Physical Determination of Organic Compounds PDF

# PHYSICAL DETERMINATION OF ORGANIC COMPOUNDS The physical determination of the organic compound gives a valuable clue about its nature and structure, and determines the criteria of its purity, such as Melting point, Boiling point, Density, Refractive Index, Optical Rotation, Surface tensions and c...

# PHYSICAL DETERMINATION OF ORGANIC COMPOUNDS The physical determination of the organic compound gives a valuable clue about its nature and structure, and determines the criteria of its purity, such as Melting point, Boiling point, Density, Refractive Index, Optical Rotation, Surface tensions and conductivity. The most important ones are M.P., B.P. and refractive index. Organic compounds, solids, liquids or gases, all possess a very different physical character than that of inorganic compounds. The inorganic compounds are ionic, and the force that holds the ions together is powerful. Thus, a very energy is needed to overcome this powerful inter ionic force, to change its physical character from one state to another. While the organic compounds are non ionic, in which the atoms are held to each other entirely by covalent bonds, forming crystals in which the structural units are molecules. These molecules are held together by an intermolecular force, which is very weak, compared to the ionic force Thus they need much less energy to overcome this force, and changes its state from one to another. ## 1- Melting Point The particles in solid organic compounds are arranged orderly in a very regular and symmetrical way, due to the intermolecular forces that hold these particles together. Melting is the change from the highly ordered arrangement of these particles to the more random arrangement that characterizes a liquid state. It occurs when a temperature is reached at which thermal energy of the particles is great enough to overcome the intermolecular forces that hold these particles in position. Such a temperature is known as Melting point or is the temperature at which the solid begins to change into liquid under a pressure of equilibrium between liquid and solid states Pure organic compound melts at a definite and sharp temperature, within a small range, while impure organic compound melts at a lower temp. and within a higher range. i.e. does not melt at a sharp temperature. (Range: is the time needed to change substance from solid to liquid state) The M.P. may be determined by several methods either using an electrical apparatus, or using the capillary tube method which is the most common method for the beginner. ## 2- The Boiling Point Liquid organic compounds consist of particles that are less regularly arranged than in solid compounds. Each particle is attracted by a number of other particles, by an intermolecular force. Boiling is the change from the liquid state to the vapour state, where the particles are freer in motion. Thus an amount of energy is needed to break off the intermolecular forces, and to bring the vapour state and the liquid state into equilibrium under the atmospheric pressure. Such an amount of energy is known as the Boiling point. A pure liquid will have similarly a sharp boiling - point which will remain constant until the whole of the liquid has boiled off, leaving no residue. Impure liquid boil at different ranges of temperature, depends largely on the physical nature of the impurities. It's define as the temperature at which it's vapour pressure equals the atmospheric pressure over the liquid, or temperature at which the vapour and liquid phases are in equilibrium at a given pressure. # PURIFICATION AND SEPARATION OF ORGANIC COMPOUNDS Organic compounds, whether solids, liquids or gaseous, when are separated from organic reactions are seldom pure. They may be contaminated with other compounds which are formed as a byproduct, owing to a side reaction. These organic compounds have to be purified before using them in other chemical reaction. Several methods of purification have to be: - Crystallization. - Sublimation. - Extraction. - Distillation. - Chromatographic adsorption. The method employed depends upon the nature of the material to be separated and purified. ## 1- Purification of organic compound by crystallization Crystallization is one phenomena which is involved in the production of pure crystals form solution, after dissolving the impure substance in a suitable solvent or mixture of solvents. The choice of the solvent is very important in this process, water or any other organic solvent (ethyl alcohol, methyl alcohol, acetone, glacial acetic acid, ethyl acetate, benzene, petroleum ether, ether, chloroform......etc.) can be used. Ether could be avoided whenever possible for its high inflammability. A distinct relation has to be existed between the constitution of the organic compound and the solvent, so they may undergo into solution. As the old rule said "like dissolves like", therefore, polar organic compound dissolve in non polar solvents and so on. It is also assumed that: - The solvent should not react chemically with the substance to be purified. - It should dissolve the impurities readily. - It should well-formed crystals of the purified compound. - It must be capable of easy removal from the crystals of the purified compound. In some cases mixture of two of the solvents might be used, especially when a substance is highly soluble in one solvent and sparingly soluble in other solvent, the mixture of the two solvents can be used for crystallization. Purification by crystallization may be improved by means of some adsorbing material such as animal charcoal or boneblack. ## 4- Separation and Purification of Organic Compound by Distillation Liquid organic compounds, can be purified and isolated from certain impurities which have been contaminated during the preparation, by distillation. Distillation is the conversion of the liquid organic compound to the vapour state with the aid of heat, and condensation of the vapours to the liquid state. The temperature at which the liquid distills, is a definite value (at a given pressure) for every pure organic compound and is known as the boiling point. Some organic compounds do decompose at high temperature or nearly their boiling points under normal pressure. Therefore, certain modification in distillation process has to be carried on to overcome the decomposition, such as decreasing the pressure which leads to lower the B.P. and avoid decomposition. There are four types of distillation commonly employed by the organic chemistry, are: - Simple distillation. - Fractional distillation. - Vacuum distillation. - Steam distillation. ### 1- Simple Distillation Is the mostly used for the liquid organic compounds, which boil at ordinary high temperature, do not decompose at this point and are not mixed with several other low boiling point substances. It consists of a distilling flask attached by a cork to a water cooled Liebig condenser, upon the end of which an adapter is fitted and immersed in a flask to receive the distillate - the receiver. The thermometer is fitted into the neck of the distilling flask by means of a well-bored cork, the bulb of the thermometer should be in the center of the neck of the flask and slightly below the level of the side tube. The distilling flask may be heated on a wire gauge with asbestos center, or in a water bath if the organic substance is of a law B.P. or on a sand bath. Few pieces of broken porcelain chips (or boiling stones) are placed in the distilling flask with the substance to be distilled. They prevent bumping, by producing a constant stream of bobbles which keeps the liquid in motion. If the organic substance is of B.P. higher than 120°c, air condenser has to be used. I.e. water should not run through the condenser during the distillation process. The high temperature vapour, when passes through the cold surface of the condenser, the glass walls will crack down, therefore the condenser should not be water - cooled, air - cooled is enough. ### 2- Fractional Distillation When the organic substance is contaminated with the impurities or mixed with other volatile organic substances of different B.P., purification or isolation of the organic substance will take place gradually by fractions, I.e. when the whole mixture is heated, the lower B.P. constituent will evaporate, condense and distil first, followed by the second higher B.P. constituent... And so on. Every constituent will be evaporated and distilled at its own B.P. When the B.P. of the constituents are close to each other, a modified condensing tube which is made of a series of bulbs or pieces of broken glass tubes. These increase the condensing surface and lessen the resistance to the passage of vapours the receiver and the separation or purification will take place by fraction. Such distillation is known as fractional distillation. There are many types of condenser that can be used in such a process, there is no need to explain these in details, since they are beyond the scope of this subject. ### 3- Vacuum Distillation Is mostly used when the organic compounds do decompose at their boiling point, or nearly to the B.P. “the B.P. may be defined as that temperature at which the vapour pressure equals the pressure of the atmosphere above the liquid.” by reducing the external pressure to 1 - 30 mm. of mercury, the B.P. is considerably reduced, and the distillation may usually be conduced without danger of decomposition. The pressure is reduced by means of a high capacity water pump, preferably constructed of metal, to 7-20 mm. ### 4- Steam Distillation Is widely used in the purification of very high B.P. organic compounds, by passing steam to the distilling flas;, the importance of the steam in this case is to distil the organic substance in a lower B.P. The vapour pressure of the steam will be added to the vapour pressure of the organic substance, producing a mixture of vapour that can be pushed to the condenser at a lower temperature than what it should have for the single component. The distillate, therefore, containing both the organic substance and water, can be isolated by a separatory funnel. Experimentally these will be done in the future experiments and several subjects.