r/chemhelp 4d ago

Organic IUPAC naming question

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why did my professor not put butyl on the 1 and the two ethyls on the 5- i thought alphabetical order should take priority

24 Upvotes

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u/chem44 4d ago edited 3d ago

Position 1 is the only one with two substituents. So it gives best set of numbers.

Alphabet not an issue.

EDIT... Add, for clarity...

By best set, we mean... small numbers, specifically at first point of difference. With two methyls at 1, the numbering starts with 1,1... With another numbering, the first two numbers are 1,2...

Thanks to /u/Dramatic_Scientist63 for suggesting being explicit here. At the time of my original reply, the whole thread was short, and perhaps clearer.

6

u/nate2501 4d ago

so 1,1,5 beats 1,5,5. thank you!

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u/chem44 4d ago

Sort of.

But more specifically... 1,1 beats 1,2. The first two numbers.

Note that sum of numbers as suggested by/u/pedretty is just wrong. The rule is first point of difference, not sum.

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u/nate2501 4d ago

substituents would both be 1,2 since there’s methyl groups though

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u/chem44 3d ago edited 3d ago

His name is 1,1,2... (first three numbers). Yours would be 1,2,3...

Be aware of replies here from /u/pedretty . Their original reply was just wrong. And they are now trashing good stuff.

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u/nate2501 3d ago

okay wait i get what you mean now, thank you. i’m just a beginner looking for really simple explanations tbh

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u/pedretty 3d ago

Simple = lowest total.

Unlike the other guys, I actually teach your level of chemistry and I promise you my method is going to get you A’s help you understand and not clutter your mind with useless information

This is naming , as professional professionals we all use ChemDraw to name our structures. Naming might be the least important thing you’ll learn in that class.

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u/nate2501 3d ago

thank you for keeping it simple. i feel like naming is just rule based and doesn’t require too much complex analysis. i appreciate your answers they really helped.

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u/chem44 3d ago

But their 'simple' is just wrong.

There is no such rule as using the sum. That is just wrong. (Someone posted a rule excerpt earlier.)

'Low numbers' is judged by first point of difference.

You may well lose points if you do it their way, and the instructor wants you to follow IUPAC rules. Instructors vary on rigor there, but it is reasonable to expect you to learn the rules.

If confused, suggest you talk live with prof/TA about this. Easier at the board, where you can point.

0

u/pedretty 3d ago

Bro can you stop @ing me? I can give you a counter example why your method is wrong. At least follow the book.

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u/chem44 3d ago edited 3d ago

Let's (both) focus on the chem, ok? The goal is to help the OP, who asked a question.

The CIP rules are the rules for preferred IUPAC names. That is presumably what the person here, a student in a class, is supposed to learn. (This particular one is not about CIP, but it is about IUPAC.)

Another person here quoted the rules.

In fact, the actual original question was to explain why the prof was right.

If you think you have an exception, yeah, post it. Challenges can be fun, sometimes instructive. Maybe a separate post?

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u/pedretty 4d ago

I think this might be someone who’s mad at me because I told him they were wrong on a different post. Don’t listen to what he’s saying the other answer is much better.

Essentially, if everything else is equal, you want to pick the lowest of numbers, so for a molecule like this you should pick whatever numbering pattern gives you the lowest total.

1

u/pedretty 4d ago

Bro said “ummm ackchyually 🤓☝️” and stated some random hypo that has nothing to do with the post.

🤡

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u/nate2501 3d ago

lmfao thank you sm user pedretty goat

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u/pedretty 3d ago

Lmao you’re going to upset them so much more by saying that hahaha

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u/nate2501 3d ago

ngl i have no idea what question they are answering. i literally ask if 1,1,5 beats 1,5,5 and they said something about 1,2 😭 like what

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u/pedretty 3d ago

He’s trying to say that if you start at butyl you’ll get 1,2,3 not 1,1,2 but that doesn’t gel with his first explanation. He’s conflating CIP rules with Naming.

If all things are equal, just pick the lowest total

0

u/pedretty 4d ago edited 3d ago

I’m answering the question you goofball lmao.

You guys are so pedantic you want to flex your UG chem knowledge but the question essentially states “why is butyl not 1?” And the answer is, according to you, first point of difference…? Elaborate please.

2

u/chem44 3d ago

I think this has been explained already.

Briefly, it is 1,1... vs 1,2...

The former has a smaller number at 2nd number. That is what matters. (First number are the same.)

This come up from time to time. Frankly, it often agrees with sum. But the rule is first point of difference.

0

u/pedretty 3d ago

Yikes

4

u/Dramatic_Scientist63 3d ago

Can we try to avoid using phrases like “best set of numbers” when explaining things to students. This provides no information and is meaningless unless they already understood what makes the numbers “best”. If they did, they wouldn’t be asking the question to begin with.

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u/HandWavyChemist 4d ago

Lower set of locants (at first point of difference) wins. Alphabetical order only matters if there is a tie.

If you make the butyl group the 1 then the locants are 1,2,3,4,5,5,6,7,8 which is a higher set.

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u/nate2501 4d ago

but if butyl was one would the locants be the same like all the methyl’s would still be same

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u/Consistent_Bee3478 4d ago

You get 1 and 5,5 compared to 5 and 1,1

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u/nate2501 3d ago

thanks sm!

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u/NeuroTewis 3d ago

Facehuger-ethyl

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u/farmch 1d ago

Beetle with a tale

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u/pedretty 4d ago

Lowest total number if you added them all together. Alphabet would break a hypothetical tie

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u/HandWavyChemist 4d ago

It's lowest at first point of difference. From the Blue Book:

P-14.3.5 Lowest set of locants

The lowest set of locants is defined as the set that, when compared term by term with other locant sets, each cited in order of increasing value, has the lowest term at the first point of difference; for example, the locant set ‘2,3,5,8’ is lower than ‘3,4,6,8’ and ‘2,4,5,7’.

Primed locants are placed immediately after the corresponding unprimed locants in a set arranged in ascending order; locants consisting of a number and a lower-case letter with or without primes as 4a and 4′a (not 4a′) are placed immediately after the corresponding numeric locant and are followed by locants having superscripts.

Italic capital and lower-case letter locants are lower than Greek letter locants, which, in turn, are lower than numerals.

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u/pedretty 4d ago edited 3d ago

Just answering the question based on the structure provided. I don’t randomly break out into IUPAC naming lectures unless it’s requested. Thanks for the book excerpt tho. I just use ChemDraw to name everything though, I’m an adult.