Synthesis of an epoxide from 1,2-diol Why is there no formation of epoxide when the two OH-groups are on different planes? I do understand the epoxidation mechanism. However, I would be grateful if someone cleared the specific requirements for epoxidation.
Thionyl chloride first reacts with the alcohol to form an alkyl chloro sulfite,which gives various stereochemical products as shown here. 



Extending this to the current question , alkyl chloro sulfite 2 is formed.${Cl^-}$ attacks in a ${S_N^2}$ mode to give 3.

Base deprotonates 3 to give 4.

Alkoxide and chloride in 4 are in correct trans configuration for a ${S_N^2}$.This results in 5(B in your scheme).



If we take C in your question , and follow the scheme above, 9  is formed.In 9 alkoxide and chloride are not in correct configuration for ${S_N^2}$.Therefore 
C in your question does not undergo epoxidation.