Naming CFCs (1 Viewer)

[mushroom_head]

a while ago, the teacher tried to confuse our whole class with this section of the module. When u name CFCs, do u name them alphabetically or according to its electronegativity?? Cuz my teacher was talking about the electronegativity but my tutor told me to ignore it and just name it according to the alphabet.

 

[Xayma]

CFC's naming is as follows:

The highest electronegative element is given preference in NUMBERING

When writing it out however, you write them out alphabetically.

Eg You can have 2-choloro-1-Fluro Ethane
but you cant have 1-choloro-2-Fluro Ethane

 

[Calculon]

Originally posted by Xayma
CFC's naming is as follows:

The highest electronegative element is given preference in NUMBERING

When writing it out however, you write them out alphabetically.

Eg You can have 2-choloro-1-Fluro Ethane
but you cant have 1-choloro-2-Fluro Ethane

I disagree with you here. As incompetent as my chemistry teacher is, hes an HSC marker and he says numbering is in alphabetical order.

 

[hipsta_jess]

we got taught to do it alphabetically.

 

[CM_Tutor]

I will check with the bible of organic chemical naming, and get back to you with the definitive answer.

 

[Xayma]

Well looking through http://www.acdlabs.com (which has the naming conventions with permission from IUPAC) the only thing close to it would be an example which had

1, Bromo-2, Chloroethane

 

[CM_Tutor]

I'm not saying that anyone is right or wrong, as I don't remember the rule, but I would point out that 1-bromo-2-chloroethane could be assigning locant priority to the bromine atom on the basis of (a) alphabetical priority, or (b) atomic number priority - which would lead to different answers for naming ClCH₂CH₂I, as Cl has alphabetical priority, but I has atomic number priority.

If, as was suggested above, we go by electronegativities, then 1-bromo-2-chloroethane should actually give the Cl priority, and thus be 2-bromo-1-chloroethane.

The problem becomes more complex with more substituents. For example, suppose we go by alphabetical priority, and thus have ClCH₂CH₂Br as 1-bromo-2-chloroethane. What do we then do with Cl₃CCH₂Br? The lowest set of locants rule surely gives priority to the 3 x Cl's over the 1 x Br, giving 2-bromo-1,1,1-trichloroethane.

As I said, I'll check the rule book, but that won't be until some time next week.

 

[TheKing]

When naming haloalkanes and CFCs, if the usual rule leads to two possible numbering systems then the lowest number is given to the more electronegative halogen.

The usual rule being all that crap about have the lowest number and yea... The bit about having it in alphabetical order still applies i'm just saying that the numbering is done according to above in that situation.

thats what i think anyway, or rather what i've been taught.
hope i helped but really people are just getting too picky

 

[mitochondria]

I really do not think that electronegativity has anything to do with naming alkyl halides. I just had a look at my organic chemistry text book for alkyl halide naming, the following are simplified steps of what described in the book (Organic Chemistry 6e):

1. numbering starts at the end closer to the first substituent ("regardless of whether it is alkyl or halo")

2. when writing the name, list the halogen substituents in alphabetical order

3. (this applies to the ethanes we are discussing) if the numbering can be properly done using the first step, "begin at the end nearer the substituent (either alkyl or halo) that has alphabetical precedence"

Thus, 2-choloro-1-fluroethane is invalid and should be 1-chloro-2-fluroethane.

This also solves the problem posted by CM_Tutor: Cl₃CCH₂Br. According to this naming system, the name for this alkyl halide will be 1-bromo-2,2,2-trichloroethane.

Just want to point out a problem (as the book is obscure on this), we know that we use the di-, tri-, tetra-... etc. prefixes when there are more than one of the same kind of substituent, do we take the prefixes into account when we are writing the names in alphabetical order? For example, should CHF₂CH₂Cl be 1,1-difluoro-2-chloroethane or 2-chloro-1,1-difluoroethane?

 

[Xayma]

Thanks for that, btw the sub are html scribts not vB code so you have to use <> instead of []

 

[CM_Tutor]

Mitochondria, I will respond to the substance of this thread once I have checked the actual answer, but in the meantime, I have some thoughts:

1. These rules are very over-simplified, as the numbering of a chain is usually not determined by the substituents - for example Cl₃C-CCl₂-CH₂-CH₂-CH₂-CH₂-CH(OH)-CH₂-CH₃ is numbered from the RHS, and is
8,8,9,9,9-pentachloro-3-nonanol.

2. All substituents are listed in alphabetical order, this is not merely a case for halogens.

3. I have seen suggestions of alphabetical, electronegativity and atomic number preference for a case like ClCH₂CH₂F. Alphabetical and atomic number priority would give 1-chloro-2-fluoroethane, electronegativity preference would give 2-chloro-1-fluoroethane. This is why I said I would check the actual rulebook. In either case, the chloro will appear before the fluoro in the name (for alphabetical reasons), and the fluoro will be spelt with two o's.

This also solves the problem posted by CM_Tutor: Cl₃CCH₂Br. According to this naming system, the name for this alkyl halide will be 1-bromo-2,2,2-trichloroethane.

This might be your opinion, but it is incorrect. The lowest overall set of locants applies here, and the name is properly 2-bromo-1,1,1-trichloroethane.

Just want to point out a problem (as the book is obscure on this), we know that we use the di-, tri-, tetra-... etc. prefixes when there are more than one of the same kind of substituent, do we take the prefixes into account when we are writing the names in alphabetical order? For example, should CHF₂CH₂Cl be 1,1-difluoro-2-chloroethane or 2-chloro-1,1-difluoroethane?

When listing substituents alphabetically, multiplying prefixes are always ignored. So, chloro is alphabetised as 'c' whether it is chloro, dichloro, trichloro, etc. In the case you give, it wouldn't matter because difluoro would come after chloro whether you alphabetised it as 'd' or 'f', but in any event, CHF₂CH₂Cl is unambiguously
2-chloro-1,1-difluoroethane, with locant set rules determining that the F's are bound to carbon # 1.

For a case like ICH₂-CHF-CH₂I, the name would be 2-fluoro-1,3-diiodopropane, as the 'f' of fluoro is alphabetised before the 'i' of diiodo.

 

[edd91]

CM tutor,
"The lowest overall set of locants applies here,"
what do you meen by that

 

[CM_Tutor]

Edd91, suppose you had the compound CH₃-CH(CH₃)-CH₂-C(CH₃)₂-CH₃. Would you name it numbering from the LHS, ie 2,4,4-trimethylpentane, or from the RHS, ie 2,2,4-trimethylpentane? The correct name is the second of these, as the locant set 2,2,4 is lower than the locant set 2,4,4.

In the case I mentioned above, Cl₃C-CH₂Br, putting carbon # 1 on the LHS gives a locant set 1,1,1,2, whereas placing carbon # 1 on the RHS gives a locant set 1,2,2,2 - the lower set of locants dictates that the name is
2-bromo-1,1,1-trichloroethane.

The topic at issue in this thread, with compounds like FCH₂CH₂Br and BrCH₂CH₂I, is what to do in cases where neither a principal functional group, nor a locant set, is able to determine the numbering of the chain.

If we go by alphabetical determination, these compounds are 1-bromo-2-fluoroethane and 1-bromo-2-iodoethane, respectively.

If we go by electronegativities, they are 2-bromo-1-fluoroethane and 1-bromo-2-iodoethane, respectively.

If we go by atomic numbers, with highest atomic numbers having priority, they are 1-bromo-2-fluoroethane and
2-bromo-1-iodoethane, respectively.

If we go by atomic numbers, with lowest atomic numbers having priority, they are 2-bromo-1-fluoroethane and
1-bromo-2-iodoethane, respectively.

 

[CM_Tutor]

I have checked the rules. The answer is that, when locant sets are the same, alphabetical priority is used to give the lowest possible locant to the alphabetically first substituent. So,

FCH₂CH₂Br is 1-bromo-2-fluoroethane

BrCH₂CH₂I is 1-bromo-2-iodoethane

ClCH₂CH₂F is 1-chloro-2-fluoroethane

I can post the actual references, should anyone want to know them.

 

[mushroom_head]

so u know if u had : Cl2FC-CClF2

would u name it as 1,2,2-trichloro-1,1,2-trifluoroethane as it says in chemistry pathways 2

or would u name it as 1,1,2-trichloro-1,2,2-trifluroethane as i thought it was??
i dunno cuz i thought u also name it from the lowest number to highest?

 

[edd91]

I'd say 1,1,2-trichloro-1,2,2-trifluroethane ^^ from CM_tutors post

 

[CM_Tutor]

so u know if u had : Cl2FC-CClF2

would u name it as 1,2,2-trichloro-1,1,2-trifluoroethane as it says in chemistry pathways 2

or would u name it as 1,1,2-trichloro-1,2,2-trifluroethane as i thought it was??
i dunno cuz i thought u also name it from the lowest number to highest?

You would name it as 1,1,2-trichloro-1,2,2-trifluoroethane, as edd91 has said.

Reason: There is no PFG present, so that will not determine numbering.
There is only one possible main chain, and it is unbranched, so no help there.
No matter which way you number the chain, you get the locant set 1,1,1,2,2,2, so that doesn't help.
So, you must look to alphabetical priority. Chloro ('c') has a higher priority than fluoro ('f'), so look at the chloro substiuents. Numbering from LHS (as written) gives 1,1,2, from RHS gives 1,2,2. The 1,1,2 numbering system is the lower set, and so is chosen.

Thus, 1,1,2-trichloro-1,2,2-trifluoroethane.

BTW, when looking at textbooks, remember that they are written by people, and people make mistakes. In this case, the same example appears on p. 243 of Smith's Conquering Chemistry, but named with the correct numbering, showing that (in this case) one of the text books MUST be wrong

 

[mushroom_head]

woohoo! so i was right after all.
so just to make things clear.... do we just ignore the whole electronegativity thing?
i don't think they'd give u some ambiguous question like that in the HSC would they?

 

[CM_Tutor]

woohoo! so i was right after all.
so just to make things clear.... do we just ignore the whole electronegativity thing?

Yes, you can ignore it, as it is not how the rules are written.

i don't think they'd give u some ambiguous question like that in the HSC would they?

They probably wouldn't mean it to be ambiguous, but it might be anyway. Eg. Draw and name an isomer of C₂H₄ClF. There is no ambiguity if you choose to draw FClHC-CH₃, ie. 1-chloro-1-fluoroethane, but there might be ambiguity if you chose FCH₂-CH₂Cl, ie. 1-chloro-2-fluoroethane.

By the way, if they asked this question, I suspect that they would accept 2-chloro-1-fluoroethane for the second one (even though its wrong, as it is an unambiguous name, and it is also an obscure rule), but I doubt they'd accept 2-chloro-2-fluoroethane for the first one.

 

[mushroom_head]

ok thanks CM_Tutor! Now i really do get it...finally!