[baylorbelle2010]

I'm supposed to draw the 9 constitutional isomers for C₇H₁₆, but when i draw them out, I get 12 isomers! Which ones am I including incorrectly? i keep checking them, and all agree with the rules of no Carbon bonded more than 4X, and 16 H fitting into the empty spaces. Please help me! my isomers are as follows:

1. Heptane
2. 2-methylhexane
3. 3-methylhexane
4. 2,2-dimethylpentane
5. 2,3-dimethylpentane
6. 2,4-dimethylpentane
7. 3,3-dimethylpentane
8. 2-ethylpentane
9. 3-ethylpentane
10. 2,2,3-trimethylbutane
11.2-ethyl-2-methylbutane
12.2-ethyl-3-methylbutane

thanks in advance for any help offered : )

[Borek]

8. 2-ethylpentane

Somehow it doesn't look OK... But it is 1 a.m. here, so could be I am missing something.

[CopperSmurf]

I found them! 

your #7 is the same as #11
#12 is the same as #5
#8 is the same as #3

only write down the longest carbon chains.
 

[AWK]

I found them! 

your #7 is the same as #11
#12 is the same as #5
#8 is the same as #3

only write down the longest carbon chains.
 

# 11 is a wrong name (though chemists understand it). The same concerns #8 and #12

[baylorbelle2010]

so why are they wrong? i would love to know so that when it comes time for a test i'm not writing down wrong information.

[Borek]

CopperSmurf already told you what you did wrong. For example your 2-ethylpentane is in fact 3-methylhexane, as you should name the isomer starting with the longest carbon chain.

That's what I hinted at in my very first post.

[baylorbelle2010]

CopperSmurf already told you what you did wrong. For example your 2-ethylpentane is in fact 3-methylhexane, as you should name the isomer starting with the longest carbon chain.

That's what I hinted at in my very first post.

so the 2-ethylpentane is wrong and the 3-methylhexane is right? im still really confused as to why you can't have an ethyl group in the same place as a methyl group by taking a Carbon off of the end.

[Borek]

It is not that you can have one and you can't have the other, it is that they are identical and rules of naming tell that you should select the longest carbon chain as a base. So name 2-ethylpentane is wrong, because this molecule contains longer, 6 atoms chain, and as such its name should be based on hexane.

Count both blue and red chain atoms.