Which of the following acids (maleic, fumaric, succinic, or malonic) has the most acidic hydrogen?
I think that malonic acid should have the most acidic hydrogen due to the presence of an active methylene group.
The extraction of hydrogen atom from malonic acid would lead to high resonance stabilization, which is not the case in other acids.
However the answer key says that maleic acid has the most acidic hydrogen. Can someone please explain it to me why?
The $\mathrm pK_\mathrm a$ of the $\ce{CH2}$ group in a dicarbonyl compound is roughly $10$, whereas the $\mathrm pK_\mathrm a$ of a carboxylic acid is roughly $5$. For example, dimethyl malonate has a $\mathrm pK_\mathrm a$ of $13$ whereas acetic acid has a $\mathrm pK_\mathrm a$ of $4.7$. So, the C–H bond acidity is not likely to be under consideration in any of your compounds.
Let's draw up some $\mathrm pK_\mathrm a$ data (only first ionisations, all data from Wikipedia):
$$\begin{array}{cc} \hline \text{Carboxylic acid} & \mathrm pK_\mathrm a \\ \hline \text{Maleic} & 1.9 \\ \text{Fumaric} & 3.03 \\ \text{Succinic} & 4.2 \\ \text{Malonic} & 2.83 \\ \hline \end{array}$$
Clearly, all of these relate to ionisation of the $\ce{CO2H}$ group; if the acidity of dimethyl malonate is anything to go by, the $\ce{CH2}$ group in malonic acid is less acidic than the $\ce{CO2H}$ group by a factor of $10^{10}$.
What remains is to analyse this trend. Why is maleic acid the most acidic? The answer is – at least partly – intramolecular hydrogen bonding. When the first carboxylic acid group is deprotonated, the resultant monoanion is stabilised as such:
You are probably familiar with the mantra that a more stable conjugate base implies a more acidic compound. This is precisely what happens here.
Obviously, for fumaric acid, this is not possible because the double bond holds the two carboxyl groups apart. One could, however, argue that similar stabilisation could be derived for malonic and succinic acids. Why is it not a factor, then? It has to do with conformation: for the malonate or succinate anions to enjoy the benefits of intramolecular hydrogen bonding, they have to twist themselves into one specific conformation where the carboxyl groups are close to each other. This results in a loss of configurational entropy (the molecule is not as "free" as it would like to be to explore different conformations). For maleic acid, though, this is not a problem because the (Z)-double bond plonks those two carboxyl groups next to each other, and they don't have a choice about it. This bears some similarity to the concept of preorganisation, where two or more groups are "organised" in a fashion which alters their properties.
That's not the only reason why maleic acid is especially acidic, however. Part of the reason is also because an $\mathrm{sp^2}$ carbon is more electronegative than an $\mathrm{sp^3}$ carbon. An $\mathrm{sp^2}$ orbital contains 33% s-character, compared to an $\mathrm{sp^3}$ orbital which has 25% s-character. s-Orbitals are closer to the nuclei than p-orbitals, and therefore, an electron in an $\mathrm{sp^2}$ orbital experiences a greater effective nuclear charge from carbon; this translates into a greater electronegativity. (For similar reasons, alkenes are more acidic than alkanes.)
So, relative to the succinate anion, both the maleate and fumarate anions are stabilised by electron withdrawal via the inductive effect. This explains why fumaric acid is more acidic than succinic acid; the intramolecular hydrogen bonding explains why maleic acid is more acidic than fumaric acid.
Lastly, of course, malonic acid is more acidic than succinic acid because the electron-withdrawing $\ce{CO2H}$ substituent is fewer bonds away. In fact, succinic acid is barely more acidic than acetic acid, which doesn't have an electron-withdrawing substituent on it.
