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When numbering substituents, do you give the substituents with the most atomic mass the lowest number even if it would give the other two substituents a higher number, or do you just number the substituents based on the fact that you just want lower numbers?
You never use atomic mass to determine anything in numbering a chain. There are two ways to number a chain: left to right, or right to left. Use the direction which gives the lowest number overall (do NOT sum). Only in the case of a tie, will you give the lower number to the alphabetically-first group.
If atomic mass is never taken into account, then how come for question 5 in the first quiz 3 preparation, the "bromo" substituent is given the bigger number (3) and the "iodo" substituent is given the smaller number (1) – even though it is a tie and if read left to right iodo would be 1 and bromo would be 3 OR if read right to left bromo would be 1 and iodo would be 3? The correct answer had bromo at 3 and iodo at 1. Why is this right if atomic mass doesn't matter and if during a tie you are supposed to prioritize the one higher on the alphabet?
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