Caproic Acid Synthesis

In this lab, caproic acid was synthesized in a multi-step process that involved the synthesis of three intermediates – diethyl n-butylmalonate, potassium n-butylmalonate, and n-butyl malonic acid respectively. An IR was used to characterize the starting material, n-bromobutane, and the first intermediate, diethyl n-butylmalonate; while IR and NMR were used to characterize the final product, caproic acid. Reactions, Mechanism, and Theory Caproic acid a. k. an n-hexanoic acid is a carboxylic acid derived from hexane which has the general formula C5H11COOH. It is a colorless oily liquid with a really pungent odor associated with goats. Caproic acid was synthesized in a multi-step process that produced three intermediates - diethyl n-butylmalonate, potassium n-butylmalonate, and n-butyl malonic acid. The synthesis of caproic acid involved alkylation, saponification, and decarboxylation reactions. The first intermediate, diethyl n-butylmalonate – a diethyl ester of malonic acid – was synthesized via an alkylation reaction. Alkylation reactions involve the formation and alkylation of an enolate. In this lab, NaOEt, a strong base was used to pull off one of the most acidic? hydrogens of the carbonyl ester – diethyl malonate – to form an enolate. In order to alkylate the alpha position which now has a negative charge, the enolate was used to attack an alkyl halide, 1-bromobutane via the Sn2 mechanism. The mechanism for the above reaction is shown below: The second intermediate, potassium n-butylmalonate, was synthesized by saponification of the first intermediate, diethyl n-butylmalonate, with potassium hydroxide.

The hydrolysis of diethyl n-butylmalonate resulted in the formation of potassium n-butyl malonic acid by losing the ethoxy group (-OCH2CH3) from both sides of the carbonyl ester. Further deprotonation of the formed acid by -OCH2CH3 forms a carboxylate, potassium n-butylmalonate. The mechanism for the above process is shown below: N-butyl malonic acid, the third intermediate, was immediately synthesized from the potassium n-butylmalonate by the protonation of potassium n-butylmalonate via the addition of excess HCl as shown below: The final step of this lab involved the decarboxylation of n-butyl malonic acid to form caproic acid. Decarboxylation is the removal of a carboxyl group, -COOH, from a carbonyl compound to form CO2 and an enol which further tautomerizes to form a ketone. In this lab, the removal of a -COOH from the n-butyl malonic acid formed 1,1-dihydroxy-hex-1-ene.  34mL of diethyl malonate was added to the solution via the separatory funnel over a period of about fifteen minutes. Next, 35mL of EtOH and 14. 5mL of n-BuBr were added to the mixture respectively. The solution was refluxed for 10 minutes using a heating mantle and then cooled on ice. The above mixture was later poured into 200ml of water and then transferred to a 500mL separatory funnel. 0mL of ether was used to extract the mixture first and then 20mL of ether each was used to extract the aqueous layer four more times. The organic layers were then combined and dried with MgSO4.

The mixture was filtered and then evaporated to ~100mL. The final solution was then transferred to a 250mL round bottom flask and simple distillation was carried out. After ether and ethanol had distilled off, the diethyl n-butylmalonate was collected. When the distillation temperature reached ~90, the process was stopped and the synthesized diethyl n-butylmalonate was stored in the round bottom flask. An IR of the starting materials, diethyl malonate and n-BuBr, and the product diethyl n-butylmalonate were run. Synthesis of potassium n-butylmalonate The diethyl n-butylmalonate was transferred to a small separatory funnel and a condenser, stir bar, Claisen adapter and the separatory flask was attached to the 250mL distillation flask. A solution of 30g KOH in 30mL of water was prepared and this solution was added to the distillation flask. 5ml of the diethyl n-butylmalonate ester was added to the solution via the dropping funnel and the mixture stirred for 5mins, the remaining ester was then added at a rapid dropwise rate. When the addition was complete, the mixture was refluxed on a heating mantle for 45 minutes, and then poured into 90mL of cold water. The mixture was then cooled on ice and NaCl was added until the mixture became saturated. Synthesis of n-Butyl malonic acid 6M HCl was added to the resulting mixture of potassium n-butylmalonate and the mixture was tested frequently using Congo Red paper until the pH was less than 1. After an oily layer formed, the water layer of the mixture was extracted four times with 30mL Et2O. The ether layers were then combined, dried over MgSO4, and filtered using gravity filtration. Synthesis of Caproic Acid: The n-butyl malonic acid synthesized from the previous lab was poured into a 250mL round bottom flask. Simple distillation of the mixture was carried out. Ether and ethanol respectively were among the first to be distilled. Finally, decarboxylation was observed, caproic acid formed and was collected using a 50ml round bottom flask and a stopper.

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Figure 1: Gas Trap Figure 2: Simple Distillation Apparatus Results

Synthesis of Diethyl n-butyl malonate: The addition of diethyl malonate to the yellow NaOEt solution formed a white solid. When EtOH was added to the mixture, the white solid dissolved, and with the addition of n-BuBr, the liquid mixture turned yellow. During reflux, however, the mixture changed to white. Extraction with ether separated the yellow organic layer from the clear aqueous layer.

During simple distillation, Et2O distilled first at ~35oC, followed by EtOH at ~76oC. At ~88oC, only a very small amount of yellow liquid was left in the boiling flask. The yellow liquid also had a slightly pungent odor. The yellow product was weighed to be 15. 40g: Boiling flask with mixture = 143. 61g Flask only = 128. 1g Product= 143. 61 – 128. 21 = 15. 40g,99g of KOH, and ~30mL of H2O produced a clear hot mixture. The addition of the KOH and H2O mixture to the yellow liquid synthesized above resulted in no color change. During reflux, no change was observed on the refluxing mixture; however, a really pungent odor could be smelled. The addition of NaCl to the mixture after reflux produced no observable changes either. The addition of excess 6M HCl formed an oily yellow layer and when the mixture was tested with Congo Red paper, the paper turned deep blue. The mixture was extracted four times with ether and the volume of the combined ether layers was approximately 120mL. The addition of MgSO4 showed no observable change.

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