It is often INCORRECTLY taught in High School that it is the lowest sum of locants that determines the numbering scheme.
See here for more discussion on the first point of difference rather than summing locants follow this link.
Here are some illustrative examples of the first point of difference and related rules:
|Here the methyl group is given the lowest number by numbering
as shown (2- rather than 4-).
|The first difference is at the second locant - by numbering as
shown this is kept lower (2- rather than 4-).
|The first difference is at the first locant - by numbering as shown this is kept lower (2- rather than 3-).
|The first difference is at the second locant (2- rather than 6-) which means 1,2- is preferred over 1,6-.
The application of this numbering is then dictated by the alphabetisation : ethyl preceeds methyl so ethyl gets the lower number.
the principle functional group, the alcohol -OH is give the lowest
locant by numbering as shown (2- rather than 4-).
no choice !
matter which way this is numbered, the -OH is at C3.
|The first difference is in the -OH locant - by numbering as shown this is kept lower (2- rather than 4-).
|The first difference is in the methyl locant - since the -OH must be at C3 either way, but by numbering as shown the methyl locant kept lower (2- rather than 4-).
|©Dr. Ian Hunt, Department of Chemistry