Centrosymmetric structures of three substituted malonic acids

Crystal structures of 2-methylmalonic acid, 2-ethylmalonic acid and 2-phenylmalonic acid, which are derivatives of malonic acid and have found usefulness in diagnostics and biochemical evaluations as markers and organic synthesis, are reported. 2-Methylmalonic acid and 2-ethylmalonic acid both crystallize in triclinic P-1 space group with cell parameters of a = 5.1033(7), b = 5.4231(7), c = 10.0887(14) Å, α = 88.074(4), β = 89.999(5) and γ = 67.955(4)° for 2-methylmalonic acid, and a = 5.2073(4), b = 7.2258(6), c = 8.5655(6) Å, α = 88.086(2), β = 75.307(2) and γ = 85.904(3)° for 2-ethylmalonic acid, 2-phenylmalonic acid on the other hand crystallizes in monoclinic P2₁/c with cell parameters a = 8.6494(4), b = 5.48733(19), c = 17.1706(6) Å and β = 90.1068(17)°. The observed topology of the hydrogen bonding network is to a large extent dictated by the symmetrical substitution pattern with an open arrangement of hydrogen bonds. Each of the C=O double bonds in both 2-methylmalonic acid and 2-ethylmalonic acid is translationally offset, forming planar centrosymmetric carboxy-dimers. Molecules related by centre of inversion in spite of the constraints imposed by substituents are linked by O-H⋯O hydrogen bonds. This results in the formation of parallel zig-zag chains running along the [101] direction. In 2-phenylmalonic acid, the zig-zag molecular residue is parallely stacked with successive catemers. These laterally displaced catemers are also coiled in a twofold twist on the screw axis (x, 1/2 + y, 1/2 - z) along the b-glide (x, 1/2 - y, 1/2 + z) when viewed along the (101) plane.