N,N′-Bis(3-methylphenyl)succinamide dihydrate

The asymmetric unit of the title compound, C18H20N2O2·2H2O, contains half a molecule with a center of symmetry at the mid-point of the central C—C bond. The N—H bonds in the amide fragments are anti to the meta-methyl groups in the adjacent benzene rings. The dihedral angle between the benzene ring and the NH—C(O)—CH2 segment in the two halves of the molecule is 5.6 (4)°. In the crystal, the packing of molecules through O—H⋯O and N—H⋯O hydrogen-bonding interactions leads to the formation of layers parallel to the bc plane. The methyl group is disordered with respect to the 3- and 5-positions of the benzene ring, with site-occupation factors of 0.910 (8) and 0.090 (8).

The asymmetric unit of the title compound, C 18 H 20 N 2 O 2 Á-2H 2 O, contains half a molecule with a center of symmetry at the mid-point of the central C-C bond. The N-H bonds in the amide fragments are anti to the meta-methyl groups in the adjacent benzene rings. The dihedral angle between the benzene ring and the NH-C(O)-CH 2 segment in the two halves of the molecule is 5.6 (4) . In the crystal, the packing of molecules through O-HÁ Á ÁO and N-HÁ Á ÁO hydrogenbonding interactions leads to the formation of layers parallel to the bc plane. The methyl group is disordered with respect to the 3-and 5-positions of the benzene ring, with site-occupation factors of 0.910 (8) and 0.090 (8).
In the C-NH-C(O)-C segment, the amide O atom is anti to the H atoms attached to the adjacent C atom. The N-H bonds in the amide fragments are also anti to the meta-methyl groups in the adjacent benzene rings, similar to that observed with respect to the ortho-methyl groups in (III) and the meta-chloro groups in (IV).
The dihedral angle between the benzene ring and the NH-C(O)-CH 2 segment in the two halves of the molecule is 5.6 (4)°, compared to the values of 62.1 (2)° in (III) and 32.8 (1)° in (IV). The striking difference may be due to the fact that the title compound is the dihydrate, i.e. is composed of the amide and lattice water molecules, which, unlike in other compounds, influence the molecular conformation through hydrogen bonding interactions.

Experimental
Succinic anhydride (0.01 mol) in toluene (25 ml) was treated dropwise with m-toluidine (0.01 mol) also in toluene (20 ml) with constant stirring. The resulting mixture was stirred for one hour and set aside for an additional hour at room temperature for completion of the reaction. The mixture was then treated with dilute hydrochloric acid to remove unreacted m-toluidine.
The resultant N-(3-methylphenyl)succinamic acid was filtered under suction and washed thoroughly with water to remove the unreacted succinic anhydride and succinic acid. The compound was recrystallized to a constant melting point from ethanol. The purity of the compound was checked by elemental analysis and characterized by its infrared and NMR spectra.
supplementary materials sup-2 The N-(3-methylphenyl)succinamic acid obtained was then treated with phosphorous oxychloride and excess of m-toluidine at room temperature with constant stirring. The resultant mixture was stirred for 4 h, kept aside for additional 6 h for completion of the reaction and poured slowly into crushed ice with constant stirring. It was kept aside for a day. The resultant solid, N,N-bis(3-methylphenyl)-succinamide dihydrate, was filtered under sucction, washed thoroughly with water, dilute sodium hydroxide solution and finally with water. It was recrystallized to a constant melting point from a mixture of acetone and chloroform. The purity of the compound was checked by elemental analysis, and characterized by its infrared and NMR spectra.
Needle-like colorless single crystals used in the X-ray diffraction studies were grown in a mixture of acetone and chloroform at room temperature.

Refinement
The H atom of the NH group was located in a difference map and later restrained to the distance N-H = 0.86 (2) Å. The stucture was modelled with stoichiometric chemical composition applying an approximation of full occupancy of H5 and no corresponding partly occupied hydrogen atom at C3. The C9'H3 group in an alternative orientation was idealized and refined using a AFIX 3 (positional optimization of the entire group only by translation, no rotations) in SHELXL. The U ij components of C9' were assumed to be identical with that of C5 (EADP C5 C9') and were restrained to approximate isotropic behaviour. Atom C9 was refined using a split model. The corresponding site-occupation factors were refined so that their sum was unity [0.910 (8) and 0.090 (8)]. A DELU restraint was used for all U ij . The water molecule was refined as a rigid group with respect to x,y,z and its orientation (AFIX 6). The other H atoms were positioned with idealized geometry using a riding model with the aromatic C-H = 0.93 Å, the methyl C-H = 0.96 Å and the methylene C-H = 0.97 Å. U iso (H) values of the methyl group and the water molecule were set at 1.5 U eq of the parent atom. The other H atoms were refined with isotropic displacement parameters (set to 1.2 times of the U eq of the parent atom).
The crystals available for X-ray studies were of rather poor quality and weak scatterers at high theta value resulting in relatively high R values.