Recrystallization - choose the most appropriate solvent to obtain a successful recrystallization of benzoic acid.

Choosing the appropriate solvent is the most challenging part of the recrystallization. In choosing a solvent, remember that "like dissolves like", which means that a nonpolar compound will dissolve well in a nonpolar solvent, and a polar compound will dissolve well in a polar solvent. Also note whether a solvent is flammable or not and find its boiling point. This will enable you to select an appropriate heating device. Generally speaking, solvents with boiling points below 100°C should be heated in a water bath (NO OPEN FLAME), above 100°C can be heated directly on a hot plate or you may use a sand bath.

Selecting a solvent is crucial for recrystallization of an organic solvent. An ideal recrystallization solvent should:

* Dissolve the entire compound at the boiling point of the solvent.

* Dissolve very little or none of the compound when the solvent is at room temperature.

* Have different solubilities for the compound and the impurities

* Have a boiling point below the melting point of the compound so that the compound actually dissolves, not melts, in the hot solvent.

* Have a relatively low boiling point

A table of solvents and their dielectric constants are as follow:-

Physical Characteristics of Solvents

Solvent

mp

bp

D420

nD20

e

RD

m

Acetic acid

7

18

.049

.3716

6.15

2.9

.68

Acetone

-95

56

0.788

.3587

20.7

6.2

2.85

Acetonitrile

-44

82

0.782

.3441

37.5

1.1

3.45

Anisole

-3

54

0.994

.5170

4.33

33

.38

Benzene

5

80

0.879

.5011

2.27

26.2

0.00

Bromobenzene

-31

56

.495

.5580

5.17

33.7

.55

Carbon disulfide

-112

46

.274

.6295

2.6

21.3

0.00

Carbon tetrachloride

-23

77

.594

.4601

2.24

25.8

0.00

Chlorobenzene

-46

32

.106

.5248

5.62

31.2

.54

Chloroform

-64

61

.489

.4458

4.81

21

.15

Cyclohexane

6

81

0.778

.4262

2.02

27.7

0.00

Dibutyl ether

-98

42

0.769

.3992

3.1

40.8

.18

o -Dichlorobenzene

-17

81

.306

.5514

9.93

35.9

2.27

,2-Dichloroethane

-36

84

.253

.4448

0.36

21

.86

Dichloromethane

-95

40

.326

.4241

8.93

6

.55

Diethylamine

-50

56

0.707

.3864

3.6

24.3

0.92

Diethyl ether

-117

35

0.713

.3524

4.33

22.1

.30

,2-Dimethoxyethane

-68

85

0.863

.3796

7.2

24.1

.71

N,N -Dimethylacetamide

-20

66

0.937

.4384

37.8

24.2

3.72

N,N -Dimethylformamide

-60

52

0.945

.4305

36.7

9.9

3.86

Dimethyl sulfoxide

9

89

.096

.4783

46.7

20.1

3.90

,4-Dioxane

2

01

.034

.4224

2.25

21.6

0.45

Ethanol

-114

78

0.789

.3614

24.5

2.8

.69

Ethyl acetate

-84

77

0.901

.3724

6.02

22.3

.88

Ethyl benzoate

-35

213

.050

.5052

6.02

42.5

2.00

Formamide

3

211

.133

.4475

11.0

0.6

3.37

Hexamethylphosphoramide

7

235

.027

.4588

30.0

47.7

5.54

Isopropyl alcohol

-90

82

0.786

.3772

7.9

7.5

.66

Methanol

-98

65

0.791

.3284

32.7

8.2

.70

2-Methyl-2-propanol

26

82

0.786

.3877

0.9

22.2

.66

Nitrobenzene

6

211

.204

.5562

34.82

32.7

4.02

Nitromethane

-28

01

.137

.3817

35.87

2.5

3.54

Pyridine

-42

15

0.983

.5102

2.4

24.1

2.37

Tetrahydrofuran

-109

66

0.888

.4072

7.58

9.9

.75

Toluene

-95

11

0.867

.4969

2.38

31.1

0.43

Trichloroethylene

-86

87

.465

.4767

3.4

25.5

0.81

Triethylamine

-115

90

0.726

.4010

2.42

33.1

0.87

Trifluoroacetic acid

-15

72

.489

.2850

8.55

3.7

2.26

2,2,2-Trifluoroethanol

-44

77

.384

.2910

8.55

2.4

2.52

Water

0

00

0.998

.3330

80.1

3.7

.82

o -Xylene

-25

44

0.880

.5054

2.57

35.8

0.62

In general, the higher the dielectric constant of the solvent, the better it will dissolve ionic compounds. Test the solvent on a small amount of the compound. If a solvent doesn't dissolve the crystals when hot, or it readily dissolves the crystals at room temperature, try another solvent. Remember to always use only a minimum amount of hot solvent! In this experiment, water is the best solvent to undergo the recrystallization process of benzoic acid. Because water's boiling point is 100 ?C and water is very polar with the formula H2O.

Once you have selected your solvent you are ready to recrystallize your sample. The process of recrystallization can be broken into the following steps:

. Selection of an appropriate solvent

2. Dissolution of the solute using a minimum amount of hot solvent

3. Decoloration of the solution if necessary (with an activated form of carbon)

4. Removal of suspended solids (through filtration of the hot solution)

5. Crystallization of the solid from the solution as it cools

6. Collecting the purified solid by filtration

7. Washing the crystals with an appropriate solvent to remove impurities

8. Drying the crystals

Choosing the solvent

The choice of solvent is crucial; by using the best solvent we can obtain the most accuracy result. An essential characteristic of a successful solvent is that the compound be soluble in the hot solvent but insoluble in the cold solvent. Tests can be performed with small amounts of material in test tubes: a few drops of a solvent are added and if the material proves insoluble then the tube is heated to see if the material will dissolve at the higher temperature. If so, then a good solvent may have been identified. Another way, if the material insoluble in the high temperature a little more solvent can be added drop by drop to ensure whether the material can dissolve or not. A solvent must be rejected if the material appears readily dissolve in the cold solvent A suitable recrystallization solvent should also be partially volatile in order to be easily removed from the purified crystals. The solvent should not react with the compound being purified and it should have the boiling point below the melting point of the compound being purified because solid melts before dissolves (boiling out). In selecting a good recrystallization solvent one should also consider flammability, toxicity, and expense. An appropriate choice of solvent for the crystallization process will pretty much ensure success, all that follows is manipulation. In this experiment, given that water is the best choice to act as solvent for crystallization process.

Dissolution of the solute using a minimum amount of hot solvent

An Erlenmeyer (conical) flask should be used of such a size that it will only be filled to around half-way when all the solvent has been added. In this experiment, we are using 125-mL Erlenmeyer (conical) flask. The solid sample is introduced, together with around 75 % of the amount of solvent thought to be required. It is always advice to use less solvent at this stage. The flask is heated on the hot plate until dissolution of the solute is complete, additional solvent can be added to the hot solution as necessary to ensure complete dissolution. By a process of gradual addition of solvent to the flask until the material has dissolved and we had ensure that the hot solution is saturated and will deposit crystalline material once it is cooled. Some insoluble impurities may be left suspended in solution, we must prepare to judge when all the desire material has dissolved and only the unwanted impurities are left as a suspension.

Decoloration of the solution if necessary

After dissolution of the solute, if some color impurities are present then charcoal is required to add. The main function of charcoal is for decolorizing. In our experiment, there are absences of color impurities, so we did not add any charcoal.

Removal of suspended solids (through filtration of the hot solution)

Once the solute is fully dissolved the remaining insoluble impurities can be removed by filtering the hot solution through a filter paper folded into a cone and placed inside a glass filter funnel. A problem here is that the solution will cool rapidly as soon as the Erlenmeyer flask is removed from the hotplate. The main purpose the filter paper is folded because we need to increase the surface area of the filter paper and hence faster the filtration process, so we can avoid the cooling process. Another way to minimize the problem, by using stemless funnel placed on top of the beaker containing few millimeters if the recrystallization solvent. The beaker is place on the hotplate and the boiling solvent serves to heat the funnel and prevent the solute from crystallizing during the filtration process.

Cooling - crystallize

Cooling the filtered solution will allow crystals to form. The rate of cooling has a role to play in determining the size of the crystal: fast cooling will tend to generate more crystals of relatively small dimensions, slow cooling might allow larger crystals to form. Usually the best compromise of speed, convenience, and crystal quality, is simply to let the solution cool to room temperature on the bench. To ensure maximum recovery of the material the solution should be cooled in an ice-water bath after the solution had been allowed to cool in room temperature.

Collecting the purified solid by cold filtration

When the crystallization process is judged to be complete the crystals need to be collected by suction filtration. Both the funnel and suction flask should be chosen so that neither will become more than half full during the filtration process. It is preferable that all of the crystalline material is transferred to the funnel as a suspension in the crystallization solvent, however it is sometimes hard to get all of the crystals moving freely by swirling the flask and occasionally it will be necessary to add more ice-cold solvent in order to transfer the last of the crystalline material. It may also be necessary to dislodge crystalline material from the sides of the flask with a spatula prior to flirtation.

Washing the crystals with an appropriate solvent to remove impurities

Once the suction filtration process is complete the collected crystals should be washed with a little more ice-cold solvent to remove final soluble impurities which should otherwise be left on the surface of the crystals. The solvent used for this final washing should be as cold as possible to minimize losses from the crystals redissolving: recall that our previous crystals are soluble in warm solvent so we'll lose material if we use not cold solvent to wash.

Drying the crystals

Once the crystals have been collected in the suction funnel they usually can satisfactorily dried by continuing to draw air over them for few minutes. The almost dry crystals should be then spread on a filter paper to allow the last traces of volatile solvent to evaporate.

Benzoic acid is widely distributed in plants, such as anise seed, cranberries, prunes, cherry bark, cinnamon, and cloves. In most plants, it acts as a natural preservative that inhibits the growth of bacteria, yeasts, and molds. Its salt, sodium benzoate, is produced by the neutralization of benzoic acid with sodium bicarbonate, sodium carbonate, or sodium hydroxide. The salt is not found to occur naturally. It is used a preservative in a variety of food products, such as jams, soft drinks, and cereals. Benzoic acid is also an effective food preservative, but its sodium salt is more popular, partly because it is more soluble in water than benzoic acid.

The technique can be carried out by dissolving a compound to be purified in a hot solvent (or solvent mixture) and then allowing the solution to cool. If the solvent or solvent mixture is properly chosen, the compound has a decreased solubility at lo lower temperatures, and it will form crystals in the solution. The purity of a solid, and therefore the success of or need for a crystallization procedure, can be determined by taking a melting point. Recall that you used melting point determination in the Thin Layer Chromatograph Chromatography and Melting Point Determination laboratory to identify an unknown nation unknown compound.

Precaution step:-

* Compound being purified must be insoluble in solvent at room temperature.

* Compound must be soluble in boiling solvent.

* Solvent's boiling point must be lower than the compound's melting point.

* An abundant quantity of crystals must be recoverable from the cool solvent.

Conclusion:-

* The step by step of the technique recrystallization had been learned. Most of the recrystallization processes undergo this technique.

* The most appropriate solvent to obtain a successful recrystallization of benzoic acid is water due to its polarity and 100 ?C boiling points.

* Benzoic acid was recrystallized with a 90% recovery using water as the solvent.

* The melting point for recrystallized benzoic acid is 119.2 ?C - 120.3 ?

References:-

. http://science.csustan.edu/nhuy/chem3012/Experiments/Recrystallization.htm

2. http://spot.pcc.edu/~sbrookha/CH241/labCH241_files/Crystallization.doc

3. http://orgchem.colorado.edu/courses/3321LMS06/CrystallizationLM21S06.pdf

4. http://www.chem.arizona.edu/courses/chem243/recrystallization.html

5. http://ice.chem.umbc.edu/Chem351l/mp_notes.html