organic papers Acta Cryst. (2005). E61, o1055–o1057 doi:10.1107/S1600536805007981 Jorge Trilleras et al.  C 11 H 9 ClN 2 O o1055 Acta Crystallographica Section E Structure Reports Online ISSN 1600-5368 5-Chloro-3-methyl-1-phenyl-1H-pyrazole- 4-carbaldehyde: sheets built from C—HO and C—Hp(arene) hydrogen bonds Jorge Trilleras, a Jairo Quiroga, a Justo Cobo, b John N. Low c and Christopher Glidewell d * a Grupo de Investigacio ´ n de Compuestos Heterocı ´clicos, Departamento de Quı ´mica, Universidad de Valle, AA 25360 Cali, Colombia, b Departamento de Quı ´mica Inorga ´nica y Orga ´nica, Universidad de Jae ´n, 23071 Jae ´n, Spain, c Department of Chemistry, University of Aberdeen, Meston Walk, Old Aberdeen AB24 3UE, Scotland, and d School of Chemistry, University of St Andrews, Fife KY16 9ST, Scotland Correspondence e-mail: cg@st-andrews.ac.uk Key indicators Single-crystal X-ray study T = 120 K Mean (C–C) = 0.002 A ˚ Disorder in main residue R factor = 0.033 wR factor = 0.085 Data-to-parameter ratio = 16.7 For details of how these key indicators were automatically derived from the article, see http://journals.iucr.org/e. # 2005 International Union of Crystallography Printed in Great Britain – all rights reserved Molecules of the title compound, C 11 H 9 ClN 2 O, are linked into sheets by a combination of one C—HO hydrogen bond and one C—H(arene) hydrogen bond. Comment The title compound, (I), was prepared under Vilsmeyer conditions in which chlorination of C5 occurs in addition to the expected formylation, giving a versatile intermediate for the synthesis of fused pyrazolo heterocycles via cyclo- condensation reactions (Paul et al., 2001). The aldehydic fragment is almost coplanar with the adjacent pyrazole ring, but the two ring planes are inclined at 71.3 (2)  (Table 1). Within the pyrazolecarbaldehyde portion of the molecule, the bonds N1—C5 and C4—C41 are both short for their types (Allen et al., 1987), while bonds C4—C5 and C41— O4 are both long for their types, suggesting some contribution to the overall molecular–electronic structure from the charge- separated form (Ia) (see scheme). The molecules of (I) are linked into sheets by a combination of one C—HO hydrogen bond and one C—H(arene) hydrogen bond (Table 2); each of these hydrogen bonds Received 11 March 2005 Accepted 14 March 2005 Online 25 March 2005 Figure 1 The molecule of compound (I), showing the atom-labelling scheme. For the sake of clarity, only one set of methyl H atoms is shown; displacement ellipsoids are drawn at the 30% probability level.