Molecular and crystal structures of N-(n-heptyl)- and N-(n-decyl)-d-gluconamide

The crystal structures are described of N-(n-heptyl)-d-gluconamide (1) [space group P1, a = 5.183(7), b = 16.18(1), c = 4.803(5) Å, α = 94.2(1)°, β = 96.1(1)°, γ = 99.0(1)°] and of N-(n-decyl)-d-gluconamide (2) [space group P2₁, a = 5.255(2), b = 35.97(1), c = 4.807(2) Å, β = 94.81(3)°], and compared with that of N-(n-octyl)-d-gluconamide (3). In contrast to other amphiphilic molecules which form bilayers and crystallise in the tail-to-tail mode, the N-(n-alkyl)-d-gluconamides form monolayers and crystallise in sheet-like head-to-tail arrangements stabilised by intra- and inter-molecular NH ··· O and OH ··· O hydrogen bonds. Compounds (2 and 3) with an even number of alkyl carbon atoms crystallise in space group P2₁ and those (1) with odd numbers in P1. This situation is correlated with a change in torsion angles around the first CC bond in the alkyl chain and the packing is such that the head CH₂OH group interacts virtually identically with the tail group CH₂CH₃ in each crystal structure. This specific interaction might explain why these molecules form gels in aqueous solution.