Naming CFCs (1 Viewer)

[Eagles]

um.. this topic seems to be finished.. but on the hsc enrichment day, I asked the lecturer and she said, name the more electro negative one first. -.-

 

[CM_Tutor]

um.. this topic seems to be finished.. but on the hsc enrichment day, I asked the lecturer and she said, name the more electro negative one first. -.-

She was wrong. I have checked with the rules published by IUPAC.

 

[mojako]

She was wrong. I have checked with the rules published by IUPAC.

what my teacher says is in line with her (the lecturer)...
suppose u have a straight chain (just for simplicity) and the total of all numbers is the same whether you number from the left or right, then
the most electronegative part has numbering priority
but, when you write the whole molecule you write it alphabetically
So:
1. does the IUPAC rule specifically say to ignore electronegativity (or is electronegativity just not mentioned)?
2. umm... this is haarrddd... which naming system do the majority of HSC markers go with?

 

[mushroom_head]

Name it alphabetically. If CM_tutor says it's right, then it's gotta be right!

 

[Xayma]

Name it alphabetically. If CM_tutor says it's right, then it's gotta be right!

No name it by the lowest set of locants first.
If locant sets are the same then you go alphabetically.

Eg. CFH-CH₂-CClH-CH

Will be 3-Chloro-1-Fluoro-Butane.
As 3,1 has a lower loctant then 2,4 even though 2,4 would be the numbering if it went purely by alphabetical numbering.

 

[mojako]

No name it by the lowest set of locants first.
If locant sets are the same then you go alphabetically.

Eg. CFH-CH₂-CClH-CH

Will be 3-Chloro-1-Fluoro-Butane.
As 3,1 has a lower loctant then 2,4 even though 2,4 would be the numbering if it went purely by alphabetical numbering.

u sure it's not by electronegativity? [if the locant sets are the same]
[ so CFH-CH2-CHCl is
3-chloro-1-fluoro-propane
and NOT 1-chloro-3-fluoro-propane ]

 

[Xayma]

No if the sets of locants are the same, then it goes alphabetically, see: CM_Tutor's post, he's a PhD student so he knows his stuff and has the textbooks.

 

[CM_Tutor]

what my teacher says is in line with her (the lecturer)...
suppose u have a straight chain (just for simplicity) and the total of all numbers is the same whether you number from the left or right, then
the most electronegative part has numbering priority
but, when you write the whole molecule you write it alphabetically
So:
1. does the IUPAC rule specifically say to ignore electronegativity (or is electronegativity just not mentioned)?
2. umm... this is haarrddd... which naming system do the majority of HSC markers go with?

First, let's address the question of authority for these rules.

The rules I am using are the rules published by IUPAC. Specifically, these come from the Commision on Nomenclature of Organic Chemistry, writing under the authority of the Organic Chemistry Division of the International Union of Pure and Applied Chemistry. The quotes below come from the book:

Nomenclature of Organic Chemistry: Sections A, B, C, D, E, F and H 1979 Edition

The book has not been recently updated. The copy I am looking at has 559 pages, including the index.

In the general principles, the book states that "[t]he rules now presented are intended to be suitable for textboks, journals and patents, for lexicons and similar compliations, and for indexes". The organisation CAS (Chemical Abstracts Service), which is a division of the American Chemical Society, uses these rules when adding new substances to its registry - this registry is the largest substance identification system in existence, listing over 23 million organic and inorganic substances.

What I am saying is that there is no room for debate as to whether these are the correct rules.

However (didn't there just have to be a however), most chemists do NOT know these rules in great detail, and frequent errors are made in naming. For the most part, these are ignored, provided the (technically incorrect) name used is unambiguous and inoffensive to basic rules. For example, for the substance:

FCH₂-CH₂-CH₂Cl

the correct name is 1-chloro-3-fluoropropane. The alternative name, 3-chloro-1-fluoropropane, is technically incorrect, for reasons I will illustrate below. However, the alternate name is unambiguous and inoffensive, and likely to be tolerated. For a case like:

ICH₂-CH₂-CHBr-CH₃

the correct name (using locant sets, as Xayma has mentioned), is 3-bromo-1-iodobutane. The name 2-bromo-4-iodobutane - which is produced from either electronegative or alphabetic priority - is unacceptable. The reason is that, althougth it is unambiguous, it is offensive in that it violates the basic rule about locant sets.

It should also be noted that common usage within some groups tolerates incorrect names. For example, consider the ester:

CH₃-C(=O)-O-CH(CH₃)₂

Common HSC usage would name this as 2-propyl ethanoate. This name is incorrect by IUPAC rules, and would be considered offensive by some. However, it is accepted practice in the HSC.

So, we have established that some incorrect names will be tolerated (in pactice), and some will not. "What do I do in an exam situation?", I hear you thinking. Hear is my advice.

School Exams: If your teacher has major issues related to electronegativity, then humour them in the exams, for cases like FCH₂-CH₂-CH₂Cl. But, I would make sure I followed locant set rules

HSC: I suspect that either would be accepted for FCH₂-CH₂-CH₂Cl. I suspect that only 3-bromo-1-iodobutane would be accepted for ICH₂-CH₂-CHBr-CH₃. I will try to confirm this with a colleague of mine, and will post more if it becomes appropriate.

Now, what about the rules themselves? The rules that bear directly on this problem are Rules C-13.1 and C-102.

"Rule C-102
102.1 - Substitutive names of halogen derivatives are formed by the addition of prefixes 'fluoro-', 'chloro-', 'bromo-', or 'iodo-' to the name of the parent compound."

This rule then offers some examples, one of which is 1-bromo-4-chlorobenzene. This example clearly shows that the numbering priority has been given to the bromine rather than the chlorine, whcih could not be the case if electronegativites were the determining factor.

To determine how locants are assigned, we look to Rule C-13.1. This rule deals primarily with the identification of the main chain, but does also deal with locants:

"Rule C-13.1
13.11 - In an acyclic compound, that chain upon which the nomenclature and numbering are based is called the 'principal chain'. When in an acyclic compound there is a choice for principal chain, the following criteria are applied successively, in the order listed, until a decision is reached:

(a) ..."

it goes on to list a whole bunch of criteria that are not relevant to this discussion, finally getting to:

"(i) lowest locants for all substituents in the principal chain cited as prefixes"

(It comments that "when series of locants containing the same number of terms are compared term by term, that series is 'lowest' which contains the lowest number on the occasion of the first difference".) This rule governs the lowest locant set mentioned above - the priority for the PFG is covered earlier. So:

F₃C-CH₂-CH(-OH)-CH₃ is 4,4,4-trifluoro-2-butanol as the PFG has priority

ICH₂-CH₂-CHBr-CH₃ is 3-bromo-1-iodobutane as the locant set 1, 3 is lower than the locant set 2, 4.

We continue:

"(j) the substituent cited first in alphabetical order"

This refers to principal chain selection, and the following example is given:

CH₂Cl
|
CH-CH₂-CH₂-CH₂-CH₂-CH₂-CHClBr
|
CH₂Br

1,8-dibromo-1-chloro-7-(chloromethyl)octane

In other words, since the substituents are located at 1, 1, 7, 8 no matter which octane chain is chosen, and no other prior part having made the determination (except to eliminate the propane chain on the grounds of length), the alphabetical priority of the bromine over chlorine determines which is the main (principal) chain, and treates the CH₂Cl as a substituent.

The final criteria is:

"(k) Lowest locants for the substituent cited first as prefix in alphabetical order"

The example offered here is fairly complex:

HOOC-CH₂-CH₂-CH(-CH₃)-CH(-NO₂)-CH₂-CH₂-COOH

which is 4-methyl-5-nitrooctanedioic acid. However, the point is straight forward. There is only one chain, the PFG's are 1 and 8 no matter what, so all we have to choose between is whether we put methyl as 4 and nitro as 5, or nitro as 4 and methyl as 5. The rule gives the priority based on the prefix cited first in alphabetical order - that is, the methyl over the nitro. Applying this rule, we get:

1-chloro-3-fluoropropane for F-CH₂-CH₂-CH₂-Cl, with the chlorine having priority for alphabetical reasons.

Now, electronegativities are not explicitly mentioned, but there is no need. The rules must be applied "in the order listed, until a decision is reached". Alphabetical priority is reached in section (k), and any electronegativity rule would come after it.

I hope this full answers this question. Anyone who is still unsure should post further questions.

 

[mojako]

Thanks heaps CM_Tutor ^_^
BTW I was just reading this part which confused me a bit...
"It should also be noted that common usage within some groups tolerates incorrect names. For example, consider the ester:

CH₃-C(=O)-O-CH(CH₃)₃"
and it's called "2-propyl ethanoate"

Is it meant to be CH₃-C(=O)-O-CH(CH₃)₂??

Anyway I did a bit of research on google and it seems that you're right.
There are a kind of amendments in 1993, if you go to here:
http://www.acdlabs.com/iupac/nomenclature/
But the 1993 recommendation also numbers alphabetically, as in here:
http://acdlabs.com/iupac/nomenclature/93/r93_370.htm

But whether teachers, and HSC markers, know this rule, is another issue

EDIT: I also noticed how they use capital at the start, eg 1-Bromo-2-chloroethane. Some other google results with keyword "IUPAC rules" also indicate the use of capitals.

 

[CM_Tutor]

Thanks heaps CM_Tutor ^_^
BTW I was just reading this part which confused me a bit...
"It should also be noted that common usage within some groups tolerates incorrect names. For example, consider the ester:

CH₃-C(=O)-O-CH(CH₃)₃"
and it's called "2-propyl ethanoate"

Is it meant to be CH₃-C(=O)-O-CH(CH₃)₂??

Yes, it should be (CH₃)₂ at the end, I have edited my original post to correct this typo.

Anyway I did a bit of research on google and it seems that you're right.
There are a kind of amendments in 1993, if you go to here:
http://www.acdlabs.com/iupac/nomenclature/
But the 1993 recommendation also numbers alphabetically, as in here:
http://acdlabs.com/iupac/nomenclature/93/r93_370.htm

I didn't know about the ammendments, but such changes usually only add new rules for new situations, and correct minor errors. They don't rewrite codified rules, so I would've been surprised if the rules had changed. I'll have a look at the sites when I get a chance. Thanks for the url's.

But whether teachers, and HSC markers, know this rule, is another issue

True - as I said above, I'll try to post more on this point if it becomes possible.

EDIT: I also noticed how they use capital at the start, eg 1-Bromo-2-chloroethane. Some other google results with keyword "IUPAC rules" also indicate the use of capitals.

Capitals are used at the start of names if they start a sentence. Ask yourself "If the word was 'the', would I put a capial letter?" If you would, then capitalise the compound. For example:

2,4,6-Trinitrotoluene is a useful indicator, but should bot be given to HSC students.

but

The ozone layer can be damaged by compounds other than CFCs, and 1,1,1-trichloroethane was banned because of its role in ozone destruction.

When a name satnds alone, most Chemists would not capitalise.

 

[mushroom_head]

i have another question:

what are locants??

 

[Xayma]

Locants are the numbers of where each atom is located.

Eg

CH₃-CCl₂-CH₃

The Cl atoms have locants of 2,2 since they are the in the middle carbon atom.

Since you could read from either side of the chain you may end up with two different sets of possible locants

Eg:
CH₃-CCl₂H

Has possible locants 2,2 or 1,1.

However, when naming you take the lowest possible locant set in this case 1,1 as 1+1<2+2

EDIT: Fixing up number of bonds, thanks CM_Tutor

 

[CM_Tutor]

Mushroom_head - as Xayma has indicated, locants are the numbers that are used to locate features on molecules.

Xayma, you mean CH₃-CCl₂-CH₃, not CH₃-CCl₂H-CH₃, as the second one has a central carbon with 5 bonds.

Also, with CH₃-CCl₂H, or with any case dealing with locants, the way to determine which is correct when no other rule intervenes should not involve summing them.

As I posted above from IUPAC, "when series of locants containing the same number of terms are compared term by term, that series is 'lowest' which contains the lowest number on the occasion of the first difference". In other words, 1, 1 is lower than 2, 2, because on the occasion of the first difference (the first term), the first sequence has the lower number. Similarly, if these were possible, 1, 1, 3, 6, 9 would be lower than 1, 1, 4, 5, 5, as the first sequence has the lower term (3 < 4) on the occasion of the first difference (3rd term).

(By some other rule intervening, I mean cases like CH₃-CH(OH)-CH₂-CH₂-CH₂-CCl₃, which is 6,6,6-trichloro-2-hexanol, even though a lower set (1, 1, 1, 5) is possible, as the PFG gets priority).

 

[mushroom_head]

ohhh...i get you! thanx~ i didn't know those numbers were called locants

 

[CM_Tutor]

Someone, who wishes to soon become anonymous, would like the following quote recorded:

my teacher asked the Board of Studies and they said both alphabetical and electronegativity-cal numberings are accepted...
but i suppose if it somehow goes into multiple choice you should choose the alphabetical.. just my personal opinion... coz in chem questions that ive seen (which are not many, actually), electronegativity is ignored

I'll soon delete this post to ensure my anonymity... so if some1 read this pls say in your reply that somebody has confirmed with the Board of Studies that......
thanks.

 

[mojako]

haha... that becomes quite funny actually...
I suppose the original poster doesn't want him/herself to be quoted.
well, he (or she) said that he doesn't want his friends to know that it's him posting. but if u quote that.. hmm.. consider how many ppl actually ask their teachers to ask the board of studies about that matter.. it's easy to identify.. I think
I guess saying that somebody has confirmed with the Board that it's ok to use either naming system would do, without saying anything else ^^

 

[Paroissien]

One question:
CH3-CCl2-CHBr-CH3

I would have named this 2-bromo-3,3-dichlorobutane, however I am wrong and it is 3-bromo-2,2-dichlorobutane as most of you can probably see.
Is this because 2+3+3 > 3+2+2 ? (if you follow me)

 

[mojako]

yes.
practically, the rule for numbering is:
1. give any functional group the lowest number. if there are more than one functional groups, some type of functional groups take priority over other types, but at the moment I can't think of an example that is in the HSC chemistry. hhmm.. actually there is... something like methyl hexanol.. the -ol takes priority (correct me if I'm wrong). generally the -ol or -ene take priority.
2. having number in such a way so that the total number is minimum.
3. if numbering from the right and from the left give the same total number, number alphabetically. (note: dichloro starts with c, not d)

The rule for writing the name haloalkanes is
write the name alphabetically (the lowest number doesn't have to be written first)

that's what I know...

BTW how do we name something like an octane with 1 methyl and 1 ethyl? which is written first and which gets numbering priority?

What about an octane with methyl and chlorine?

 

[CM_Tutor]

One question:
CH3-CCl2-CHBr-CH3

I would have named this 2-bromo-3,3-dichlorobutane, however I am wrong and it is 3-bromo-2,2-dichlorobutane as most of you can probably see.
Is this because 2+3+3 > 3+2+2 ? (if you follow me)

No - but you're close.

Quoting myself from the long post above regarding the IUPAC rules:

When series of locants containing the same number of terms are compared term by term, that series is 'lowest' which contains the lowest number on the occasion of the first difference.

In other words, the locant set numbering left to right is: 2, 2, 3
Numbering right to left, it is: 2, 3, 3

The occasion of the first difference is the second term, where L-->R is 2 and R-->L is 3.

As 2 < 3, the L-->R numbering is correct.

So, the name is 3-bromo-2,2-dichlorobutane.

 

[CM_Tutor]

BTW how do we name something like an octane with 1 methyl and 1 ethyl? which is written first and which gets numbering priority?

What about an octane with methyl and chlorine?

Mojako, I'm not sure what you are asking here. You refer to an octane with a 1-methyl and a 1-ethyl. Do you mean something like:

CH₃
|
CH-CH₂-CH₂-CH₂-CH₂-CH₂-CH₂-CH₃
|
CH₂-CH₃

If you do, then this would be named 3-methyldecane, as the '1-ethyl' is not a side chain, it is part of the main chain.

If not, then could you clarify?