2-ethylhexanol aldol condensations

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. 

Ethylhexanol is produced hy the aldol condensation of hutyralde-hyde. The reaction occurs in presence of aqueous caustic soda and produces 2-ethyl-3-hydroxyhexanal. The aldehyde is then dehydrated and hydrogenated to 2-ethylhexanol ... 

Figure 8-6. The Hoechst AG process for producing 2-ethylhexanol from n-butyraldehyde (1) Aldol condensation reactor, (2) separation (organic phase from liquid phase), (3) hydrogenation reactor, (4) distillation column.

Hydroformylation is an important industrial process carried out using rhodium phosphine or cobalt carbonyl catalysts. The major industrial process using the rhodium catalyst is hydroformylation of propene with synthesis gas (potentially obtainable from a renewable resource, see Chapter 6). The product, butyraldehyde, is formed as a mixture of n- and iso- isomers the n-isomer is the most desired product, being used for conversion to butanol via hydrogenation) and 2-ethylhexanol via aldol condensation and hydrogenation). Butanol is a valuable solvent in many surface coating formulations whilst 2-ethylhexanol is widely used in the production of phthalate plasticizers. 

The n-butyraldehyde is treated with a 2 per cent w/w aqueous sodium hydroxide and undergoes an aldol condensation at a conversion efficiency of 90 per cent. The product of this reaction, 2-ethylhexanal, is separated and then reduced to 2-ethylhexanol by hydrogen in the presence of a Raney nickel catalyst with a 99 per cent conversion rate. In subsequent stages of the process (details of which are not required), 99.8 per cent of the 2-ethylhexanol is recovered at a purity of 99 per cent by weight. 

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. 

Ethylhexanol is produced by aldol condensation of butyraldehyde followed by reduction. It can also be made in one step from propylene and synthesis gas converted to butanols and 2-ethylhexanol without isolating the butyraldehydes. See Chapter 10, Section 6. 

Ethyl alcohol has been made by the hydration of ethylene (9) since 1930. Like isopropyl alcohol, part of the output is used as a solvent, but most is converted to other oxygenated chemicals. Its most important raw material use is conversion to acetaldehyde by catalytic air oxidation. Acetaldehyde, in turn, is the raw material source of acetic acid, acetic anhydride, pentaerythritol, synthetic n-butyl alcohol (via aldol condensation), butyraldehyde, and other products. Butyraldehyde is the source of butyric acid, polyvinyl butyral resin, and 2-ethylhexanol (octyl alcohol). The last-named eight-carbon alcohol is based on the aldol condensation of butyraldehyde and is used to make the important plasticizer di-2-ethylhexyl phthalate. A few examples of the important reactions of acetaldehyde are as follows ... 

Another oxo plant, now being constructed, will make butyl compounds (88). These may be the source of butyl alcohol, butyl acetate, butyric acid for the manufacture of cellulose acetate butyrate and other products, butyraldehyde for polyvinyl butyral, and the eight-carbon compounds including 2-ethylhexanol. All these will add to the present production of the same compounds made by the older methods from acetaldehyde via aldol condensation. 

Ethylhexanol is usually produced by subsequent aldolization of butyraldehyde produced in the oxo reaction followed by hydrogenation of the intermediate unsaturated aldehyde.89 In Esso s Aldox process, however, in situ aldol condensation is effected by suitable promoters.11 Magnesium ethoxide and soluble zinc compounds are recommended to promote controlled aldolization during the oxo reaction. The Shell variant uses potassium hydroxide. Serious disadvantages (mixed aldolization with the branched aldehyde, problems associated with recycling of the additives), however, prevented wider use of the Aldox process. 

One exception to the general application of these ketone syntheses was failure of compounds having an alpha-substituted carbon atom such as isobutyl alcohol or 2-ethylhexanol to undergo the dehydrogenation (7) condensation reaction. This failure of alpha-substituted reactants to undergo the ketone synthesis was unexpected as the aldol condensation of alpha-substituted aldehydes with one labile hydrogen atom occurs readily. 

The primary aldehyde product is reduced to the desired butanol, or it is subjected to a base-catalyzed aldol condensation and then hydrogenated to give 2-ethylhexanol. The phthalic ester of the latter is used as a plasticiser in PVC. The first process was based on a Co2(CO)s catalyst, a precursor of HCo(CO)4. The pressure is high, ca. 200-300 bar, in order to maintain the catalyst s stability. In the 60s Shell developed a process using phosphine ligands which allowed the use of lower pressures. The catalyst is less active but it directly produces alcohols with a somewhat higher linearity. 

The largest volume hydroformylation reaction converts propylene into n-butyraldehyde, from which is made 1-butanol for solvents, or 2-ethylhexanol (the phthalate ester of which has been widely used as a plasticizer for PVC) via an aldol condensation. Estimated world production of butanol is approaching 2 Mt/a. 

A remarkable example of the cooperation of different active sites in a polyfunctional catalyst is the one-step synthesis of 2-ethylhexanol, including a combined hydroformylation, aldol condensation, and hydrogenation process [17]. The catalyst in this case is a carbonyl-phosphine-rhodium complex immobilized on to polystyrene carrying amino groups close to the metal center. Another multistep catalytic process is the cyclooligomerization of butadiene combined with a subsequent hydroformylation or hydrogenation step [24, 25] using a styrene polymer on to which a rhodium-phosphine and a nickel-phosphine complex are anchored (cf Section 3.1.5). 

The primary aldehyde product is reduced to the desired butanol, or it is subjected to a base-catalysed aldol condensation and then hydrogenated to give 2-ethylhexanol. The phthalic ester of the latter is used as a plasticiser in PVC. The... 

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... 

As for condensation reactions. King ei al. [1 discussed on the search for solid base catalysts suitable for industrial aldol condensation of aldehydes (n-butanal and n-hexanal. mainly). Moreover, the authors made emphasis on the industrial importance of such processes to obtain a number of key compounds such as 2-ethylhexanol (which can be converted into dimethylhexylphtalate. a plasticizer for PVX ), methyl isobutyl-kctone (MIPK. an excellent solvent for cellulose and resin-based Ci>atings) or Ouerbet alcohols, used in cosmetics, textiles, lubricants and surfactants. In any case, condensation reactions will be more extensively discussed on the following section, devoted to Fine Chemistry. 

The principal industrial application is the hydroformylation of propene, where the primary product n-butyraldehyde may be converted via an aldol condensation followed by hydrogenation to give 2-ethylhexanol which is used as a plasticiser alcohol in dialkylphthalate formulations. Alternatively butyraldehyde is converted to butanol which is used as a solvent. 

U.S. production of w-butanol has increased to 600 kt per annum, largely for conversion to unsaturated esters and saturated ester and ether solvents, while 2-ethylhexanol (about 300 kt per annum in the U.S., but much higher in Europe with exports at 250 kt per annum) is used mainly for the phthalate ester as a plasticizer for PVC. The n- and iso-butyraldehydes are also subjected to aldol condensation/crossed Cannizzaro reactions with formaldehyde, to give the polyols trimethylol-propane (2, 2-bishydroxymethyl-l-butanol) and neopentyl glycol (2,2-dimethyl-1,3-propanediol). 

The rate of the reaction changes depending upon the partial pressure of CO it reaches a maximum at ca 3 MPa. The yield of the reaction decreases as the temperature increases. At low temperatures the rate of the hydroformylation is small and therefore higher temperatures are applied. Hydroformylation allows the preparation of various valuable products because the oxo synthesis may utilize different compounds containing a carbon-carbon double bond, for example, dienes, polyenes, and unsaturated aldehydes, ketones, nitriles, alcohols, esters, etc. For example, dienes may afford dialdehydes. A substantial amount of aldehydes is converted to alcohols which find considerable application in the preparation of detergents, plasticizers, and lubricants. The aldol condensation generally is not desired in hydroformylation processes. Nevertheless, via aldol condensation followed by hydrogenation, 2-ethylhexanol is obtained from n-butyraldehyde see equation (13.117). 

There are certain cases, however, where aldol condensation is desired. For instance, by the aldol condensation of butyraldehyde, followed by hydrogenation, one obtains 2-ethylhexanol [893,894,952], a very important... 

The operating plants produce aldehydes in the range Cg-Cjg which are either hydrogenated as such to give the corresponding alcohols or subjected to aldol condensation prior to the hydrogenation. In the latter case the resulting alcohols contain double the number of carbon atoms as the aldehyde used for the aldol condensation (e. g. 2-ethylhexanol is made from two moles of n-butyraldehyde via 2-ethylhexenal). 

From 1974 onwards, Rh-based hydroformylation became industrial. The use of a catalyst metal that is about 1000-times more expensive than cobalt was driven by several reasons. First, Rh-hydroformylation is more active and thus requires much lower process pressures (lower energy consumption in compression units) and smaller reactors. Second, Rh-hydroformylation shows a very high selectivity to the aldehyde product with only minimal hydrogenation activity being observed. This is of particular importance for propylene hydroformylation where butyl alcohol is not the principle market use. In contrast, for the desired end-use of w-butyraldehyde in the form of its aldol condensation product 2-ethylhexanol a pure aldehyde feed is required as hemiacetals (formed by reaction of aldehyde and alcohol) complicate product purification and add to operating costs. 

The aldehydes can be the desired product but often they are reduced to the alcohol or oxidized to the carboxylic acid. Other reactions, such as aldol condensations, can be employed as illustrated with butanal. Reduction of the aldol product gives the commodity chemical, 2-ethylhexanol, used to make plasticizers such as dioctyl phthalate (dioctyl phthalate is the common name for the diester of phthalic acid with 2-ethylhexanol, but more precisely, it can be called di-2-ethylhexyl phthalate). 

Aldehydes fiad the most widespread use as chemical iatermediates. The production of acetaldehyde, propionaldehyde, and butyraldehyde as precursors of the corresponding alcohols and acids are examples. The aldehydes of low molecular weight are also condensed in an aldol reaction to form derivatives which are important intermediates for the plasticizer industry (see Plasticizers). As mentioned earlier, 2-ethylhexanol, produced from butyraldehyde, is used in the manufacture of di(2-ethylhexyl) phthalate [117-87-7]. Aldehydes are also used as intermediates for the manufacture of solvents (alcohols and ethers), resins, and dyes. Isobutyraldehyde is used as an intermediate for production of primary solvents and mbber antioxidants (see Antioxidaisits). Fatty aldehydes Cg—used in nearly all perfume types and aromas (see Perfumes). Polymers and copolymers of aldehydes exist and are of commercial significance. 

Example 11.2. Streamlined network for hydroformylation of n-heptene catalyzed by phosphine-substituted cobalt hydrocarbonyl. In hydroformylation of straight-chain olefins with a phosphine-substituted cobalt hydrocarbonyl catalyst, the model must account for three complications that are absent with cyclohexene isomerization by migration of the double bond along the hydrocarbon chain, formation of isomeric aldehydes and alcohols, and condensation of the straight-chain aldehyde to "heavy ends (chiefly an alcohol of twice the carbon number, such as 2-ethylhexanol from propene via n-butanal and a C8 aldol). A streamlined network for n-heptene is ... 

---