[nameless74]

I have a test on Monday and I'm so behind on naming hydrocarbons. Teacher gave us a work sheet to study on the weekend but I'm having a problem. He did 1 question which i'm trying to understand.

                 CH3
                  |
CH3 - CH2 - CH - CH - CH3
                         |
                        CH2
                         |
                        CH3

Okay, I know that the longest chain of carbon is 6 which is hexane. Well the next step I'm kind off confused about where to start. Is it going to be from left to right. I've been searching some example about naming hydrocarbons and some they start from left to right or right to left. I get some right but mostly wrong. Like the one example that i gave you. My answer would be:

3rd carbon there is 1 carbon so it would be 3 methyl
4th carbon there is 2 carbons so it would be 4 ethyl

so my answer would be 3-methyl, 4-ethyl hexane
which is wrong the answer should be 3, 4 dimethyl hexane

another example from the book:

             CH2CH3
             |
CH3 - CHCHCHCH3
         |      |
   CH3CH2   CH3

the longest chain of carbon is 6 which is hexane.
i startet the carbons from left to right so
2nd carbon has 2 carbons - 2 ethyl
3rd carbon has 2 carbons - 2 ethyl
4th carbon has 1 carbon - 4 methyl

so the answer would be 4 methyl, 2 ethyl, 2 ethyl hexane or 4 methyl, diethyl hexane

and the right answer was 2,4-dimethyl-3-ethylhexane.

i spent the night trying some example and its frustrating just spending 2 hours for 1 example that i couldn't get the right answer. i need helpppppp .   

[orgopete]

This is where models could sometimes help. Even though you see the structure as written on paper, if you used the model representation, you could imagine it being in any direction in space and on paper. Then, if you numbered the atoms of the longest chain, that would give the parent hydrocarbon name. The substitutents should have the lowest locants (numbers corresponding to the carbon chain). Sometimes that requires reversing the numbering to get the lowest locants.

The error you made in the first example was to change the numbering after you initially assigned it. Simply write the atom numbers on the carbons for the longest chain as you determine it and you will arrive at the correct locants, 3,4-dimethyl.

In the second example, you would have a choice of 3-isopropyl-4-methylhexane or 2,4-dimethyl-3-ethylhexane. Both names could be understood, but the IUPAC rules prefers the name with the larger number of locants. (I didn't look up this rule and cannot state it precisely.)