Butane-1,4-diyl bis(chloroacetate)

Polyesters have a wide range of applications as biodegradable materials. Medical uses, such as bioabsorbable surgical sutures and drug delivery systems, are a good example. Most of the speciality polymers actually commercialized as sutures are based on glycolide, which is their major component (Chu, 1997). This group includes sutures such as Dexon or Sa®l (Schmitt & Polistina, 1967), Vicryl (Schneider, 1955), Maxon (Rosensaft & Webb, 1981), Monocryl (Bezwada et al., 1995) and Monosyn (Erneta & Vhora, 1998), which differ slightly in composition, and obviously in properties (thermal and mechanical) and degradation rates. All of these polymers are prepared by ring-opening polymerization, with the high cost of the cyclic glycolide monomer being one of the most limiting factors. Considerable efforts are currently focused on identifying alternative syntheses and on obtaining related polymers with enhanced properties. Thus, the solid-state polycondensation reaction of halogenated carboxylates appears to be an interesting method for the synthesis of polyglycolide (Herzberg & Epple, 2001), which could then generate the glycolide ring by pyrolysis. We have recently demonstrated that poly(ester amides) of high molecular weight could also be prepared by a condensation reaction between N,N0-bischloroacetyldiamines and dicarboxylate salts (Vera et al., 2004). The driving force of these polymerizations corresponds to the formation of metal halide salts (Epple & Kirschnick, 1997). A similar process could be extended to prepare new polyesters containing glycolic acid residues, characterized by the sequence OCH2COO(CH2)nOCOCH2OCO(CH2)mÿ2CO. The title compound, alternatively called 1,4-bis(chloroacetoxy)butane, (I), is one of the monomers that could be employed to prepare the series derived from 1,4-butanediol (n = 4). It is of interest to determine the crystalline structure of various monomers, since these kinds of reactions sometimes occur in the solid state. Furthermore, knowledge of their molecular conformations is a useful tool for the determination of the polymer structure, since they correspond to small fragments of its sequence.

The title compound, C 8 H 12 Cl 2 O 4 , lies about an inversion centre. The molecular conformation is characterized by a tgtgt conformation for the butanedioxy moiety and a trans conformation for the ClCH 2 ÐC( O)O bond. The molecular packing is stabilized by a network of weak CH 2 Á Á ÁO C intermolecular hydrogen bonds, where each molecule interacts with its four closest neighbours.

Comment
Polyesters have a wide range of applications as biodegradable materials. Medical uses, such as bioabsorbable surgical sutures and drug delivery systems, are a good example. Most of the speciality polymers actually commercialized as sutures are based on glycolide, which is their major component (Chu, 1997). This group includes sutures such as Dexon or Sa®l (Schmitt & Polistina, 1967), Vicryl (Schneider, 1955), Maxon (Rosensaft & Webb, 1981, Monocryl (Bezwada et al., 1995) and Monosyn (Erneta & Vhora, 1998), which differ slightly in composition, and obviously in properties (thermal and mechanical) and degradation rates. All of these polymers are prepared by ring-opening polymerization, with the high cost of the cyclic glycolide monomer being one of the most limiting factors.
Considerable efforts are currently focused on identifying alternative syntheses and on obtaining related polymers with enhanced properties. Thus, the solid-state polycondensation reaction of halogenated carboxylates appears to be an interesting method for the synthesis of polyglycolide (Herzberg & Epple, 2001), which could then generate the glycolide ring by pyrolysis. We have recently demonstrated that poly(ester amides) of high molecular weight could also be prepared by a condensation reaction between N,N H -bischloroacetyldiamines and dicarboxylate salts (Vera et al., 2004). The driving force of these polymerizations corresponds to the formation of metal halide salts (Epple & Kirschnick, 1997). A similar process could be extended to prepare new polyesters containing glycolic acid residues, characterized by the sequence OCH 2 -COO(CH 2 ) n OCOCH 2 OCO(CH 2 ) mÀ2 CO.
The title compound, alternatively called 1,4-bis(chloroacetoxy)butane, (I), is one of the monomers that could be employed to prepare the series derived from 1,4-butanediol (n = 4). It is of interest to determine the crystalline structure of various monomers, since these kinds of reactions sometimes occur in the solid state. Furthermore, knowledge of their molecular conformations is a useful tool for the determination of the polymer structure, since they correspond to small fragments of its sequence.
The molecule of (I) is shown in Fig. 1, and selected torsion angles and the hydrogen-bond geometry are reported in Tables 1 and 2, respectively. The ester group is planar within experimental error, with an r.m.s. deviation of 0.0041 A Ê for atoms C2, C3, O3 and O4 from the best plane passing through them. The molecule lies on an inversion centre and, consequently, the molecular conformation is symmetric (symmetry code: 1 À x, 1 À y, 1 À z).
The conformations of the chloroacetyl unit and the butanediol moiety, which is a constituent of some synthetic polyesters of commercial interest, such as poly(tetramethylene terephthalate) and poly(tetramethylene succinate), are interesting. A tgtgt conformation was found in (I) for the tetramethylene moiety, a fact that is in agreement with structural studies carried out on poly(tetramethylene succinate). This polymer exists as two polymorphs, the form (Chatani et al., 1970), where the butanediol residues adopt a kinked conformation, and the less predominant form, characterized by an all-trans conformation (Ichikawa et al., 1994). However, the reported structures for poly(tetramethylene terephthalate) show different conformations for the butanediol unit, namely ggtgg and tstst for the (Mencik, 1975;Yokouchi et al., 1976) and forms (Yokouchi et al., 1976), respectively. The shorthand nomenclature (Tadokoro, 1979) refers to the sequence of torsion angles with gauche (g), trans (t) or skew (s) conformations. Furthermore, potential energy calculations (Palmer et al., 1985) demonstrate that in the case of poly(tetramethylene terephthalate), the ggtgg conformation is stabilized with respect to the tgtgt conformation.
A survey of the Cambridge Structural Data Base (CSD, ConQuest Version 1.6; Allen, 2002;Bruno et al., 2002) shows 12 crystal structures containing XCOOCH 2 CH 2 CH 2 CH 2 OCOX units, with X being an aromatic group (phenyl, chlorophenyl or nitrophenyl) in the majority of cases. The tgtgt conformation was only found in two compounds, viz. tetramethylene glycol o-chlorobenzoate (Bocelli & Grenier-Loustalot, 1984) and tetramethylene glycol p-nitrobenzoate (Palmer et al., 1985). The all-trans conformation was observed in four compounds, with the remainder corresponding to asymmetric conformations where, in general, one of the two OÐCH 2 ÐCH 2 ÐCH 2 torsion angles is close to a gauche conformation. Among the known crystal structures of this class, compound (I) is a unique linear molecule containing aliphatic ester groups, with an observed molecular conformation in agreement with the determined structure of the form of poly(tetramethylene succinate).
The ClÐCH 2 ÐC( O)ÐO torsion angle has a trans conformation, which places the electronegative Cl and O(CH 2 ) atoms as far apart as possible. In fact, 65 crystal structures containing a total of 96 chloroacetoxy fragments have been solved, with the trans conformation observed for the majority (61, 24, 13 and 2% for the trans, cis, gauche and skew conformations, respectively). It should be pointed out that this bond tends to a cis conformation in chloroacetamide fragments [ClCH 2 ÐC( O)NH] because of the possibility of intramolecular NÐHÁ Á ÁCl hydrogen bonds (Rao & Mallikarjunan, 1973;Kalyanaraman et al., 1978;Urpõ Â et al., 2003).
The packing in (I) is characterized by a network of weak intermolecular CH 2 Á Á ÁO C hydrogen bonds (Fig. 2), where each molecule interacts with its four closest neighbours (Table 2). Hydrogen bonds are established along a direction which, on average, runs parallel to the crystallographic b axis. The methylene and carbonyl groups that interact belong to asymmetric units related by a twofold screw axis.

Experimental
The title compound was synthesized by the dropwise addition of a chloroform solution of 2.2 equivalents of chloroacetyl chloride (0.22 mol in 100 ml) to a chloroform solution of 1,4-butanediol (0.1 mol in 150 ml). The reaction mixture was stirred at room temperature for 3 h and then repeatedly washed with water. Finally, the chloroform solvent was evaporated under reduced pressure. The white solid obtained was recrystallized from ethanol to give colourless rhombic crystals of (I) (yield 85%, m.p. 349 K). 1 H NMR (CDCl 3 , TMS, internal reference): 4.26 (m, 4H, OCH 2 ), 4.09 (s, 4H, ClCH 2 ), 1.81 (m, 4H, OCH 2 CH 2 ); 13 C NMR (CDCl 3 , TMS, internal reference): 167.33 (CO), 65.53 (OCH 2 ), 40.84 (ClCH 2 ), 25.04 (OCH 2 CH 2 ). H atoms were placed in calculated positions and were re®ned isotropically riding on their attached C atoms, with CÐH distances of 0.97 A Ê . All H atoms belong to CH 2 groups, of which there are three in the asymmetric unit of (I). The displacement parameters of the two H atoms of each CH 2 group was re®ned as a free variable.