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Crystals of p-chloroaniline are orthorhombic; a = 8.67, 6 = 9.28, c = 7.40 A, Z = 4, space group Pnam. The structure was determined with visual intensity data from the three-dimensional Patterson function, and refined by least-squares, with anisotropic thermal parameters for the chlorine, nitrogen, and carbon atoms, and isotropic thermal parameters for the hydrogen atoms ; the final R is 0.1 1 for 482 reflexions. The aromatic ring is planar within experimental error, but the chlorine and nitrogen are displaced from the plane in the same direction, by 0.03 and 0.06 A, respectively. The bond distances indicate electron-donation from the amino -group into the ring, the lengths being CI-C(4) = 1.75. C(l)-C(2) = 1.42, C(2)-C(3) = 1.39, C(3)-C(4) = 1.40. C(1)-N = 1.39 A.The intermolecular distances correspond to van der Waals interactions. THE C-NH, bond distances in aromatic amines are very significantly shorter than the single-bond distance, and these contractions are usually accounted for, in terms of valence-bond theory, by contributions to the f resonance-hybrid molecule of structures with a carbonnitrogen double b0nd.l This situation may be contrasted with that in nitro-compounds, where such canonical structures are " sacrificial '' and the C-N bond distances are not significantly different from the singlebond 1ength.l L4s a further contribution to these studies we have examined the structure of $-chloroaniline (I), and the results indicate C-N bond shortening, and hence contribution from structures such as (11). EXPERIMENTAL Crystals of p-chloroaniline from aqueous ethanol are colourless octahedra bounded by { 11 l } . The unit-cell and space-group data were determined from various rotation, Weissenberg, and precession photographs. Crystal Data (A, Cu-K, = 1.5418 A; A, hlo-h', = 0.7107 A) .-p-Chloroaniline, Cl-C,H4-SH,, ;14 = 127.6, m. p. 71". (all i 0 . 0 1 A), u = 595 A3, D, (flotation in aqueous KI) = 1.43, 2 = 4, D, = 1.42. Absorption coefficients, ~(CU-K,) = 47 cni.-l, p(RIo-K,) = 5 cm.-l. F(000) = 264. Absent reflexions: 0KZ absent when (k + I) = (2n + l ) , h0Z absent when h = ( 2 n + 1 ) . Space group PnaB, (CZgg) or Pnam (D2h16). Pnawz from structure analysis.The intensities of the reflexions were measured visually from Cu-K, lzKl Weissenberg films, K -1 0 7. The layers were correlated by intensity measurements of Mo-K, OkZ and kkO precession films. The crystal used was approximately spherical with diameter 0.2 mm., and no absorption corrections were considered necessary. The structure amplitudes were derived as usual; 482 reflexions were observed, Orthorhombic, a = 8.67, b = 9.28, c = 7.40 STRUCTURE ASALYSIS vectors in the three-dimensional Patterson function. These peaks could be interpreted in terms of space group Pnam, with C1, C(4), C(1), and N in positions (4(c) and the other carbon atoms in 8(d), that is, with the mean molecular plane perpendicular to the space-group mirror plane. However, to avoid any assumption about space group, the molecule was rotated about the C1-N axis, giving a structure ...

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