[eRReR]

I have a question about the rule about the first point of difference. In high school, I learned it as the "lowest sum" rule, where you add up the locants and the path giving the lowest sum was used for naming.
Then, I heard about the first point of difference, which makes sense. My question is this: I found a website talking about it, and it names the following molecule 5-ethyl-2-methylheptane.

Wouldn't the first point of difference rule name it in the reverse direction, making it 3-ethyl-6-methylheptane? Isn't "3" less than "5?"
Some help, please!

[fledarmus]

The first point of difference rule basically says that you go down the chain from both ends looking for the first substituent. So compare the two ends of the chain - what you have labeled 1 and 7. Any difference? No. Look at the next carbon in the chain - 2 and 6. Any difference? Yes. 2 has a substituent. So that end should get the lowest numbering, and your compound as drawn is appropriately numbered.

I think you are confusing the alphabetical order of the naming with the numerical order of the numbering. You find the lowest numbers using the first point of difference rule, and you find that 2 is less than 3 (you are not comparing 3 and 5, as you tried to do.) So your substituents would be a 2-methyl group and a 5-ethyl group, rather than a 3-ethyl group and a 6-methyl group. Then you put the substituents in alphabetical order - ethyl comes before methyl - and you end up with 5-ethyl-3-methyl.

[eRReR]

Thank you so much! That really clears things up for me. I believe that I get it now. It's just that all of the websites that tried to explain it never started with the part about counting along the parent chain and comparing the FIRST substituent. They just say "the first point of difference" and I thought it meant the first difference in the locant for the first alphabetical substituent, but now I know.

[jason.ridler]

Thank-you so much for this! I teach Chemistry at high school and have spent about an hour trying to find out how the first point of difference rule works. I had the same misunderstanding as you did eRReR. Thanks heaps fledarmus! I registered just so that I could post to say thanks