Diacetone alcohol from aldol condensation

With concentrated alkali, a resin is formed from repeated aldol condensations between aldol, crotonaldehyde and acetaldehyde. A similar condensation occurs with acetone (b.p. 56°), but the equilibrium mixture contains only a few per cent, of diacetone alcohol (III), b.p. 166° ... 

As in the case of homogeneous acids as catalyst, we would also benefit from using solid ba.ses instead of dissolved bases as catalyst. A number of industrially important reactions are carried out with bases as catalyst. A well know example is the aldol condensation of acetone to diacetone alcohol, where dissolved NaOH in ethyl alcohol is u.sed as a catalyst at about 200 to 300 ppm level. However, heterogeneous pelleted sodamide can be used as a catalyst for this reaction and it obviates the problem of alkali removal from the product, which would otherwise lead to reversion of diacetone alcohol to acetone during distillation of the product mixture. 

Diacetone alcohol is a solvent used in hydraulic fluids and printing inks. Recall that the aldol condensation is an example of a variety of carbanion reactions used to make large molecules from smaller ones. An aldehyde or a ketone with at least one hydrogen on the carbon next to the carbonyl will react to give the aldol condensation. The mechanism is given as follows. 

One of the difficulties in performing aldolization is the reversibility of the reaction, which limits the equilibrium conversion. The thermodynamic equilibrium has been investigated by Craven [2] for the industrially important aldolization of acetone to diacetone alcohol (DA). Because the reaction is exothermic, the yield of aldol obtained from pure acetone decreases with temperature 23.1 % at 273 K, 16.9 % at 283 K, 12.1 % at 293 K and 9.1 % at 303 K. The same conclusions can be drawn from the work of Guthrie [3-6] who reported equilibrium constants in the aqueous phase, in relation to the of the substrates, for a series of aldol condensations at room temperature. For the aldolization of aeetaldehyde at 298 K the values relative to the reaction are AG° = -2.4 kcal mor , A77 = -9.84 kcal mol , and equilibrium constant K = 51 m . The results for a few specific reactions performed in the aqueous phase are reported in Table 1 where Ky and K2 represent the equilibrium constants for the formation of the aldol and for its dehydration, respectively. 

According to these thermodynamic data, significant conversion can be obtained for the condensation of acetone with formaldehyde. Diacetone alcohol can be formed in low yield only and has a lower tendency to dehydrate than the aldols formed from benzaldehyde. The energy balance of the process is so displaced by the formation of water that the unsaturated ketone is usually obtained. The... 

As an example, the dehydration of diacetone alcohol, the )3-hydroxy ketone produced during the self-condensation of acetone leads to mesityl oxide, the a,p-imsatruated ketone dimer of acetone, which can further react with another acetone molecule, as shown in Scheme 6. Since both, mesityl oxide and acetone are able to form a carbanion by a-hydrogen abstraction, two trimers of acetone can be obtained in a consecutive aldol-dehydration sequence 4,6-dimethyl hepta-3,5-dien-2-one (carbanion supplied by acetone) and phorone (carbanion formed from mesityl oxide). A water molecule is lost in both reactions, and both products are a,/ -unsaturated ketones (11,19). A third tiimer is also possible. A review on the operating conditions and catalysts needed for the production of a, -tmsaturated ketones from acetone can be formd in Ref (11), whereas a discussion on the reaction pathways on different catalysts is reported in Ref (20). 

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