toluic acid obtained at the terminal of the reaction are impure as the experimental thaw point is much lower than the theoretical value. The experimental thaw point of the compound is of wider scope than the theoretical scope which is 178oC to 182oC. This might be due to the wrong finding of the runing point of the crystals at the start.
Wurtz Coupling side reaction reaction may happen if the add-on of p-bromotoluene and anhydrous quintessence is added excessively fast to the Mg, the reaction possibly overheated. The side reaction may originate by extremist yoke or by reaction of the ab initio formed organometallic with more organic halide. The metallation reaction consists of a rate finding measure which involve the individual negatron transportation ( SET ) from metallic Mg to the ?* orbital of the C-X bond of the organohalide.This transportation gave rise to a radical-anion or radical-cation brace at the Mg surface. The transportation of halide anion to Mg•+ to give XMg• , followed by prostration of XMg• and R• require RMgX. If diffusion of R• from a adjacent site takes topographic point, dimer ( R-R ) formation might happen. This dimer formation is known as Wurtz yoke.
This may give some unconsumed Mg at the terminal of the reaction. This reaction involves the reaction between the extra p-bromotoluene and the Grignard reagent that is formed. To minimise the side reaction from happening, the mixture is added easy drop wise to the mixture of Mg and I in the unit of ammunition underside flask to guarantee that no extra p-bromotoluene is added to respond with the Grignard reagent.
The per centum output for 4-toluic acid is non really high. This might due to the loss of the crystals during the transportation of the mixture from one setup to the following, some crystals remained on the walls of the setup. As the drying of crystals utilizing IR lamp is really slow and this might hold contribute to the higher mensural weight due to the staying H2O in the compound, a vacuity oven can be used alternatively to rush up the drying procedure. Besides, during the initial phases of separation, there may non be a complete separation even though extraction was done several times. This is because organic compounds have certain affinity for organic dissolvers like methylene chloride. However, as an excess 40ml of dissolver has been used for the extraction, and the extraction procedure has been repeated for another 2 times, the output might be somewhat higher. Besides, the side merchandises produced might hold contributed to mass besides.
3 ) Discussion
Formation of Grignard reagent
Grignard reagent is formed by responding Mg turnings with alkyl halides ( 4-bromotoluene ) to organize alkylmagnesium halide ( 4-tolylmagnesium bromide ) . It is made by infixing Magnesium in to the CBr bond in the presence of Mg, I2
anhydrous THF. The formation of the Grignard reagent is an exothermal reaction.
When a grignard reagent with low solubility is formed, a crust may construct up over the top of the reaction. Therefore, a magnetic scaremonger is added to forestall the crust from organizing. Iodine crystals is used as color index to bespeak the extent of the reaction, and used to interrupt up the Mg and take the oxide bed from the Mg. Hence, Mg 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 of the Apostless as a nuceophile, the Mg within it form coordination bond with the lone brace of O on the C dioxide. Therefore, conveying the Grignard reagent molecule closer to the C dioxide. At the terminal of the reaction, 4-toluic acid would be obtained.
Grignard reagent donates negatrons to the electrophilic C of CO2 to organize a carboxylate salt. The usage of dry ice as the beginning of C dioxide helps modulate the reaction, as its highly low temperature of this solid moderates Grignard add-ons that are extremely exothermal.
During acid work up, deionised ice H2O, concentrated hydrochloric acid is used to protonate the 4-toluic acid.
3.3 The system has to be kept perfectly dry
Grignard reagent is a really strong base, with approximative pKa45. It can cut down C dioxide to carboxylic acid and aldehyde, ketone and esters to alcohol.
Grignard reagent must be kept perfectly free from H2O as it would respond really rapidly and exothermically with H2O to bring forth methane seriess and this destroys the Grignard reagent. Therefore, this introduces drosss to the compound such as hydroxide MgBrOH and methylbenzene in this instance, which would either rise or lower the thaw point. Besides, the per centum output might be affected besides. C: Documents and SettingsNg FamilyDesktop ( 2 ) .gif
Besides, if the reaction is non kept anhydrous, H2O molecule will besides respond with C dioxide to carbonaceous acid, which would be farther cut down to a carboxylic acid in the presence of a nucleophile, in this instance 4-tolymagnesium bromide.
Grignard reagent, RMgX, is really polar and therefore needs a coordinating dissolver to maintain it in solution. Tetrahydrofuran ( THF ) is suited due to the handiness of lone-pair negatrons for coordination to the Mg ion and ensuing solubilisation in organic medium.
3.31 To minimise exposure to H2O during the synthesis of Grignard reagent
This can be done by guaranting that all the setup are dried before use. Acetone is used to rinse all the setup during the the synthesis of Grignard reagent every bit good as the reaction with dry ice. This is because propanone has low boiling point, so it would be evaporated off easy and will besides take any H2O nowadays. The Ca chloride guard tubings should be placed at the gap of the unit of ammunition underside flask during the intermission between each add-on of the Grignard reagent to the dry ice. Therefore, it can absorb the wet form the surrounding environment so as to forestall H2O vapour from responding with the synthesized Grignard reagents. The dry ice used should be crushed and frost should be wiped off with fabric before usage to cut down contact with H2O.
3.4 Solvent extraction
In this experiment, 4-toluic acid is extracted by solvent extraction. An organic dissolver and an aqueous dissolver are used ; the latter is used to pull out 4-toluic acid. The organic dissolver should hold a lower boiling point than the runing point of the 4-toluic acid that would be recrystallized subsequently. If the dissolver has a higher boiling point than the 4-toluic acid, the substance may oil out as a liquid alternatively of crystalline solid. Dichloromethane chosen for this experiment has a low boiling point of 400C, therefore, the dissolver is able to purify off easy during recrystallization measure. Besides, the organic dissolver used should non be mixable with H2O so as to organize two different beds during extraction. A somewhat higher concentration of 10 % NaOH is chosen for the extraction is to maximise the the formation of salt of 4-toluic acid which would in bend maximize the output for 4-toluic acid extracted. The add-on of dilute NaOH is needed to do 4-toluic acid more soluble in the aqueous bed as it forms salt of 4-toluic acid which are ions and is soluble in H2O. 4-toluic acid reacted with 10 % NaOH to organize a Na salt which is soluble in H2O, therefore 4-toluic acid is present in the aqueous bed.
Solvation of 4-toluic acid salt in NaOH. ( where R is toluene substituent ) .
When methylene chloride is added to the petroleum merchandise, the 4-toluic acid dissolved in it. The separatory funnel was shaked to increase the surface country of contact between the two different liquid stages, so that the solute will travel to the bed in which it is more soluble in. Sodium salt of 4-toluic acid will travel to the aqueous bed. 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 organize dipole-dipole interaction and H bonds with H2O. Therefore, it moves from organic bed into the aqueous bed and two beds are formed- the aqueous bed at the top ( contained the desired compound, but in the signifier of salt of 4-toluic acid ) and the organic bed at the underside. The aqueous bed is paler xanthous in coloring material and clearer while the organic bed is darker xanthous and is turbid.
The two different beds are formed due to the difference in denseness and mutual opposition ; they can be separated easy. As 4-Toluic acid reacted with NaOH to organize an inorganic Na salt, it would be in the aqueous bed as it does non fade out in the organic methylene chloride. On the other manus, methylene chloride can non fade out in aqueous dissolver so it would be in the organic bed. The advantage of this method is that it does non affect the apparatus of bulky setup, so it is easy to transport out the experiment.
The extraction of the aqueous bed is done a twosome of times because there might be uncomplete separation of the aqueous bed from the organic bed. 10 % NaOH is used to pull out the 4-toluic acid twice so as to maximise the extraction of salt of 4-toluic acid. Deionised H2O is used as the concluding dissolver for extraction to pull out any staying salt of 4-toluic acid and separate any organic drosss from the aqueous bed. The three aqueous beds incorporating the salt of 4-toluic acid are combined.Multiple extractions would be more efficient. The output of 4-toluic acid will be higher with increased figure 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 non be able efficient plenty to pull out the compound.
During the last measure of extraction, p-toluic acid reacts with NaOH to organize Na salt which is basic. To recover the acidic 4-toluic acid, concentrated HCl is added to the combined aqueous bed. Congo Red paper is used to look into for sourness by observing any free H+ ions nowadays in the solution. Besides, it ensures that there is more than sufficient H+ nowadays to protonate the carboxylate ion of the 4-toluic acid after neutralizing all the OH- ions ( H+ + OH- i? H2O ) . An option is to utilize pH index paper as it can give a more accurate finding of the sourness of the solution like the Congo ruddy paper ( index for pH degree 3.0-5.0 ) .
After acidification, 4-toluic acid is non ionic and is indissoluble in the aqueous dissolver, so, it would be precipitated out.
Protonation of carboxylate salt
The precipitate is xanthous in coloring material, this is from the dissolved I. The mixture is so filtered utilizing vacuity filtration to divide the filtrate from the precipitate incorporating the 4-toluic acid. Vacuum filtration is more efficient manner to run out off the solution to obtain the crystals as it removes the H2O faster from the mixture solution. Hence, this would cut down the clip required to dry the crystals under the IR lamp. However, a few beds of filter paper is needed for this filtration procedure as a thin bed of filter paper possibly easy torn by force per unit area exerted by the vacuity pump.
Last, the crystals are washed in cold deionised H2O to take any H2O soluble or really polar drosss such as strong acids ( HCl ) , strong bases ( NaOH ) , inorganic salts. A lower temperature will cut down the rate of dissolution of the crystals, so this will ensue in a higher output. However, the organic drosss produced by Wurtz matching reaction would non be able to be removed by the purification procedure, and therefore will lend to an inaccurate thaw point.
Crude merchandise incorporating 4-toluic acid is first dissolved in a hot boiling ethyl alcohol. This is to let the 4-toluic acid to be solubilized in H2O and take the indissoluble drosss. Then H2O is added bead wise to the boiling ethyl alcohol until it becomes turbid. The turbidness is the consequence of little crystals coming out of the solution. Besides, during the measure of H2O add-on, as H2O has greater ability to fade out drosss, so H2O soluble drosss will be dissolved. Ethanol is so added bead wise until the cloudy solution becomes clear wholly to coerce the crystals back into the solution. The solution is cooled for recrystallization to happen. Pure 4-toluic acid is obtained and soluble drosss remain in the cold dissolver after recrystallization. All the boiling solution contains boiling french friess. The boiling french friess are made of Ca carbonate have really little nucleating pores on it to ease the minor hot musca volitanss as vapour can be formed at the pores and cut down the loss of merchandise through bumping. The french friess should be added to the solution before it is heated to avoid the loss of merchandise. When left to chill at room temperature, crystals started to organize in the light xanthous solution. When placed in an ice bath, more crystals were formed. To increase output of the crystals, scratch the underside of the flask as it increases the glass surface country, supplying a chapped surface on which the solid can crystallise. The crystals are recovered through vacuity filtration and were white in coloring material. Crystals are so placed under infra-red lamp to take any H2O nowadays as the presence of H2O would hold a damaging consequence on the thaw point finding and the mass weighed.
During the recrystallization procedure, some safeguards need to be taken to forestall the taint of the crystals. A 100 milliliter conelike flask with a smaller gap should be used alternatively of a beaker to cut down taint from the air. The boiling french friess added during recrystallization provide a surface for the seed crystals to organize. Therefore, it is important that boiling french friess are added into the conelike flask. The conelike flask should foremost be allowed to chill to room temperature before seting into the ice bath to foster lower the temperature. This is because a sudden temperature bead would do drosss to be trapped within the crystals, therefore lending to dross of merchandises. However, if the crystals are allowed to chill for excessively long, it may besides pin down drosss. Therefore, the solution should be cool down for 10min to room temperature before undergoing the ice H2O bath for another 10min.
4 ) Decision
In decision, 4-toluic acid has been synthesized utilizing Grignard reagent and purified several times via solvent extraction and recrystallization. However, the obtained compound is still non really pure and the per centum output is non really high.