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- Gracin, Sandra
(författare)
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Polymorphism and Crystallization of p-Aminobenzoic Acid
- 2004
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Polymorphs are solid phases where the chemical composition isequal but the crystal structure differs. Many organic compoundsmay appear in more than one crystalline structure. The differentpolymorphs of a givensubstance may have significantly differentphysical properties (packing, thermodynamic, spectroscopic,kinetic, surface and mechanical properties). Sometimes the moststable polymorph is difficult to produce or a metastable form hasfavourable properties. This thesis focuses on the crystallization of p-aminobenzoicacid. This model compound crystallizes in two differentpolymorphic forms: 1) the α-polymorph, which isthe commercially available form and appears as long, fibrousneedles 2) theβ-polymorph, that appears in the form ofprisms. The thermodynamic stability and crystallization from differentsolvents have been studied experimentally. The system is found tobe enantiotropic with a transition temperature of 25 °C,below which theβ-form is the stable polymorph. The compoundhas been crystallized from thirteen different solvents, either byslow cooling after which the product is allowed to mature insuspension, or by rapid cooling followed by immediate isolation.Needles were obtained from all solvents by both methods. In waterand ethyl acetate below 20°C the prismaticβ-form isobtained however, often together with the needles. Theβ-formcrystals usually needed hours or days to grow at the very slowcooling used, while needles usually appeared in seconds. Bycareful control of supersaturation and temperature coolingcrystallization can be performed to produce the pure β-formin water and in ethyl acetate. The influence of the solvent isexplained by analysis of the crystal structures versus thepossible interaction of the solvent molecules with the solute insolution. The α-form structure is governed by dimers and iskinetically favoured because the dimers easily form in thesolution especially in less polar solvents. The crystal structureof the β-modification is not based on dimers but on fourmembered rings with alternating amino and carboxyl groups. Keywords:controlled crystallization, polymorphism,p-aminobenzoic acid
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| 2. |
- Gracin, Sandra, et al.
(författare)
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Polymorphism and Crystallization of p-Aminobenzoic Acid
- 2004
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Ingår i: Crystal Growth & Design. - : American Chemical Society (ACS). - 1528-7483 .- 1528-7505. ; 4:5, s. 1013-1023
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Tidskriftsartikel (refereegranskat)abstract
- p-Aminobenzoic acid (PABA) crystallizes in two different polymorphic forms: the alpha-polymorph, which is the commercially available form and appears as long, fibrous needles, and the beta-polymorph, which appears in the form of prisms. The thermodynamic stability and crystallization from different solvents have been studied experimentally. The system is found to be enantiotropic with a transition temperature of 25degreesC, below which the beta-form is the stable polymorph. The compound has been crystallized from 13 different solvents, either by slow cooling after which the product is allowed to mature in suspension, or by rapid cooling followed by immediate isolation. Needles were obtained from all solvents by both methods. In water and in ethyl acetate, at slow cooling below 20degreesC, the prismatic beta-form is obtained, however, often together with the needles. The beta-form crystals usually needed hours or days to grow at the very slow cooling used, while needles usually appeared in seconds. By careful control of supersaturation and temperature, cooling crystallization can be performed to produce the pure beta-form in water and in ethyl acetate. The influence of the solvent is explained by analysis of the crystal structures versus the possible interaction of the solvent molecules with the solute in solution. The alpha-form structure is governed by carboxylic acid dimers and is kinetically favored since it is believed that the corresponding dimers easily form in the solution, especially in less polar solvents.
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