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Topic: First point of difference rule  (Read 535 times)

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Offline jason.ridler

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First point of difference rule
« on: July 17, 2022, 09:57:52 PM »
Hi, I teach Chemistry at a high school in Auckland, New Zealand. A colleague of mine made a PowerPoint presentation in which he named the compound below 1,1,3-tribromo-4,4-difluorobutane. Using the first point of difference rule, shouldn't this be: 2,4,4-tribromo-1,1-difluorobutane?
Numbering from the left:   1,1,2,4,4
Numbering from the right: 1,1,3,4,4
The first point of difference is the third number, so shouldn't the numbering start from the left, since 2 is lower than 3?

Or am I misunderstanding the rule?

Thanks for your help

Jason

Offline Hunter2

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Re: First point of difference rule
« Reply #1 on: July 18, 2022, 06:57:10 AM »
In my opinion   2( R,S),4,4-tribromo-1,1-difluorobutane is the right naming, because the lower number get the element with the highest electronegativity. And this is flourine in this case. Also the second C is a chiral one. You can Name it with R or S.

Offline Borek

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Re: First point of difference rule
« Reply #2 on: July 18, 2022, 12:20:56 PM »
ChemSketch confirms 2,4,4

(this is actually something I suggest you add to your toolbox - you can download it and use it for free, it is a perfect tool for checking naming issues, my understanding is ACDLABS works on their naming system in close cooperation with IUPAC)
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Offline jason.ridler

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Re: First point of difference rule
« Reply #3 on: July 19, 2022, 12:39:22 AM »
Brilliant! Thanks Hunter 2 and Borek - much appreciated. Thanks for suggesting ChemSketch as well. I've downloaded and installed it, and this should help me in the future with these sort of queries.

Offline jason.ridler

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Re: First point of difference rule
« Reply #4 on: July 19, 2022, 01:32:16 AM »
Actually, one more thing. Hunter2, you said that the naming was right because of the electronegativities of the elements involved. Was the logic I was using incorrect?

Thanks

Jason

Offline Hunter2

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Re: First point of difference rule
« Reply #5 on: July 19, 2022, 01:51:29 AM »
The logic is half correct. The naming numbers should be low as possible that is correct, but counting from right or left doesnt matter, because you can turn the molecule. By accident you choose the right naming.

Offline jason.ridler

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Re: First point of difference rule
« Reply #6 on: July 19, 2022, 07:05:17 PM »
Hmmm, I'm still a bit confused. Can you have a look at fledarmus's reply in this post: https://www.chemicalforums.com/index.php?topic=58973.0

Is he correct? It was his logic that I was using for the naming of that molecule.

Offline Borek

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Re: First point of difference rule
« Reply #7 on: July 20, 2022, 02:25:04 AM »
Nothing wrong with your approach.
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Offline jason.ridler

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Re: First point of difference rule
« Reply #8 on: July 20, 2022, 08:41:10 PM »
Great! Thanks Borek. I'm glad to know that I'm on the right track :)

Offline hollytara

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Re: First point of difference rule
« Reply #9 on: July 25, 2022, 08:19:02 PM »
Here is an interesting exercise - run these through your IUPAC namer:

CHF2 - CH2 - CH2 - CHCl2       CHF2 - CH2 - CH2 - CHBr2       CHF2 - CH2 - CH2 - CHI2       

You get these names:  1,1-dichloro-4,4-difluorobutane, 1,1-dibromo-4,4-difluorobutane, and
1,1-difluoro-4,4-di-iodobutane. 

IUPAC picks the priority for 1 vs 4 in this case by ALPHABETICIZATION.  Halogens are all equivalent - we don't worry about atomic mass, atomic number, electronegativity... just first letter of the name!  The one that comes first in the alphabet is 1 the one that comes second is 4. 

(for another example - make a cyclohexane with CF2  CCl2 and CBr2 at 1,3, and 5.  The naming is 1,1-dibromo-3,3-dichloro-5,5-difluorocyclohexane.  All alphabetical. 

This is different than the priority rules for R/S or E/Z where atomic number and mass can be used. 

But if we put a second substituent on one of the inner carbons - the first point of difference rule kicks in and whatever end has the substituent close to it is numbered "1" - no matter what is on it!

Alkyl, alkoxy, and halo substituents have no preferences or priorities for numbering.  If there is no first point of difference it goes by alphabetical.


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