Experiment #1 Recrystallization and Melting Points

Pure compound

A homogenous sample consisting only of molecules having the same strucutre.

A compound formed in a chemical reaction or extracted from some natural source is _______ _________ when initially isolated.

rarely pure

Chemical transformation intended to produce a single product almost invariably yield a reaction mixture containing sa number of contaminants.

My include the products of
-side reactions
-unchanged starting materials
-inorganic materials
-solvents

Are chemicals that have been purchased pure?

NO, they are needed in the purification process or to decomposition that may occur during storae and shipment.

What is the goal of organic chemists in purifying products?

To obtain a subtance that cannot be further purified by any known experimental techniques.

What is recrystaliization of solids?

A valuable techniqur to master, it is one of the most common methods used to purify solids.

What are three other methods of purifying solids?

-Sublimation
-Extraction
-Chromatography

Explain the process of recrystallization. (SOLUTION RECRYSTALLIZATION TECHNIQUE.

Dissolving the solid in an appropriate solvent at an elevated tepeature and allowing the crystal to re-form on cooling, so that any impurities remain in solution.

Describe an alternate approach of recrystallization of a solution.

Melting the solid in the absence of solent and then allowing the crystals to re-form so that impurities are left in the melt.

Is the alternate methid often used?

NO, because the crystals often form from a viscous oil that contains the impurities and it is difficult to separate the desired pure solid.

Are solids MORE soluble in a hot than a cold solvent?

Solution cystallization takes advantage of solids are MORE soluble in a HOT solvent.

*The extent to which a solid PRECIPITATES depends on thr difference in its solub9ility in thr particular solvent at temperatures between the extremes used.

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What is the UPPER extreme determined by?

By the BOILING point of the solvent.

What is the LOWER extreme determined by?

Usually dictated by EXPERIMENTAL CONVENIENCE.

What are ice-water baths often used for?

To cool the solution to 0 degrees Celsius.

What are ice-salt and dry ice-acetone baths commonly used for?

To cool solutions to -20 degrees Celsius and -78 degrees Celsius.

What should the soid be after this?

It should be recovered with reater efficiency at these teperatures, provided the solvent itself does not freeze.

Describe the impurities present in the original solid mixture that have dissolved.

If they have dissolved and remain dissolved after the solution is cooled, isolation of the crystals that have formed should ideally provide PURE material.

What could happen alternatively???

The impurities may not dissolve at all in the hot solutoin and may be removed by FILTRATION before the solution is cooled.

What should the crystals formed be?

The crystals that form should be PURER than the original solid mixture.

After a solid has been recrystallized, what may it not be?

It may still not be PURE.

What is the EASIEST METHOD for determining THE PURITY OF THE SAMPLE?

The melting point of the solid.

What is the technique of solution recrystallization?

-SELECTION of an appropriate solvent
-DISSOLUTION of the solid to be purified in the solvent near or at its boiling point.
-DECLORATION with an activated form of carbon, if necessary to remove colored impurities and FILTRATION of the hot solution to remov

What are the next three techniques of solution recrystallization?

-FORMATION of crtstalline solid form the solution as it cools.
-ISOLATION of the purified solid by filtration.
-DRYING the crystals.

SDDFID

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Selection of Solvent

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What are the five criterias of the use of a solvent in recrystallization?

1.) -SOLUBLE in the hot solvent and INSOLUBLE in the COLD solvent (room temp.)

*This combination of solvent and solute allows dissolution to occur in an amount o fsolvent that is not unduly large and also permits recovery of the purified product in high yield.

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Favorable TEMPERATURE COEFFICIENT.

A solvent having these solubility properties as a function of temperature would be said to have a favorable temperature coefficient.

2.) - the IMPURITIES should be either insoluble in the solvent at ALL TEPMERATURES or must remain at least MODERATELY soluble in the cold solvent.

if the impurities are SOLUBLE, the temperature coefficient must be UNFAVORABLE. Otherwise, the DESIRED PRODUCT and the impurities would both crystallize simultaneously from solution.

3.) - The boiling point of the solvent should be LOW ENOUGH so that it can be READIKY REMOVED from the crystals.

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4.) - The BOILING POINT of the solvent should generally be LOWER than the melting point of the SOLID being purified.

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5.) - The solvent should not REACT CHEMICALLY with the substance being purified.

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What should be used to find the appropriate solvent for recystallization?

trail-and-error techniques

What can be used for the process of selection for solutes?

Generalizations about solubility characteristics for classes of solutes.

Polar compounds are normally soluble in polar solvents and INSOLUBLE in NONPOLAR solvents.

Nonpolar compounds are more SOLUBLE IN NONPOLAR SOLVENTS. :LIKE DISSOLVES LKIKE

A highly POLAR compound is unlikely to be SOLUBLE in a HOT, nonpolar solvent, but it may be ......

VERY SOLUBLE in a COLD, very POLAR SOLENT.

What is the best choice for a satisfactory recrystallization?

A solvent of intermediate polarity.

How are the solvents commonly used in recrystallizations measured by?

DIELECTRIC CONSTANTS
-Measures in range of recrystallizations in polarity.

What are the solvents in the dielectric constants of 2-3 considered?

NONPOLAR

What are the dielectric with constants above 10 considered?

POLAR

What are solvents in the range of 3-10?

INTERMEDIATE POLARITY.

What is the special case with Petroleum?

- It doesn't contain the either FUNCTIONAL group at all.
- It is a mixture of VOLATILE ALIPHATIC HYDROCARBONS obtained from refining petroleum.

What does the composition and boiling point of the mixture of petroleum depend on?

It depends on the particular distillation "cut" obtained.
-The BOILING range of this type of solvent is ually "bp 60 to 80 dergrees Celsius)

Name chemical strucutres of Common Recrystallization Solvents found in the table 3.1 in the book.

-Ethanol
-Methanol
-Cyclohexane
-Toluene
-Diethyl ether
-Tetrahydrofuran
-Dioxane
-Dichloromethane
-Ethyl Acetate
-Acetone

This experiment involves the handling of large volumes of volatile solvents.

-Most of the solvents used are of relatively LOW TOXICITY.
-AVOID INHALING their vapors.
-PERFORMED in the hood.
Alternative: Clamping an INVERTE funnel over the recrystallization flask and connecting the stem of the efunnel to a soure of a vacuum.
-HIGHL

PERFORMING dissolutions in SINGLE solvents at mini and microsales differ from usin mixed solvents.

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Describe the Miniscale Technique.

The solid to be PURIFIED is WEIGHED and PLACED in an Erlenmeyer flask of appropriate size.

Why can't a beaker be used as a vessel for recrystallization?

It has a relatively LARGE SURFACE area to which RECRYSTALLIZED PRODUCT may stick, lowering the efficiency of recovery.

Why should some crystals of the IMPURE SOLID always be retained?

-They may be needed as "seeds" to INDUCE crystallization.

What should the flask be equipped with?

It should be equipped with MAGNETIC STIRRING. Or contain either boiling stones or a wooden stick to prevent bumping of the solution while boiling.

*Milliliters of solvent are added ot the flask and the mixture is heated.

More solvent is added to thr mixture in small poritions jusing a Pasteur pipet until the solid just dissolves.

Let BOILING RESUME after eac haddition so that a MINIMUM AMOUNT of solvent is used to effect DISSOLUTION.

Using excessive amounts of SOLVENT decreases the RECOVERY OF SOLUTE.
-May need to perform a HOT FILTRATION.

Also, ADD AN ADDDITIONAL 2-5% of solvent to prevent premature crystallization during this operation.

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What is likely to occur if adding solvent fails to dissolve any more solid?

-It is LIKELY that insoluble IMPURITIES ARE PRESENT.
-These impurities can be removed by HOT FILTRATION.

Why is being careful with using too much solvent important?

This is rising POOR RECOVERY of the PURIFIED SOLUTE. This is imortant when only a SMALL quantity of solid remains, this may be the material that is INSOLUBLE.

Describe the Microscale Technique?

The protocol for DISSOLUTION at the microscale level mimcs that at larger scales.

-Solid is weighed and placed in a test tube with a few crystals of IMPURE MATERIAL being retained as "seeds" TO INDUCE CRYSTALLIZATION.

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Few drops of solvent and the mixture is heated to the boiling point. Additional solvent as needed is added dropwise using a pipet.

The DIFFERENCE is that the MICROSCALE mimics that at LARGER SCALES.

MIXED SOLVENTS DISSOLUTION

Two options for effecting the dissoluton once the solvents have been selected.

FIRST OPTION.) -Solid to be PURIFIED is first DISSOLVED IN A MINIMUM VOLUME of the hot solvent in which it is soluble.

The SECOND SOLVENT is thn added to the BOILING SOLUTION until the mixture turns cloudy.

What does the cloudiness signal?

Signals initial formation of CRYSTALS. Caused by the fact that the addition of a SECOND SOLVENT results in a solvent mixture in which the solute is less soluble.
- MORE of the FIRST SOLVENT is added dropwise until the solution clears.

TWO FURTHER ASPECTS OF THIS OPTION SHOULD BE NOTED.

-The solution should be COOLED SLIGHTLY below the LOWER boiling point before the second solvent is added IF THIS SOLVENT HAS A BOILING point lower than the first.
-OTHERWISE this addition could cause SUDDEN boiling of the mixture.

-Also, hot filtration should be performed if needed BEFORE ADDITION of the second solvent.

...This will PREVENT crystallization during the filtration step.

What is the potential disadvantage of this method for mixed-solvent recrystallization?

Is that unduly large volume of the secod solvent may be required if excessive amounts of the initial solvent have been used.

What is the second option for effecting dissolution once the solvents have been selected?

The solute is ADDED to the solvent in which iti s INSOLUBLE and the mixture is heated near the BOILING POINT of the solvent. The second solvent is then added in small portions until the solid just dissolves.

What happens if you use too much of the first solvent?

The disadvantage is that using too much of the first solvent will require the addition of the undesirably large volumes of the second one.

Decoloration and Hot Filtration

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After dissolution of the solid mixture, what does the change of solution color signal?

It signals the presence of impurities if the desired compound is known to be colorless.

What if the compound is colored?

Contaminants may ALTER the color of the solution.

Ex. impuritiies should be suspected if the substance is yellow but the solution is green.

The DECOLORATION step is unecessary if the solution is COLORLESS.

How can colored impurities be removed?

By adding a small amount of decolorizing carbon, to the HOT but not BOILING solution.

What would hapen if it were added to a boiling solution?

The liquid wiukd likely frith over the top of the flask, resulting in the LOSS of product and possible injury.

What do you do with the solution once the decolorizing carbon is added?

The solution is heated to boiling for a few minutes while being continuously stirred or swirled to prevent bumping.

Why is removing decolorizng carbon by hot filtration dificult powdered form is used rather than the pelletized form is used?

The finely divided solid particles may pass through filtre paper or the cotton of a Pasteur filterating pipet.

What are the steps to remove the decolorizing carbon during filtration?

The hot decolored solution is allowe to cool slightly below its boiling points, a small AMOUNT OF FILTER-AID such as Celite is added to ABSORB the caron.
-The mixture is then REHEATED to boilin. Then, iti s subjected to a hot filtration.

Why does this technique for decolorizing solutions work?

It works because colored impurities as wellas the compound being PURIFIED are absorbed on the surace of the CARBON PARTICLES. Less product will be recovered if you use TOO MUCH decolorizin carbon.

What is removed by gravity filtration of the hot solution?

Insoluble impurities such as dust and decollorizng carbon are removed, but this IS NOT NECESARY if the solution is clear and homogeneous.

Why is vacuum filtration preffered for gravity filtration?

Because the latter technique may cause cooling and concentration of solution, leading to EVAPORATION of the solvent, and this may result in PREMATURE CRYSTALLIZATION.

What should be used to minimize crystalization in the funnel?

-A short stemmed or stemless glass funnel and using FLUTED filter paper will MINIMIZE crystaqllization on the filter

CRYSTALLIZATION

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What is the hot solution of solute allowed to do?

It is allowed to cool slowly to room temperature, this will allow crystallization TO OCCUR.

What should the solution be protected from and how during COOLING AND CRTSALLIATION?

Should be proteected from AIRBORNE CONAMINANTS by convering the opening with FILTER PAPER, INVERTED BEAKER, or by loosely plugging it with a clean cork.

How to do rapid cooling?

IMMERSING THE FLASK in water or ICE WATER bath.

What does this do?

Tends ot ead to VERY SMALL CRYSTALS that may absorb IMURITIES from solution.

Why should the formation of crystals larger than about 2 mm be aboided?

Because some of hte slution may become TRAPPED within the crystals. The drying of these crystals masy be MORE DIFFICULT AND IMPURITIES may be left in them.

How to fix large crystals?

Gentle agitation of the solution normally induces production of smaller crystals.

What does failure of crystalliation to occur after the solution has cooled usually mean?

Means that either too much solvent has been used or that the soution is SUPERSATURATED.

SUPERSATURATED SOUTION

Can be made to produce crystals by SEEDING.

SEEDING

A crystal of the original solid is ADDED to the solution to INDUCE CRTSTALLIZATION which may then be quite rapid.

If not SOLID is available and a VOLATILE SOLVENT is being used, IT IS SOMETIMES POSSIBLE TLO PRODUCE A SEED CRYSTAL by immersing hte tip of a glass stiring rod and alloing ht solvent to EVA[PRATE.

The crystals that form on the END OF THE ROD are reinserted into the solution to initiate crystallization.

DEFINE OILING OUT

The solute will sepearate from the SOLUTION as an oil rather than a solid.

Why is oiling out undesired?

The OILS usually contain signifianct aounts of impuritiites.

How to solve oiling out?

- Oils may persist on COOLIN Gwith no evidence of crytstallization. May be often INDUCED to crystallize by scratching the OIL AGAINST THE SIDE of the flask with a glass stirring rod at he interfac of th oil and the solution.

- Oils may form from th ehot solution and then SOLIDIFY to an amorphous mass at LOWER TEMPEATURES.

The crystals o the solute may precipitate from the mother liquor. The oil is not PURE LIQUID, the solid mass produced from it will be IMPURE. A usualy remedy is to REHEAT THE ENTIRE MIXTURE to effect dissolution, ADDA FEW ML of additional pure solvent and

How is the crtstalline product with filtration?

it is ISOLATED.

The technique for doing this varies depending on the scale on which the crystallization was performed.

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FILTRATION AND SOLVENT REMOVAL

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MINISCALE PROCEDURE

-The SOLID is isolated by VACUUM FILTRATION usin a Buchner or Hirsch funnel and a CLEAN DRY FILTER FLASK.
-The crystals are normally washed with a small amount of PURE COLD SOLVENT with the vacuu off.
-The vacuum is then REAPPLIED ot remove as much solven

MICROSCALE TECHNIQUE

-The crystalline product is isolated by CENTRIFUGATION. (sedimentation of particles under the influence of the centrifugal force and it is used for separation of superfine suspensions. )

-The solid is SCRAPED OFF THE PLUG BACK into the Craig tube and washed by STIRRING IT BRIEFLY with a small amount of pure, cold solvent.
-The plug s REINSERTED and the solvent is once again removed by centrifugation.

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What is the filtrate sometimes called?

The mother liquor

Further cooling of hte iltrate in an ice water bath may allow ISOLATION of a second crop of crystals.

The fitrate can also be CONCENTRATED by evaporating part of hte solvent and cooling the residual solution. These crystals are likely to be LESS PURE than the first. They should not be comvined until their PURITY HAS BEEN ASSESSED by comparison of their me

DRYING THE CRYSTALS.

AT THE MINISCALE level, the FINAL TRACES of solvent are removed by transferring the crystals from the filter paper o the BUCHNER funnel to a watchglass o vial.

Solids may also be TRANSFERRED TO FRESH PIECES OF filter or weighin paper for dryin.

This is NOT DESIRABLE because fibers or paper may CONTAMINATE THE PRODUCT when it is transerred.

FOR MICROSCALE OPERATIONS.

The solid may be left in the CRAIG TUBE used for the crystallization. It is good practice to protect the crystals from airborne contaminant by COVERING THEM with a piece of filter paper or using loosley cotton or cork.

How to remove the last traces of solvent from the crystalline product?

By AIR OR OVEN DRYING. The temperature MUST BE 20-30 degrees Celsius below the melting point of the product or the crystals will turn into a PUDDLE.

MELTING POINT SAND APPARATUS

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How to determine the melting point of a compound?

Involves heating a small amount of a solid and determining the temperature at which it melts. Most use a CAPILLARY TUBE to contain the sample.

What is the first tep in determining a melting point?

Transgerring the sample into a melting point capillay tube. These tubes have ONE SEALED END AND ARE COMMERCIALLY AVAIABLE.