rac-(1R,2R,4S)-1,2-Dibromo-4-[(1R)-1,2-dibromoethyl]cyclohexane

In the title compound, C8H12Br4, the cyclohexane ring exhibits a chair conformation. The C—Br distances range from 1.964 (6) to 1.985 (5) Å and the C—C distances range from 1.496 (6) to 1.543 (7) Å. Short intermolecular Br⋯Br contacts [3.467 (4) Å] occur in the crystal.


Related literature
The title compound is an environmentally novel brominated flame retardant de Wit et al., 2010), also known as TBECH, which was recently identified in beluga whales and in the eggs of herring gulls and double-crested cormorants Gauthier et al., 2009). There is relatively little information available concerning the persistence of TBECH in environmental media, its bioaccumulation in food webs and the toxicity of the pure stereoisomers (Rattfelt et al., 2006;Muir et al., 2007;Khalaf et al., 2009;Nyholm et al., 2009Nyholm et al., , 2010. The BrÁ Á ÁBr contacts in the crystal structure can be classified according to Ramasubbu et al. (1986). Data collection: SMART (Bruker, 2001); cell refinement: SAINT (Bruker, 2001); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008) and ORTEPIII (Burnett & Johnson, 1996); software used to prepare material for publication: SHELXTL.

Experimental
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: SJ5056). Comment 1,2-Dibromo-4-(1,2-dibromoethyl)cyclohexane, also known as tetrabromoethylcyclohexane (TBECH), is a cycloaliphatic brominated flame retardant used as an additive to flammable materials (e.g., polystyrene and polyurethane) to decrease the risk of accidental fire , de Wit et al. 2010. Due to the presence of 4 chiral carbons (C1, C2, C4, C5) TBECH can exist as four diastereomeric pairs of enantiomers . The structural differences between these stereoisomers lead to concomitant variability in physicochemical properties such as hydrophobicity and water solubility, resulting in variable propensities for biological uptake and metabolism. In this respect, the complex stereoisomerism of TBECH is a challenge for its trace quantification in relevant environmental matrices and in the food chain. Recently TBECH has been found to bioaccumulate in fish after dietary exposure (Rattfelt et al. 2006, Nyholm et al. 2009) and it was identified as a possible persistent, bioaccumulative and endocrine disrupting organohalogen chemical (Muir et al. 2007, Khalaf et al. 2009). Calculated half-lives of technical TBECH in activated aerobic and anaerobic soil at 20 °C were estimated to be 21 and 23 days, respectively (Nyholm et al. 2010). In the same study much slower degradation was observed during incubation at 8 °C (half-life: 120 days), suggesting that TBECH will persist in temperate climate zones for an extended period. However, the findings of TBECH in a maritime species (beluga whale) ) as well as seabirds (herring gulls and double-crested cormorants) (Gauthier et al. 2009) were reported for the first time.

Structure Reports Online
The molecular structure of the compound and the atom-labeling scheme are shown in Fig 1. The compound crystallises as a racemate. and each molecule is involved in two intermolecular Br···Br contacts [d(Br1-Br2): 3.467 (4)Å] below the sum of their van der Waals radii, which influence the molecular packing and lead to the formation of chains along the b axis.
Generally, halogen···halogen contacts C-X···X-C are defined as type I if the C-X···X angle θ1 is equal or nearly equal to the X···X-C angle θ2. If σimeq 180° and σimeq 90°, the contact is defined as type II (Ramasubbu et al. 1986). For the title compound the respective values amount to θ1(C1-Br1···Br2) = 161.2 (2)° and θ1(C2-Br2···Br1) = 137.3 (2)° These values are in acoordance with type I contacts arise as a result of close packing about an inversion center.

Experimental
In a 2 L two-necked round bottom flask equipped with a thermometer and a 50 ml dropping funnel, 20.1 g (186 mmol) 4vinylcyclohexene were dissolved in 1000 mL of dichloromethane. Bromine (19.5 ml, 381 mmol) was slowly added through the dropping funnel within 60 min. Light was excluded from the flask and the reaction mixture was stirred for 20 hrs at ambient temperature. Then excess bromine and dichloromethane were removed by rotary evaporation and the white residue was recrystallied from methanol. For single-crystal x-ray crystallography colourless crystals of the title compound were grown by slow solvent evaporation from methanol at ambient temperature in the absence of light.

Refinement
The C-H hydrogen atoms were located in difference maps and and fixed in their found positions with U iso (H) = 1.2 of the parent atom U eq or 1.5 U eq (C methyl ). Fig. 1. ORTEP representation of the title compound with atomic labeling shown with 30% probability displacement ellipsoids.  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 > σ(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.