tert-Butyl 2-(4-chlorobenzoyl)-2-methylpropanoate

The title compound, C15H19ClO3, is bent with a dihedral angle of 72.02 (9)° between the mean planes of the benzene ring and a group encompassing the ester functionality (O=C—O—C). In the crystal, molecules related by inversion symmetry are connected by weak C—H⋯O interactions into infinite chains. These interactions involve H atoms from a methyl group of the dimethyl residue and the O atoms of the ketone on one side of a molecule; on the other side there are interactions between H atoms of the benzene ring and the carbonyl O atoms of the ester functionality. There are no directional interactions between the chains.

The title compound, C 15 H 19 ClO 3 , is bent with a dihedral angle of 72.02 (9) between the mean planes of the benzene ring and a group encompassing the ester functionality (O C-O-C). In the crystal, molecules related by inversion symmetry are connected by weak C-HÁ Á ÁO interactions into infinite chains. These interactions involve H atoms from a methyl group of the dimethyl residue and the O atoms of the ketone on one side of a molecule; on the other side there are interactions between H atoms of the benzene ring and the carbonyl O atoms of the ester functionality. There are no directional interactions between the chains.

Comment
Treatment of 2,2-disubstituted t-butyl β-keto esters with trifluoroacetic acid at room temperature quantitatively generates the corresponding 2,2-disubstituted β-keto acids, which were used to probe the nature of the transition state for the thermal decarboxylation of β-keto acids (Logue et al., 1975). Structurally similar indanone-derived β-keto ester derivatives have been prepared recently (Mouri et al., 2009;Noritake et al., 2008;Rigby & Dixon, 2008). The directing nature of weak C-H···O H-bonds has been noted to be of importance to afford the three dimensional structure observed in these kinds of molecules (Karle et al., 2009).
In this contribution we present the solid state structure of one such 2,2-disubstituted β-keto acid, i.e. the title compound being the para-chlorobenzene derivative. This is the third paper in a series of four dealing with substituted derivatives (H-,  8, 27.6, 54.0, 81.9, 130.2, 133.6, 139.0, 173.8, 196.8.

Refinement
All H atoms were placed at calculated positions, with C-H = 0.93 Å (aromatic) or 0.96 Å (methyl) and refined using a riding model with U iso (H) constrained to be 1.5 U eq (C) for methyl groups and 1.2 U eq (C) for all other C atoms. The quality of the data as reflected by only 58% of the reflections observed, large ADP's and inaccurate C-C bond lengths is low. The data had been collected on a 30 year old single point detector instrument not equipped with a low temperature device as part of a class project with undergraduate students. Due to the time constraints imposed by the class schedule a maximum exposure time of 60 s had to be alloted for measuring each reflection.  Fig. 1. ORTEP-3 (Farrugia, 1997) drawing of the title compound with displacement ellipsoids drawn at the 50% probability level.

Special details
Experimental. Number of psi-scan sets used was 3. Theta correction was applied. Averaged transmission function was used. No Fourier smoothing was applied.
Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.