Equivalent numbering - meaning?

[Davis Chong 4H]

Hey y'all,

On page 6 of the organic chem book given out (green) it says:

When equivalent numbering is possible (here is is 3,5 or 5,3 {the example was 3-ethyl-5-methylheptane}) then the first substituent in the name is assigned the lower number.

What does it mean by equivalent numbering? If the numbers are equal, there wouldn't be a lower number, right? Dreadfully confused out of my socks,

dc

[Catherine_L_1C]

I think the phrasing is referring to the fact that if you started numbering at the other end the methyl substituent would be at position 3 and the ethyl substituent at 5, so there are 2 constituents both at the 3 or 5 position either way you start from (making them "equivalent"). The example just wanted to make a point about alphabetizing and assigning the lower number to the first substituent. Hope that cleared things up!

[Davis Chong 4H]

bless u