The Dominant Parameter is Electronic

The electron-attracting nitro group in the antibiotic chloramphenicol (Tab. 1.1) has been replaced by other electron-attracting functions such as methyl-sulfonyl (thiamphenicol) or an acetyl (or cetophenicol). Apparently, the dominant feature is of electronic nature. 

Similar electronic effects are found in the benzodiazepines series in which the chlorine atom of diazepam can be exchanged with a bromine (bromazepam) or with a nitro group (nitrazepam). 

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