Physical Pharmacy: Crystalline Solids - Polymorphs and Allotropes

ability for a compound to exist in more than one crystal form with different unit cell parameters

Polymorphism

Polymorphs have different

-stabilities

-MP

-x-ray crystal diffraction patterns

-solubilities

Theobroma cacao aka

-theobroma oil

-cacao butter

Theobroma cacao polymorphic forms

-Gamma

-alpha

-beta prime

-beta

the unstable form of theobroma oil

Gamma

MP of gamma form theobroma oil

18C

the stable form of theobroma oil

Beta

MP of Beta form theobroma

34.5C

lowest temperature for melting to not destroy the beta form

33C

Allotropy aka

Elemental polymorph

special case of polymorphism in elemental substances

Allotropy

Allotropes of carbon

-graphite

-diamond

soft and less organize carbon allotrope

graphite

Hardest mineral in earth

diamond

in Mohs scale of hardness this carbon allotrope =10

diamond

Allotrope of oxygen

-nascent oxygen

-molecular oxygen

-ozone

nascent oxygen is in

peroxides

molecular oxygen is the

diatomic oxygen (O2)

oxygen allotrope that protects use against excessive UV light

Ozone (O3)

Classification of polymorphs based on the direction of change

-enantiotropic

-monotropic

change of one form to another is REVERSIBLE

Enantiotropic

with more than one stable form

Enantiotropic

example of enantiotropic

sulfur

≥ 2 stable crystalline form, they can interchange because all forms are stable

Enantiotropic

Change takes place in ONE DIRECTION ONLY

Monotropic

Metastable -> stable

Monotropic

metastable means

unstable

with only one stable form

Monotropic

Which is stable and metastable between diamond and graphite?

diamond: metastable

graphite: stable

so the direction of stability of carbon allotropes is

Diamond -> graphite

_____ form have lowering melting points and enthalpies of fusion and so greater solubilities

Metastable (unstable)

in terms of drug solubility, this is the preferred form

metastable