Benzylammonium hexanoate

A binary mixture of benzylamine and hexanoic acid has been reacted to form the title salt, C7H10N+·C6H11O2 −. This crystal has a 1:1 stoichiometry of acid- and amine-derived species which contrasts with other related species which can have a number of other integer ratios of acid and amine components. The diffraction data indicate complete transfer of a proton from the acid to the amine to give the salt, comprising a cation and anion combination, with the formation of three hydrogen bonds around each ammonium group. This contrasts with other related species.

A binary mixture of benzylamine and hexanoic acid has been reacted to form the title salt, C 7 H 10 N + ÁC 6 H 11 O 2 À . This crystal has a 1:1 stoichiometry of acid-and amine-derived species which contrasts with other related species which can have a number of other integer ratios of acid and amine components. The diffraction data indicate complete transfer of a proton from the acid to the amine to give the salt, comprising a cation and anion combination, with the formation of three hydrogen bonds around each ammonium group. This contrasts with other related species.

Experimental
Data collection: COLLECT (Nonius, 1998); cell refinement: SCALEPACK (Otwinowski & Minor, 1997); data reduction: DENZO (Otwinowski & Minor, 1997) and SCALEPACK; program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXL97. Several studies, mainly spectroscopy-based, have reported the existence of stable complexes formed between simple fatty acids and amines, both alkyl and aromatic-based e.g. . Numerous 1:1 acid:amine complexes have been identified; in addition, various examples of 2:1 and 3:1 adducts have been discovered, usually in an acid-rich environment Kohler et al., 1981). Interestingly, no amine-rich complexes have yet been observed; indeed, it has been proposed that these would be highly unstable were they to form (Paivarinta et al., 2000), although there is a report of a diamine complex formed between methylamine and dnsa (3, 5-dinitrosalicyclic acid) due to deprotonation of the phenolic group in the acid (Smith et al., 2001;Smith et al., 2002).
The 1:1 acid:amine complexes are generally considered to derive their stablity from the complete transfer of a proton from the acid to the amine with subsequent cation-anion electrostatic interaction and strong hydrogen-bond formation. In 2:1 or high stoichiometry complexes, the hydrogen bond is considered to extend over the three (or more) species involved.
The 1:1 complex of hexanoic acid and benzylamine forms by reaction of the two species with complete proton transfer from the acid to the base. Each ammonium ion in this salt can now form three hydrogen bonds, one of which is shown in Fig. 1 and all three in Fig. 2. This work follows from similar findings reported by (Jefferson et al., 2011) who report the structure of a 1:1 complex of octanoic acid and decylamine using the same experimental method of preparation. This work differs from the previous study concerning complex formation with an aromatic amine, rather than an alkyl amine reported previously. In general, few examples of such single-crystal data exist for such complexes, due mainly to the difficulty of growing suitable crystals. The molecular arrangement of the alkyl and aromatic groups is also somewhat surprising. One might have imagined the aromatic rings interacting strongly together and 'stacking′ separately from the alkyl chains of the hexanoic acid. However, they appear to be arranged adjacent to each other in the 1:1 crystal, with the planes of the aromatic ring and the alkyl chain backbone essentially parallel, Fig. 2.

Experimental
Hexanoic acid and benzylamine, with purities of 99.5% and 99.7% respectively as determined by titration and GC, were purchased from Sigma Aldrich and used without further purification. The crystals were grown by pipetting a small volume (approximately 1 ml) of each into two small vials, and leaving both within a larger vial over a number of weeks all under an inert atmosphere of nitrogen. After this period numerous crystals were observed, with particularly abundant growth on a polypropylene surface that had been left therein as a nucleating surface. The inert atmosphere was employed to minimize reaction of the amine with atmospheric CO 2 , which can make such complexation studies difficult ).
Elemental analysis gave values of 69.85%, 6.22%, 9.42% and 14.52% for carbon, nitrogen, hydrogen and oxygen respectively. For a 1:1 complex these values are expected to be 69.92%, 6.27%, 9.48% and 14.32%, in excellent supplementary materials sup-2 Acta Cryst. (2012). E68, o3004 agreement. The 1:1 stoichiometry also agrees with the crystal structure determination given here. The experimental sample temperature 180 K represents a compromise of several factors. It is selected as the temperature which is cold enough to get improved thermal factors but not so cold that the crystals fracture and it is a temperature at which the cryostream can run efficiently for an extended period.

Special details
Experimental. The data is moderately weak at high angle (66% observed), a fact reflected in the rather large K value in the analysis of variance. Absorption correction: multi-scan from symmetry-related measurements Sortav (Blessing, 1995) Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. 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 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 N1 0.2211 (