Why Do Organic Compounds Crystallise Well or Badly or Ever so Slowly? Why Is Crystallisation Nevertheless Such a Good Purification Technique?

Observation of the comparative crystallisation behaviour of over 400 related acylanilides has allowed some understanding of the role of conformation and molecular symmetry in determining the time taken by organic compounds to crystallise. This may be related to the number of alternative orientations in which a molecule can attempt to dock at a crystal site. Two- to 3-fold slowing of crystal growth compared with molecules with conformationally symmetric groups would be expected if the process involves simple acceptance of a single molecule at a growth site. The observed reduction in rate is much larger than this. It is suggested that it may be around x(3) where x is the number of alternative orientations. An attempt is made to account for the failure of some compounds to form extended domains of crystal perfection, by considering the fitting of impurity molecules or the misfitting of correct molecules in the lattice. As an explanation of the role of crystallisation in purification, a proposal is made as to why an identical molecule is more likely to fit into an existing crystal lattice than any foreign molecule.