From monochlorination of (2R)−2−bromobutane at C1 is formed (2S)-2-bromo-1-chlorobutane.
From monochlorination of (2R)−2−bromobutane at C4 is formed (2R)-3-bromo-1-chlorobutane.
According to Cahn-Ingold-Prelog rules, the priority of halogen substituents decreases in the following sequence:
-I > -Br > -Cl > -F
According to the rule, chlorine has priority over bromine, so the configuration changes at C1 substitution.
