{HEAD ~ X} in formula represents {TAIL ~ a hydrogen atom}, a halogen atom, a hydrocarbon group having 1 to 20 carbon atoms, an alkoxy group or an amido group 
represents
{ HEAD ~ X } in formula represents { TAIL ~ a hydrogen atom } , a halogen atom , a hydrocarbon group having 1 to 20 carbon atoms , an alkoxy group or an amido group
HEAD HEAD HEAD HEAD HEAD OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH