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Topic: How to figure out longest chain?  (Read 8211 times)

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Offline xoannnnna

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How to figure out longest chain?
« on: September 28, 2010, 03:56:42 PM »
I attached a picture of a structure in which I thought the name was 2-methyl-3-isopropylpentane. But, it really is 3-ethyl-2,4-dimethylpentane.

Can anyone tell me how you figure this out and why it is like that? If I look at my orginal answer, it still seems correct to me.

Thanks!

Offline sjb

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Re: How to figure out longest chain?
« Reply #1 on: September 28, 2010, 04:09:06 PM »
Seems fine to me too, but I seem to recall that isopropyl is not really an IUPAC approved name.

Offline monica

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Re: How to figure out longest chain?
« Reply #2 on: September 28, 2010, 04:49:53 PM »


according to the rules we take the chain which has more branching ...because of stability...so its 3-ethyl because of -CH2CH3 group and two CH3  groups on 2 and 4carbon so dimethy and lastly whole chain has 5 carbons so pentane.

Offline azmanam

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Re: How to figure out longest chain?
« Reply #3 on: September 28, 2010, 05:30:21 PM »
yes, take the chain with the larger number of gross substituents (2 vs 3 in your example)
Knowing why you got a question wrong is better than knowing that you got a question right.

Offline energetica

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Re: How to figure out longest chain?
« Reply #4 on: October 05, 2010, 02:32:43 PM »
I wonder then why my professor told me this morning that's incorrect the name 2-methyl propane? He said that correct is isobutane. I know both are correct as one is named according to IUPAC nomenclature and other is a common name for the same structure.
I mean if I have to name a structure should I use IUPAC nomenclature or their common names like it's in case with isobutane?  ???





Offline xoannnnna

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Re: How to figure out longest chain?
« Reply #5 on: October 05, 2010, 03:30:01 PM »
That structure's IUPAC name would be 1,1-dimethylethane anyway. I guess you can use either one, but he probably said the first one is wrong because it is wrong, lol.

Offline Doc Oc

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Re: How to figure out longest chain?
« Reply #6 on: October 05, 2010, 03:58:05 PM »
That structure's IUPAC name would be 1,1-dimethylethane anyway. I guess you can use either one, but he probably said the first one is wrong because it is wrong, lol.

Actually, 2-methyl propane is correct in terms of rigorous IUPAC convention.  Isobutane is correct, but it's not the IUPAC name.

Offline energetica

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Re: How to figure out longest chain?
« Reply #7 on: October 05, 2010, 04:01:35 PM »
So which one should I use when I name the structure? IUPAC?

Offline Jzalkm

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Re: How to figure out longest chain?
« Reply #8 on: October 05, 2010, 10:15:47 PM »
the free wonderful chemsketch software at www.acdlabs.com is helpful for IUPAC naming. helps cross-check the name. It is easy to draw and generate the name of the molecule. besides, you may view in 3-D the molecules as well among other features.
all the best. (link to software: http://www.acdlabs.com/resources/freeware/chemsketch/)

Offline Doc Oc

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Re: How to figure out longest chain?
« Reply #9 on: October 06, 2010, 08:10:25 AM »
So which one should I use when I name the structure? IUPAC?

IUPAC is the most foolproof way to name something so go with that.  At some point if you start to really delve into organic chemistry you will find more utility in learning the "common" names for things.

Offline orgopete

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Re: How to figure out longest chain?
« Reply #10 on: October 06, 2010, 10:25:55 AM »
I'm with J-bone on this one. I have seen a number of different changes in nomenclature. I actually began with CAS nomenclature, Beilstein, common, German, French, etc. During my stint with teaching, I was uncomfortable with IUPAC because I did not have a purpose in using it. However, the advantage that it has is that it is systematic. I began to compare common and IUPAC. By doing that, I thought I began to cover nomenclature reasonably well.

Which nomenclature do you use? It depends. I doubt many chemists could draw cholesterol from its IUPAC name. The IUPAC name is long. The kinds of names you are likely to find would be something like 2-fluorocholesterol. This combines the systematic name with a common name. The common names for many chemicals are more likely to be understood for large and unwieldy names. If you understand the principles of nomenclature, then you should be able to handle common, IUPAC, CAS, and many other names.
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Offline energetica

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Re: How to figure out longest chain?
« Reply #11 on: October 06, 2010, 05:12:32 PM »
So maybe it wouldn't be bad to while using IUPAC nomenclature to write and try to memorize even common names of structures. I mean I don't have problems with IUPAC nomenclature, but I guess my prof thinks I should learn even common names (as we have few levels of organic chemistry and probably later we would need common names more than IUPAC ones).
Well anyway thanks for advices and resolving my "doubts".  :)

Offline karlosshughes

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Re: How to figure out longest chain?
« Reply #12 on: October 27, 2010, 01:41:13 AM »
Because you can't use isopropyl as a constituent of a systematic name as isopropane is not a systematic name.

Therefore you replace isopropyl with (2-methylethly).
Chemistry student & organic kid.
Dabbles in other chemistry too (but only because I have to XD).

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