How is chemical interesterification initiated: Nucleophilic substitution or α‐proton abstraction?

Abstract

Esters of carboxylic acids including 2-methylhexanoic, 2-methylbutyric, 2,2-dimethyl-4-pentenoic, palmitic, and oleic acids were tested as substrates in methoxide-catalyzed interesterification and transesterification. The aliphatic acid esters participated in the ester-ester interchange upon addition of catalytic sodium methoxide. Their isopropyl esters also produced methyl esters on heating with sodium methoxide. The esters of α-methyl-substituted acids did not participate in the ester-ester interchange. Their isopropyl esters did not react with methoxide to produce methyl esters. However, upon addition of methanol with sodium methoxide, their methyl esters were produced. These results indicate that the first step in interesterification is possibly that methoxide abstracts the α-hydrogen of an ester to form a carbanion. Interesterification is then likely completed via a Claisen condensation mechanism involving the β-keto ester anion as the active intermediate. The β-keto ester anion contains positively charged ketone and acyl carbons that are active sites for nucleophilic attack by anions such as methoxide and glycerinate, which would produce a methyl ester or rearrange acyls randomly. On the other hand, transesterification is a nucleophilic substitution by methoxide at the acyl carbon in the presence of methanol.Chemical interesterification rearranges the distribution of FA in TG. This rearrangement changes the TG composition, and therefore the physical properties of the oils, and does not lead to the formation of trans FA.Interesterification can occur without catalysts at very high temperatures, such as 250°C, or catalytically under milder conditions, such as 60°C. Many compounds have been patented as interesterification catalysts including metal salts, alkali hydroxide, alkoxide (alkylates), and alkali metals (1-3). Sodium methoxide is the most commonly used catalyst for chemical interesterification. The reaction is commonly divided into three periods: induction, interchange, and completion. During the induction period (ranging from seconds to minutes), sodium methoxide reacts with glycerides to produce an intermediate commonly referred to as the "real catalyst" (1); no interchange of acyl groups takes place during this period. Although the existence of this real catalyst is not doubted by those skilled in the art, experimental evidence revealing the identity or structure of the catalytic species is still lacking. Once the real catalyst is formed, the interchange period of the reaction begins. Acyl groups continue to interchange until thermodynamic equilibrium is reached and no further net change in the distribution of FA occurs. At this point, the reaction reaches the third period, completion, which is usually terminated out by quenching with water to destroy the real catalyst.Two reaction mechanisms have been proposed for interesterification. Weiss et al. (4) suggested that interesterification was initiated when methoxide abstracted the α-hydrogen of an ester to form an enolate anion (α-carbanion ester) tha...

Interesterificação química de óleo de soja e óleo de soja totalmente hidrogenado: influência do tempo de reação

“…logo, a reação é randômica e entropicamente dirigida até que o equilíbrio termodinâmico seja alcançado. 5,6 Embora existam diversos estudos sobre o mecanismo da interesterificação química e produtos interesterificados, a cinética da reação ainda é pouco explorada na literatura. 7 Neste sentido, apenas a temperatura de reação se revela como parâmetro praticamente consolidado.…”

“…Contudo, com a diminuição desse tempo de reação haveria, conseqüentemente, diminuição de custo do processo e melhor aproveitamento da capacidade dos reatores, além de diminuir a probabilidade de formação de sabões, principal subproduto da reação. 5,10,11 Ainda, a importância de um estudo minucioso do tempo de reação reside no fato de que a intensidade de formação de novas espécies triacilglicerólicas pode afetar consideravelmente as propriedades físicas das bases oleosas. 12 gorduras parcialmente interesterificadas apresentam teor de sólidos, composição triacliglicerólica e comportamento de cristalização que diferem substancialmente da mistura inicial e do produto completamente randomizado.…”

Revisiting the mechanisms of low-temperature, base-catalysed ester interchange reactions

“…Besides the observation by Liu [13] that without an a-hydrogen, there is no ester interchange, which observation cannot be explained by the glycerolate mechanism, further objections against the glycerolate mechanism were put forward [14]: it does not explain why an amount of soap or FFA is formed that is equivalent to the amount of sodium methanolate catalyst used, and it does not explain why compounds like acetone [15] or "an aprotic substance selected from the group consisting of dimethyl formamide, dimethyl acetamide, dimethyl sulphoxide, dimethyl cyanaminde, 3,3-dimethylaminopropionitrile, and mixtures thereof" [16] accelerate the ester interchange reaction. According to Seestrom et al [17] an equivalent amount of soap or FFA is formed during interesterification according to: 2 triglycerides + 1 sodium methoxide → 1 soap + 1 methyl esters + 2 diglycerides This observation has been confirmed independently by several authors and also forms the basis of cost comparisons [18, p. 155].…”

“…In 2004, Liu [13] asked how the chemical interesterification is initiated: by a nucleophilic substitution or by the abstraction of an a-proton? By observing that a-substituted fatty acids did not participate in ester-ester interchange on the addition of sodium methanolate and only formed methyl esters on the subsequent addition of methanol, Liu [13] concluded that the first step in the methanolate catalysed interesterification is the abstraction of an a-hydrogen from a fatty acid moiety to form an enolate carbanion.…”

“…By observing that a-substituted fatty acids did not participate in ester-ester interchange on the addition of sodium methanolate and only formed methyl esters on the subsequent addition of methanol, Liu [13] concluded that the first step in the methanolate catalysed interesterification is the abstraction of an a-hydrogen from a fatty acid moiety to form an enolate carbanion. He then assumed the interesterification to be completed via a Claisen condensation mechanism involving the b-keto ester as the active intermediate.…”