Re-crystallization Of Benzoic Acid
Re-crystallization Of Benzoic Acid
Apparatus:
Bunsen
burner, beaker, tripod stands, match box, beaker, china dish, stirrer, fluted
filter paper, funnel, spatula, water bath, guaze wire,
Required chemicals:
Distilled
water, salt of benzoic acid,
Re-crystallization
Definition:
A technique for purifying a crystalline sample where the impure sample is dissolved in a small volume of solvent, forming a supersaturated solution. As the temperature of the solution drops, pure crystals form, the impurities remaining dissolved in the solvent.
There are also different re-crystallization techniques that can be used such as:
Single-solvent re-crystallization:
The
mixture of "compound A" and "impurity B" are dissolved in
the smallest amount of hot solvent to fully dissolve the mixture, thus making
a saturated solution. The solution is then allowed to cool. As the solution
cools the solubility of compounds in solution
drops. This results in the desired compound dropping (recrystallizing) from
solution. The slower the rate of cooling, the bigger the crystals form.
→ Solvent added (clear) to compound → Solvent heated to give saturated compound solution → Saturated compound solution allowed to cool over time to give crystals and a saturated solution.
Multi-solvent re-crystallization:
This
method is the same as the above but where two (or more) solvents are used. This
relies on both "compound A" and "impurity B" being soluble
in a first solvent. A second solvent is slowly added. Either "compound
A" or "impurity B" will be insoluble in this solvent and
precipitate, whilst the other of "compound A"/"impurity B"
will remain in solution. Thus the proportion of first and second solvents is
critical. Typically the second solvent is added slowly until one of the
compounds begins to crystallize from solution and then the solution is cooled.
Heating is not required for this technique but can be used.
→
Solvent added (clear) to compound → Solvent heated to give saturated compound
solution→ Second solvent added to compound solution to give mixed solvent
system→ Mixed solvent system allowed to cool over time to give crystals and a
saturated mixed solvent system.
Hot filtration-re-crystallization:
Hot
filtration can be used to separate "compound A" from both
"impurity B" and some "insoluble matter C". This technique
normally uses a single-solvent system as described above. When both
"compound A" and "impurity B" are dissolved in the minimum
amount of hot solvent, the solution is filtered to remove "insoluble matter
C". This matter may be anything from a third impurity compound to
fragments of broken glass. For a successful procedure, one must ensure that the
filtration apparatus is hot in order to stop the dissolved compounds
crystallizing from solution during filtration, thus forming crystals on the
filter paper or funnel.
→
Solvent added (clear) to a mixture of compound + insoluble substance → Solvent
heated to give saturated compound solution + insoluble substanc → Saturated
compound solution filtered to remove insoluble substance → Saturated compound
solution allowed to cool over time to give crystal and a saturated solution.
Benzoic acid
Introduction:
Occurrence:
First described in the 16th century, benzoic acid exists in many plants; it makes up about 20 percent of gum benzoin, a vegetable resin. It was first prepared synthetically about 1860 from compoundsderived from coal tar. It is commercially manufactured by the chemical reaction of toluene (a hydrocarbon obtained from petroleum) with oxygen at temperatures around 200° C (about 400° F) in the presence of cobalt and manganese salts as catalysts. Appreciable amounts have been found in most berries (around 0. 05%). Cranberries contain as much as 300-1300 mg free benzoic acid per kg fruit. Benzoic acid is a byproduct of phenylalanine metabolism in bacteria. It is also produced when gut bacteria process polyphenols (from ingested fruits or beverages).
Chemical names:
Benzenecarboxylic acid; Dracylic acid; Carboxybenzene; Benzeneformic acid, phenylcarboxylic acid, Benzoic acid.
Chemical formula:
1. Its chemical formula is C7H6O or C6H5COOH.
Structural formula:
Its structural formula is
Formula weight:
Its formula weight is 122.123 g/mol.
Properties:
It is a white crystalline solid(White scales or needle crystals).
Slightly soluble in water.
Odorless or with a slight benzaldehyde odor.
Its boiling point is 249.2 deg C at 760 mm Hg.
Its melting point is 121.5 °C -123.5 °C.
Its density is 1.3 g/cm³.
When heated to decompose it emits acrid smoke and irritating fumes.
Its pH is about 4 (solution in water).
Uses:
Benzoic acid helps prevent infection caused by bacteria.
It is used to treat skin irritation and inflammation caused by burns, insect bites, fungal infections, or eczema.
Benzoic acid can be used as emulsion, toothpaste, jam or other food’s bacteriostatic agent.
It can also be used as a mordant in dyeing it can be use for the production of plasticizer and spices, also used as steel equipment’s rust-proofing agents and printing.
It is used in the production of the food and industrial preservatives sodium and potassium benzoate as a building block in the synthesis of alkyd resins.
Harmful effects of benzoic acid:
Immediately or shortly after exposure to benzoic acid, the following health effects can occur:
Eye damage.
Irritation of the skin, resulting in a rash, redness and a burning feeling.
Irritation to the nose, throat and lungs if inhaled, which may cause coughing, wheezing and/or shortness of breath.
Benzoic acid can cause drying and cracking of the skin with redness and itching.
Exposure to benzoic acid in high concentrations, particularly in susceptible individuals, may cause a skin allergy.
Principle:
Solution is more soluble in a solvent at its boiling point. Excess solute is given out in the form of crystals when hot saturated is cooled.
Purpose of recrystalization of benzoic acid:
- To purify samples of organic compounds that are solids at room temperature.
·
To
dissociate the impure sample in the minimum amount of an appropriate hot
solvent.
Procedure:
- First, prepared a saturated solution of given salt of benzoic acid.
- Filtered it through fluted filter paper.
- Cooled it at moderate temperature.
- Crystals of benzoic acid appeared.
- Crystals were separated by filter paper again.
- Washed with cold water.
Yield:
Gained yield=0.37g
Actual yield=1.0g
Yield= 0.37g/1.0g X
100=37g
Precautions:
Wash hands thoroughly
after handling.
Use only in a
well-ventilated area.
Minimize dust generation
and accumulation.
Avoid contact with eyes,
skin, and clothing.
Avoid breathing dust.
Confirmation:
Observed melting point of
benzoic acid=
Result:
Hence the purified
crystals of benzoic acid was formed dissociating the impurities.
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