text: Disulfides have a C-S-S-C dihedral angle approaching 90°.  The S-S bond length is 2.03 Å in diphenyl disulfide,  similar to that in elemental sulfur.
  Two kinds of disulfides are recognized, symmetric and unsymmetric. Symmetrical disulfides are compounds of the formula . Most disulfides encountered in organo sulfur chemistry are symmetrical disulfides.  Unsymmetrical disulfides (also called heterodisulfides or mixed disulfides) are compounds of the formula .  Unsymmetrical disulfide are less common in organic chemistry, but many disulfides in nature are unsymmetrical.  Illustrative of a symmetric disulfide is cystine.