Different between Doebner condensation of dimethyl acetonedicarboxylate and cyanoacetic acid

How to make Malonic Acid in laboratory？

There are a number of methods you could follow to make malonic acid, however they all depend on you having other reactants available. 

The first method requires acetaldehyde (acetic acid may also work), acetone, hydrogen peroxide, a hydroxide base, borane and permanganate/dichromate. This would involve a reaction known as aldol condensation in which the carbonyl oxygen of the acetone would be replaced by the alpha carbon on the acetaldehyde, forming a CC double bond. This is then reacted with H2O2, BH3 and OH- to perform an 'anti-markovnikov' hydration reaction, adding the OH group the the gamme carbon. This is then oxidised by the ox. agent to form malonic acid. 

An alternative method (proposed on wikipedia) requires chloro-acetic acid, sodium carbonate, sodium cyanide and sodium hydroxide. In this reaction, the initial acid is deprotonated by the carbonate before being reacted with the cyanide anion, to form cyanoacetic acid(CAS NO.:372-09-8). This is oxidised by hydroxide or one of the ox. agents used above before finally being reprotonated to form malonic acid. 

It would perhaps be easier to try to substitute the malonic acid in the experiment for something you do have access to. This would depend on what you need the malonic acid for though since sometimes it may not be able to be substituted.

What is the different between Doebner condensation of dimethyl acetonedicarboxylate and cyanoacetic acid?

The Doebner condensation is a variation of the Knoevangel condensation when there is a carboxylic acid group that can be oxidized to CO2. The hydrogens that are vicinal to two carbonyl groups are fairly acidic, because the resulting anion is stabilized both by two electronegative oxygens and by resonance between the carbanion and its adjacent carbonyl carbons. In both Knoevangel and the Doebner variation, a base B: abstracts an acidic hydrogen to create a resonance-stabilized carbanion and the conjugate acid H:B. The carbanion attacks the electrophilic carbonyl carbon of an aldehyde, creating an alkoxide that then abstracts a proton from H:B to form the alcohol and recreate the base :B. 

Now, in Knoevangel the base abstracts the other acidic proton, the twin of the proton that was abstracted to form the carbanion, to eliminate HOH and form a double bond between the carbanion carbon and the electrophilic carbonyl carbon that the carbanion attacked. In the Doebner variation, that proton stays put. Instead, the adjacent carboxylate is oxidized to CO2, and the pair of electrons in the bond between the carbanion and carboxylate carbons becomes a pi bond between the carbanion carbon and the electrophilic carbonyl carbon, kicking out -OH. 

Although it's conceivable that you might cleave one of the esters to form a carboxylate for Doebner, you'd end up with -CH2-CN replacing -O-CH3, which is not the product you want. Instead perhaps the nitrile and carboxylate of cyanoacetate, both electron withdrawing groups and both capable of resonance-stabilizing a beta anion, serve in place of the two carbonyl groups in Knoevangel as described above. (Any base will abstract the carboxylic proton of acetic acid before it grabs the beta proton, so let's assume you're either starting with the carboxylate, or else have plenty of base in the pot.) Now base :B abstracts the beta proton from cyanoacetate, the carbanion attacks the central carbonyl of dimethyl-1,3-acetonedicarboxylate, creating a symmetric alkoxide. The alkoxide abstracts a proton from H:B, then a pair of non-bonding electrons on the carboxylate hydroxy O forms a bond to its carbon, oxidizing the carboxylate to CO2, while the bond from the carboxylate to the beta carbon breaks and reforms as a pi bond to the original acetone carbonyl in a concerted reaction step. Now you have =C-CN replacing the original acetone =O, double bonded to the original central carbonyl carbon. 


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