Looking At Synthesize Grignard Reagent Biology Essay

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toluic acid obtained at the end of the reaction are impure as the experimental melting point is much lower than the theoretical value. The experimental melting point of the compound is of wider range than the theoretical range which is 178oC to 182oC. This might be due to the incorrect determination of the melting point of the crystals at the start.

Wurtz Coupling side reaction reaction may occur if the addition of p-bromotoluene and anhydrous ether is added too fast to the magnesium, the reaction maybe overheated. The side reaction may arise by radical coupling or by reaction of the initially formed organometallic with more organic halide. The metallation reaction consists of a rate determining step which involve the single electron transfer (SET) from metallic magnesium to the σ* orbital of the C-X bond of the organohalide.This transfer gave rise to a radical-anion or radical-cation pair at the magnesium surface. The transfer of halide anion to Mg•+ to give XMg•, followed by collapse of XMg• and R• require RMgX. If diffusion of R• from a neighboring site takes place, dimer (R-R) formation might occur. This dimer formation is known as Wurtz coupling.

This may give some unconsumed magnesium at the end of the reaction. This reaction involves the reaction between the excess p-bromotoluene and the Grignard reagent that is formed. To minimize the side reaction from occurring, the mixture is added slowly drop wise to the mixture of magnesium and iodine in the round bottom flask to ensure that no excess p-bromotoluene is added to react with the Grignard reagent.

The percentage yield for 4-toluic acid is not very high. This might due to the loss of the crystals during the transfer of the mixture from one apparatus to the next, some crystals remained on the walls of the apparatus. As the drying of crystals using IR lamp is very slow and this might have contribute to the higher measured weight due to the remaining water in the compound, a vacuum oven can be used instead to speed up the drying process. 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. However, as an extra 40ml of solvent has been used for the extraction, and the extraction process has been repeated for another 2 times, the yield might be slightly higher. Also, the side products produced might have contributed to mass also.

3) Discussion

Formation of Grignard reagent

Grignard reagent is formed by reacting magnesium turnings with alkyl halides (4-bromotoluene) to form alkylmagnesium halide(4-tolylmagnesium bromide). It is made by inserting Magnesium in to the CBr bond in the presence of Mg, I2

Anhydrous ether

anhydrous THF. The formation of the Grignard reagent is an exothermic reaction.

When a grignard reagent with low solubility is formed, a crust may build up over the top of the reaction. Thus, a magnetic stirrer is added to prevent the crust from forming. Iodine crystals is used as colour indicator to indicate the extent of the reaction, and used to break up the magnesium and remove the oxide layer from the magnesium. Hence, magnesium become more reactive. In this reaction, MgI2 is formed.

3.2 Formation of 4-Toluic Acid from Grignard reagent

Carbon dioxide is a carbonyl compound and it acts as an electrophile. In R-MgBr acts as a nuceophile, the magnesium within it form coordination bond with the lone pair of oxygen on the carbon dioxide. Thus, bring the Grignard reagent molecule closer to the carbon dioxide. At the end of the reaction, 4-toluic acid would be obtained.

Grignard reagent donates electrons to the electrophilic carbon of CO2 to form a carboxylate salt. The use of dry ice as the source of carbon dioxide helps regulate the reaction, as its extremely low temperature of this solid moderates Grignard additions that are highly exothermic.

During acid work up, deionised ice water, concentrated hydrochloric acid is used to protonate the 4-toluic acid.

3.3 The system has to be kept absolutely dry

Grignard reagent is a very strong base, with approximate pKa45. It can reduce carbon dioxide to carboxylic acid and aldehyde, ketone and esters to alcohol.

Grignard reagent must be kept absolutely free from water as it would react very quickly and exothermically with water to produce alkanes and this destroys the Grignard reagent. Thus, this introduces impurities to the compound such as hydroxide MgBrOH and toluene in this case, which would either raise or lower the melting point. Also, the percentage yield might be affected also. C:\Documents and Settings\Ng Family\Desktop\(2).gif

Also, if the reaction is not kept anhydrous, water molecule will also react with carbon dioxide to carbonic acid, which would be further reduce to a carboxylic acid in the presence of a nucleophile, in this case 4-tolymagnesium bromide.

Grignard reagent, RMgX, is very polar and thus needs a coordinating solvent to keep it in solution. Tetrahydrofuran (THF) is suitable due to the availability of lone-pair electrons for coordination to the magnesium ion and resulting solubilisation in organic medium.

3.31 To minimize exposure to water during the synthesis of Grignard reagent

This can be done by ensuring that all the apparatus are dried before usage. Acetone is used to wash all the apparatus during the the synthesis of Grignard reagent as well as the reaction with dry ice. This is because acetone has low boiling point, so it would be evaporated off easily and will also remove any water present. The calcium chloride guard tubes should be placed at the opening of the round bottom flask during the pause between each addition of the Grignard reagent to the dry ice. Thus, it can absorb the moisture form the surrounding environment so as to prevent water vapor from reacting with the synthesized Grignard reagents. The dry ice used should be crushed and frost should be wiped off with cloth before use to reduce contact with water.

3.4 Solvent extraction

In this experiment, 4-toluic acid is extracted by solvent extraction. An organic solvent and an aqueous solvent are used; the latter is used to extract 4-toluic acid. The organic solvent should have a lower boiling point than the melting point of the 4-toluic acid that would be recrystallized later. If the solvent has a higher boiling point than the 4-toluic acid, the substance may oil out as a liquid instead of crystalline solid. Dichloromethane chosen for this experiment has a low boiling point of 400C, thus, the solvent is able to distill off easily during recrystallization step. Also, the organic solvent used should not be miscible with water so as to form two different layers during extraction. A slightly higher concentration of 10% NaOH is chosen for the extraction is to maximize the the formation of salt of 4-toluic acid which would in turn maximize the yield for 4-toluic acid extracted. The addition of dilute NaOH is needed to make 4-toluic acid more soluble in the aqueous layer as it forms salt of 4-toluic acid which are ions and is soluble in water. 4-toluic acid reacted with 10%NaOH to form a sodium salt which is soluble in water, thus 4-toluic acid is present in the aqueous layer.

Solvation of 4-toluic acid salt in NaOH. (where R is toluene substituent).

When dichloromethane is added to the crude product, the 4-toluic acid dissolved in it. The separatory funnel was shaked to increase the surface area of contact between the two different liquid phases, so that the solute will move to the layer in which it is more soluble in. Sodium salt of 4-toluic acid will move to the aqueous layer. Upon shaken with 10% NaOH, salt of 4-toluic acid is formed as the OH- ion reacts with the 4-toluic acid such that it becomes a polar salt and form dipole-dipole interaction and hydrogen bonds with water. Thus, it moves from organic layer into the aqueous layer and two layers are formed- the aqueous layer at the top (contained the desired compound, but in the form of salt of 4-toluic acid) and the organic layer at the bottom. The aqueous layer is paler yellow in colour and clearer while the organic layer is darker yellow and is turbid.

The two different layers are formed due to the difference in density and polarity; they can be separated easily. As 4-Toluic acid reacted with NaOH to form an inorganic sodium salt, it would be in the aqueous layer as it does not dissolve in the organic dichloromethane. On the other hand, dichloromethane cannot dissolve in aqueous solvent so it would be in the organic layer. The advantage of this method is that it does not involve the setup of bulky apparatus, so it is easy to carry out the experiment.

The extraction of the aqueous layer is done a couple of times because there might be incomplete separation of the aqueous layer from the organic layer. 10% NaOH is used to extract the 4-toluic acid twice so as to maximize the extraction of salt of 4-toluic acid. Deionised water is used as the final solvent for extraction to extract any remaining salt of 4-toluic acid and separate any organic impurities from the aqueous layer. The three aqueous layers containing the salt of 4-toluic acid are combined.Multiple extractions would be more efficient. The yield of 4-toluic acid will be higher with increased number of extractions. However, the efficiency of multiple extractions is related to the distribution coefficient, D. Extremes ratio like larger than 100 or lesser than 0.01would not be able efficient enough to extract the compound.

During the last step of extraction, p-toluic acid reacts with NaOH to form sodium salt which is basic. To regain the acidic 4-toluic acid, concentrated HCl is added to the combined aqueous layer. Congo Red paper is used to check for acidity by detecting any free H+ ions present in the solution. Also, it ensures that there is more than sufficient H+ present to protonate the carboxylate ion of the 4-toluic acid after neutralizing all the OH- ions (H+ + OH-  H2O). An alternative is to use pH indicator paper as it can give a more accurate determination of the acidity of the solution like the Congo red paper (indicator for pH level 3.0-5.0).

After acidification, 4-toluic acid is not ionic and is insoluble in the aqueous solvent, so, it would be precipitated out.



Protonation of carboxylate salt

The precipitate is yellow in colour, this is from the dissolved iodine. The mixture is then filtered using vacuum filtration to separate the filtrate from the precipitate containing the 4-toluic acid. Vacuum filtration is more efficient way to drain off the solution to obtain the crystals as it removes the water faster from the mixture solution. Hence, this would reduce the time required to dry the crystals under the IR lamp. However, a few layers of filter paper is needed for this filtration process as a thin layer of filter paper maybe easily torn by pressure exerted by the vacuum pump.

Lastly, the crystals are washed in cold deionised water to remove any water soluble or very polar impurities such as strong acids(HCl), strong bases(NaOH), inorganic salts. A lower temperature will reduce the rate of dissolving of the crystals, so this will result in a higher yield. However, the organic impurities produced by Wurtz coupling reaction would not be able to be removed by the purification process, and thus will contribute to an inaccurate melting point.

3.5 Recrystallization

Crude product containing 4-toluic acid is first dissolved in a hot boiling ethanol. This is to allow the 4-toluic acid to be solubilized in water and remove the insoluble impurities. Then water is added drop wise to the boiling ethanol until it becomes turbid. The turbidity is the result of small crystals coming out of the solution. Also, during the step of water addition, as water has greater ability to dissolves impurities, so water soluble impurities will be dissolved. Ethanol is then added drop wise until the turbid solution becomes clear totally to force the crystals back into the solution. The solution is cooled for recrystallization to occur. Pure 4-toluic acid is obtained and soluble impurities remain in the cold solvent after recrystallization. All the boiling solution contains boiling chips. The boiling chips are made of calcium carbonate have very small nucleating pores on it to ease the minor hot spots as vapor can be formed at the pores and reduce the loss of product through bumping. The chips should be added to the solution before it is heated to avoid the loss of product. When left to cool at room temperature, crystals started to form in the light yellow solution. When placed in an ice bath, more crystals were formed. To increase yield of the crystals, scratch the bottom of the flask as it increases the glass surface area, providing a roughened surface on which the solid can crystallize. The crystals are recovered through vacuum filtration and were white in colour. Crystals are then placed under infra-red lamp to remove any water present as the presence of water would have a detrimental effect on the melting point determination and the mass weighed.

During the recrystallization process, some precautions need to be taken to prevent the contamination of the crystals. A 100 ml conical flask with a smaller opening should be used instead of a beaker to reduce contamination from the air. The boiling chips added during recrystallization provide a surface for the seed crystals to form. Thus, it is crucial that boiling chips are added into the conical flask. The conical flask should first be allowed to cool to room temperature before putting into the ice bath to further lower the temperature. This is because a sudden temperature drop would cause impurities to be trapped within the crystals, thus contributing to impurity of products. However, if the crystals are allowed to cool for too long, it may also trap impurities. Thus, the solution should be cool down for 10min to room temperature before undergoing the ice water bath for another 10min.

4) Conclusion

In conclusion, 4-toluic acid has been synthesized using Grignard reagent and purified several times via solvent extraction and recrystallization. However, the obtained compound is still not very pure and the percentage yield is not very high.