Butanal aldol condensation

To illustrate how aldol condensation may be coupled to functional group modifi cation consider the synthesis of 2 ethyl 1 3 hexanediol a compound used as an insect repellent This 1 3 diol is prepared by reduction of the aldol addition product of butanal... 

The largest commercial process is the hydroformylation of propene, which yields n-butyraldehyde and isobutyraldehyde. n-Butyraldehyde (n-butanal) is either hydrogenated to n-butanol or transformed to 2-ethyl-hexanol via aldol condensation and subsequent hydrogenation. 2-Ethylhexanol is an important plasticizer for polyvinyl chloride. This reaction is noted in Chapter 8. 

Butyl alcohol is not the principal use of butanal obtained by propene hydro formylation. Rather its major market is 2-ethylhexanol that is prepared via aldol condensation followed by hydrogenation. [4] Thus formation of alcohols when aldehydes are desired is not only a direct efficiency loss, but also the alcohol impurity will form hemiacetals and acetals that complicate refining and lead to increased operating costs. 

Equation 2.10. Formation of 2-ethylhex-2-enal via aldol condensation of butanal... 

Microorganisms have also been developed to produce alternative products, such as lactic acid [65], propane-1,3-diol [67], 3-hydroxypropionic acid [68], butane-2,3-diol [69] and numerous other intermediates. For instance, bacteria such as the Clostridium acetobutylicum ferment free sugars to C4 oxygenates such as butyric acid or butanol. They form the C4 oxygenates by Aldol condensation of the acetaldehyde intermediates. The Weizmann process exploits this property to ferment starch feedstock anaerobically at 37 °C to produce a mixture of w-butanol, acetone and ethanol in a volume ratio of 70 25 5 [3],... 

Ex situ formation of (33) followed by addition to a mixture of CHCI3 and aldehyde (aliphatic or aromatic) in DMF at low temperature leads to deprotonation of CHCI3 but not of the aldehyde, and formation of the trichlorocarbinol takes place without formation of side products by aldol condensation, Scheme 21 [99]. The reaction could also be carried out (75-86% yield for butanal) using polymeric amides as a PB and with similar conditions [74]. 

Write structural formulas and names of four possible aldol condensation products from propanal and butanal. In each case, indicate which aldehyde acts as nucleophile and which as electrophile. 

Write structural formulas for the )3-hydroxycarbonyl compounds and their dehydration products formed by aldol condensations of (a) butanal, (6) phenylacetaldehyde, (c) diethyl ketone, (d) cyclohexanone, (e) benzaldehyde. 

Di- and polysubstituted pyrrolizines are uncommon. The only pyrrolizine isolated from the intramolecular aldol condensation of 2-acetyl-l-(butan-3-on-l-yl)pyrrole was the 6,7-disubstituted compound 223.122 A small amount of the 5,6-disubstituted pyrrolizine (145d) was obtained when the 2-formylpyr-ryl anion was condensed with l,2-di(phenylsulfonyl)ethene.90 By contrast with the production of the single isomer (223), the homologous diketone 224... 

The carbon skeleton of 2-ethyl-1-hexanol is the same as that of the aldol condensation product derived from butanal. Hydrogenation of this compound under conditions in which both the carbon-carbon double bond and the carbonyl group are reduced gives 2-ethyl-l-hexanol. 

Formation of aldehydes by the reaction of alkene, CO and H2 catalysed by Co2(CO)8 was discovered by Rolen in 1938 [25]. This is the 1,2-addition of H and CHO to alkenes, and hence called hydroformylation or the oxo reaction. Production of butanal, (33) from propylene as a main product is an important industrial process. Aldol condensation of butanal, followed by hydrogenation affords 2-ethyl-1-hexanol (34), which is converted to phthalate, and used as a plasticizer of poly(vinyl chloride). 

Reaction of aliphatic aldehydes with alkali acetylides in liquid ammonia gives the carbinols in very small amounts, even when the aldehyde is added to a strongly cooled solution of lithium acetylide. The predominant reaction presumably is formation of the enolate and the aldol condensation product As shown on p. 21, a suspension of LiOCH in THF can be obtained by gradually replacing the ammonia of an ammoniacal solution of the acetylide by THF. The lithium acetylide obtained in this way probably thanks its stability to the complexed ammonia. In the procedure described below, butanal is added to the suspension to give the acetylenic carbinol in a reasonable yield. Since this compound is rather volatile, it is essential to remove the greater part of the THF, before the hydrolysis is carried out. The main solvent which then has to be removed in the isolation procedure is the diethyl ether, used for the extractions. During the addition of the aldehyde, acetylene is introduced to suppress the formation of the diol RCH(OH)C=CCH(OH)R. 

Condensation of butanal has been carried out on alkaline earth metal oxides at 273 K[52 53] yielding 2-ethyl-3-hydroxy-hexanal as a main product the order of activity per unit surface area was equal to that in the case of self-condensation of acetone and in agreement with the order of basicity of the solids, namely, SrO > CaO > MgO. The authors found that for aldol condensation of w-butyraldehyde, the active sites are the surface O2 ions and the rate-determining step is the z-hydrogcn abstraction. The differences in rate-determining step and active sites in the condensation of butyraldehyde and self-condensation of the acetone were attributed to differences in acidity of the z-hydrogcn in the two molecules. CaO was slightly... 

The epoxide can be prepared from an alkene and the amide from a carboxylic acid. The new target. 2-ethyl-2-hexenoic acid, has a CC double bond in conjugation with the carbonyl group of the carboxylic acid. Whenever a compound with an ,/3-unsaturated carbonyl group is encountered, it is worthwhile to consider the possibility of using an aldol condensation (see Section 20.5) or a related reaction to prepare it. To examine this possibility, the aldehyde that will provide the carboxylic acid upon oxidation is disconnected at the double bond. Because both fragments produced by this disconnection are the same, it is apparent that an aldol condensation of butanal can be employed to prepare this compound. The synthesis was accomplished as shown in Figure 23.5. 

The example was chosen to illustrate pathway reduction and the resulting simplification of mathematics. So as not to distract from that message, complications encountered in practice with this particular reaction were disregarded. These include isomerization of 2-cis- to 2-tram- and 1-butene, conversion of 1-butene to 2-methyl butanal and n-pentanal, and aldol condensation of the latter (see also Example 11.1 in Section 11.2). 

Example 8.2. Aldol condensation of n-butanal in alcohol solvent [15], Results obtained for aldol condensation of n-butanal in n-octanol are shown in Table 8.1. The reaction is second order in aldehyde. Its apparent second-order rate coefficient, km = r,ic / Ca,j, is tabulated as a function of buffer composition. Although the reaction is second order in aldehyde, its rate is not proportional to the B -to-HB ratio and so is not first order in OH. Equation 8.10 thus cannot represent the results. 

Table 8.1. Observed second-order rate coefficients for aldol condensation of n-butanal (ll%wt in n-octanol, potassium-octoate buffer, 171 °C) [16],...

The use of alkali metal oxide catalysts for aldol condensation reactions has been examined for the production of 2-ethylhexenal from butanal [34]. When coupled to a hydrogenation catalyst the system can produce the plasticizer alcohol 2-ethyl-hexanol directly. When isobutyraldehyde was used as the feed to a silica-supported sodium oxide catalyst, no products were formed but a significant amount of carbon was deposited on the catalyst and in the reactor (Scheme 21.2). 

A key issue in the hydroformylation reaction is the ratio of linear to branched product produced. Figure 6.1 explains this colloquial expression. The linear product is the desired product, since the value of butanal is higher also because this is the product which can be converted to 2-ethylhexanol via base-catalyzed aldol condensation and hydrogenation. Sometimes the aldol condensation reaction is carried out simultaneously with hydroformylation. When butanal is the desired product the condensation reaction is suppressed. The detergent alcohols... 

Example 8.2. Aldol condensation of n-butanal in alcohol solvent [15], Results obtained for aldol condensation of n-butanal in n-octanol are shown in Table 8.1. The reaction is second order in aldehyde. Its apparent second-order rate coefficient,... 

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