Numbering the Carbon

JukaKim_1D
Posts: 24
Joined: Fri Sep 25, 2015 3:00 am

Numbering the Carbon

How do you know which Carbon is carbon one, for example on 1-ethoxy-3-methylpentane? why is the carbon to the left of the oxygen 1 and not the one on the right? also, if the carbon to the left of the oxygen is the first carbon, why is the carbon to the right also a 1 when naming it "1-ethoxy"?

Chem_Mod
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Re: Numbering the Carbon

Because the other option is 5-ethoxy-3-methylpentane which uses higher numbers.

Manuel Gonzalez_1L
Posts: 17
Joined: Fri Sep 25, 2015 3:00 am

Re: Numbering the Carbon

What I do is, I right them out both ways and then I add the number of Carbons and chose the one with the least. For example, 1+3 is 4 while 5+3 is 8. So the first one was the correct one because 4 is smaller than 8. Hopefully that helps a little.

Johnchou1A
Posts: 37
Joined: Fri Sep 25, 2015 3:00 am

Re: Numbering the Carbon

Manuel Gonzalez_1L wrote:What I do is, I right them out both ways and then I add the number of Carbons and chose the one with the least. For example, 1+3 is 4 while 5+3 is 8. So the first one was the correct one because 4 is smaller than 8. Hopefully that helps a little.

I thought that we aren't supposed to use the "sum rule" to decide numbering.

Chem_Mod
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Re: Numbering the Carbon

Do not EVER take the sum of numbers.

1,3 is preferable to 3,5 for the following reason: 1 is less than 3 (and that's it. If the first numbers were tied, then you move on to compare the second ones)

Taking the sum CAN result in the wrong name.