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Organic Synthesis
Published in Suresh C. Ameta, Rakshit Ameta, Garima Ameta, Sonochemistry, 2018
Chetna Ameta, Arpit Kumar Pathak, P. B. Punjabi
An aldol condensation is a condensation reaction in organic chemistry in which an enol or an enolate ion reacts with a carbonyl compound to form a P-hydroxyaldehyde or P-hydroxyketone, followed by dehydration to give a conjugated enone. Aldol condensations are important in organic synthesis, because they provide a smooth way to form carbon-carbon bonds (Carey and Sundberg, 1993). These reactions are usually catalysed by strong acids or bases, and a variety of different Lewis acids have been evaluated in this reaction (Reeves, 1966). Unfortunately, the presence of a strong acid or base promotes the reverse reaction (Hathaway, 1987) and this leads to the self-condensation of the reacting materials to give the corresponding byproducts in low yields (Nakano et al., 1987).
Enolate Anions and Condensation Reactions
Published in Michael B. Smith, A Q&A Approach to Organic Chemistry, 2020
The term simply means that the enolate of one ketone or aldehyde reacts with another molecule of the same ketone or aldehyde. Under equilibrium conditions both the enolate anion and the starting aldehyde or ketone are present and can react with one another in an aldol reaction, a “self-condensation.” What is a “mixed” aldol condensation?
Acid–base bifunctional magnesium oxide catalyst prepared from a simple hydrogen peroxide treatment for highly selective synthesis of jasminaldehyde
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2020
Self-condensation of heptanal is minimized as the polarization of the carbonyl group is less likely to occur in heptanal due to its relative weaker interaction with the catalyst, as deduced from the TGA-MS studies (Figure 9). Moreover, the surface of the catalyst would be expected to have a significantly excess of benzaldehyde over heptanal and the stronger binding of benzaldehyde at the surface (Sharma, Parikh, and Jasra 2008a). Calcination of the sample at 400°C and higher resulted in the opening up of more basic sites that would result in a higher heptanal surface concentration. Hence, the selectivity to jasminaldehyde is decreased although the conversion is increased due to the higher density of basic sites.
Rapid microwave-assisted liquid phase conversion of bio-ethanol to n-butanol over a heterogeneous catalyst
Published in Biofuels, 2021
From a comparison of the product concentration (Figure 5) and ethanol conversion (Figure 6) profiles it can be qualitatively deduced that ethanol conversion at a CSF between 4.6 and 5.7 is mostly due to the formation of acetaldehyde and acetic acid which increases logarithmically with an increase in severity of reaction conditions. Formation of acetaldehyde from dehydrogenation of ethanol is the first step in the Guerbet coupling mechanism. Low amounts of butanol and butyraldehyde were formed here, indicating that not enough energy was available to catalyse the conversion of acetaldehyde to butanol. As reaction severity was increased, acetaldehyde concentration increased exponentially up to a CSF of 6 while acetic acid concentration decreased, and both butanol and butyraldehyde were formed in small quantities through aldol condensation of acetaldehyde to butanol (Guerbet mechanism) and self-condensation of acetaldehyde to butyraldehyde [20,29,39]. Acetic acid formation is suppressed at more severe reaction conditions, with a decrease in acetic acid concentration observed with an increase in CSF. Acetic acid can be formed from ethanol in the presence of sufficient water [40], but since the reaction was started with pure ethanol and conducted in an inert atmosphere, the water for the formation of butyraldehyde could only have originated from the formation of crotonaldehyde and water during the aldol condensation of acetaldehyde. Since no crotonaldehyde was detected in any of the sample during analysis, it was concluded that the reaction did follow the crotonaldehyde route, but that ethanol reacted with acetaldehyde to form butanol [29]. At a CSF above 6, a rapid decrease in acetaldehyde concentration is observed, while butanol concentration increases linearly. The results confirmed that the overall reaction of bio-ethanol to bio-based butanol did follow the first step in the Guerbet mechanism, but that butanol was formed mostly through the reaction of ethanol and acetaldehyde. Furthermore, from a product mass balance, it was observed that at a high enough CSF, more butanol was formed than could have been obtained from acetaldehyde (see Figure 7).
Studies on synthesis of environment-friendly products for paint and coating applications
Published in Indian Chemical Engineer, 2020
Appala Naidu Uttaravalli, Srikanta Dinda
From the thorough literature search pertaining to ketonic monomers, it has been observed that most of the studies were carried out on dimerisation, trimerization and tetramerization reactions of cyclohexanone or alkyl cyclohexanone. Melvin and Murry [4] have studied the self-condensation of various aldehydes and ketones in the presence of Amberlite IR-120 and Dowex-50 as catalysts. The author found that Dowex-50 offered comparatively better conversion over Amberlite IR-120. Takashi and Kazuo [5] have studied the kinetics of self-condensation of cyclohexanone to yield 2-(1-cyclohexenyl) cyclohexanone in the presence of sodium ethoxide catalyst at a temperature range of 0.6–10°C and a pressure range of 1–2000 kg/cm2(g) using a high-pressure apparatus. Joseph et al. [6] have patented a process for the preparation of 2-(1-cyclohexenyl) cyclohexanone by self-condensation of cyclohexanone in the presence of Lewatite SP-120 resin catalyst at a temperature range of 80–110°C. The reported conversion of cyclohexanone was around 30% with a yield of around 95% of 2-(1-cyclohexenyl) cyclohexanone. Muzart [7] has studied the self-condensation reaction of cyclopentanone and cyclohexanone in the presence of basic aluminium oxide (Al2O3) at room temperature to prepare dimer products. Jose et al. [8] have studied the kinetics of self-condensation of cyclohexanone in the presence of Amberlyst-15 at a temperature range of 70–110°C. It was reported that the conversion of cyclohexanone was around 35% more when the condensed water was continuously removed from the reaction mixture. Yogesh et al. [9] have studied the self-condensation of cyclohexanone to yield dimer, trimer and tetramer products in the presence of different acidic ion exchange resin catalysts such as Amberlyst-15, Amberlyst-45, T-63, T-66, ZSM-5 and Zeolite β at a temperature range of 80–100°C under atmospheric pressure. Ying-Ling et al. [10] have patented a process for producing 2-(1-cyclohexenyl) cyclohexanone by self-condensation of cyclohexanone in the presence of solid acidic (AlxSyOz) catalyst at a temperature range of 130–150°C using a stainless steel reactor. Lorenzo et al. [11] have studied the kinetics of self-condensation of cyclohexanone in the presence of Amberlyst-15 as a catalyst. The study was performed in the temperature range of 70–110°C.