Organic Chem 3

Chemistry Department

University of Malaya

Organic Chemistry Laboratory Manual

Level 3

Organic Chemistry Level 3____________________________

Organic Chemistry

CONTENTS Page Introduction 2 1. Conversion of Cholesterol to Cholestenone 3

2. Formation of carbon-carbon bonds – The Wittig Reaction 6

3. Lactone Synthesis 10

4. Analysis of a Binary Mixture 12

5. Appendices 15

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SCES/P3120 INTRODUCTION

Laboratory Notebook:

One laboratory notebook or jotter (e.g. inexpensive exercise book ) is required for recording

all primary experimental data. All reaction products must be submitted.

Course Content

The Course is designed so that on completion you should know of the following:

(a) How to separate and identify organic compounds by classical and spectrophotometric

techniques and chromatographic techniques including LC and TLC

(b) How to do routine operations e.g. crystallization, distillation, sublimation, “vacuum”

distillation, and extractions.

(c) How to synthesize compounds and study reaction mechanisms

The following is an approximate guide as to how you should spend your time in the lab.

1. Two organic synthesis experiments 50% Credit

2. Spectral and Qualitative analyses of a binary mixture 50% Credit

This is a final Year Organic chemistry course and you are expected to do most of the

laboratory operations on your own. As a matter of routine you may have to perform

recrystallizations, distillations, extractions, steam distillation, vacuum distillation and other

things learnt during the earlier years. You have no excuse if you do not know them. If you are

unsure or have forgotten refer back to your manuals or texts. You are also reminded not to

neglect safety precautions (e.g. in anticipation of fire hazards, in handling corrosive

chemicals, recovery/disposal of solvents, in handling toxic/carcinogenic/explosive

chemicals). As a matter of good personal habits expected of a potential UM graduate you are

to be tidy, save on chemicals and handle equipment or glassware with intelligence and care.

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Experiment 1

CONVERSION OF CHOLESTEROL TO CHOLESTENONE

Preparation of Δ5-cholesten-3-one and Δ4-cholesten-3-one from cholesterol – use of a

protecting group in organic synthesis

In an attempt to oxidize cholesterol (I) to the corresponding ketone Δ5-cholesten-3-

one (II), it was found (L. F. Fieser, J. Am. Chem. Soc., 1953, 75, 4377, 4386) that it was not

possible to obtain pure Δ5-cholesten-3-one. Due to the presence of a double bond β, γ- to the

alcohol group, an array of products were obtained:

HO(I)

O(II)

O OO

OO

HOOCHOOC O

OHO

HO O

To avoid this difficulty it is necessary to temporarily remove the double bond and to

regenerate it after oxidation. This is carried out by adding bromine across the double bond,

and later debrominating using zinc (Note: modern reagents for carrying out the oxidation

directly without affecting the double bond include PCC, TPAP/NMO, and (COCl)2/DMSO;

Et3N or the Swern oxidation).

HO HO OBrBr BrBr

Br2 Na2Cr2O7

(III) (IV)

O(II)

O

Zn (CO2H)2

(V)

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Under the mild condition to be used here for debromination, it is possible to isolate

the unconjugated ketone Δ5-cholesten-3-one (II). Subsequent treatment of this with acid,

however, will cause migration of the double bond, resulting in formation of the α,β-

unsaturated ketone Δ4-cholesten-3-one (V).

Procedure

Preparation of Cholesterol Dibromide (III)

Add a mixture of 2.5 g bromine (caution), 0.2g anhydrous sodium acetate and 25 mL

glacial acetic acid to 5 g cholesterol in 50 mL ether. After a few minutes, the mixture

will precipitate as crystals. Collect these cholesterol dibromide (III) crystals by

suction filtration and wash with a small amount of glacial acetic acid.

Oxidation – 5α,6β-Dibromcholesten-3-one (IV)

Transfer the cholesterol dibromide (III) into a flask containing 70 mL glacial acetic

acid. Prepare a solution of 2.7g sodium dichromate in 70 mL glacial acetic acid

which has been warmed to 90 oC and pour this mixture, with stirring, into the flask

containing the cholesterol dibromide (III) suspension. The temperature of the mixture

must be kept between 55-58 oC and the solid must dissolve within 5 minutes (if not,

warm the mixture to 65 oC and most of the solids should dissolve within 10 minutes.

Discard any impurities that did not dissolve). Leave the mixture to cool to room

temperature (about 30 minutes). Cool the mixture in an ice-bath and add (dropwise)

25 mL water: EtOH (2:1; v/v) mixture while stirring with a magnetic stirrer for 30

minutes at 0 – 10 oC. 5α,6β-Dibromcholesten-3-one (IV) is obtained as fine white

crystals, m.p. 72-76 oC. Proceed to the next step, or, if time does not permit, store

these crystals in the fridge.

(Note: Cholesterol could be oxidised directly using the chromium complex,

CrO3.2(C5H5N))

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Debromination to Δ5-cholesten-3-one (II)

Transfer 2 g (IV) prepared earlier to a 50 mL conical flask and add 20 mL ether and 5

mL glacial acetic acid. Add 1.2 g zinc dust (freshly grounded) in 3-4 portions over 15

min while stirring with a magnetic stirrer. Heat will be generated and zinc acetate

will precipitate out. Stir for 15 minutes, and monitor the reaction progress by TLC.

After reaction has completed, add 1 mL water to dissolve the zinc salts. Add some

ether to the mixture and wash with water followed by 10% NaOH solution. Filter the

solution through cotton and Na2SO4 (1-2 g) and wash the residue with 5 mL ether.

Add 10 mL MeOH to the filtrate and evaporate the ether on a steam bath until crystals

appear. Collect the Δ5-cholesten-3-one (II) crystals (~60 %, m.p. 122-125 oC, vmax

1720 cm-1).

Isomerisation Δ4-cholesten-3-one (V)

Heat a mixture of 1 g Δ5-cholesten-3-one (II), 0.1 g anhydrous oxalic acid and 8 mL

95% ethanol on a steam bath or a hot plate till all the crystals dissolve (about 15

minutes) and continue heating for another 10 minutes. Leave to cool until product

crystallises and collect (m.p 81-82 oC, vmax 1620, 1670 cm-1).

Question:

1. Calculate the λmax for the compounds I – V. Compare the IR and NMR

spectra for both ketones (II) and (V).

2. Write mechanisms for the transformations that you have carried out.

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Experiment 2

WITTIG REACTION

THE SYNTHESIS OF 3-PHENYLPROPENOIC ACID

Background

The Wittig reaction is used to convert the carbonyl group of aldehydes and ketones

into an alkene group. This reaction is named after Georg Wittig who won the Nobel

Prize for the reaction in 1979. In a typical Wittig reaction, triphenylphosphine (1)

reacts with an alkyl halide (2) to first form a phosphonium halide (3). Subsequently,

addition of a strong base eliminates the hydrogen halide to form an ylide,

alkylidenephosphorane (4) (Scheme 1).

Scheme 1

Ph3P: + R'H2C X Ph3P CH2R' Ph3P CHR'

Ph3P CHR'

X

Base

(1) (2) (3)

(4)

The carbon of the ylide acts as a nucleophile and adds to the carbonyl group (5) to

form a betaine intermediate which undergo an in situ 1,2-elimination to give the

triphenylphosphine oxide (6) and an alkene (7) as the product (Scheme 2).

Scheme 2

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Ph3P CHR'R

R''O

Ph3P O

R'HC CR''

R

Ph3P OR

R''CHR' +

+

(4) (5)

(8)(7)

This experiment illustrates the general procedure with the preparation of (E)-3-

phenylprop-2-enoate (10) from the reaction of the ylide, methyltriphenyl-

phosphoranylethanoate (9) and benzaldehyde. The ester which is formed (10) is then

hydrolyzed with base and 3-phenylpropenoic acid (11) is then isolated as crystalline

solids (Scheme 3).

Scheme 3

Br CO2Me Ph3P CO2Me Ph3P CO2Me

Ph3P CO2Me

OPh PhCO2Me

PhCO2H

(8)(9)

(9)

(10) (11)

Br

Procedure

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(I) Preparation of triphenylphosphoranyl ethanoate (4) from methoxycarbonyl

- methylenyltriphenylphosphonium bromide.

Dissolve triphenylphosphine (5 g, 0.02 mol) in dry toluene (50 mL) in a clean

and dry 250-mL round-bottomed flask. Add methyl bromoethanoate (3 g, 0.02

mol) [Note 1] to the mixture and warm the mixture to 60 oC. Shake the

mixture once in a while, keeping the temperature constant at 60 oC for 10

minutes. Cool the reaction mixture to room temperature and leave for 1 hour

at this temperature. Collect the phosphonium salt (3) (~4 g) by filtration [Note

2]. Weigh the product and record its melting point.

Dissolve the phosphonium salt (3) (4.2 g, 0.01 mol) in toluene (100 mL) and

add to an aqueous solution of 0.38 M sodium hydroxide (100 mL). Stir the

mixture vigorously for 1 hour (or until the liquid phase becomes clear).

Separate and dry the toluene layer using anhydrous Na2SO4. Remove the

solvent using the rotary evaporator to obtain the crude product (4). Record the

weight and melting point of the product. If necessary, the crude product could

be recrystallised using ethyl acetate/petroleum ether (40-60 0C) mixture as the

solvent.

(II) Preparation of (E)-3-phenylpropenoic acid

Method 1

Dissolve the crude product (4) (3 g, 0.0089 mol) in dichloromethane (10 mL)

and add to it a solution of benzaldehye (1.0 g, 0.0094 mol) in dichloromethane

(10 mL). Reflux the mixture in a water bath for 1 hour. Remove the solvent

to obtain a yellowish white solid containing methyl 3-phenylpropenoate. Add

saturated sodium carbonate solution (20 mL) and water (100 mL) to the solid

and stir the mixture vigorously for 30 min to hydrolyze the ester to an acid

salt. At this stage, it is suitable to leave the mixture overnight before

proceeding to the next step.

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Reflux the mixture and distill off the excess benzaldehye [Note 3]. Cool the

mixture and add water to replace the amount that was distilled off. Filter the

mixture to remove the crude triphenylphosphine oxide. Acidify the filtrate

with concentrated hydrochloric acid to precipitate out the 3-phenylpropenoic

acid. Recrystallize the 3-phenylpropenoic acid from a mixture of water-

ethanol solution (3:1, v/v). Weigh and take the melting point of the acid.

Record its UV and IR spectra.

Method 2:

Add dichloromethane (10 mL) to benzaldehyde (1g, 0.0094 mol) in a round-

bottomed flask. Loosely stopper the flask, place it in an ice bath, and stir for at

least ten minutes. After this time, slowly stir the crude product (4) (3 g, 0.0089

mol) and continue to stir in the ice bath for an additional ten minutes. Then

allow the solution to warm up to room temperature while continuing to stir.

Retrieve the stir bar from the flask and remove the solvent from the reaction

mixture using a rotary evaporator. Add hexane (20 mL) to the residue in the

flask and gently stir with a glass rod. The product ester is soluble in hexane

but the triphenylphosphine oxide is not. Vacuum filter the solution using

another 20 mL hexane to rinse the flask. Transfer the filtrate to another dry

and clean round-bottomed flask. Remove hexane using a rotary evaporator.

Weigh and record the melting point of the product. Characterize the product

using UV and IR.

Notes:

1. Methyl bromoethanoate is a lachrymator. Please use the fume-cupboard to weigh.

2. More precipitate could be obtained if the filtrate is kept overnight.

3. Distilling of about 50 mL solution is sufficient to remove any volatile compound

from the reaction mixture.

QUESTIONS

1. What is the structure of the triphenylphosphoranyl ethanoate (4)?

2. Write a mechanism for formation of (E)-3-phenylpropenoic acid.

3. What is the limiting reagent in the above Wittig reaction above?

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Experiment 3

LACTONE SYNTHESIS VIA A REFORMATSKY REACTION- CORRECTING THE

LITERATURE

As part of a conformational study, it was desirable to obtain a γ-lactone with a bulky

substituent at the β-position. According to the literature, the synthesis of β-t-butyl-γ-

butyrolactone was an easy matter, consisting of two simple steps. However when this was

carried out, the product had an anomalous NMR spectrum. In this experiment, the lactone is

prepared, the NMR spectrum obtained, the product structure deduced, and the reaction

mechanism reinterpreted.

Me3CCOCH3 Me3C COH

CH3

CH2CO2MeReformatsky

Zn/C6H6BrCH2CO2Me+

O

O

Me3C

50% H2SO4

Procedure

Preparation of Methyl-3-hydroxy-3,4,4-trimethylpentanoate

Weigh 8g of granulated zinc into a 250 mL 3-necked flask with two necks closed by

stoppers. Add 25 mL of bench dilute sulphuric acid to the zinc, and allow the steady reaction

to proceed for 5 min by which time the metal surface should appear bright. Then, decant off

the acid and wash the metal successively with water (2 x 10 mL) followed by a good grade of

acetone (1 x 10 mL). After decanting off as much solvent as possible, add 30 mL of benzene,

fit the flask for downward distillation and distil to a final volume of approximately 5 mL.

While the flask remains warm, fit it with a mechanical stirrer, a reflux condenser and

dropping funnel (Note 1), and place in the dropping funnel a mixture of pinacolone (10g),

methyl bromoacetate (15.4g) in dry (Note 2) benzene (50 mL). Run in 10 mL of this solution,

and observe carefully for the reaction to begin (Note 3). After the reaction has started, the

remainder of the solution should be added at a rate to maintain gentle reflux ( cf. Grignard

Reaction, Level II Practical ). When addition is complete, reflux the reaction until almost all

the zinc has disappeared (2 – 3 hr). Then, cool the flask in an ice bath, and with stirring, add

enough 10% H2SO4 to dissolve the Zn(OH)2.

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Wash the organic layer with 5% H2 SO4 , (2 x 50 mL), 10% Na2 CO3 (25 mL) and finally

with water (2 x 25 mL) and dry over anhydrous magnesium sulphate.

Remove as much as possible of the solvent by distillation at atmospheric pressure,

and subsequently distil under reduced pressure. The β – hydroxyester has a b.p of 82 -84 0C

@ 10 – 11 mmHg. If the water pressure is good you could distil at ~ 90 – 120 0C depending

on the pressure. Yield : 7 – 9 g ( 39 – 50% ). Record the IR and 1H NMR spectra.

Cyclisation to γ-lactone

Carefully add the β-hydroxyester to 100 mL of 50% H2SO4 at room temperature, and

reflux the mixture for 30 min. After cooling, extract the product into chloroform (2 x 20 mL).

Wash the chloroform layer successively with sodium bicarbonate (1 x 10 mL), water (1 x 10

mL), dry (magnesium sulphate) and evaporate to dryness (rotary evaporator). Recrystallise

the product from petroleum spirit, and record the m.p , IR and NMR spectra. Yield : 4 – 4. 5

g ( 76 – 86% ).

Notes

1. All apparatus used in this step must be thoroughly oven dried before use, and

assembled while warm. Wet apparatus will result in poor yields.

2. Dried with molecular sieves or sodium

3. Gentle warming may be required. Do not add any further reagent until the reaction

has begun.

Questions

1. Based on the spectra obtained, assign a reasonable structure to the product, and

propose a mechanism for its formation ( Hint : acid catalysed rearrangement ).

2. What is the purpose of distilling benzene from the zinc before starting the

Reformatsky reaction ?

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Experiment 4

ANALYSIS OF A BINARY MIXTURE

You are provided with a mixture comprising about equal quantities of two components. You

are required to separate the mixture into the individual pure components and then identify the

two compounds using a combination of classical qualitative analysis and spectroscopy. You

are also required to submit any derivatives prepared as well as to return any unused samples.

Guide to Qualitative Analysis of Mixtures

It is necessary to separate a mixture before the individual components can be

identified. Observe the physical state of unknown mixture. Do simple tests on very small

amounts, e.g. solubilities in polar and nonpolar solvents, acidic or basic aqueous salutions.

Observe carefully if the mixture is partially soluble and if reaction occurs. These properties

could form a basis for separation as well as an idea of the nature of the compound.

The following are brief descriptions of possible methods of separation:-

(a) Solubility Differences

This involves extraction and is probably the easiest way but it is unlikely that one

encounters such a simple mixture. One component can be extracted into a solvent, the

other component being insoluble. This component can then be isolated by distillation

or evaporation of solvent. Normally solvent with wide differences in polarity are tried

first, e.g. water and petroleum ether. Some examples where this method will work are

sucrose/naphthalene, acetamide/ diphenyl and glycine hydrochloride/benzophenone,

where contrasting differences in polarity of the components exist.

(b) Chemical Differences

A derivative is made and this is separated from the other components by precipitation

or extraction.* The usual type of derivative is formation of a salt, e.g. if given an

acidic/neutral mixture insoluble in water, the acidic component can be extracted into

aqueous basic solution. Similary, a phenol/neutral mixture can be treated in the same

way as most phenols are sufficiently acidic to be extracted into aqueous sodium

hydroxide. For a basic/neutral mixture, extraction into aqueous acid will effect a

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separation. For example , a mixture of benzoic acid benzophenone can be separated

by aqueous NaOH/ether; a mixture of aniline and chlorobenzene can be separated by

aqueous HCl/ether. The separated components are then recovered and purified.

Elaborate schemes can be read from your text.

* Caution: It is wise to try out on a small scale first. Water soluble organic

compounds may be troublesome, so it is best to avoid too much of aqueous reagents.

A mixture of a phenol and a ketone with α- hydrogens would both dissolve in NaOH.

Solution of the phenol may be effected with very dilute NaOH and done rapidly to

avoid decomposition of the carbonyl compound. Some amines form salts which may

not dissolve readily in H2O, a different acid may be used.

(c) Boiling Points Differences

Two liquids which do not form a constant boiling point mixture and have widely

separated boiling points can be separated by fractional distillation using a simple

fractionating column. The boiling points must be at least 25 0C apart. For high boiling

mixtures this can be done under reduced pressure, preferably under nitrogen if the

compound decomposes.

( d) Recrystallization

Often one component can easily be obtained from a mixture by a suitable choice of

solvents for recrystallization

(e) Chromatography Methods.

Generally TLC, LC, GLC and other chromatographic methods are extremely effective

separation methods but they can be long and tedious and furthermore are limited to

small scale work involving less than 1g. unless appropriate facilities are available.

The above methods should only serve as a guide as there can be no hard and fast

rules. Usually, methods (b) and (c) would be suitable. Note also that many problems

could arise in the process of recovering the separated components (refer text). In

principle, all the given amounts should be recoverable unless they are accidently

thrown away or are extremely volatile. It may be wise to try a small scale separation

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first to check on your separation method before scaling up. In small scale work avoid

filtration but use the centrifuge.

The compounds after separation and purification should have good physical

constants, e.g., m.p , b.p , etc. , before it is sent for spectra. Use your text and proceed

to determine the elements present, run the IR spectra and do some wet tests to

determine the functional type. Run or ask for the NMR spectra. Make at least one

good derivative. If no derivative is possible do a chemical transformation to give a

product that can be easily characterized by NMR. Comparison of Rf values or

retention times with standards under different conditions is also acceptable.

References:

1. Shriner, Hermann, Morrill, Curtin, and Fuson, The Systematic Identification of

Organic Compounds (Wiley).

2. Furniss, Hannaford, Smith, and Tatchell, Vogel’s Textbook of Practical Organic

Chemsitry (Longman).

3. Williams and Fleming, Spectroscopic Methods in Organic Chemistry (McGraw-Hill).

4. Silverstein and Webster, Spectrometric Identification of Organic Compounds (Wiley).

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Appendix 1-8

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Organic Chemistry Level 3______________________________

Appendix 1

CHROMATOGRAPHY

The various types of chromatography is summarized in the Diagram below.

Chromatography

GLiquid Chromatography

as chromatography

Gas – Liquid Chromatography GLC

Gas – Solid chromatography

Ion exchange Exclusion

Liquid/ liquid chromatography LLC

Liquid/ solid chromatography LSC

Thin layer TLC

Paper PC

Gel permeation Gel filtration

Liquid – liquid chromatography LLC is partition chromatography. The sample is

retained by partitioning between the mobile liquid and the stationary liquid, e.g in paper

chromatography the stationary phase is water held on the fibres of cellulose.

Liquid – solid chromatography LSC is adsorption chromatography. In column or

thin layer TLC the sample is adsorbed on the adsorbent silica gel or alumina. In practice

silica gel or alumina has varied amounts of moisture so that the chromatography is a

combination of LLC and LSC.

Exclusion chromatography uses a highly porous material or gel which separates

compounds (usually polymers) according to the molecular size.

Ion exchange uses ionic groups bonded into a polymeric resin. Ionic coumpounds

( e.g. amino acids ) have different affinities to the resin and can be separated.

Bonded phase chromatography BPC is similar LSC or LLC except that the

absorbent material is chemically modified silica gel or similar substance. The OH groups

of silica gel can be silated and various organic groups can be attached as shown below

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16


Bonded phases are advantageous as different polarity types may be synthesized

and both organic or aqueous solvents can be used. When using aqueous organic solvent

mixture it may be noted that the less polar material absorb on the stationary phase while

the more polar compounds are eluted by the polar aqueous eluant . This is referred to as

reverse phase chromatography. A summary of the LC types and adsorbents is given in

the tables below.

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17


MODES OF LIQUID CHROMATOGRAPHY

Modes Abbreviation Predominant Mechanism Common Names Liquid – solid LSC Adsorption of Surface Adsortion chromatog-

raphy, liquid – solid chromatography, linear elution adsorption chromatography

Liquid – liquid LLC Partition in Liquid Phase Partition chromato – graphy, Sorption chromatography

Bonded Phase BPC Partition and / or adsorption

Gel chromatography

Reverse Phase RPC Paritition and / or adsorption

Sorption chromatography

Ion Exchange None Adsorption on Fixed Ionic Site

Cation or Anion Exchange

Steric Exclusion

None Diffusion into Pores Gel permeation (GPC) ,molecular exclusion, gel filtration ( GFC )

LIQUID – SOLID ADSORBENTS

Adsorbent Chemical

Structure Estimated Usage %

Surface Properties Application

Silica ( SiO2 )x 70 Slightly acidic General purpose Alumina ( Al2 O3 )x 20 Silightly basic * General purpose Charcoal 1 Graphitized –

nonpolar , Oxidized – Polar (silightly basic)

Sample cleanup

Florisil Magnesia – Silica

Coprecipitate

2 Strongly acidic General purpose adsorbent

Polyamides 2 Basic Phenols and aromatic nitro compounds

Others ( clay, Kieselguhr, diatomaceous earth, Celite )

5 Relatively nonpolar Very polar compounds

* depends on method of preparation

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18


A guide to a selection of the type of LC is given below

High Pressure Liquid Chromatography ( HPLC )

The obvious drawback of column chromatography is the tedium of eluting the

sample out of the column . To speed up the process high pressure may be used to

force in the eluant through the column. By using high pressure up % to 4000 psi. it is

necessary to use stainless steel columns.

A high pressure liquid chromatography system consists of a solvent pump, plumbing

connections, injection port for a sample introduction, a detector to detect the eluted

samples and a recorder to give a graphic output representation. For compounds

absorbing UV radiation a UV detector is convenient. Read up on the instrument, and

the operating instructions.

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19


Caution:

Read & understand the instrument first. Consult lecturer & lab. Assistant in charge.

Handle microsyringe with loving care.

Column Chromatography ( CC ) or Liquid Chromatography ( LC ). CC or LC is an example of adsorption chromatography. It consists of a stationary

solid phase, known as the adsorbent, supported in a column and of mobile liquid

phase which is allowed to flow down the column. This is referred to as the eluant.

The mixture to be separated is introduced onto the adsorbent at the top of the column

in solution form. The two components will be adsorbed to different extents depending

upon the polarity of the molecules. The eluant is introduced and allowed to flow

down the column. The components of the mixture will undergo many adsorption

desorption processes as they pass down, the least polar molecule moving more

quickly.

Compound Elution Sequences*

Hydrocarbons

Olefins

Ethers

Halogen Compounds

Aromatics General order of elution

Ketones

Aldehydes

EstersAlcohol, amines, mercaptans

Acid and strong bases.

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20


Increasing the polarity of the eluant well increase the speed at which the components

move down the column. It is usual to begin with a solvent of low polarity and

gradually change to more polar solvents

Eluotropic Series*

Petroleum ether Cyclohexane Carbon tetrachloride Benzene Methylene chloride Chloroform (alcohol free) Diethyl ether Increasing Polarity Ethyl acetate Pyridine Methanol Acetone n- Propanol Ethanol Acetic Acid

The type of adsorbent chosen also effects the speed at which the compounds are

eluted. The more active the adsorbent the more slowly the compounds will move

down the column.

Adsorbents for Adsorption Chromatography* Cellulose Starch Sugars Magnesium silicate Calcium sulphate general order of increasing activity Silicic acid Florisil Magnesium oxide (magnesia) Aluminium oxide (alumina) Activated Chareoal By suitable choice of adsorbent and eluant it is possible to separate compounds of

similar chemical constitution. If two molecules are very similar it is necessary to

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21


choose conditions whereby they move slowly down the column. This ensures they

reach equilibrium in more adsorption desorption processes which facilitates

separation.

Thin layer chromatography (TLC) is a modification of column chromatography. In

this case the adsorbent is supported on a flat surface. To develop the chromatogram

the plate is placed vertically in a tank containing the eluant which flows upwards by

capillary action. Unlike column chromatography it is not possible to alter the polarity

of the solvent during development of the chromatograph but by using suitable

proportions of mixed solvents a separation is usually achieved.

Silica-gel and alumina are the most common adsorbents. For T.L.C. those are

usually combined with Plaster of Paris which acts as a binding agent.

As in paper chromatography, a Rf value can be calculated where

Rƒ : distance moved by compound

distance moved by solvent front.

If the components are colourless they can be detected by various means, eg using

U.V. light, iodine vapour or charring with sulphuric acid.

The great advantage of T.L.C. is its short development time, usually of the order

of 30 mins.

* From Pasto and Johnson, Organic Structure Determination

Gas-liquid Chromatography (GLC) proceeds mainly by partition chromatography

i.e. separation depends upon the different solubilities of the components at

equilibrium between the gas and liguid phase. The stationary phase is a liquid

supported on a solid contained in a tube. The mobile phase is an inert gas which also

acts as a carrier of the mixture. This mixture can be solid, liquid or gaseous.

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22


The mixture in solution is injected by means of a syringe and at a temperature

high enough to ensure vapourisation of the components. It is carried by means of the

inert gas through a heated long tube containing the liquid phase and is subject to

many solubility equilibria.

After separation the components & carrier gas pass through a detector which

indicates when a compound is being eluted by drawing a curve on chart paper. The

area under the curve can be considered to be proportional to the number of moles

eluted.

The compounds can be characterized by their Retention Times i.e. the time taken

from being injected to being eluted under set conditions.

s - moment of injection of sample b – Retention time of compound

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23


Appendix-2

CRYSTALLIZATION

The solute is dissolved in a solvent to give a hot saturated solution. This is filtered

to remove insoluble contaminants. On allowing to cool slowly to R.T. the solution

becomes supersaturated and deposits crystals.

The solution is filtered through a fluted filter paper under gravity. While filtering

this crystals may be deposited. This can be avoided by (a) Warning the funnel and

receiving conical flask. (b) Having prepared a hot saturated solution, adding more

solvent, filtering and evaporating until the original volume is obtained.

After cooling to R.T. crystallization can be encouraged by (a) cooling in ice

(b) scratching and (c) seeding – addition of the crystal of solute.

The crystallized compound is collected by suction filtration and washed to

remove mother liquor.

From Mixed Solvents The two solvents must be miscible and are chosen such that

the compound is soluble in solvent I and insoluble or appreciably less soluble in

solvent II. A hot solution is prepared in solvent I. After filtering solvent II is added

dropwise until the solution just turns cloudy. On cooling crystals are formed.

Choosing a solvent A suitable solvent is found by trial and error, and this should be

conducted on a small scale. The aim is to find a solvent or combination of solvents in

which the solute is soluble at high temperatures and much less soluble at room

temperature.

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24


Generally polar solvent will dissolve polar compounds. It is better if the boiling

point of the solvent is lower than the melting-point of the solute, otherwise an oil may

be formed. The solvent should be sufficiently volatile so that the crystals scan be

easily dried.

Recrystallisation is used for purifying a compound which is contaminated with one

more other compounds. These may have similar solubilities to the major component

but as they are present in smaller amounts they can be separated by one or more

recrystallisations

Fractional crystallization is the separation of two components of similar solubility

and present in comparable amounts by successive crystallizations and recombination

of fractions. During this process the first component becomes more and more

concentrated in the crystals and the mother liquor becomes more enriched with the

second component. This is a tedious way of separating two components.

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Appendix-3

Distillation

Fractional Distillation is used to separate liquid mixtures which cannot be separated

by ordinary distillation because their boiling points are too close.

It consists of a continual process of repeated vapourisation and condensation and

is based on the principle that when a liquid mixture boils, the vapour will be richer in

the more volatile component and when this vapour condenses, the less volatile

component will condense first. Liquids which do not form an azeotropic mixture and

whose boiling points are separated by at least 30°C can be separated this way.

See Renfrow Hawkins, Organic Chemistry Laboratory Operations p 13, 147 Louis F. Fieser, Organic Experiment p.30

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Small scale fractional distillation at low pressure

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Distillation

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Flash Distillation

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Vacuum Distillation A liquid begins to boil when is vapour pressure equals

atmospheric pressure. By reducing the external pressure the boiling point is reduced.

This technique is employed with liquids having very high boiling points or liquids

which decompose before their boiling point is reached at atmospheric pressure. The

vacuum is applied to the apparatus before the flask is heated and on completing the

distillation the source of heat is removed first. Do not apply a vacuum to a hot

solution.

Because liquids bump when boiled under reduced pressure a claisen head is used

and an air-leak. The air leak allows a stream of small bubbles to be sucked through

the solution. The air-leak must reach as close to the bottom of the flask as possible

and be flexible.

See: Renfow Hawkins, Organic Chemistry Laboratory Operation, m.s. 13, 147

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Louis F. Fieser Organic Experiments, m.s. 30.

Louis F. Fieser Organic Experiments p. 250

Renfrow Hawkins Organic Chemistry Laboratory Operations p.143

The pressure at which the liquid boils is measured by a manometer. The most

common type is a closed-end monometer.

The pressure in the closed arm is zero. This manometer is kept closed off from the

system as solvent vapour can dissolve in the mercury. Then the pressure in the closed

arm will no longer be zero, resulting in an uncorrect reading. This type of manometer

is accurate up to a few millimeters of mercury.

For lower pressure, 1-10¯² mm, an Adwards’ Vacustat is used.

J.A. Elvidge and P.G. Sammes. A course in Modern Techniques of Organic

Chemistry p. 115.

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Appendix- 4

SUBLIMATION

Sublimation occurs when a compound passes directly from the solid phase to

vapour phase when heated. This process is usually performed under reduced pressure

and is used as a method of purification.

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Appendix -5

N.M.R

Nmr samples Clean nmr tubes are available from the lab.asistant. To clean tubes use organic solvent e.g. acetone , ethanol , water etc. Rinse with acetone and finally dry in an oven (60°C) Your compound should be soluble in CDCl3. If not consult the lecturer. For mixtures or impure samples instruct the spectroscopist of your requirements.

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Appendix – 6

I.R IR samples All samples must be DRY. Test first to ensure that your sample is NOT water or in an aqueous solution. Water will dissolve away the NaCl cells which are rather expensive (RM 600) . Solids can be dried in a vacuum dessicator. Liquids can be used neat between NaCl plates. Solids can be micro-pelleted with KBR. Liquids and solids if soluble in CCl4 or CS2 can be used in solution cells. Spectral grade CHCl3 may be used for less soluble compounds. Note the bands cut off when solvents are used. Solids can be mulled in Nujol but note the bands cut off by Nujol ( hydrocarbon oil ) If you get poor spectra go to the instrument room and instruct the lab. assistant as to your requirements.

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