The Benzil−Cyanide Reaction and Its Application to the Development of a Selective Cyanide Anion Indicator

Abstract

The benzil-cyanide reaction is a cyanide-specific reaction that has been exploited to produce a colorimetric indicator for this toxic anion. This was done by producing a pi-extended analogue of benzil, 7, which is soluble in a 70:30 (v/v) mixture of methanol-water. In this medium, dilute solutions of 7 are yellow but produce colorless products when exposed to low concentrations of cyanide anion (> or = 1.7 microM; added as an aqueous NaCN solution), but no other common anions (e.g., OH(-), F(-), N3(-), benzoate(-), and H2PO4(-)). On the basis of these observations and supporting mechanistic analyses, it is concluded that the modified benzil system 7 is a promising cyanide anion indicator that is attractive in terms of its selectivity, ease-of-use, water compatibility, and the low, naked-eye discernible cyanide detection limit it provides.

Recent Advances in the Colorimetric Detection of Cyanide

“…[33] Sessler et al explored the benzil-rearrangement and the benzil-cyanide reaction for cyanide detection. [27,36,55] Both reactions proceed via the same intermediate to either a rearranged product in aprotic solvents or after cleavage in protic solvents to the corresponding benzaldehyde and benzoate ester. Especially the latter reaction, the socalled benzil-cyanide reaction seems to be very attractive since only catalytic amounts of cyanide are potentially required to trigger the reaction (Scheme 2D).…”

“…Since the product of the benzil- 1), [25] organic-based sensor (2), [28] boron-based sensor (3) [38] and metal-based sensor (4). Cyanide mediated reactions with organic-based sensors (A, [28] B, [29] C, [33] D [27] ).…”

“…[36] The benzil reaction leads to a remarkable enhanced cyanide sensitivity (Scheme 2D). [27] The reaction occurs in methanolwater mixtures, but the addition of base (NaOH, 5 mM) is necessary for the reaction to be completed within a reasonable time. A blue shift in the absorption spectra maximum (Δλ max = 43 nm) is observed after C-C bond breakage and is reflected in a colour change from yellow to colourless.…”

“…[15][16][17][18][19][20] Ideally, these systems show a colour change in the presence of the target molecule which is easily detectable with the 'nakedeye'. [21][22][23][24] For cyanide sensing, their mode of action is based on hydrogen bonding, [25,26] bond-forming reactions between either the nucleophilic cyanide and an electrophilic carbon [27][28][29][30][31][32][33][34][35][36] or boron centre [12,[37][38][39] or metal coordination [40][41][42][43] (Scheme 1). Progress is observed for all of these different systems, but a sensor that meets all of following three criteria has not yet been synthesized: i) optical detection below the maximum permissible level of cyanide in drinking water (1.7 μM); ii) demonstrating a fast and unambiguous response in pure water (<5 sec) without the need for special reaction conditions or sample pre-treatment; iii) displaying high selectivity in the presence of other anions.…”

Teaching a Known Molecule New Tricks: Optical Cyanide Recognition by 2-[(9-Ethyl-9H-carbazol-3-yl)methylene]propanedinitrile in Aqueous Solution

“…8 Consequently, considerable efforts have been devoted to the detection of cyanide by using colorimetric and fluorescent probes. 9 Currently, there are several methodologies and detecting principles that have been applied to cyanide ion recognition, which includes metal ion displacement, 10,11 demetalation of preassembled complexed sensor, [12][13][14][15][16] utilizing nucleophilic attack of CN − on activated carbonyl groups [17][18][19][20][21][22][23][24][25][26][27][28][29][30] or Michael acceptor type of activated C=C double bond. [31][32][33][34][35] Nevertheless, the innate nucleophiphilic nature of cyanide ion can minimize or avoid the interference by other anions such as fluoride and acetate 29 during the detection and hence, it is widely used.…”