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Organic Chemistry‎ > ‎Laboratory‎ > ‎

Purification of Organic Compounds


INTRODUCTION  

            In this experiment distillation which is a seperation and purification method of compounds was used. Distillation is based on the fact that the vapour of a boiling mixture will be richer in the components that have lower boiling points.Therefore, when this vapour is cooled and condensed, the condensate will contain more volatile components. At the same time, the original mixture will contain more of the less volatile material. Distillation columns are designed to achieve this separation efficiently.  Although many people have a fair idea what “distillation” means, the important aspects that seem to be missed from the manufacturing point of view are that:

            - distillation is the most common separation technique    - it consumes enormous amounts of energy, both in terms of cooling and heating requirements   -it can contribute to more than 50% of plant operating costs

The best way to reduce operating costs of existing units, is to improve their efficiency and operation via process optimisation and control. To achieve this improvement, a thorough understanding of distillation principles and how distillation systems are designed is essential.

Simple distillation is good for separating relatively low-boiling liquids (less than 150 degrees C) from nonvolatile solids or other liquids with significantly different boiling points.  A 25 degree difference is generally separable from solvent.

            Higher boiling solvents can be distilled if pressure over the distillation system is reduced.  This method is called vacuum distillation. 

Steam distillation is a method for separating water-insoluble or slightly-soluble substances from water mixtures.  This is used frequently for isolating flavor or fragrance oils from natural products (lavender, mint, etc.).

            Some mixtures that seem like they should distill well just won’t.  Those liquids that don’t behave like they ought to (according to Raoult’s Law that describes vapor pressures in mixtures), that have a constant composition and a constant boiling point as they distill, are called azeotropes.  95% ethanol behaves this way.  Most mixtures will undergo composition changes as they distill, so the temperature of the distilling vapor will change with time.  Such is not true for an azeotrope.  The temperature you record during the distillation of one of these special mixtures remains constant.

MATERIALS

            Acetone , water, simple distillation mechanism, beaker, steam distillation mechanism, coffee, thermometer.

PROCEDURE

            First simple distillation was done.Simple distillation mechanism was installed.Thirty five mL acetone and 35 mL water were placed in distillation flask.Then the mixture was heated.The thermometer was not allowed to show 100°C.Then at 56°C acetone boiled.And 32 mL acetone was collected in the graduated cylinder.

            Secondly steam distillation mechanism was set up.Our sample is coffee.Water was heated and then a liquid which smell like coffee was collected in beaker.

POSTLAB QUESTIONS

1- Fractional distillation is used to separate mixtures of miscible liquids, such as ethanol and water. The process depends on the components of the mixture having different boiling points. The liquid is heated so that it turns into a gas. The vapours pass up a fractionating column where they are gradually cooled. As each of the components of the mixture cools to its boiling point, it turns back into a liquid. The different components of the mixture condense at different levels in the fractionating column and thus may be separated 

2-At the boiling point of such a mixture,both components will contribute some molecules to the vapor.The more volatile component,because it is more easily vaporized,will have a larger fraction of its molecules in the vapor state than will less volatile component.When this vapor is condensed into another container,the resulting liquid (the distillate) will be rich in the more volatile component then was the original mixture.As distillation continiues,the boiling point rises until,finally,the boiling point of the less volatile liquid is reached.The first portion of the distillate is has the greater concentration of the more volatile component in the distillate increases.Thus,the more volatile liquid in the mixture can be obtained by collecting only the first portion of the distillate.

3-The packed column,usually glass beads,helices or fingers,gives a large surface area for contact of the ascending vapours and descending liquid.After the first fraction has distilled,the temperature will rise and the rate of distillation will slow.This is an intermediate fraction containing a little of both component of the mixture.Finally,the temperature will become constant and the pure higher boiling compound will distill.

DISCUSSION

            During the distillation, the boiling point (temperature) of the liquid can be observed. Once the boiling point has been obtained, comparison of this number with literature values can help establish the identity of the unknown liquid.The central premise of distillation is that a mixture of liquids can be separated from each other by taking advantage of differences in boiling points between the compounds in the mixture. In order to do this, the components of the mixture are slowly vaporized, condensed, and collected.

Since the components of the original mixture presumably have different boiling points they vaporize at different temperatures, and consequently, are separated by collecting fractions boiling at different temperatures. Normally, if the impurities and desired compounds have boiling point differences of greater than 50°C, the separation is easily accomplished using the simplest of distillation equipment.At first glance, distillation seems like an ideal method, certainly a lot easier than recrystallization. However, it does have its problems. While the boiling points of pure compounds are fairly sharp, generally within a range of 1-2°C, they are not as sensitive to impurities as melting points. In fact, a distillate containing impurities of several percent will not usually have a boiling point significantly different than the pure compound. Constant boiling mixtures, call azeotropes are also possible and may appear to be pure compounds when in fact they are mixtures of compounds.

Some organic compounds decompose with excessive heat. Therefore traditional distillation is not always the best method for separating liquids. In some cases, steam distillation can be used to separate high boiling components from liquid and even solid mixtures. In this experiment, you will be using steam distillation to isolate the essential oils found in spices bought from the grocery store. These spices are complex mixtures of chemicals. The goal is to collect the essential oil from your assigned spice without destroying the chemical nature and physical properties of the compounds. You will use gas chromatography to identify the major components in the essential oil that you separate from your spice. You will be assigned a spice the week before this lab begins, if you have any special requests for spices please let your instrcutor know at that time.The fundamental principle that lies behind distillation is that the composition of a liquid mixture (ie two different liquids mixed together) is different from the composition of its vapour mixture, due to differences in volatility of the liquids in the mixture.                                                                      The fundamental principle that lies behind distillation is that the composition of a liquid mixture (ie two different liquids mixed together) is different from the composition of its vapour mixture, due to differences in volatility of the liquids in the mixture. If you mix two liquids (one boils at 100°C, the other at 50°C) then the vapour above the liquid will have much more of the 50°C substance than the 100°C substance. If you then remove the vapour and cool it, so that it condenses, then the new liquid mixture will have much more 50°C substance than the 100°C substance. If you start again heating the new liquid condensed from the vapour (at a slightly lower temperature because it has more of the substance that boils at a lower temperature), then the vapour will again have even more 50°C substance than the 100°C substance, and so on. If you do this enough times, then you can get all of the 50°C substance separate from the 100°C substance. You can get this to happen without having to remove the vapour each time, and you then have continuous distillation, and there is a temperature gradient up the column from hot at the bottom to cool at the top.

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