and selective or

precipitation if the process is rapid and nonselective. Since the impurities are usually

present in much smaller amounts than the compound being crystallized, most of the

impurities will remain dissolved in the solvent even when it is cooled. The purified

substance can then be separated from the solvent and impurities by filtration.

n this experiment, you will carry out a crystallization of impure sulfanilamideusing !"# ethyl alcohol as the solvent. Sulfanilamide is one of the sulfa drugs, the first

generation of antibiotics to be used in successfully treating many ma$or diseases such as

malaria, tuberculosis, and leprosy.

H2N S

O

O

NH2

Sulfanilamide

The solubility of sulfanilamide in !"# ethyl alcohol is given in the following table%

Temperature Solubility (mg/mL

&'( )*

+&'( +**&'( *

&'(

'( +)&

/otice that the solubility increases significantly as the temperature increases.

Therefore, !"# ethyl alcohol is an excellent solvent for crystallizing sulfanilamide.


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0 graph of these data is shown below%

0

50

100

150

200

250

0 20 40 60 80

Temperature in Degrees Celsius

S o l u b i l i t y i n m g / m l

1ou will crystallize a sample of impure sulfanilamide by dissolving it in the

minimum amount of boiling !"# ethyl alcohol 2 '(3 and then cooling the solution,first to room temperature, and then to & '( in an ice-water bath.

The purity of the final material after crystallization will be determined byobserving the color of your crystals and by performing a melting point on your sample.

1ou will also weigh your sample and calculate the percent recovery. t is not li4ely thatyou will obtain a )& recovery. This is true for several reasons% experimental loss, the

original sample is not )& sulfanilamide, and some sulfanilamide is soluble in thesolvent even at & '(. 5ecause of this latter factor, some sulfanilamide will remain

dissolved in the mot!er li"uor 2the liquid remaining after crystallization has ta4en

place3.

The melting point of a substance is often used to determine purity. The melting

point of a sample is usually expressed as two numbers called the melting point range,such as ))+ 6 ))*'(. The first number is the temperature at which the substance begins

to melt 2when liquid is first observed3 and the second number is the temperature at which

the sample has completely melted 2no solid left3. 0 very pure sample will have a narrowmelting point range that will be close to the literature value 2supposedly determined on avery pure sample3. 0n impure sample will have a lower melting point and the range will

be bigger. The literature melting point of sulfanilamide is )*." 6 )."'(.

t is instructive to loo4 at the structure of sulfanilamide and as4 whether or not

!"# ethyl alcohol should be a reasonable solvent for crystallizing this substance. This

question can be answered by referring to the guidelines for predicting polarity and


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solubility behavior 2see 7ab 89, Solubility3. There are several polar bonds in

sulfanilamide, the /: and the S; bonds. 5ecause of these bonds, sulfanilamide has

some polar character. n addition, the /:+ groups and the oxygen atoms in sulfanilamide

can form hydrogen bonds with ethyl alcohol. So even though, it is li4ely that

sulfanilamide would be soluble in !"# the benzene ring part of sulfanilamide is quite

nonpolar, sulfanilamide has an intermediate polarity because of the polar groups.


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+. :eat the solvent bea4er on a 'arm hotplate until it is almost boiling 2you may need to

turn up the hot plate to reach the boiling point of ethyl alcohol but turn it up slowly3.

Since !"# ethyl alcohol boils at a relatively low temperature 2'(3, it evaporates quiterapidly. Setting the temperature of the hotplate too high will result in too much loss of

solvent through evaporation. )o not !eat t!e bea&er o% sul%anilamide yet

9. 5efore heating the flas4 containing the sulfanilamide, add enough hot solvent with a

Pasteur pipet or a plastic pipet to barely cover the crystals. Then heat the flas4 containing

the sulfanilamide until the solvent is boiling. 0t first this may be difficult to see becauseso little solvent is present.

*. 0dd another small portion of solvent 2about &." m73, continue to heat the flas4, and

swirl the flas4 frequently. 1ou may swirl the flas4 while it is on the hotplate or, for morevigorous swirling, remove it from the hot plate for a few seconds while you swirl it.

Bemember that the temperature of the solvent should be at or near the boiling point of

!"# ethyl alcohol during this entire process. ?hen you have swirled the flas4 for )&-)"

seconds, chec4 to see if the solid has dissolved. f it has not, add another portion of solvent. :eat the flas4 again with swirling until the solvent boils. Then swirl the flas4

for )&-)" seconds, frequently returning the flas4 to hotplate so that the temperature of themixture does not drop.

". (ontinue repeating the process of adding solvent, heating, and swirling until all of thesolid has dissolved completely. *ote t!at it is essential to add just enoug! solvent to

dissolve t!e solid + neit!er too muc! nor too little 5ecause !"# ethyl alcohol is very

volatile, you need to perform this entire procedure fairly rapidly. ;therwise, you may

lose solvent nearly as rapidly as you are adding it and this procedure will ta4e a very longtime. The time from the first addition of solvent until the solid dissolves completely

should not be longer than )"-+& minutes.

. Bemove the flas4 from the heat and place it on an insulating surface such as a

noteboo4 or a cor4 ring. The purpose of the insulating surface is to prevent the solution

from cooling too rapidly. (over the flas4 with a watch glass or inverted bea4er to reducethe rate of cooling. (rystallization should begin by the time the flas4 has cooled to room

temperature. f it has not, gently scratch the inside surface of the flas4 with a glass rod to

induce crystallization. ?hen it appears that no further crystallization is occurring at room

temperature, place the flas4 in an ice-water bath using a bea4er. 5e sure that both water and ice are present and that the bea4er is small enough to prevent the flas4 from tipping

over.

. ?hen crystallization is complete, vacuum filter the crystals using a 5uchner funnel

2this will be demonstrated in the pre-lab lecture3. Coisten the filter paper with a few

drops of !"# ethyl alcohol and turn on the vacuum 2or aspirator3 to the fullest extent.=se a spatula to dislodge the crystals from the bottom of the flas4 before transferring the

material to the funnel. Swirl the mixture in the flas4 and pour the mixture into the funnel,

attempting to transfer both crystals and solvent. Therefore, you need to pour the mixture

quic4ly, before the crystals have completely settled to the bottom of the flas4. 21ou may


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need to do this in portions, depending on the size of your funnel and how s4illed you are

at this procedure.3 ?hen the liquid has passed through the filter, repeat this procedure

until you have transferred all the liquid to the 5uchner funnel. 0t this point there willusually be some crystals remaining in the flas4.

. =sing your spatula, scrape out as many of the crystals as possible from the flas4. 0ddabout + m7 of ice+cold !"# ethyl alcohol 2measured with a plastic pipet3 to the flas4.

Swirl the liquid in the flas4 and then pour the remaining crystals and alcohol into the

:irsch funnel. /ot only does this additional solvent help transfer the remaining crystalsto the funnel, but the alcohol also rinses the crystals already on the funnel. This washing

step should be done whether or not it is necessary to use the wash solvent for transferring

all the crystals. f necessary, repeat with another +-m7 portion of ice-cold alcohol. 1ou

should wash the crystals with a total of about * m7 of ice-cold solvent.

!. (ontinue drawing air through the crystals on the 5uchner funnel by suction for about

five minutes. Transfer the crystals onto a pre+'eig!ed watch glass for air-drying.

Separate the crystals as much as possible with a spatula. The crystals should becompletely dried within )&-)" minutes and you may let them sit overnight. 1ou can

usually determine if the crystals are dry by observing whether or not they stic4 to aspatula or stay together in a clump. ?eigh the dry crystals and calculate the percent

recovery. Becord the appearance of the crystals 2color, shape, size of crystals3. 1our

crystals will most li4ely loo4 li4e needles or plates.

)&. Determine the melting point of the pure sulfanilamide and the original impure

material. Bemember that the melting point of each sample should be reported as a range.

1our instructor will show you how to prepare a sample for doing a melting pointdetermination and how to use the melting point apparatus.


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Results (Please fill out and submit with the lab report. Recopy if messy)

1. Weight of impure sulfanilamide (to three decimal points):

0.820g

2. Appearance of pure sulfanilamide after crystallization (color, shape and approximate

size of crystals):

Clear, white, round, sparkly

3. Weight of purified sulfanilamide (to three decimal points):

0.387g

4. Calculate the % recovery.

0.387g/0.820g x 100 = 47.2%

5. Melting point of impure sulfanilamide (should be a range):

157ºC - 160ºC

Melting point of pure sulfanilamide:

The literature (theoretical) melting point of pure sulfanilamide is 164.5ºC – 166.5ºC

The experimental melting point of pure sulfanilamide is 162ºC – 164ºC


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Lab Report Guide: THIS IS DONE, RESULTS ARE ABOVE- 1. Resuts !" pts#

o Resuts s$eet %eat& 'ed out (it$ data

o )roper si*%i'+a%t '*ures

o Le*ibe +a+uatio% o per+e%t re+o-er&

- . Resuts A%a&sis ! pts# / )LEASE HEL) ELABORATE ON THISo Brie t&ped dis+ussio% o t$e per+e%t re+o-er& a%d

+o0pariso% o t$e 0easured 0eti%* poi%t to t$eiterature -aue. $at do t$ese -aues te us about t$e

su++ess o t$is e2peri0e%t or a+3 t$ere o4Erom the bac4ground explanation above% Ft is not li4ely that you will obtain a )&

recovery. This is true for several reasons% experimental loss, the original sample is not)& sulfanilamide, and some sulfanilamide is soluble in the solvent even at & '(.G

- ". )ost Lab 5uestio%s !6 pts# )LS SEE BELOo T&ped a%s(ers to t$e )ost Lab 7uestio%s. Note t$at single

sentence ans'ers 'ill not su%%ice State t!e ans'er to t!e "uestion %ollo'ed

by a brie% description o% t!e evidence supporting t!at ans'er

Post-Lab Questions PLEASE ANSWER THESE

1. Based on the appearance of the sulfanilamide before crystallization, do you think

it is pure? Explain.

2. Give three reasons why the recovery was not 100%. Be specific.

3. In the crystallization procedure, the solution should be heated to the boiling point

of 95% ethyl alcohol (78°C). If the solution is only heated to 40°C, how would

this affect the results? Be specific. I think it would have not dissolved

completely, please elaborate

4. In this problem, a solid is given with two possible solvents for crystallizing theimpure solid. Which solvent would work best? Explain your choice. The solid is

listed first.

Benzophenone Water or 95% Ethyl alcohol


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O

Urea Water or HexaneNH2

O

H2N

5. Listed below are solubility-vs-temperature data for an organic substance A

dissolved in water.

Temperature Solubility of A in

(°C) 100 mL of water (g)

0 1.5

20 3.0

40 6.5

60 11.0

80 17.0

a. Suppose 0.1 g of A and 1.0 mL of water were mixed and heated to 80°C. Would all

of the substance A dissolve? Explain.

b. The solution prepared in (a) is cooled. At roughly what temperature will crystals of

A appear?

c. Suppose the cooling described in (b) were continued to 0°C. How many grams of A

would come out of solution? Explain how you obtained your answer.

131n - lab4 - recrystallization erika.doc

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