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|Molar mass||144.126 g·mol−1|
|Melting point||94 to 95 °C (201 to 203 °F; 367 to 368 K) (decomposes)|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
Meldrum's acid or 2,2-dimethyl-1,3-dioxane-4,6-dione is an organic compound with formula C
4. Its molecule has a heterocyclic core with four carbon and two oxygen atoms; the formula can also be written as [–O–(C(CH
The compound can easily lose an hydrogen ion from the methylene element CH
2 in the ring (carbon 5); which creates a double bond between it and one of the adjacent carbons (number 4 or 6), and a negative charge in the corresponding oxygen. The resulting anion [C
4]− is stabilized by resonance between the two alternatives, so that the double bond is delocalized and each oxygen in the carbonyls has a formal charge of −1/2.
The ionization constant pKa is 4.97; which makes it behave as a strong monobasic acid. In this and other properties, the compound resembles dimedone and barbituric acid. However, while dimedone exists in solution predominantly as the mono-enol tautomer, Meldrum's acid is almost entirely as the diketone form.
The high acidity of this compound was long considered anomalous—it is 8 orders of magnitude more acidic than the closely related compound dimethyl malonate. In 2004, Ohwada and coworkers determined that the energy-minimizing conformation structure of the compound places the alpha proton's σCH orbital in the proper geometry to align with the π*CO, so that the ground state poses unusually strong destabilization of the C-H bond.
Alkylation and acylation
The acidity of carbon 5 (between the two carbonyl groups) allows simple alkylation and acylation of Meldrum's acid at this position. For example, deprotonation and reaction with a simple alkyl halide (R–Cl) attaches the alkyl group (R–) at that position:
These two reactions allow Meldrum's acid to serve as a starting scaffold for the synthesis of many different structures with various functional groups. The alkylated products can be further manipulated to produce various amide and ester compounds. Heating the acyl product in the presence of an alcohol leads to ester exchange and decarboxylation in a process similar to the malonic ester synthesis. The reactive nature of the cyclic-diester allows good reactivity even for alcohols as hindered as t-butanol. Ketoesters of this type are useful in the Knorr pyrrole synthesis.
Synthesis of ketenes
These ketenes can be isolated using flash vacuum pyrolysis (FVP). Ketenes are highly electrophilic and can undergo addition reaction with other chemicals. With this approach it is possible to form new C–C bonds, rings, amides, esters, and acids:
Alternately, the pyrolysis can be performed in solution, to obtain the same results without isolating the ketene, in a one-pot reaction. The ability to form such diverse products makes Meldrum's acid a very useful reagent for synthetic chemists.
The compound is named after Andrew Norman Meldrum who reported its synthesis in 1908. He misidentified its structure as a β-lactone of β-hydroxyisopropylmalonic acid; the correct structure, the bislactone of 1,3-dioxane was reported in 1948.
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