Extract of Acta Cryst. 19, (1965), 482

Twinning in p-aminobenzoic acid
by R.C.G.Killean, P.Tollin, D.G.Watson, D.W.Young


[...]
Groth (1908) has quoted that crystals of p-aminobenzoic
acid belong to the monoclinic prismatic class with axial
ratios 1.4403:1:0.7312, ß=100°10'and density 1.393 g.cm-3
at 20°C. This observation was verified by Prasad, Kapadia
& Thakar (1937) who published unit cell dimensions a=
12.26, b=8.61, c=6.30, ß=100°10', which gave axial ra-
tios 1.424:1:0.732 and calculated density 1.38 g.cm-3. Pra-
sad et al mention crystallization difficulties and indicate
that the crystals were rhombic plates obtained by repeated
slow crystallization from aqueous alcohol. The present 
authors have attempted to repeat this work with the purpose
of verifying that p-aminobenzoic acid molecules associate
as dimers. Recrystallization of commercially available p-
aminobenzoic acid from the solvents listed in Table 1 gave
three distinct crystallographic forms. Despite repeated re-
crystallization the rhombic plate modification has not been
obtained. [...]

Table1. Crystallographic modifications of p-aminobenzoic acid

Solvent       Morphological   Crystallographic  Comments
                  habit         modification


Methanol        Acicular            I           White crystals which
Water             and               I           become straw colour-
Acetone         fibrous             I           ed on exposure to the
                                                atmosphere.


Acetone acid    Blades and fibrous  II          Straw coloured rap-
                  spherolitic                   idly decomposing to
                                                white powder unless
Dioxan          (a) Prismatic       III         sealed in glass tubes
                (b) bladed                      II and III have differ-
                                                rent systematic ab-
                                                sences.