TLC Polarity

HONORS ORGANIC CHEMISTRY Thin Layer Chromatography (TLC) I. Background TLC is a laboratory method used to separate mixtures of substances into their components and to identify or confirm the identity of the components. All forms of chromatography work on the same principle. They all have a stationary phase (a solid, or a liquid supported on a solid backing) and a mobile phase (a liquid or a gas). The mobile phase flows through the stationary phase and carries the components of the mixture with it. Different components travel at different rates. We'll look at the reasons for this further down the page. Thin layer chromatography is done exactly as it says - using a thin, uniform layer of silica gel or alumina coated onto a piece of glass, metal or rigid plastic. The silica gel (or the alumina) is the stationary phase. The stationary phase for thin layer chromatography also often contains a substance which fluoresces in UV light - for reasons you will see later. The silica gel of the stationary phase is very polar, which will ultimately affect the rates at which the components of the mixture separate. The mobile phase is a suitable liquid solvent or mixture of solvents. The polarity of the mobile phase can be adjusted from very polar to very nonpolar (and anywhere in between). II. The Two Phases ➢ The stationary phase - silica gel Silica gel is a form of silicon dioxide (silica). The silicon atoms are joined via oxygen atoms in a giant covalent structure. However, at the surface of the silica gel, the silicon atoms are attached to -OH groups. So, at the surface of the silica gel you have Si-O-H bonds instead of Si-O-Si bonds. The diagram shows a small part of the silica surface. The surface of the silica gel is very polar. As a result, the more polar a compound is, the more strongly it will be attracted to the stationary silica phase and the smaller the distance it will travel up the plate. The other commonly used stationary phase is alumina - aluminum oxide. The aluminum atoms on the surface of this also have -OH groups attached. Anything we say about silica gel therefore applies equally to alumina. ➢ The mobile phase – a liquid solvent(s) The mobile phase is composed of one or more solvents in which the mixture is soluble. As the mobile phase absorbs onto the stationary silica plate, the components of the mixture travel with the mobile phase. How does the liquid travel up the plate? This occurs by a process known as capillary action- for example think about how water is drawn up through a piece of paper towel. The polarity of the mobile phase can be adjusted by selecting which solvent/ combination of solvents to use. III. How Does Separation Occur? As the solvent (mobile phase) begins to adsorb up the plate, it first dissolves the compounds in the spot that you have put on the origin (to be discussed below). The compounds present will then tend to get carried up the chromatography plate as the solvent (mobile phase) continues to move upwards. Chromatography works on the principle that different compounds will be attracted to the different phases to different extents. This attraction is typically affected by the polarity of the molecules as well as the polarities of the two phases. • If a compound is very polar, it will be more strongly attracted to the polar silica stationary phase and less attracted to the less polar mobile phase. o Therefore, the more polar a compound, the smaller the distance it travels up the TLC plate. • If a compound is less polar, it will be less strongly attracted to the polar silica stationary phase and more attracted to the less polar mobile phase. o Therefore, the less polar a compound, the greater the distance it travels up the TLC plate. IV. Setting Up , Developing, and Visualizing the TLC Plate Now that you understand the basics of why TLC works, you can begin to understand how the method is performed in the laboratory. If the mixture you wish to separate is a solid, it must first be dissolved in a suitable liquid. The liquid is then spotted along a line at the bottom of the TLC plate known as the origin. The origin is simply a line, drawn in pencil, along the bottom of the TLC plate. It is always important to draw the origin with a pencil as dyes from ink may dissolve in the liquid mobile phase and travel up the TLC plate, thereby affecting the separation of the mixture you wish to test. Next, the TLC plate is placed inside of a developing chamber containing a small amount of the liquid mobile phase at the bottom. When placing the TLC plate inside the developing chamber, always be sure that the origin is above the level of the mobile phase otherwise the mixture will be washed off the plate before it has a chance to separate. The TLC plate is then allowed to develop- as the mobile phase is adsorbed onto the stationary phase, it will travel up the TLC plate carrying the components of the mixture along with it. As the mobile phase reaches the top of the TLC plate, the plate is carefully removed from the TLC chamber and another line, called the solvent front, is drawn in pencil. The solvent front simply indicates how far up the plate the mobile phase has traveled. It is important to mark the solvent front quickly as the solvent will dry and you will not be able to visualize how far it traveled unless the line is marked. Many organic compounds are colorless and you will likely not notice anything particularly exciting occurring along your TLC plate unless you can properly visualize the spots. As mentioned at the beginning of this background, the stationary phase for thin layer chromatography also often contains a substance which fluoresces in UV light. When your TLC plate is placed underneath a UV light, the components of your mixture will appear as dark spots on the TLC plate. You can simply circle them to mark for future reference. 2 V. The Rf Value Now that your mixture has been separated on the TLC plate, how do you analyze the data to determine what is in your mixture? A key to the analysis of TLC data is the calculation of the retention factor, or Rf, which is defined as the distance traveled by the compound divided by the distance traveled by the solvent. For example, if a compound travels 2.1 cm and the solvent front travels 2.8 cm, the Rf is 0.75: The larger an Rf of a compound, the larger the distance it travels on the TLC plate. When comparing two different compounds run under identical chromatography conditions, the compound with the larger Rf is less polar because it interacts less strongly with the polar adsorbent on the TLC plate. Conversely, if you know the structures of the compounds in a mixture, you can predict that a compound of low polarity will have a larger Rf value than a polar compound run on the same plate. The Rf can provide corroborative evidence as to the identity of a compound. If the identity of a compound is suspected but not yet proven, a known sample of the compound, or standard, is spotted and run on a TLC plate side by side (or on top of each other) with the compound in question. If two substances have the same Rf value, they are likely (but not necessarily) the same compound. If they have different Rf values, they are definitely different compounds. Note that this identity check must be performed on a single plate, because it is difficult to duplicate all the factors which influence Rf exactly from experiment to experiment. 3