Benzylethyldimethylammonium bromide

The crystal structure of the title compound, C11H18N+·Br−, has been determined as part of an ongoing study of the influence of the alkyl chain length on amphiphilic activity of quaternary ammonium salts. The title salt forms a three-dimensional network of ionic contacts through weak C—H⋯Br hydrogen bonds, with donor–acceptor distances in the range 3.757 (2)–3.959 (2) Å, in which methyl groups serve as donors.

The crystal structure of the title compound, C 11 H 18 N + ÁBr À , has been determined as part of an ongoing study of the influence of the alkyl chain length on amphiphilic activity of quaternary ammonium salts. The title salt forms a three-dimensional network of ionic contacts through weak C-HÁ Á ÁBr hydrogen bonds, with donor-acceptor distances in the range 3.757 (2)-3.959 (2) Å , in which methyl groups serve as donors.

Comment
Quaternary alkylammonium salts are widely used to modify natural clay minerals into hydrophobic organo-clays which exhibit high capability to remove hydrophobic contaminants from aqueous solutions (Ogawa & Kuroda, 1997). From the systematic study of the relation between the crystal structures of chosen homologous benzyldimethylalkylammonium bromides and their cations' ability for sorption on clay minerals (Kwolek et al., 2003;Hodorowicz et al., 2003Hodorowicz et al., , 2005, it became obvious that the hydrophobic interactions are responsible for an alkyl-chain bilayer formation when the long-chain (n = 8-12) ammonium cations are adsorbed on montmorillonite (Hodorowicz et al., 2005), whereas a different way of cation packing seems to dominate in the case of short-chain ammonium cations (Kwolek et al., 2003). The crystal structure analysis of benzyldimethylethylammonium bromide was performed to find out the influence of molecular geometry, and the length of the alkyl chain in particular, on the packing properties of the ammonium cations. The structure of the title compound is shown in Fig. 1. The asymmetric unit is composed of a quaternary ammonium cation and a bromide counterion (N + ···Br − = 4.439 (2) Å). The bond lengths and angles indicate the typical tetrahedral arragement of the substituents at the N atom. The molecular dimensions are comparable with the values reported in the literature (Allen et al., 1987). Methyl and methylene groups of the quaternary ammonium cation as well as C-H of the benzene ring are involved in weak intermolecular interactions of the C-H···Br − type (Table 1). There are also relatively strong interactions of the C-H···π type observed between the C2 methyl group and the π system of the benzene ring, which result in cation chains along [100] (Fig. 2). The chains are joined into layers parallel to (010) due to C-H···Br − interactions (Fig. 3). The interactions are also responsible for packing of the layers along [010], as shown in Fig. 4. Each layer consists of cations inclined to the anionic layer and arranged in a zig-zag 'head-to-tail' system. The thickness of the layer is b/2. The observed architecture of the short-chain ammonium cation layers, best seen in Figs. 3 and 4, could be considered as a model for the organic cation layers intercalated into the montmorillonite structure (Kwolek et al., 2003).

Experimental
The title compound was prepared by dissolving a 1:1 mixture of bromoethane and N,N-dimethylbenzylamine in acetone at 273 K. The solution was slowly heated to room temperature to give colourless single crystals of the title compound.
Recrystallization from acetone afforded crystals suitable for X-ray measurements.

Refinement
All hydrogen atom positions were observed in a difference Fourier map. Nevertheless, in the refinement procedure the hydrogen atoms were positioned geometrically and refined using a riding model, with C-H = C-H = 0.97 Å for CH 2 groups, 0.96 Å for CH 3 groups, and 0.93 Å for aromatic CH, and with U iso (H) = 1.5U eq (C) for methyl groups and U iso (H) = 1.2U eq (C) for all other H atoms.  Fig. 1. ORTEP-3 (Farrugia, 1997) drawing of the asymmetric unit with atom labels. Displacement ellipsoids of non-H atoms are drawn at the 30% probabilty level.    ing R-factors(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Br1 0.29436 (