DEPARTMENT OF PURE AND APPLIED CHEMISTRY Visayas State University, Baybay, Leyte CHEM 126 Organic Chemistry II Laboratory Report Name: Balagao, Sherey Mae U. Course/ Yr: BS Chemistry 2 Group No.: 3 Date Performed: December 16, 2014 Date Submitted: January 13, 2014 Rating: Experiment No. 4 I. PREPARATION OF ADIPIC ACID FROM CYCLOHEXENE Introduction Dicarboxylic acids are organic compounds that contain two carboxyl functional groups. These compounds are sometimes written as HO2C-R-CO2H. Adipic acid, a dicarboxylic acid with IUPAC name Hexanedioic acid is an organic compound with the formula (CH2)4(COOH)2. From an industrial perspective, it is the most important dicarboxylic acid: About 2.5 billion kilograms of this white crystalline powder are produced annually, mainly as a precursor for the production of nylon. The great majority of the 2.5 billion kg of adipic acid produced annually is used as monomer for the production of nylon by a polycondensation reaction with hexamethylene diamine forming 6,6-nylon. Other industrial uses of adipic acid are the production of adhesives, plasticizers, gelatinizing agents, hydraulic fluids, lubricants, emollients, polyurethane foams, leather tanning, urethane and also as an acidulant in foods. Adipic acid is used after esterification with various groups such as dicapryl, di(ethylhexyl), diisobutyl, and diisodecyl. Adipic acid otherwise rarely occurs in nature. Nylon 6,6- a polymer of adipic acid and 1,6-diaminohexane Adipic acid was commonly obtained by oxidation of castor oil with nitric acid (splitting of the carbon chain close to the OH group), but it is also obtained by oxidation of cyclohexanone or cyclohexene. [O] Just as cyclohexene Adipic acid the word reduction usually refers to the addition of hydrogen to a molecule, the word oxidation usually means the addition of oxygen. The oxidation of an alkene is an example of an addition reaction. Oxygen atoms in the oxidizing agent add to the carbon-carbon double bond. As a result, the ring opens and the dicarboxylic acid is formed. The purpose of this experiment is to synthesize adipic (hexanedioic) acid through the permanganate oxidization of cyclohexene and also to gain familiarization of the laboratory method for following the progress of permanganate oxidations. II. Results Tabulated results obtained from the procedure, including any necessary computations made with respect to the data is shown below. Accuracy of the overall procedure is important to yield maximum amount of the product. At first, the original procedure required chromic acid as the oxidizing agent but with chromic acid being dangerous if not carefully handle, potassium permanganate was used as an oxidizing agent instead. Reaction Equation + cyclohexene MW= 82g/mol [O] oxidizing agent ….(KMnO ) 4 Volume of Cyclohexene used adipic acid MW 146 g/mol 2 mL Moles of Cylohexene used 0.019 mol Density of Cyclohexene 0.81 g/cm3 Moles of adipic acid produced 0.019 mol Actual Yield 1.1 g Theoretical Yield 2.77 g Percent Yield 39.71% Melting point of adipic acid 149 oC -152oC Calculations: No. of moles of cyclohexene used: It was given that 2 mL of cyclohexene was used in the experiment obtained from the previous experiment where it was produced from the dehydration of cyclohexanol. Using the given volume of 2 mL, the molecular weight (82g/mol), and the density of cyclohexene (0.81g/cm3), the theoretical number of moles of cyclohexene used can be calculated. 0.81gcyclohexene 1 mol cyclohexene x 1 mL cyclohexene 82 g cyclohexene = 0.019 moles cyclohexene No. of moles of cyclohexene = 2 mL cyclohexene x 0.019 moles of cyclohexene produces 0.019 moles of adipic acid because cyclohexene has a ratio of 1:1 with adipic acid Theoretical yield of adipic acid: The theoretical amount of product obtainable in grams can be computed by multiplying the moles of adipic acid produced to the molecular weight of adipic acid which is 146 g/mol. This is by the knowledge that 0.019 moles adipic acid can be obtained and hence just by converting it to grams, the theoretical yield is known. Theoretical yield = 0.019 mol adipic acid x 146 g adipic acid . 1 mol adipic acid = 2.77 g adipic acid Percentage yield of adipic acid: To compute the percentage yield, actual yield which was the yield obtained from the experiment is divided by the theoretical yield which was calculated above, multiplied the whole to 100. The yield obtained from the experiment is 1.1 grams. Percentage yield = = Actual yield Theoretical yield 1.1 grams 2.77 grams Percentage yield = 39.71% x 100% III. Discussion This experiment is in connection with the last experiment because it uses cyclohexene as one of its starting material. This experiment carried out the oxidative cleavage of cyclohexene to produce adipic acid. Cyclohexene, an alkene, is susceptible to oxidation because it has a carbon-carbon double bond which is a site of relatively high electron density. Oxidative cleavage of alkenes is a well-known reaction. Several reagents are known to react with alkenes which result in complete breaking of both bonds to the carbon atoms. With relatively mild oxidation, only the pi bond of the alkene is cleaved, producing epoxides and 1,2-diols. More vigorous oxidation can result in the complete cleavage of the carbon-carbon double bond, leading to the formation of various carbonyl compounds, with the specific product dependent on the substitution pattern of the alkene and on the nature of the oxidant used. Reaction of cyclohexene with potassium permanganate under basic conditions results in oxidation. The cyclohexene is oxidized by the permanganate which leads to a ring opening reaction producing adipic acid as the final product. The mechanism of this type of reaction is shown below MnO2 Brown precipitate The first step of the reaction is the interaction of the oxidizing agent which is the potassium permanganate with the cyclohexene, which occurs at the double bond of cyclohexene. MnO4 ion attaches to the carbons in the double bond creating two separate Carbon-Oxygen bonds. Then, the resulting molecule undergoes a reaction with water in a series of several steps which are quite complex, to produce a diol molecule (1,2cyclohexanediol) and the compound Manganese Oxide (MnO2) which is a brown precipitate. But before the 1,2-cyclohexanediol is further oxidized (which is the mechanism shown above), the manganese product actually loses water to form MnO3 ion which is unstable. The unstable MnO3 ion undergoes a reaction with water to regenerate the permanganate ion. The permanganate ion that was regenerated is the one that oxidized the diol further. To better understand the said reaction, it is illustrated below: + 2 OH The permanganate ions in the solution further oxidize the 1,2-cyclohexanediol as being a strong oxidizing agent thus forcing a ring opening reaction and converting the cyclic structure into an alkyl chain with two terminal aldehyde groups, a di-carbonyl compound. Possible mechanism: The di-carbonyl compound produced from previous step is still oxidized by the permanganate ions in solution, forming a product that is figuratively close to being a dicarboxylic acid but without hydrogen. The negatively charged oxygen from the produced molecule attracted the free potassium ions in the solution from potassium permanganate (KMnO4). The reaction is illustrated below: [O] The di-carboxylic salt product is then converted into adipic acid through the addition of concentrated Hydrochloric acid (HCl) to the solution and then cooling it in a waterbath. The solution is then acidified in order to induce protonation and break the bond it formed with potassium ions. This is the final step of the reaction mechanism and it is shown below: + 2KCl As the mechanism of the reaction is discussed already, parts of the procedure that was done is also to be discussed. At the start of the procedure where cyclohexene was added by potassium permanganate, an excess permanganate ions were form at the solution and this excess permanganate ions need to be removed because it could possibly affect or interfere the reactions. Methanol was added to the mixture which is being heated in a water bath in order to remove the excess permanganate ions in the solution. Through spot tests on filter paper, the removal of excess permanganate ions were being observed. Presence of puple color ring around a brown color spot (MnO2) indicates that excess permanganate ions were still present. Addition of methanol to the mixture was gradually done until the purple color ring disappeared. Potassium permanganate (KMnO4) as a very strong oxidant able to react with many functional groups, such as secondary alcohols, 1,2-diols, aldehydes, alkenes, oximes, sulfides and thiols. Under controlled conditions, KMnO4 oxidizes very efficiently primary alcohols to carboxylic acids. Methanol as a primary alcohol was partially oxidized first into aldehyde before being fully oxidized to formic acid, the carboxylic acid product. Illustration of oxidation stages of methanol is shown below: [O] Methanol [O] Methanal Formic acid When a primary alcohol is converted to a carboxylic acid, the terminal carbon atom increases its oxidation state by four. It was observed in the experiment that the flask used was completely covered with the brown color Manganese dioxide (MnO2). Sodium hydroxide (NaOH) was added to the flask. Sodium hydroxide did two important reaction to flask. First was the rinsing measure where it is intended to maximize the yield of adipic acid. There is a partial oxidation of MnO2 by NaOH that is why MnO2 was removed from the sides of the flask. The other possible use of NaOH is the neutralization of formic acid which was formed during the oxidation of methanol. The reaction of NaOH and formic acid is an acid-base reaction forming the salt sodium formate. The oxidation reaction and acidbase reaction of NaOH with MnO2 and formic acid respectively are shown below: Oxidation reaction 2MnO2 + 4NaOH + O2 2Na2MnO4 + 2H2O Neutralization of formic acid HCOOH (aq) + NaOH (aq) NaCOOH (aq) + H2O ( l ) After the acidification of the dicarboxylate salt which it forms the adipic acid, boiling procedure was done to remove any excess water present in the mixture as well as any formic acid left behind with a boiling point close to water at 100.8oC. It also remove any unreacted methanol from the previous step having a boiling point at 64.7oC. Then next after the boiling procedure was the step common in obtaining the purest possible final product, recrystallization, was done where 1.1 g pure white adipic acid was obtained (39.7% yield). Amount of solvent used was calculated in advance because using too much solvent leads to a loss of product. Discussion on Yield The group obtained 39.77% of the final product. This percentage yield is higher that the yield obtained by the group from the previous experiment. Still human error is a factor in obtaining this yield. The yield may be higher if the group did not broke the beaker containing the mixture during the boiling procedure. Other reason of the loss of product could be the addition of the initial reactants and the start of the experiment, proper swirling of the mixtures affect the yield of the product. Despite those some circumstances, it’s still a success as the group obtained pure, white crystals of adipic acid. IV. Conclusion Therefore, gaining familiarization of the procedure on how to synthesize adipic acid was achieved by this experiment. It also gave insights into one of the important reaction of organic chemistry, oxidation reaction, which define as the addition of oxygen or the removal of hydrogen where in the experiment, cyclohexene was oxidized to produce adipic acid. As of today the mechanism is not yet fully specified because some pathways take several steps that are not yet fully understood. Though the students might not also understand it fully but at least there is a better understanding on how the reaction mechanism undergoes. V. Answer to questions 1. What reaction is effected by the addition of methanol to unreacted permanganate? -Addition of methanol affects the further oxidation of the diol intermediate to remove any excess permanganate in the solution. 2. Does this reaction affect your product? -Addition of methanol to unreacted permanganate don’t have much effect on the product or even with the yield because at that point, the excess permanganate oxidized those needed to be oxidized. VI. References http://wwwchem.uwimona.edu.jm/lab_manuals/c10expt12.html http://www.uvm.edu/~mcase/courses/chem143/adipic_acid.pdf http://www.researchgate.net/publication/229106098_Clean_synthesis_of_adipic _acid_from_cyclohexene http://en.wikipedia.org/wiki/Adipic_acid https://www.thechemco.com/chemical/adipic-acid/ http://pages.towson.edu/jdiscord/www/332_lab_info/332labsirpmr/expt2oxidati on.pdf http://bowluss.faculty.yosemite.edu/uploads/11_Preparation%20of%20adipic%2 0acid%20from%20cyclohexene.pdf http://oatao.univ-toulouse.fr/9999/