Why do we call now a substituent or a group in organic chemistry “R” instead of a radical?
Paula Bruice discusses this in "Organic Chemistry", 6th ed.
It used to be that "R" groups were radicals, yes. It was once said that -OH in Ethanol (EtOH) was attached to an ethyl "radical". Consequently, we decided to distinguish between them and call a group with an unpaired electron a "free radical".
Historically, we progressed to calling "R" a group , or substituent , and so we abandoned the words "free radical" and just call "R" a group/substituent and "radical" a group with an unpaired electron .