Why do we use column chromatography?
Column Chromatography is a preparative technique used to purify compounds depending on their polarity or hydrophobicity. In column chromatography, a mixture of molecules is separated based on their differentials partitioning between a mobile phase and a stationary phase.
It turns out that carbon and hydrogen don't have significantly different electronegativity values. Basically, that means that in our fluorene molecule the electrons (in the form of the covalent bonds) are being shared equally among all atoms. This makes fluorene a non-polar molecule.
- Explain relative Rf values of fluorene and fluorenone. Fluorenone is more polar than fluorene because of its C=O bond. Fluorenone's spot will be lower than fluorene because of its "stickness." Flask three was only supposed to have Fluorenone but this shows contamination of the other compound.
- Acetone is another molecular material with both polar and nonpolar characteristics. Here acetone has been added to water (left tube) and carbon tetrachloride (right tube). As you can see, it has mixed with both the polar water molecules and with the nonpolar carbontetrachloride molecules.
- IODINE: Iodine forms a diatomic non-polar covalent molecule. The graphic on the top left shows that iodine has 7 electrons in the outer shell. Since 8 electrons are needed for an octet, two iodine atoms EQUALLY share 2 electrons.
A mixture of fluorene, fluorenone and fluorenol is examined by TLC and gives the following Rf values: 0.3, 0.5, 0.8. -0.8 is fluorene, 0.5 is fluorenone, 0.3 is fluorenol. -Fluorenol is an alcohol, so it is more polar than the ketone fluorenone, giving it a lower Rf value.
- Naphthalene is non-polar. Its dipole moment is zero. The naphthalene molecule is planar with D2h symmetry, like ethylene. This accounts for its zero dipole moment.
- O is more electronegative than H and C so it draws electron density from both, increasing the difference in charge between O-H and O-C, hence increasing polarity. In propanone there is no H so the Oxygen can only withdraw electron density from the carbon so the molecule is less polar.
- Because water is polar and oil is nonpolar, their molecules are not attracted to each other. The molecules of a polar solvent like water are attracted to other polar molecules, such as those of sugar. This explains why sugar has such a high solubility in water.
Water is very polar and therefore absorbs very strongly on the polar stationary phase (alumina). Since benzil is less polar than benzoin, a polar solvent is used as solvent for the recrystallization. Benzil is less soluble in cold 95 % ethanol than benzoin causing the benzil to precipitate first.
Names Boiling point 346.0 to 348.0 °C; 654.8 to 658.4 °F; 619.1 to 621.1 K Solubility in water insoluble Solubility in ethanol soluble Solubility in diethyl ether soluble
- Recrystallization is the process in which deformed grains of the crystal structure are replaced by a new set of stress-free grains that nucleate and grow until all the original grains have been consumed. The process is accomplished by heating the material to temperatures above that of crystallization.
- Crystallisation is a separation technique that is used to separate. a solid that has dissolved in a liquid and made a solution. The solution is warmed in an open container, allowing the solvent to evaporate, leaving a saturated solution. A solution that has as much solid dissolved in it.
Updated: 14th October 2018