Packing conflicts in the Z′ = 5 structure of CF₃(CF ₂)₃(CH₂)₁₀COOH

Dimers of the partially fluorinated amphiphile 1-(perfluorobutyl)undecanoic acid, CF₃(CF₂)₃(CH₂) ₁₀COOH, crystallize in the same basic packing arrangement found for many n-alkyl carboxylic acids. The unit cell of the fluorinated compound, however, contains five independent molecules rather than the one found for most n-alkyl carboxylic acids including CH₃(CH₂) ₁₃COOH. The structures of the two C₁₅ acids are compared. The structural modulations that lead to the large value of Z′ can be understood as resulting from the conflict between the necessity of filling space densely and uniformly and the tendency of unlike groups to be segregated spatially. The few other known structures of compounds with both (CF ₂)_n and (CH₂)_m regions show evidence of the same conflict.