3-Methylideneoxolane-2,5-dione

The title compound (itaconic anhydride), C5H4O3, consists of a five-membered carbon–oxygen ring in a flat envelope conformation (the unsubstituted C atom being the flap) with three exocyclic double bonds to two O atoms and one C atom. In contrast to the bond lengths, which are very similar to those in itaconic acid in its pure form or in adducts with other molecules, the bond angles differ significantly because of the effect of ring closure giving rise to strong distortions at the C atoms involved in the exocyclic double bonds. In the crystal, C—H⋯O interactions link the molecules, forming an extended three-dimensional network.

The title compound (itaconic anhydride), C 5 H 4 O 3 , consists of a five-membered carbon-oxygen ring in a flat envelope conformation (the unsubstituted C atom being the flap) with three exocyclic double bonds to two O atoms and one C atom. In contrast to the bond lengths, which are very similar to those in itaconic acid in its pure form or in adducts with other molecules, the bond angles differ significantly because of the effect of ring closure giving rise to strong distortions at the C atoms involved in the exocyclic double bonds. In the crystal, C-HÁ Á ÁO interactions link the molecules, forming an extended three-dimensional network.

3-Methylideneoxolane-2,5-dione Uwe Beginn, Martin Frosinn, Martin Reichelt and Hans Reuter
Comment 3-Methylenedihydrofuran-2,5-dione represents the anhydride of 3-methylendihydrufuran-2,5-carbonic with the trivial name itaconic acid. From this, the trivial name itaconic anhydride of the title compound is derived. Itaconic anhydride was synthesized for research projects on its polymerization to a homo-polymer (Otsu & Yang, 1991) or together with other monomers with special focus on the properties of the resulting products and the reactions of the anhydride function of the polymers with other substances. Due to the problems (hydration, decay, isomerization) that occur if itaconic anhydride is stored a longer time or under wrong conditions and to ensure its purity, the anhydride was directly synthesized from the itaconic acid and purified before polymerization.
The asymmetric unit of the title compound consists of one molecule ( hybridized carbon atoms of the ring. All in all, bond lengths are very similar to those of the acid in its pure state (Harlow & Pfluger, 1973) or in adducts with other molecules like 2,2′-dipyridyl-N,N′-dioxide (Smith et al., 1997) or urea (Baures et al. 2000).
Bond angles within the ring vary between 103.3 (1)° at C4 to 110.7 (1)° at O1 indicating small differences to the angles within a regular pentagon (108°). With respect to the carbon atoms C2, C3 and C5 that are involved in an exocyclic double bond to oxygen, respectively carbon this endocyclic bond angles are very unfavorable because they prefer bond angles of 120°. As a consequence, one of the two exocyclic bond angles at these atoms is widened Without the possibility of forming classical (O-H···O) bonds and in the absence of a π-ring system for π-π-interactions, intermolecular interactions are restricted to van der Waals ones (Fig. 2), dominated by C-H ···O distances from 2.48 to 2.73 Å (Fig. 3, Tab. 1). It is worthwhile to notice that succinic anhydride that differs from itaconic anhydride by replacing the exocyclic C=CH 2 fragment by a second methylene group crystallizes in the same chiral orthorhombic space group P2 1 2 1 2 1 with similar dimensions of the unit cell. Within its solid state structure Ferretti et al. (2002) have identified as a key feature of the crystal packing the interaction of the negatively charged carbonyl oxygen atoms with the ring atoms of two neighbouring molecules. This structure motif is also present in solid state structure of itaconic anhydride (Fig. 5) supplementary materials sup-2 Acta Cryst. (2013). E69, o321 with O···C contacts in the range of 3.083 (2) to 3.419 (2) Å and O···O contacts of 3.097 (2) Å, respectively 3.389 (2) Å.

Refinement
Hydrogen atoms were clearly identified in difference Fourier syntheses. Their positions were idealized and refined at calculated positions riding on the carbon atoms with C-H = 0.99 Å for CH 2 (sp 3 ) and C-H = 0.95 Å for CH 2 (sp 2 ).
In the absence of suitable anomalous scattering, Friedel equivalents could not be used to determine the absolute structure. Refinement of the Flack parameter led to inconclusive values [0 (10)] for this parameter. Therefore, Friedel equivalents (716) were merged before final refinement.