Lab Report on Aldol Condensation

The lab report below was submitted as part of the coursework for CM1121 Basic Inorganic Chemistry. Please do not plagiarise from it as plagiarism might land you into trouble with your university. Do note that my report is well-circulated online and many of my juniors have received soft copies of it. Hence, please exercise prudence while referring to it and, if necessary, cite this webpage.

1.     Abstract

The experiment involves the separation of two organic compounds which consist of an acidic and neutral compound, their purification as well as their identification by use of their melting points. Firstly, the separation is done by solvent extraction which involves the addition of dichloromethane (CH₂Cl₂) and 2% NaOH. The compounds dissolve in them, forming two layers of liquids which are separated using a separating funnel. Secondly, the compounds are then recovered. The neutral compound is obtained through simple distillation by distilling CH₂Cl₂ of lower boiling point off. The acidic compound is acidified by concentrated HCl, forming a precipitate which can be recovered by vacuum filtration. Thirdly, the acidic and neutral compounds are purified by single solvent and solvent pair method respectively. The methods employed are determined by solubility tests. Lastly, the compounds are identified by their melting points by use of the melting point apparatus. The acidic compound identified is salicylic acid and the neutral compound is dibenzalactone. 

2.     Introduction

Solvent extraction is one of the most important and common method used for separation in laboratory and industrial scales. It has an extensive application in the separation of radioactive materials such as the reprocessing of nuclear fuels¹.  In organic chemistry, solvent extractions done in labs are usually small-scale, using a separatory funnel. In industrial applications, batch-mode extraction is used on a much larger scale than laboratory solvent extractions ².

All scientists like to observe systems, make observations and draw conclusions in their results. it is like a protocol which if rigorously adhered to in e lab, is most likely to turn a chance observation into an important discovery. In organic chemistry, the fact that a sample has been taken from bottle on a shelf is no assurance of purity or even that the substance is what the label says it is. Labeling mistakes occur too frequently, especially in samples which have been relabeled after purchase. Organic compounds also degrade on storage and its stability under specific conditions of storage is impossible to estimate. This self evident precaution is often overlooked y even the most experienced research chemists, often to their downfall. The identification of an unknown compound encapsulates the events which occur during any scientific investigation, no matter how short or how grand.

In this experiment, two organic compounds, one acidic compound and another neutral compound are being separated. The possible neutral organic compounds are hydrocarbons, alcohol, ketone or nitrile and acidic organic compounds are carboxylic acids. The physical properties such as their densities and solubilities and chemical properties such as acid-base reactions and reactivity of both the acidic and neutral compounds must be taken into account to determine their method of extraction.

The carboxylic acid in the unknown compound is a long chain one (as seen from the possible compounds in the lab manual). They are generally insoluble in water and soluble in organic solvents. They can also react with a base to form a carboxylate anion. RCO₂H + NaOH RCO₂ ^- Na⁺ (water soluble salt) + H₂O After reaction with an OH^- ion, the solubility properties of the carboxylic acid changes. The carboxylate anion is soluble in water but is insoluble in organic solvents. Neutral compounds, on the other hand, are insoluble in water but they are soluble in organic solvents. Solvent extraction, or liquid-liquid extraction is used to capitalize on the each of their different solubilities. The organic solvent chosen is dichloromethane and aqueous solution of 2% NaOH. When CH₂Cl₂ is added to the unknown compounds, both the carboxylic acid and the neutral compound would be dissolved in it. When NaOH is added and shaken, the OH^- ion reacts with the carboxylic acid to form a carboxylate anion which is no longer soluble in CH₂Cl₂ but soluble in water (in NaOH) due to the polar bonds form with water, causing the carboxylic acid to move to the aqueous layer. As the organic solvent and alkaline solution are immiscible, two layers of liquids are formed based on differences in polarity and density. The extractions of both the organic and aqueous layer are done multiple times as extraction done for the first time may not lead to complete separations as explained by the partition coefficient (in Discussion section). CH₂Cl₂ is extracted twice with the addition of NaOH to remove any acid and water impurities from the organic layer. CH₂Cl₂ is then extracted once more with addition of deionised water to remove any water soluble impurities. The two NaOH and the deionised water layers are combined to form the aqueous layer containing the carboxylate anion and the CH₂Cl₂ layer forms the organic layer containing the neutral compound.

The carboxylic acid is precipitated with the use of concentrated HCl. The H⁺ ion in the acid would first react with the OH- ion in NaOH (H⁺ + OH^- à H₂O).

After all the OH^- ions have been converted into water, the carboxylate ion, RCOO- would react with the extra H+ to yield the carboxylic acid (RCOO^- + H⁺ à RCOOH). Net equation is RCO₂^- Na⁺ + HCl àRCO₂H _(s) + NaCl.

Congo Red paper is used to check if the solution contains excess H⁺. Congo red paper turns blue when it detects free H⁺ ions. Since the long chain carboxylic acid is insoluble in water, a precipitate is formed in the solution.  The solution is then filtered to remove the aqueous solution of HCl from the carboxylic acid compound. Recrystallisation of the carboxylic acid compound needs to be done to remove the impurities in the compound.

The organic layer is dried using a drying agent, anhydrous Na₂SO₄, to remove any traces of water. This is done to achieve high purity of the organic compound and to ensure smooth recrystallisation. Simple distillation is carried out to distill off CH₂Cl₂, which has a boiling point of 40 ̊C³, lower than that of the neutral compound (whose melting point is above 60 ̊C which means its boiling point is above 60 ̊C as well). Two pieces of boiling chips placed in the round bottom flask during the distillation process. They are small, insoluble, porous stones made of calcium carbonate⁴ or silicon carbide⁴. The pores in it trap air and provide spaces where bubbles of solvent vapor can form. When a boiling chip is heated in a solvent, it releases tiny bubbles, ensuring smooth and even boiling and prevent bumping and boiling over and loss of the solution.

There are two types of recrystallisation methods, namely, recrystallisation using a single solvent or using a solvent pair. Solubility tests are conducted to ascertain the method of recrystallisation that is to be used. The sample is added to either water or ethanol. It is crucial to take note never to add too much sample as saturated solution may not be able to dissolve the excess sample, causing ourselves to mistaken the sample to be insoluble in the solvent.

After recrystallisation is completed, the crystals are placed under the infra-red lamp to remove any water content that may still be in the crystals. This step is essential because the water content in the crystals has an impact on the range of melting point of the compounds, causing an inaccurate deduction of the identity of the compounds later on. After the removal of water in the crystals, the melting points of the two compounds are determined using the melting point apparatus.

3.     Experimental

The apparatus that are needed are two dry 250mL conical flask, 4 test tubes and test tube rack, retort stand, electronic balance, clamp for retort stand, metal ring to support separatory funnel, separatory funnel, glass funnel, vacuum flask, glass rod, beaker, infra-red lamp, distillation materials which include dry round bottom flask, thermometer, thermometer holder, west condenser, boiler, 2 vacuum tubes, water pump, 3 way connecting tube, 2 clamp holders and straight tube adapter. The chemicals and materials needed are 80mL dichloromethane, 80mL 2% NaOH, 20mL of deionised water, one spoonful of anhydrous Na2SO4, boiling chips, Congo Red paper, a few filter papers, ice, 5 mL concentrated HCl, ethanol and cold deionised water.

Separation by solvent extraction: Place unknown sample and 70mL CH₂Cl₂ into 250mL conical flask, swirling it to ensure all the solids have dissolved. Then transfer the contents to the separatory funnel. Rinse the conical flask with 10mL of CH₂Cl₂ then pour it into the separatory funnel. Add 40mL of 2% NaOH into the separatory funnel and shake it contents vigorously for 3 times, releasing the stopcock each time. Allow it to stand for two layers to form. Drain the lower organic layer into a conical flask and pour the aqueous layer into a beaker. Place the organic layer back into the separatory funnel and repeat the steps again with another 40mL of 2% NaOH and once more with 20mL of deionised water, combining the drained NaOH and deionised water into one beaker and the organic layer to a conical flask.

Recovery of neutral compound: Add a spoonful of anhydrous Na₂SO₄ into the organic layer to remove any water impurities in it. Using a folded filter paper and a filter funnel, filter the dried organic layer and place it into a dry round bottom flask. Set up a simple distillation to remove CH₂Cl₂ as distillate and stop the distillation process once no more distillate is produced. Recover crude product N from the round bottom flask.

Recovery of acidic compound by acidification: Place the beaker containing the aqueous layer into an ice bath and add 5mL of concentrated HCl. Check to see if the mixture is acidic by use of Congo paper which should turn blue. Using the vacuum filtration method, collect precipitate A.

Solubility test: Using a small amount of each of the two compounds and add each compound to a test tube with 2mL of deionised water or ethanol. The solubility of the compounds in deionised water and ethanol at room and hot temperatures are recorded. The test results can be found on the Results section of this report. From the test results, the acidic and neutral compound would have to undergo recrystallisation by single solvent and solvent pair techniques respectively.

Recrystallisation by single solvent: Place the acidic compound A into a 100mL conical flask and a small amount of 100% deionised water is added. The solution is heated until all the solid have dissolved. The solution is left to cool at room temperature and then in ice water bath to allow crystallization. The crystals are filtered by vacuum filtration and wash with cold solvent. Place the crystals under an infra-red lamp to dry it further.

Recrystallisation by solvent pair: Place neutral compound N into a 100mL conical flask with 10mL ethanol and boil it. Boiling deionised water is added drop-wise until a turbid solution is formed. Then add ethanol drop-wise until the turbidity disappears. The mixture is allowed to cool at room temperature then in ice water bath to allow crystallization. The crystals are filtered by vacuum filtration and wash with cold deionised water. Place the crystals under an infra-red lamp to dry it further.

Identification of compounds: The mass of the plastic bag and two crystal compounds with the plastic bags are then weighed separately and the readings are recorded. The results are shown in the Results section in this report. Fit a minute portion of each of the compounds into two capillary tubes. The first tube is used to measure the approximate melting range with rapid heating of 20 ̊C gradient and the second tube to find a more accurate melting range with slower heating of 1 ̊C gradient.  The range of the melting points should be recorded between the temperature at which liquid is observed and the temperature at which all the solid becomes a liquid. The melting point ranges of the acidic and neutral compounds are recorded and their identities are obtained when the melting points ranges are compared with those in the laboratory manual.

4.     Results

Table 1: Solubility test results for compound A and N in water and in ethanol.

Solvent	Acidic compound (A)		Neutral compound (N)
	Room temp	Hot	Room temp	Hot
Water	NS	S	NS	NS
EtOH	S	S	S	S
Solvent used for recrystallization	Single solvent(water)		Solvent pair

NS: not soluble / poorly soluble           S: soluble

From this solubility tests, the solvent used for recrystallisation is single solvent for compound A and solvent pair for compound N.

Table 2: Mass of crystals A and N obtained after purification.

Mass (g)	Acidic compound (A)	Neutral compound (N)
Plastic bag	0.65	0.65
Plastic bag + purified product	2.07	0.94
Purified product	1.42	0.29

Table 3:  Melting point of crystals A and N their identification.

	Acidic compound (A)	Neutral compound (N)
Rough melting-point range (trial run)/ 0C	157.3-159.0	107.5-113.5
Accurate melting-point range (accurate run)/ 0C	158.4-158.9	110.1-110.4
Compound identified	Salicylic acid	Dibenzalactone

The recrystallised acidic compound identified is salicylic acid which has a melting point range of 158.4 ̊C -158.9 ̊C, which does not correspond to the experimental value of 159.0 ̊C sharp⁵. Its recrystallised appearance is white, which corresponds to its actual appearance of white⁶ as well. The compound A is insoluble in water at room temperature, as indicated in Table 1, corresponds to salicylic acid actual property that it is poorly soluble in water at room temperature with a solubility of 0.2g/100mL of H₂O at 20 ̊C⁵.

The recrystallised neutral compound identified is dibenzalactone which has a melting point range of 110.1 ̊C -110.4 ̊C, which correspond to the experimental value of 110 ̊C to 112 ̊C⁷. Its recrystallised appearance is dull light brown, which does not correspond to its actual appearance of bright yellow⁷ in colour. The compound N is insoluble in water at room temperature, as indicated in Table 1, corresponds to dibenzalactone actual property that it is insoluble in water but soluble in ethanol⁷.

Percentage Yields of Compounds A and N

Percentage Yield of Compound A

=  × 100%

= ×100%

= 71.0 %

Percentage Yield of Compound N

= × 100%                                                     

=×100%

= 14.5%

Percentage Yield of Sample recovered

=  × 100%

= 43.0%

5.     Discussion

Percentage yield for compound A and N

The yield from compound A can be considered good⁸ as it has a percentage yield of approximately 71%. The yield is good because care was taken during vacuum filtration by ensuring that the Buchner funnel fully covers the rubber adaptor so that there is no leakage of the aqueous layer into the filter flask. I have also made sure that the filter paper is wet as this would cause the paper to adhere to the plate and keeps materials from passing under the paper during filtration.   Care was also taken to make sure that the filter paper is secure on the filter, that air is being drawn through the paper, and that all of your apparatus is securely clamped. The mixture of concentrated HCl and the aqueous is then added slowly to the Buchner funnel, ensuring that the pressure of the fluid passing through is not focused on one point, but evenly spread out on the entire filter paper. This is to ensure that the filter paper does not break and the particles from the aqueous layer do not flow through the holes. Extra care was also taken to ensure that the particles also do not creep under the edges of the filter paper.

However, the yield could have been higher as there might be minute amounts of the crystals could be lost during the processes of transferring them from one container to another, when weighing them, or even blown away by the wind. This error is rather inevitable to a large extent, even though we may be extremely careful. We can try to make as little mass of the crystals lost as possible, and avoiding being near fans which can blow the crystals away if we are not careful.

The yield from compound N is considered poor⁸  as it has a percentage yield of 14.5%. The brownish colour of compound N also has a stark difference from its actual colour of bright yellow. This could be because I added a drop of concentrated HCl into the solvent during recrystallisation by solvent pair by mistake. This caused the ethanol to turn to a dark brown solution while heating on the hot plate, instead of a white turbid. The dark brown solution is also unable to turn turbid with addition of boiling water. With the help of a lab assistant, simple distillation was done and the rotary evaporator was used to obtain the solid for melting point analysis. The addition of a drop of concentrated HCl might have introduced a substantial amount of impurities that causes the differences in the colour and yield of the compound. Due to the large differences in the colour and the possibility that impurities are introduced, the compound might not even be dibenzalactone. This means that the drop of concentrated HCl might have also changed the chemical structure of the true neutral compound, even though it does not have any basic properties that could possibly react with an acid. This could mean that the compound N identified is not dibenzalactone, even though their melting points correspond. The experiment could be repeated to obtain a more accurate deduction.

The process of drying of both compounds also posed another problem, which is the loss of product. As the product was left in the open, under infra-red lamp, the wind might blow away some of the smaller crystals thus affecting the resulting yield of the product. One method of improvement is to use a Petri dish instead of a watch glass for the drying process. As the Petri dish comes with a cover, the product is more protected. Another method would be to dry the crystals using a vacuum oven, not only would this method effectively remove water from the crystals, it is also relatively a faster method of drying crystals.

Other reasons for the loss of compounds could be due to spillage during the separating steps causing the loss of compounds or that they were lost during transfer from one container to another because some were stuck on the walls of the containers as the complete transfer of mixture was not possible. There were also crystals were lost as they were stuck onto the filter paper after suction filtration, and not all were collected. Also, some of the compound did not crystallize out during cooling, meaning to say that crystallization was incomplete and some of the compound was still dissolved in the solution.

There are also inaccuracies in the readings taken for the mass of the compounds and this could be due to having water trapped between the crystals due to insufficient drying, leading to bigger mass obtained and higher expected mass of the compounds recovered, assuming other factors remaining a constant. Also, during the initial stages of separation, there may not be a complete separation even though extraction was done several times. This is because organic compounds have certain affinity for organic solvents like dichloromethane.

 Purity of the compounds

The products obtained are generally considered quite pure, as the melting ranges obtained are near to their literature values. Also, the melting range is narrow (approximately 2°C)⁹, the products can be considered quite pure based on this small range. However, there are still limitations to the melting ranges readings obtained. A very small portion of the crystals was used to determine melting point, hence, the results may not accurately reflect the purity of the actual product.

The recrystallised acidic compound identified is salicylic acid which has a melting point range of 158.4 ̊C -158.9 ̊C, which does not correspond to the experimental value of 159.0 ̊C sharp⁵. The melting point range is lower than expected because there could be impurities present in the sample. Impurities that are present in the sample may lead to lower than expected melting point range.

There could also be errors that may affect purity of compounds collected at the end, and affecting the melting points eventually. When the melting range of the crystals is determined, the change of the state of crystals between the first appearance of drops of liquid within the sample to the disappearance of the last trace of solid of melting is not very obvious, and therefore, making it difficult to judge the accurate range. However, repeated readings were taken to narrow down the melting range and obtain more accurate readings. Additionally, dust particles from the surrounding may inevitably land in the mixtures to lower the purity of the crystals when they are brought to test for their melting points.

However, if a broad melting point range is obtained, this indicates a strong evidence for a lack of purity which could mean that the pure substance decomposes on heating, introducing impurities into it. The darkening of the sample or the evolution of gas is an indication that this is occurring. Dissolution of the compound in residual or occluded recrystallisation solvent will also give rise to a broad melting point range⁹.

To verify our results that the melting point range of our sample is similar to that of the pure compound, we can make an intimate mix of the unknown sample with a pure sample of the proposed material in equal proportions and obtain the melting point of the mixture. If the substances are identical, then the melting point with not change. However, if the melting point is different, this means that the proposed compound is incorrect or that impurities are added that lowers and broadens the melting point range. This technique is useful when several possible candidate compounds within one or two degree Celsius of the observed value which is evident for dibenzalactone and acetanilide melting point range of 110 ̊C -112 ̊C and 113 ̊C to 115 ̊C respectively whose values are quoted from the lab manual.

Discussion of Methods Used

Extraction using the differences in density of the two layers

One advantage of this method is that two layers are formed which can be separated easily. Acidic compound A reacted with aqueous NaOH to form a sodium salt. It does not dissolve in the organic CH₂Cl₂ solvent as it is inorganic, and hence it remained in the aqueous layer. For compound N, it will dissolve in organic CH₂Cl₂ solvent and remained in the organic layer. Since organic layer with compound N has a higher density, it would be the lower layer. The upper aqueous layer, being less dense, would consist of the compound A.

Another advantage of this method is that it does not involve the setup of bulky apparatus, giving the experiment accessibility. However, the handling part of the separatory funnel can be quite difficult. It was important to release the built up pressure inside the funnel after each gentle shake to mix the contents inside with the stopper being held tightly. If not, the stopper, together with the content would be expelled out of the funnel due to the immense pressure built up. Besides, when the lower layer was drained out, the solvent should be drained out as much as possible, so that the upper layer would not contain the organic layer which will in turn affect the purity of the compound at the later part of the experiment and affecting the melting point of the compounds eventually. Furthermore, although two solvents are separated in two layers in the separatory funnel, they may have the same color. And thus, no clear distinction can be shown between the two layers of solvents.

Separatory funnel was shaked a few times

Shaking the separatory funnel is also important to increase the contact area between the two liquid phases. The solute will move to the layer in which it is most soluble in. Compound N will move to the organic layer whereas sodium salt of compound A will move to the aqueous layer.

Choosing of an organic solvent

Firstly, the organic solvent chosen must have a boiling point lower than the melting point of the substance that is to be recrystallised later.  If the boiling point of the solvent is too high, the substance may come out of the solution as a liquid rather than as a crystalline solid, causing the solid to oil out¹⁰. For this experiment, an organic solvent of low boiling point must be chosen. Dichloromethane has a low boiling point of 40⁰C, enabling the solvent to be distilled off easily during the isolation process of the compound from the solvent. Secondly, the organic compound that is extracted using the organic solvent must be more soluble in the organic solvent as compared to water. As such, the organic compound must have high solubility in the organic solvent. Most importantly, the organic solvent used must not be miscible with water so as to form two different layers during extraction, enabling the separation of the two compounds.

The use of dilute 2% NaOH

Most organic compounds are more soluble in organic solvents as compared to water. Initially, compound A may be more soluble in the organic solvent as compared to water. Dilute NaOH must be added to react with Compound A in order to form a sodium salt. Salts are ionic and most of them (especially sodium salts) are soluble in the aqueous layer and not in water-immiscible organic solvent. Therefore, the addition of dilute NaOH is needed to ensure that compound A is more soluble in the aqueous layer. Thus, separating compound A and N through liquid-liquid extraction is possible.

Extraction is done thrice (more than once)

In the presence of 2 solvents, compounds A and N would have their own distribution or partition coefficient with the following formula: Partition coefficient= solubility of compound in dichloromethane/ solubility of compound in 2% NaOH. The partition coefficient tells us the fraction of solute which is present in the two solvents after extraction. If the partition coefficient for compound A is 1/7, 87.5% of A would be in the aqueous layer whereas the other 12.5% would still be in the organic layer. As such, when we do extraction once, the yield of compound A would only by 87.5%.

By extracting compound A the second time by adding more NaOH, 87.5% of the remaining 12.5% of A in the organic layer would be extracted into the aqueous layer. This means that another 10.9% of A would be in the aqueous layer. A third extraction would take 87.5% of the remaining 1.6% of A present in the organic layer. The organic solvent would have 0.2% of compound A, which is a very small amount. All in all, more than 99.8% A would be extracted into the aqueous layer. Therefore, with increased number of extractions, the yield of the extracted solute is actually greater.

Suction Filtration

It is an effective method in separating liquids from solids as compared to the simple filtration with the use of filter funnel. This is because suction filtration filters the crystals faster as it sucks out the solvent into the filter flask by removing its internal pressure. It also allows crystals to be washed easily. Less time would also be spent drying the crystals under the Infra-Red Lamp as compared to simple filtration as more solvent is being separated from the crystals in suction filtration.

However, there are disadvantages to this method. Firstly, it needs the vacuum machine to do the operation, this means that suction filtration cannot be done if no such apparatus are available. Also, a thicker filter paper would be needed to withstand the pressure from suction filtration. If the filter paper was torn during suction filtration due to the paper being not thick enough, there would be a loss of crystals (residue), and suction filtration would have to be repeated to obtain the maximum yield of crystals.

Excess concentrated HCl is added to the aqueous mixture

During extraction, compound A reacts with NaOH to form a water-soluble salt. Compound A is converted to its original form as a carboxylic acid by an excess of concentrated HCl. It must be added carefully until the mixture becomes acidic. After acidification, compound A no longer dissolve in the aqueous solvent as it is no longer ionic and thus, it is precipitated out of the solution. Suction filtration is used to obtain compound A. Acidification must be done carefully, pouring a small amount of acid and swirling each time because the acid-base neutralization is exothermic. The change in temperature may affect the solubility of compound A in the aqueous solvent which may affect the yield of the compound obtained.

Anhydrous Na₂SO₄ is added to the organic mixture

Anhydrous Na₂SO₄ is a drying agent which would absorb all the water present in the organic mixture. The lab manual mentioned to add a spoonful of it into the solution of interest, to remove water from it. However, the more correct procedure of drying the organic layer should be first adding a spoonful of anhydrous Na₂SO_4. If all the drying agent clumps, add another spoonful. To determine if it has clumped, stir the mixture with a clean and dry spatula or swirl the mixture rapidly. If any portion of the drying agent flows freely on the bottom of the container when stirred or swirled, enough drying agent is used. Otherwise, it is necessary to continue adding the drying agent until the clumping stops¹⁰. However, adding too much drying agent may cause all the liquid to be absorbed. If this occurs, additional solvent will have to be added to recover the product from the drying agent.

Use of boiling chips

The stones have tiny nucleating points on the chip where vaporization can take place. The chip will relieve minor hot spots and prevent loss of product through bumping. The introduction of a boiling chip into a hot liquid may result in instant vaporization and loss of product.

Recrystallisation

Compound A is not soluble in room temperature water but is soluble in hot water. As such, a single solvent is used for recrystallisation of compound A. For purification to occur, the impurities present must either be soluble in cold water or insoluble in hot water. If the impurities are soluble in cold water, the dissolved impurities would be removed in the final filtration of the cooled solution to collect the crystals of the pure compound. If the impurities are not soluble in hot water, insoluble impurities are removed by filtering the hot mixture. Compound A is then cooled and crystallized and pure compound A can be obtained.

Compound N is not soluble in both cold and hot water but is soluble in both cold and hot ethanol. Thus, a solvent pair of both water and ethanol must be used as a mixture of ethanol and water provides a medium in which compound N is very soluble in the hot medium but shows low solubility in the cold. This is useful because at high temperatures, the ethanol-water mixture behaves like alcohol and at low temperatures, it behaves like water. If the material is soluble in ethanol, not many crystals come back from the alcohol alone. If the material is insoluble in water, it is not possible to dissolve it. If a mixed solvent is used, the properties of both solvents can be incorporated. Compound N is first dissolved in a hot boiling ethanol. Water, in which compound N is insoluble to, is slowly added drop wise to the boiling solution until it becomes turbid, the turbidity is tiny crystals of compound coming out of the solution. Then, ethanol is added drop wise until solution until the solution turns completely clear. This is done to force the crystals back into the solution. The solution is cooled for recrystallisation to occur. Pure compound N can be obtained. The disadvantage of using this method is that sometimes the recrystallised compound does not form crystals but oil instead. Compounds usually oil out if the boiling point of recrystallisation solvent is higher than the melting point of the compound. If the oil solidifies, the impurities are trapped in the solid ‘oil’ and the solid has to be purified again.

Washing the crystals with cold deionised water

Deionised water is used to remove the high polar molecules that could be on the crystals. Deionised water is capable of attracting inorganic salts, strong acids, strong bases, low-molecular weight organic compounds with less than 5 carbons such as alcohols and amines¹⁰.

Cold deionised water is used as the solubility of the compound is lower at cold temperature (0^oC) than at warm temperature (25^oC), this implies that less amount of the sample will be dissolved at the former temperature when washed, leading to more crystals being crystallised.

Use of Congo red paper

It is used to check for acidity, to ensure that there is excess H+ available to protonate the carboxylate ion after reacting with all the OH- ions. Other forms of tests such as the use of pH indicator paper, pH meter or litmus paper can be used.

Questions in manual

1.      [I.4] Why rinse the conical flask with 10ml of dichloromethane? How many mL of organic solution is there?

Some of the solid sample might be still undissolved in the 70mL of dichloromethane that was added and some of the sample might be still on the sides of the conical flask instead of dissolving. Therefore, an extra of 10mL of dichloromethane was added to dissolve the remaining undissolved solid sample. The total amount of organic solvent is 80mL.

2.      [I.6]Why the organic layer is the lower layer?

The two layers that are formed are dichloromethane solution and 2% NaOH. The organic layer of dichloromethane solution has a higher density than 2% NaOH, which contains mostly water.

3.      [I.9] How many mL of organic layer is there in the conical flask? How many mL of aqueous layer is there in the beaker?

There are 80mL of organic layer in the conical flask and 100mL of aqueous layer in the beaker.

4.      [II.2] Why should the organic layer be dry?

The procedure of separating the organic compound from the two layers may have caused it to be contaminated with traces of water. Drying is done to further separate the aqueous layer from the organic layer and to remove the water contamination. This is done to achieve high purity of the organic compound and to ensure smooth recrystallisation.

5.      [IV.2] Why for successful recrystallisation, the compound must be soluble in hot solvent, but relatively insoluble in cold solvent.

The compound has to be soluble in a hot solvent so that it can dissolve in it. The compound has to be relatively insoluble in cold solvent so that it can crystallize into its pure form from the solution.

6.      Can ethanol be used as the organic solvent for the extraction in this experiment?

Ethanol cannot be used as the organic solvent for the extraction in this experiment. For ethanol to be used as an organic solvent, it must not be miscible with water so that two layers of mixture would be able to form and the compounds can be separated. However, ethanol is miscible in water as it is able to form hydrogen bonds with water and hence, it would be able to dissolve in water. This would be a problem as there would not be a clear separation between the organic layer and the aqueous layer as both layers would mix together. As such, by using ethanol as an organic solvent, it is impossible to separate compound A and N. Furthermore, ethanol can react with the organic acid (Compound A) to form esters through esterification. This may lower the yield of compound A and the esters form may even serve as impurities. Thus, the compound A that would be obtained may not be as pure if ethanol was used.

7.      Outline in a flowchart how you would separate p-bromobenzoic acid and p-cresol (m.p. 252-254 ºC,  m.p. 32-34 Cº)

6.     Conclusion

The sample was extracted, separated, recrystallised, both the acidic and neutral compounds were determined by melting points and respective percentage yields were calculated and are summarised below.

	Neutral compound	Acidic compound
Mass obtained (g)	0.29	1.42
Melting point (oC)	110.1-110.4	158.4-158.9
Literature M.P. (oC)	110.0-112.0	159.0
Possible Compound	Salicylic acid	Dibenzalactone
Percentage Yield (%)	14.5	71.0

7.     References

1.      The Complex Formation- Association models of solvent extraction of ions, Journal of Radioanalytical Chemistry, Vol 31(1976) S. Siekierski.

2.       Illustrated Guide to Home Chemistry Experiments, Robert Bruce Thompson(April 1, 2008)

3.      http://cartwright.chem.ox.ac.uk/hsci/chemicals/dichloromethane.html

4.      http://orgchem.colorado.edu/hndbksupport/dist/boilchip.html

5.      http://en.wikipedia.org/wiki/Salicylic_acid

6.      http://avogadro.chem.iastate.edu/MSDS/salicylic_acid.htm

7.      http://en.wikipedia.org/wiki/Dibenzylideneacetone

8.      Vogel, A.I., Tatchell, A.R., Furnis, B.S., Hannaford, A.J. and P.W.G. Smith. Vogel's Textbook of Practical Organic Chemistry, 5th Edition. Prentice Hall, 1996.

9.      Experimental organic chemistry by L.M Harwood, C.J Moody and J.M Percy

10.  Organic Laboratory Techniques, a small scale approach by Donald L.Pavia, Gary M lampman, George S.Kriz and Randall G.Engel