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Post lab discussion 1

MJP

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Experiment 1: Solubility

2: Isolation of caffeine

3: Recrystallization and Melting

point determination

4: Steam distillation

5: Chromatography

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Solubility

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Solubility of Organic Compounds

Strong base (Amine) Weak acid (Phenol)

Strong Acid (Carboxylic acid)

Neutral Substance (Aldehyde, ketone, alcohol, ester)

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Solubility category

(1) Solutions in which both the solvent and the

solute are covalent (molecular).

(2) Ionic solutions in which the solute ionizes and

dissociates.

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General guidelines

covalent bonding

All hydrocarbons are non polar Compounds with electronegative atoms (O, N, S) are

generally polar

Check for the presence of C=O, C-O, OH, NH and CN

The presence of electronegative halogens (Cl,I,Br) does not

alter the polarity of an organic compound in a significant way

(slightly polar)

When comparing compounds of the same family,

adding carbon to the chain decreases the polarity

Hydrogen bonding increases solubility in water

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Solubility in 5% HCl

The possibility of amine should be considered if a

compound is soluble in 5% HCl.

R NH2 + HCl R-NH3+ + Cl-

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Solubility in 5% NaHCO3 and 5% NaOH

Compounds that dissolve in sodium bicarbonate(weak

base) are strong acid

Compounds that dissolve in sodium hydroxide(strong

base) may either be a strong or weak acid.

Strong acids

Soluble in both NaOH and NaHCO3

Weak acids

Soluble in NaOH but not in NaHCO3

Sulfonic acids R-SO3H

Carboxylic acids RCOOH

Phenols ArOH

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Solubility in Concentrated sulfuric acid

Alcohols, ketone, aldehydes and esters, alkenes,

alkynes, ethers, nitroaromatics and amides Compounds that are soluble in concentrated sulfuric

acid but not in dilute acid are extremely weak bases.

Almost any compound with N, O or S can be

protonated.

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Inert compounds

Compounds that are not soluble in concentrated

sulfuric acid or any other solvent are INERT.

Alkanes, simple aromatics, alkyl halides.

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N HCO3

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NaHCO3

Acetone S1 + +

Aniline B _ _ +

Benzyl alc N _ _ _

Benzaldehyde N _ _ _ +

Benzoic acid A1 _ + +

Benzamide M _ _ _

Ethyl alcohol S2 + _

Hexane I _ _ _ _

Phenol A2 _ + _

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Isolation of caffeine

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Solid Liquid extraction

Solid Liquid Extraction Caffeine is soluble in hot water as well as tannins and

other impurities

Tannins are acidic compounds

Calcium carbonate (as a base) calcium salts are formed;

removed by filtration

Water insoluble compounds: cellulose, proteins and lipidmaterials

Over 200 compounds are extracted in hot water

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Liquid liquid extraction

Liquid-Liquid Extraction

Caffeine is extracted with chloroform or dimethylchloride

The solute (caffeine) distributes itself between two

immiscible liquids (Chloroform and water)

The equilibrium is:

A (aq) A (org)

K = [A] org / [A] aq

Why is it advisable to perform liquid-

liquid extraction using small portions of

solvents rather than one large volume?

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Good qualities of solvent for liquid-liquid

extraction

Immiscible with the first solvent (usually water)

Target compound should be soluble in extracting

solvent

Major impurities should not be soluble

should be sufficiently volatile

Non toxic

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Extraction efficiency

1.0g of organic compound is dissolved in 100.0mL of

water. Compare the extraction efficiency with two

30.0mL portions of ether versus one time 60.0mL

extraction. The distribution coefficient at 298K is 8.36.

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Emulsions

Colloidal suspension of one liquid to another;minute droplets of an organic solvent often are heldin suspension in an aqueous solution when the twoare mixed vigorously.

How to minimize emulsions

1. Add sodium chloride

2. Add water soluble detergent

3. Gentle swirling or shaking of the sep funnel4. Filter the mixture

5. Separate by using a centrifuge

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Steam distillation

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Steam distillation is the distillation of a mixture of water

(steam) and an organic compound (or mixtures of it)

Criteria for compounds to be steam distillation:

1. Must have a high boiling point

2. Immiscible with water

3. May decompose if direct distillation is attempted

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Principle

Raoults Law: = * for miscible liquids

= + for immiscible liquids

During boiling, The total vapor pressure of themixture is equal to the atmospheric pressure. The

temperature boils at a lower temperature than the

boiling points of any one components.

=

0

0

The composition ofthe distillate and the

boiling point is

constant throughout

the process.

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BP and composition of distillates

mixture Boiling point ofpure substance

Boiling point of

mixture

Composition (%

water)

Benzene-Water 80.1 69.4 8.9

Octane- water 125.7 89.6 25.5

1-Octanol-water 195.0 99.4 90.0

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At 760 mmHg, bromobenzene (C6H5Br, bp 155C) steam

distills at approximately 95C. Calculate the

approximate amount of water needed to steam distill20 g of bromobenzene.

(at 95C, the vapor pressure of water is 634mmHg)

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Recrystallization

and

Melting point determination

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Crystallization vs precipitation

Precipitation rapid formation of an amorphous solid

- occurs when a hot saturated solution is

cooled too quickly

Crystallization slow formation of crystalline solid

- impurities tend to be excluded from thegrowing crystal structure

(why?)

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Crystallization a purification technique for impurities that

have a different solubility with your target compound.

Recrystallization further crystallization to obtain a purer

compounds.

Steps in crystallization:

1. Choose the best solvent

2. Dissolving the compound in a minimum amount of hot

solvent.

3. Filter to remove impurities

4. Cool the crystal slowly.

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What is the best solvent?

(C)

(A)

Grams

Soluble

(B)

temperature

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Solvent for crystallization

Ideal solvent:

1. Does not react with the compound

2. Boils at a temperature below the compounds melting

point3. Dissolves a large amount of compound when hot and

small amount at room temperature

4. Moderately volatile

5. Non toxic, non flammable and inexpensive

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Supersaturated solution

A hot solution cools to room temperature yet there is no

crystal formation.

What will you do?

1. Is the solution supersaturated?

2. Too much solvent?

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Melting point

Primary index of purity for crystalline compounds

MP is a physical property of that substances.

MP can be used to identify a substance

The purer the material, the higher is the melting point

The purer the material, the more narrow is the melting

point range

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Effect of impurities

*pure organic compounds has a sharp meltingpoint (range of 1.0C or less)

*impure compound a broader range (3.0C or more)

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Chromatography

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Chromatography

Mikhail Tsweet coinedthe termCHROMATOGRAPHY,

from the Greek words

chromacolor andgrapheinwriting.

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Chromatography is a physical method of separationin which the components to be separated aredistributed between two phases, one which isstationary (stationary phase), while the other (themobile phase) moves in a definite direction.IUPAC

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Mobile phase: moving fluid liquid or gas orsupercritical fluid carries the components ofthe sample through volume or layer ofparticles of solid materials

Stationary phase: solid support, sorbent,

static phase, packing material

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Classification

Based on the physical means by which the stationary and

mobile phases are brought into contact

1. Column chromatography

= stationary phase is held in a narrow tube throughwhich the mobile phase is forced underpressure/gravity

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2. Planar chromatography

= stationary phase is supported on a flat plateor in the interstices of paper; here, themobile phase moves thru the stationaryphase by capillary action or under theinfluence of gravity

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Based on the types of mobile and stationary phases and

the kinds of equilibria involved in the transfer of solutes

between phases.

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Distribution constant

Kc= distribution constant;

partition ratio; partition coefficient

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If the stationary phase is POLAR: the least polar

substance moves out of the column last (Column

Chrom) or the spot is on top of the paper (PaperChrom)

If the stationary phase is NON-POLAR: the most polar

subtances moves out of the column first (Column

Chrom)

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Paper chromatography liquid-liquid

partitioning technique

Stationary phase adsorbed water molecules cellulose

Mobile phase Solvent 1: pet ether:diethyl ether-

acetone

- Solvent 2: pet ether:acetone

(which of the two solvent system is able to separate

many pigments in the leaf?)

- AA: 1:2 2% ammonium hydroxide:

isopropyl alcohol

* Solvent moves through capillary action*

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Cellulose absorbs 22% water.

Most polar substance is strongly held, least polarsubstance moves up the top of paper

Paper chromatography is applied to the separation ofhighly polar substance or those that are polyfunctional.

(sugars, amino acids, natural pigments)

f l ( h )

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Rf value (paper chrom)

Rf values (ratio to front or retention factor)

Rf = distance travel by substance/distance travelledby solvent

Low Rf value ---- strongly bonded to the stationaryphase

High Rf value ------strong attraction to the solvent

(least bonded to the stationary phase)

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Thin layer chromatography- solid-liquid

partitioning technique

Stationary phase silica or alumina on a support

(polar)

Mobile phase ------- solvent system 6:2:2

(v/v/v) n-butanol-ethanol-NH3

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Important uses of TLC in organic chemistry

1. To establish if two compounds are identical

2. To determine the number of components in a mixture

3. To determine the appropriate solvent for columnchromatography

4. To monitor column chromatography separation

5. To check the effectiveness of separation

6. To monitor the progress of reaction

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Rf value for TLC

1. The solvent system

2. Adsorbent

3. Thickness of the adsorbent layer

4. Relative amount of material spotted

Rf values can be used to identify the unknown only when

the conditions are completely specified.

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Techniques and problems encountered

1. Spotting --- the smaller the spot, the better the

separation

2. Development ---- the chamber should be lined

with filter paper to saturate

the air with solvent vapors

for speedy development

3. Solvent System --- trial and error