Expt 3 and 4 Sublim and Recryst

7/29/2019 Expt 3 and 4 Sublim and Recryst

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- definite and constant composition throughout the

system

molecules which upon exhaustive fractionation fails to

yield fractions with different properties

- free from other substance (impurities)

Solids melting point, freezing pt, solubilityLiquids

- refractive index, boiling point, density


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1. Governed by the solubility of a solid in a solvent

increases with increase in temperature and decreaseswith decrease in temperature.

2. Purification is based on differences in the solubility

of the solids and impurities in a given solvent.

PURIFICATION TECHNIQUE:

RECRYSTALLIZATION


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1. Choosing the appropriate solvent

2. Dissolving the impure sample in a suitable solvent at or near the boiling point

to form a nearly saturated solution.

3. Decolorization: Addition of activated carbon/charcoal is used to absorb all

colored impurities.

4. Filtering the hot solution to remove any insoluble impurity.

Allowing the hot solution to cool slowly thus causing some of the dissolvedsubstance to crystallize out. Crystals are well formed in slow cooling, whereas at

rapid cooling, precipitation occurs.

If crystals do not form:

Seeding (to serve as nuclei for crystal growth)

Scratching (disturb equilibrium of the supersaturated solution)

5. Separating the crystals from the supernatant solution (also called mother

liquor) by filtration.

6. Test for purity melting point. If found to be impure, recrystallization can be

repeated until pure.

Steps of purification through crystallization


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Choice of Solvent1. Non-reactive with solutes

2. Dissolves most substance to be crystallized inhot solution and crystallize most of the

substance when cold.

3. It should dissolve either the substance of

interest or only the impurities, but not both.

4. Boils at a temperature below the compounds

melting point.

5. It should yield well formed crystals of the

desired substance.

6. Easy to separate from the crystals usingfiltration. Moderately volatile, so crystals can

readily be dried.

7. Nontoxic, nonflammable and inexpensive.


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1. If the sample dissolved immediately in 1 mL of the solvent

or upon slight warming, the solvent is unsuitable.

2. If all solids does not dissolved even on heating, another ml

of the solvent is added and heat is again applied. If until

after 3 mL of the solvent and is still undissolved even when

heated, the solvent is unsuitable.

3. If the solid dissolves in the hot solvent, the test tube is

cooled to determine whether crystallization occurs. If no

crystals appear, this maybe due to the absence of nuclei

for crystal growth. The glassware can be scratched with a

glass rod below the surface of the solution or seeded with a

few crystals. If no crystals form, the solvent is not suitable.4.


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Synthesis of Acetanilide

(Reaction mechanism: Nucleophilic Acyl Substitution)


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Synthesis of Acetanilide

(Reaction mechanism: Nucleophilic Acyl Substitution)

Clear,light red oily liquid Clear colorless liquid White glistening crystals

with aromatic odor with strong vinegar odor

d= 1.02 g/mL 1.08 g/mL 1.219 g/mLMW= 93.13 g/mole 102.9 g/mole 135.17 g/mole


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PURIFICATION TECHNIQUE: SUBLIMATION

A purification technique for organic solids using heat to increase the

vapor pressure of the solid, to convert it directly to gas without undergoing

the liquid phase. Upon cooling, the crystals are of higher purity.

Figure 1. Phase Diagram


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Factors affecting sublimation:

1. Desired compounds must have relatively high vaporpressure naphthalene, anthracene, benzoic acid,

salicylic acid, beta-naphthol, camphor, iodoform, and

acetanilide.

2. Desired compound does not decompose upon heating.3. Sample must be able to be recondensed.


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Advantages of sublimation technique

1. Less tedious and shorter to perform compared to

recrystallization.2. Less contamination in the desired product.

Disadvantages of sublimation technique

1. A considerable amount of solute may be lost while

heating.

2. Limited to a few compounds with have relatively high

vapor pressure.

3. Impurities may also sublime with the desired substance,

even if repeatedly done to purify mixture.


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Identification of Substance/Purity Test

Benzoic acid is a weak acid that's used as a preservative. It's naturally

ocurring in many fruits in trace amounts. The largest natural source ofbenzoic acid is from the gum of the Chinese Balsam tree, called gum

benzoin. Gum benzoin is typically 13-20% benzoic acid.

Physical properties:

Color and Nature - White, crystalline flakes

Melting Point 1220

C Boiling Point 2490

CSpecific Gravity 1.321 Solubility in water 0.29 g/100 ml


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TEST for PURITY using Melting Point Determination

1. Melting point temperature range at which thecrystals start to melts and the last crystal disappear.

Melting range for pure substances - =/< 0.5 0C.

2. Heat of fusion amount of energy needed to

convert the sample from solid to its liquid states, bydisrupting the intramolecular forces that are taking

place in the crystal lattice.

3. A pure substance has a sharp and narrow meltingpoint. Impurities can lower the melting point

(colligative properties) and broaden the melting

point range.


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Factors to ensure proper melting point determination

1. Well-grounded/pulverized

2. Well-packed3. No spaces (absence of air/moisture)

4. Dry sample

5. Small amount of sample, to ensure uniform heating

Upon melting, the following phenomenon can take place

1. Decomposition2. Oxidation

3. Rearrangement

4. Hydration

5. Polymorphs (oxalic acid-118/1210C mpt)

Mixed melting point method

Expt 3 and 4 Sublim and Recryst

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