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    THIN LAYER

    CHROMATOGRAPHY

    VISUALISATION

    REAGENTS

    ELBERTUS KRUISWIJK

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    Forword The idea for this book was born in 2004 when I was coming to the end of writing my named

    organic reactions book. This book is a collection of reagents that I have collected of the last 5

    years working as a chemist. It is definitely not complete and does not come near the classic and

    very good book by Egon Stahl.

    I hope that organic chemists find this book useful. The layout is as follows. The first 20

    pages deal with the background of chromatography.

    There are more than 250 different ‘dips’ mentioned in this book in alphabetical order. If you

    use the electronic version you can search with the search option in Adobe acrobat. Otherwise the

    index is a possible starting point. In the index the reagents are written in small letters, and the

    compounds you are trying to detect in capital letters. At each page the preparation of the spray or

    dip solution is described in detail. Under the heading treatment, you can find how to use the

    reagent and what colour of spots you can expect. The necessary chemicals are given and

    referenced to the 2005 – 2006 Aldrich catalogue, in some cases to other catalogues. I have

    referenced to the Aldrich catalogue, because in most organic labs this is the most commonly used

    catalogue. I like to make clear that I have not been sponsored by Aldrich. Supplier codes and CAS

    number are also provided. References are given to the Merck index or Beilstein and selective

    journal publications. Under the heading comments some additional information has been given if

    necessary and there is some room left under notes to add your own comments. Structures are

    provided where necessary.

    This book can be used by anyone who is active in practical organic chemistry.

    None of the mentioned reagents have been specially tested during the preparation of this

    book nor by me nor by the proof readers. If there are any comments about the entries, please

    contact me at tlc123@tiscali.co.uk and please do contact me.

    Of course, I am indebted to the following group of people who were willing to volunteer to

    proof read this book. In random order many thanks to Jelle Brinksma, Kiadis, Groningen, The

    Netherlands, thank you Jelle you are always very supportive and Richard Tucker, West Monmouth

    School, Pontypool. Thank you all for your time and fruitful discussions.

    Bert Kruiswijk

    Aberaman, 27-08-2005.

    mailto:tlc@tiscali.co.uk

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    CHROMATOGRAPHY The term chromatography is derived from the Greek word chromagraphein, i.e. chroma =

    colour, graphein = write. Chromatography was invented by the Russian botanic Mikhail Tswett

    who using this method proofed the existence of different kinds of chlorophyll. He used a leaf

    extract on a column filled with calcium carbonate. Coloured bands were made visible after eluting

    the column with solvent. It took until 1931 when Kuhn, Winterstein and Lederer used this method

    again for the separation of carotenoids. In 1941 Martin and Synge developed partition

    chromatography and were awarded the Novel prize in 1952. However, already in 1948 A. Tiselius

    received the same prize in recognition of his contribution to electrophoresis and adsorption

    chromatography.

    There are several techniques to separate substances, all of the techniques depend upon

    the difference in distribution of the various compounds in the applied mixture between the mobile

    phase and the stationary phase. This book will only consider thin layer chromatography (analysis),

    normally abbreviated as TLC. If the reader is interested in the mathematical background of

    chromatography it is recommended to consult specialist handbooks.

    THE STATIONARY PHASE Many different materials are capable of retaining both solvents and solutes. The two most

    commonly used as stationary phase (adsorbent) are silica gel (SiO2) and alumina (Al2O3). Both

    compounds are supplied highly purified and finely powdered. They are readily dispersed into the

    atmosphere and inhaled. Therefore they should always be handled in the fume hood and if this is

    not possible the use of a facial dust mask is recommended. The active centres on the surface of

    the adsorbents do not possess the same adsorption power. This is a result of the special

    orientation, the chemical character and the conformation of the adsorption places. In the

    adsorption processes of silica gel and alumina are not only electrostatic interactions important but

    also hydrogen bonding plays an important role. Both silica gel and alumina can be purchased in

    large quantities, figure 1.

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    Figure 1

    Silica gel Silica gel has a good linear capacity and hardly shows any catalytic character that can lead

    to the decomposition of certain compounds. It has a large specific surface area (300 – 800 m2/g)

    and a large pore volume (> 0.7 ml/g). The pore diameter and specific surface can be changed

    during the preparation of the silica gel. For spherical particles the ratio between surface and

    volume equals 6/d. If the diameter (d) decreases, the specific surface area increases. Vicinal and

    geminal hydroxyl groups are responsible for the adsorption process, figure 2.

    Si O

    OH

    Si OH Si HO OH

    Vicinal Geminal

    Figure 2

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    Furthermore, these hydroxyls can form hydrogen bonds in different manners, figure 3.

    Si OH O

    Si OH

    Free

    Si O O

    Si O H

    H

    Vicinal OH

    Si O

    O

    Si O

    H

    H O

    H

    H

    Hydrogen bonded with water

    Figure 3

    Alumina On the surface of aluminium oxide are acidic (Al3+) and basic (O2-) groups present, as

    result acidic compounds will be strongly adsorbed. The specific surface are is smaller than for

    silica gel (100 – 200 m2/g) and the pore volume is also smaller (0.2 – 0.3 ml/g). Alumina can

    however catalytically decompose acidic compounds, furthermore chemisorption can take place.

    Like silica gel alumina may be regarded as a typical polar sorbent and the order of separation of

    compound classes in alumina and silica gel is generally similar. Carbon-carbon double bonds

    contribute somewhat more to compound adsorption energy on alumina as compared to silica gel,

    and hence compounds differing only in relative unsaturation (e.g. aromatic hydrocarbons) are

    generally better separated on alumina than on other polar adsorbents such as silica gel. Active

    aluminas are markedly sensitive to the differing shapes of various aromatic hydrocarbons and

    some of their derivatives permitting an excellent separation of many aromatic isomers. The

    preferential adsorption of acidic substances on alumina offers many useful separation possibilities

    in the case of weak acids and this effect can be further enhanced by the use of basic solvents.

    Stronger acids however tend to chemisorb on alumina, requiring the use of acid treated aluminas.

    Acidic and neutral aluminas are useful for the separation of base sensitive materials. It should be

    noted that separation order maybe considered as distinct adsorbent subtypes.

    Activity The activity is governed by the amount and the adsorption power of the active centres on

    the surface of the adsorbent. First of all the activity is dependent on the nature of the adsorbent.

    For a certain adsorbent the activity is not constant. The amount of water plays an important role.

    On the surface of polar adsorbent is normally a variable amount of water molecules bound. These

    water molecules form hydrogen bonds with the hydroxyl groups of the adsorbent, as result that

    adsorption places are being blocked. Because the active centres differ in adsorption power, the

    amount of water has not only influence on the amount of possible adsorption places but also on

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    the activity. The most active places are occupied first. The adsorption power of the remaining

    places will decrease and the retention value increases. The remaining places form a more

    homogenous group, which is important for the quality of the separation. The more homogeneous

    the adsorption power of the active sites, the larger the linear capacity. This is the reason why it is

    not always good to use an adsorbent with the highest activity.

    Activity grade There are five activity grades depending on the amount of water present. In activity grade I

    all the water has been removed. The higher the number of the activity grade the higher the

    amount of water and the lower the activity is, table 1. The adsorbed water can be removed by heating. Up to 150 oC only the adsorbed water will be removed. Other reactions take place above

    200 oC. The geminal hydroxyl groups split off water with as result that the amount of adsorption

    places decreases. The vicinal groups split off water above 500 oC and the specific surface area

    decreases. Above 1200 oC all remaining water is removed and silica gel will have a hydrophobic

    character. The activity g