Purification of Organic Compounds PDF

 INTRODUCTION – Purification of organic compounds Organic compounds are isolated either from natural sources or from reactions mixtures. These compounds are seldom pure and are usually contaminated with small amounts of other similar compounds, which are found to exist together o...

 INTRODUCTION – Purification of organic compounds Organic compounds are isolated either from natural sources or from reactions mixtures. These compounds are seldom pure and are usually contaminated with small amounts of other similar compounds, which are found to exist together or formed during the reaction. In order to characterize them, it is important to purify them. 3  Methods of purification of solid organic compounds Recrystallization Sublimation  1. CRYSTALLIZATION The method of purification of solid organic compounds depends on the difference in solubility of solvent for compound and impurities. Aim To separate a solid compound in pure and geometrical form. Principle A saturated solution of the impure substance in a suitable solvent is made at a temperature higher than the room temperature. On cooling this solution, the substance reappears in the form of well shaped crystals. Process Purification by crystallization involves the following steps: Choice of solvent Preparation of solution Filtration of the solution Separating the crystals Drying of crystals 5 The most important criteria for selecting the recrystallization solvent Dissolve all the compound at the boiling point of the solvent. Has boiling point below the melting point of the compound so that the compound dissolves not melts in the hot solvent. Has relatively low boiling point (60-100oC) so that the solvent is easily removed from the crystals and crystals are easy to appear. To be non-reactive with the compound and non- toxic. To be relatively inexpensive. What happens if there is no solvent that dissolve the compound on heating and reprecipitate on cooling? -You should use solvent pair that consists two miscible solvents, one in which the compound soluble and the other in which the compound insoluble. Ex: Ethanol-water -What is the cloud point when doing a solvent pair recrystallization? It is the point at which the crystals are beginning to come out of a solution. - We take the compound in a beaker then add dropwise the hot solvent in which the compound is more soluble then add drop wise the solvent in which compound is insoluble. If the solution turns cloudy due to the recrystallization of solid, this will be an acceptable solvent pair to use for recrystallization. Method of recrystallization One gram of impure Apart of recrystallization substance + suitable solvent amount of recrystallization solvent  After complete precipitation, filter the residue (pure compound), Dry it then weight it. Calculate the percent recovery. Percent recovery= Mass of pure crystals/Mass of impure crystals X 100 Describe the reasons of the low percent recovery? Due to using wrong solvent, too much solvent, incomplete crystallization (not enough time and temperature), using inefficient technique.  2. SUBLIMATION It is the conversion of substance from solid state to gaseous state without passing through liquid state Principle The vapor pressure of the substance that sublime must be more than the atmospheric pressure. The weaker the intermolecular attractions and the lower the molecular weight , the higher the vapor pressure will be. 11 Fig :-Sublimation 12  Melting point It is a temperature at which the substance converts from solid state to liquid state. At this temperature, solid and liquid phases exist in equilibrium. We use H2SO4 in the apparatus that measure the melting points of organic compounds because its boiling point is 300oC. 13  Melting range give indicator of the purity of substance. The purer the compound, the higher its melting point and the narrower its melting range. Impurities lower the melting point of organic compound because the impurities disturb the crystal lattice, so less energy is required to break the intermolecular attraction forces between molecules 14  Methods of purification of liquid compounds 3. DISTILLATION Aim To separate a solution of two liquids whose boiling points are different. Principle Distillation involves the conversion of a liquid into its vapors upon heating and then cooling the vapors back into the liquid. Depending on the difference in boiling points of liquids. Types of distillation  Simple Distillation  Fractional Distillation  Distillation Under Reduced Pressure or Vacuum Distillation  Steam Distillation 15  SIMPLE DISTILLATION Principle It is used for separating liquids having boiling points differing by 10-20 degrees. The liquid having the lower boiling point distills over first, and the other liquid component is left behind. In this process, vaporization and condensation occur side by side. Process Example Simple distillation of a Cyclohexane- Toluene mixtures 16 17  FRACTIONAL DISTILLATION Principle It is used for separating two liquids in any mixture, which have boiling points within a narrow range of temperatures. In such cases, simple distillation does not give complete separation and a modified version called fractional distillation is employed. Process Example Fractional Distillation of a Cyclohexane- Toluene mixtures 18