Crystal Structure of 2-Phenylazulene

Abstract

AFTER a long controversy, the space group of azulene is now apparently established as the centro-symmetrie P2₁/a, with two non-centro-symmetric molecules in the unit cell^1,2, a result which leads to a disordered structure in which successive azulene molecules in the crystal are reversed in direction at random. Two previously observed cases of random positioning, namely, between chlorine and bromine in p-chlorobromobenzene³ and between methyl and chlorine in 2-amino-4-methyl-6-chloropyrimidine⁴, involve atoms or groups of comparable size, and are not as surprising as is the case of azulene, where there is a rather large difference in size between the five- and seven-membered rings.