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Author Topic: 5-ethyl-2,6-dimethylnonane  (Read 5103 times)

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Agent-X

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5-ethyl-2,6-dimethylnonane
« on: June 26, 2009, 09:14:36 PM »

I'm looking at 5-ethyl-2,6-dimethylnonane.

What I don't get is why at the 6 locant it's considered a methyl instead of something like a propyl.
I notice that it's more than a methyl. It looks like it extends out further. Why is this considered a methyl group?
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macman104

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Re: 5-ethyl-2,6-dimethylnonane
« Reply #1 on: June 26, 2009, 09:36:29 PM »

Is answer 1a in this pdf file the structure you are talking about?

How are you numbering it, that you are getting the 6 location more than a methyl?
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Agent-X

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Re: 5-ethyl-2,6-dimethylnonane
« Reply #2 on: June 26, 2009, 09:47:58 PM »

Is answer 1a in this pdf file the structure you are talking about?

How are you numbering it, that you are getting the 6 location more than a methyl?

I can't open that .pdf.
The picture should help explain things.

Going back from the 9 locant, the group looks like a propyl group: CH3CH2CH2-R
From the 2 locant, it's obvious that it's a methyl group.
I don't understand the reason for the branch leading off of the 6 locant to be called a methyl group.
Am I to assume that the branch going northeast of the 6 locant is the methyl group?
« Last Edit: June 26, 2009, 10:15:07 PM by Agent-X »
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Agent-X

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Re: 5-ethyl-2,6-dimethylnonane
« Reply #3 on: June 26, 2009, 10:25:44 PM »

i seem to be having tech. difficulties, but here is the pic:
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ugbede

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Re: 5-ethyl-2,6-dimethylnonane
« Reply #4 on: June 27, 2009, 12:09:00 AM »

Your numbering of the carbon to carbon bond mighty be wrong. It from left to right or from the part that has the smallest number of carbön. Try reading the rules of organic numencleture.
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alphahydroxy

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Re: 5-ethyl-2,6-dimethylnonane
« Reply #5 on: June 27, 2009, 02:26:32 AM »

You seem to have numbered it right, and I'm not sure what the problem is...

Positions 1 and 9 represent the longest linear carbon chain, and at positions 2 and 6 you have a methyl group...
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azmanam

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Re: 5-ethyl-2,6-dimethylnonane
« Reply #6 on: June 27, 2009, 06:44:08 AM »

I suspect what's throwing you is your identification of the longest linear chain, the "parent alkane chain."  In the examples in class, it always works out that the longest linear chain happens to be the horizontal one... but it doesn't have to be.  I think you're numbering left to right and assuming that the horizontal carbon chain must be the parent alkane.  In which case, it absolutely does look like a propyl group is coming off of carbon 6.  The key here is that the longest linear chain does not have to be horizontal.  The longest linear chain starts at the left, goes to carbon 6, then kinks and continues to carbon 9.

If you were assume the propyl group is the substituent, then your parent chain would only be 7 carbon atoms long, and you would name it 5-ethyl-2-methyl-6-propylheptane.  but if you kink the parent chain, then it becomes 9 carbon atoms long.  Then the substituent at carbon 6 is the methyl group and you end up with the name 5-ethyl-2,6-dimethylnonane.

see the image for the two ways of naming the molecule.  This is a common 'gotcha' question professors love.  The longest linear chain does not have to be horizontal, and you should not assume it always will be.
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Agent-X

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Re: 5-ethyl-2,6-dimethylnonane
« Reply #7 on: June 27, 2009, 11:02:14 AM »

...I think you're numbering left to right and assuming that the horizontal carbon chain must be the parent alkane.  In which case, it absolutely does look like a propyl group is coming off of carbon 6.  The key here is that the longest linear chain does not have to be horizontal.  The longest linear chain starts at the left, goes to carbon 6, then kinks and continues to carbon 9.

If you were assume the propyl group is the substituent, then your parent chain would only be 7 carbon atoms long, and you would name it 5-ethyl-2-methyl-6-propylheptane.  but if you kink the parent chain, then it becomes 9 carbon atoms long....

So, you're saying that what I didn't think about is identifying the longest chain with consecutive carbons, which would be considered the parent alkane?
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azmanam

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Re: 5-ethyl-2,6-dimethylnonane
« Reply #8 on: June 28, 2009, 12:36:19 AM »

right.  it sounded like you were considering heptane to be the parent alkane (probably because it is horizontal, a common rookie mistake).  rather, the longest linear chain is the parent alkane - in this case there is one turn in the longest linear chain, and the parent alkane is  nonane.
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Agent-X

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Re: 5-ethyl-2,6-dimethylnonane
« Reply #9 on: June 28, 2009, 12:36:11 PM »

Right.  It sounded like you were considering heptane to be the parent alkane (probably because it is horizontal: a common rookie mistake).  Rather, the longest linear chain is the parent alkane -- in this case there is one turn in the longest linear chain -- and the parent alkane is nonane.

Ok, thanks.   8)
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