Lecture 2 - Structural Determination of Organic Compounds PDF

ORGANIC CHEMISTRY (ACMP102) Accra College of Medicine Structure of Organic Molecules Dr Richard Amewu Structural Determination of Organic Compounds Introduction. Isolation and Purification of Organic Compounds. Tests for Purity. Qualitative Analysis of Elements in an Organic...

ORGANIC CHEMISTRY (ACMP102) Accra College of Medicine Structure of Organic Molecules Dr Richard Amewu Structural Determination of Organic Compounds Introduction. Isolation and Purification of Organic Compounds. Tests for Purity. Qualitative Analysis of Elements in an Organic Compound. Determination of Empirical Formula and Molecular Formula from Analytical Data. Structural Information from Physical Properties. Structural Information from Chemical Properties.  Structural Determination of Organic Compounds The general steps to determine the structure of an organic compound Isolation and Purification of Organic Compounds  Isolation and Purification of Organic Compounds Technique Aim 1. Filtration To separate an insoluble solid from a liquid (slow) 2. Centrifugation To separate an insoluble solid from a liquid (fast) 3. Recrystallization To separate a solid from other solids based on their different solubilities in suitable solvent(s) 4. Solvent extraction To separate a component from a mixture with a suitable solvent 5. Distillation To separate a liquid from a solution containing non-volatile solutes  Isolation and Purification of Organic Compounds Technique Aim 6. Fractional distillation To separate miscible liquids The mixture boils below with widely different boiling 100°C points 7. Steam distillation To separate liquids which are immiscible with water and decompose easily below their b.p. 8. Vacuum distillation ditto 9. Sublimation To separate a mixture of solids in which only one can sublime 10. Chromatography To separate a complex mixture of substances (large/small scale)  Isolation and Purification of Organic Compounds Tests for Purity If the substance is a solid, è its purity can be checked by determining its melting point If it is a liquid, è its purity can be checked by determining its boiling point  Isolation and Purification of Organic Compounds Isolation and Purification of Organic Compounds The selection of a proper technique è depends on the particular differences in physical properties of the substances present in the mixture Isolation and Purification of Organic Compounds Filtration To separate an insoluble solid from a liquid particularly when the solid is suspended throughout the liquid. The solid/liquid mixture is called a suspension. Isolation and Purification of Organic Compounds Filtration The laboratory set-up of filtration Isolation and Purification of Organic Compounds Filtration There are many small holes in the filter paper. è allow very small particles of solvent and dissolved solutes to pass through as filtrate. Larger insoluble particles are retained on the filter paper as residue. Isolation and Purification of Organic Compounds Centrifugation When there is only a small amount of suspension, or when much faster separation is required; è Centrifugation is often used instead of filtration.  Isolation and Purification of Organic Compounds Centrifugation The liquid containing undissolved solids is put in a centrifuge tube. The tubes are then put into the tube holders in a centrifuge. A centrifuge Isolation and Purification of Organic Compounds Centrifugation The holders and tubes are spun around at a very high rate and are thrown outwards. The denser solid is collected as a lump at the bottom of the tube with the clear liquid above. Isolation and Purification of Organic Compounds Crystallization Crystals are solids that have è a definite regular shape. è smooth flat faces and straight edges. Crystallization is the process of forming crystals. Isolation and Purification of Organic Compounds Crystallization by Cooling a Hot Concentrated Solution To obtain crystals from an unsaturated aqueous solution è the solution is gently heated to make it more concentrated. After, the solution is allowed to cool at room conditions. Isolation and Purification of Organic Compounds Crystallization by Cooling a Hot Concentrated Solution The solubilities of most solids increase with temperature. When a hot concentrated solution is cooled. è the solution cannot hold all of the dissolved solutes. The “excess” solute separates out as crystals.  Isolation and Purification of Organic Compounds Crystallization by Cooling a Hot Concentrated Solution Crystallization by cooling a hot concentrated solution Isolation and Purification of Organic Compounds Crystallization by Evaporating a Cold Solution at Room Temperature As the solvent in a solution evaporates, è the remaining solution becomes more and more concentrated è eventually the solution becomes saturated è further evaporation causes crystallization to occur Isolation and Purification of Organic Compounds Crystallization by Evaporating a Cold Solution at Room Temperature If a solution is allowed to stand at room temperature, è evaporation will be slow. It may take days or even weeks for crystals to form. Isolation and Purification of Organic Compounds 2. Crystallization by Evaporating a Cold Solution at Room Temperature Crystallization by slow evaporation of a solution (preferably saturated) at room temperature Isolation and Purification of Organic Compounds Solvent Extraction Involves extracting a component from a mixture with a suitable solvent. Water is the solvent used to extract salts from a mixture containing salts and sand. Non-aqueous solvents (e.g. 1,1,1- trichloroethane and diethyl ether) can be used to extract organic products.  Isolation and Purification of Organic Compounds Solvent Extraction Often involves the use of a separating funnel. When an aqueous solution containing the organic product is shaken with diethyl ether in a separating funnel, è the organic product dissolves into the ether layer. Isolation and Purification of Organic Compounds Solvent Extraction The organic product in an aqueous solution can be extracted by solvent extraction using diethyl ether Isolation and Purification of Organic Compounds Solvent Extraction The ether layer can be run off from the separating funnel and saved. Another fresh portion of ether is shaken with the aqueous solution to extract any organic products remaining. Repeated extraction will extract most of the organic product into the several portions of ether. Isolation and Purification of Organic Compounds Solvent Extraction Conducting the extraction with several small portions of ether is more efficient than extracting in a single batch with the whole volume of ether These several ether portions are combined and dried è the ether is distilled off è leaving behind the organic product Isolation and Purification of Organic Compounds Distillation A method used to separate a solvent from a solution containing non-volatile solutes. When a solution is boiled, è only the solvent vaporizes. è the hot vapour formed condenses to liquid again on a cold surface. The liquid collected is the distillate. Isolation and Purification of Organic Compounds Distillation The laboratory set-up of distillation  Isolation and Purification of Organic Compounds Distillation Before the solution is heated, è several pieces of anti-bumping granules are added into the flask. è prevent vigorous movement of the liquid called bumping to occur during heating. è make boiling smooth.  Isolation and Purification of Organic Compounds Distillation If bumping occurs during distillation, è some solution (not yet vaporized) may spurt out into the collecting vessel.  Isolation and Purification of Organic Compounds Fractional Distillation A method used to separate a mixture of two or more miscible liquids  Isolation and Purification of Organic Compounds Fractional Distillation The laboratory set-up of fractional distillation Isolation and Purification of Organic Compounds Fractional Distillation A fractionating column is attached vertically between the flask and the condenser è a column packed with glass beads è provide a large surface area for the repeated condensation and vaporization of the mixture to occur. Isolation and Purification of Organic Compounds Fractional Distillation The temperature of the escaping vapour is measured using a thermometer When the temperature reading becomes steady, è the vapour with the lowest boiling point firstly comes out from the top of the column Isolation and Purification of Organic Compounds Fractional Distillation When all of that liquid has distilled off, è the temperature reading rises and becomes steady later on. è another liquid with a higher boiling point distils out. Fractions with different boiling points can be collected separately. Isolation and Purification of Organic Compounds Sublimation Sublimation is the direct change of è a solid to vapour on heating, or è a vapour to solid on cooling. è without going through the liquid state. Isolation and Purification of Organic Compounds Sublimation A mixture of two compounds is heated in an evaporating dish. One compound changes from solid to vapour directly. è The vapour changes back to solid on a cold surface. The other compound is not affected by heating and remains in the evaporating dish. Isolation and Purification of Organic Compounds Sublimation A mixture of two compounds can be separated by sublimation Isolation and Purification of Organic Compounds Chromatography An effective method of separating a complex mixture of substances. Paper chromatography is a common type of chromatography. Isolation and Purification of Organic Compounds Chromatography The laboratory set-up of paper chromatography Isolation and Purification of Organic Compounds Chromatography A solution of the mixture is dropped at one end of the filter paper  Isolation and Purification of Organic Compounds Chromatography The thin film of water adhered onto the surface of the filter paper forms the stationary phase. The solvent is called the mobile phase or eluent. Isolation and Purification of Organic Compounds Chromatography When the solvent moves across the sample spot of the mixture, è partition of the components between the stationary phase and the mobile phase occurs Isolation and Purification of Organic Compounds Chromatography As the various components are being adsorbed or partitioned at different rates, è they move upwards at different rates The ratio of the distance travelled by the substance to the distance travelled by the solvent è known as the Rf value è a characteristic of the substance Isolation and Purification of Organic Compounds A summary of different techniques of isolation and purification Technique Aim (a) Filtration To separate an insoluble solid from a liquid (slow) (b) Centrifugation To separate an insoluble solid from a liquid (fast) (c) Crystallization To separate a dissolved solute from its solution (d) Solvent To separate a component from a extraction mixture with a suitable solvent (e) Distillation To separate a liquid from a solution containing non-volatile solutes Isolation and Purification of Organic Compounds A summary of different techniques of isolation and purification Technique Aim (f) Fractional To separate miscible liquids with distillation widely different boiling points (g) SublimationTo separate a mixture of solids in which only one can sublime (h) To separate a complex mixture of Chromatography substances