Don't sum locants

Basic IUPAC Organic Nomenclature

Don't sum locants!

Simply put, there is NO IUPAC rule based on summing the locants despite what you may have been taught elsewhere!

However, to be honest, the sum idea works in most simpler cases (unfortunately). But, learning the "sum rule" typically means one doesn't take the time to learn the first point of difference rule which is required in several other situations. Therefore, it is recommended that you learn the first point of difference rule as you will probably need to learn it at some point anyway, and it really isn't any more difficult to learn!

Mathematically, it is summing a list of numbers versus comparing a list of numbers written in numerical order (which only requires kindergarden math !)

HINT:

for each possible numbering scheme, list out the locants in numerical order
compare the "lists" to locate the first point of difference, i.e. the first time you encounter a low number
at the first point of difference, select the lower locant choice.

Here are some examples of the application of the first point of difference rule to generate the correct name where the mythical "sum rule" fails:

Consider the following example (from an IUPAC nomenclature website):

		Longest chain is C10
First, look at numbering the longest chain from left to right		Locants would be 3,4,9
Second, look at numbering the longest chain from right to left		Locants would be 2,7,8

If we were to sum, then the total from the first numbering scheme (left to right) 3 + 4 + 9= 16, the second scheme (right to left) 2 + 7 + 8 = 17
Which means if the sum rule existed, it would make scheme 1 the answer. But it's not...

However, let's follow a logical method for identifying the first point of difference...
(1) list out the locant set for each scenario in numerical order, then (2) compare the lists looking for the first time the numbers are different:

list out the locant set for each scenario with the locants in numerical order compare the lists looking for the first time the numbers are different (mouseover to highlight) pick the scenario with the lower number at the first difference (here 2 is less than 3)
The correct name is 2,7,8-trimethyldecane

Consider the following example (from Carey "Organic Chemistry" by McGraw-Hill):

		Longest chain is C8
First, look at numbering the longest chain from left to right		Locants would be 2,3,3,7,7
Second, look at numbering the longest chain from right to left		Locants would be 2,2,6,6,7

If we were to sum, then the total from the first numbering scheme (left to right) 2 + 3 + 3 + 7 + 7 = 22, the second scheme (right to left) 2 + 2 + 6 + 6 + 7 = 23 which means if the sum rule existed, it would make scheme 1 the answer, but it's not...

However, let's follow a method for identifying the first point of difference... (1) list out the locant set for each scenario in numerical order then (2) compare the lists looking for the first time the numbers are different:

list out the locant set for each scenario with the locants in numerical order compare the lists looking for the first time the numbers are different (mouseover to highlight) pick the scenario with the lower number at the first difference (here 2 is less than 3)
The correct name is 2,2,6,6,7-pentamethyloctane

More advanced example:		C6 ring with an alkene as the principal functional group, so it's a cyclohexene system
First, look at numbering clockwise from the top alkene C		Locants would be 1,2,2
Second, look at numbering counter clockwise from the top alkene C		Locants would be 1,6,6
Third, Second, look at numbering clockwise from the bottom alkene C		Locants would be 2,3,3

As you will learn later, for cycloalkenes, the two carbon atoms in the alkene C=C must be numbered 1 and 2. Therefore scheme 1 is incorrect because this is not the case (since the C at "2" is not part of the alkene).
In schemes 2 & 3 the C=C are numbered 1 and 2.

Schemes 2 & 3 need to be distinguished based on the first point of difference rule:

list out the locant set for each scenario with the locants in numerical order compare the lists looking for the first time the numbers are different (mouseover to highlight) pick the scenario with the lower number at the first difference
The correct name is 1,6,6-trimethylcyclohexene