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Carboxylic Acids, Carboxylic Acid Derivatives, and Acyl Substitution Reactions
Published in Michael B. Smith, A Q&A Approach to Organic Chemistry, 2020
When a nucleophile (Y–) reacts with an acid derivative, acyl addition leads to an alkoxide intermediate (A) known as a tetrahedral intermediate. The key feature of A is the presence of a leaving group X in the tetrahedral intermediate. The alkoxide unit “kicks out” the leaving group (X) to give a new acyl derivative. Since X has been replaced with Y in the final product, this reaction is an acyl substitution reaction. What is the mechanism for the acid catalyzed esterification of butanoic acid with ethanol?
Biodiesel production from non-edible crops using waste tyre heterogeneous acid catalyst
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2022
Sakthi Saravanan Arumugamurthi, Periyasamy Sivanandi, Senthilkumar Kandasamy
Protonation of the carbonyl group of free fatty acid is the initial step in acid-catalyzed esterification. It produces carbon II, weakening π bonds of the C=O bond and transforming it into a powerful electrophile. Methanol reacts with carbon II to form the tetrahedral intermediate III, which is then rearranged into an ester, water, and H+ (Tang et al. 2019). To assess biodiesel conversion, 1 wt% catalyst load was used first, and then gradually increased to 5 wt%. It can be noticed that the yield of biodiesel was enhanced by up to 72.8 wt% when the catalyst load was increased gradually. As the catalyst dosage is increased, so does the reaction speed and conversion. Figure 7 depicts the optimal biodiesel yield as a function of catalyst load. Catalyst loading beyond 4 wt% reduces the yield of biodiesel because bulk mass fraction diffusion rate decreases and results in saturation of the product. The optimal catalyst dose for getting the highest biodiesel yield was found to be 4 wt%, beyond which the biodiesel yield drops. Excess catalysts produced the development of an emulsion, which increased the viscosity of the solution and made the separation of water and esterified oil phase problematic. Furthermore, employing more catalysts than the recommended dosage raises the viscosity of the reaction mixture, resulting in higher mass transfer resistance and higher manufacturing costs. Similar results were found for a sulfonated catalyst made from rice husk (Chen, Wang et al. 2013).
Purification technology for renewable production of fuel from methanolysis of waste sunflower oil in the presence of high silica zeolite beta
Published in Green Chemistry Letters and Reviews, 2021
Leila Fereidooni, Mojtaba Enayati, Alireza Abbaspourrad
Figure 12(b) displays the trans-esterification reaction mechanism for biodiesel production using HSZB treated with KOH. In the first step of reaction, CH3O− ion is formed from the reaction between the active site of K2O with methanol. This CH3O− ion is a strong base which has high catalytic activity in the trans-esterification reaction (18). In the second step, the reactive CH3O− ion attacks the carbonyl carbon atom of WSO to form tetrahedral intermediate. Further rearrangement of tetrahedral intermediate produced one methyl ester molecule and diglyceride anion. In the third step, diglyceride molecule reconstruction began with the reaction with H+ from the catalyst. In step 4, the diglyceride anion may also react with methanol and generate reactive CH3O− ion. The catalytic reaction was then followed by the reaction between diglyceride and other CH3O− ion to produce monoglyceride molecule and one methyl ester molecule. Finally, CH3O− ion attacked carbonyl carbon atom of monoglyceride to produce one methyl ester molecule and one glycerol molecule.
Ultrasonic and microwave effects on Prussian blue catalysed high-quality biodiesel production using Watermelon (Citrullus vulgaris) seed oil and alcohol extract (from fibrous flesh) as an exclusive green feedstock
Published in Biofuels, 2021
K. C. Rajanna, G. Krishnaiah, Srinivas Pasnoori, P. S. Santhoshi, Y. Rajeshwer Rao, K. S. K. Rao Patnaik
The alcohol reacts with the fatty acids to form the mono-alkyl ester (biodiesel) and crude glycerol. It is a reversible reaction. Excess of alcohol is used in order to achieve complete conversion, because equilibrium is shifted towards forward direction. In a base catalyzed transesterification reaction, a base molecule (e.g. NaOH, KOH, sodium methoxide etc.) deprotonates alcohol (R2OH) to generate alkoxide (R2O ̅) nucleophile, which in turn attacks the carbonyl carbon of the reactant ester(RCOOR2) to give a transient tetrahedral intermediate. The transient species, thus formed either reverts to the starting material, or proceeds to the transesterified product (RCOOR2) as shown the following sequence of steps.