Haloalkane naming question (1 Viewer)

[porcupinetree]

https://imgur.com/rVbvxpi

My teacher says that for haloalkanes, we should number the carbons so that the first alphabetical halogen (in this case, bromine) has priority, rather than numbering them in a way so that the all of the halogens have equal priority in the numbering process. However, I've read some online pages which seem to suggest the opposite. Either way, when it comes to actually naming, we should put the first alphabetical one first (I think?), which means options B and D are out.

So which is the correct way to order the carbons?

Btw, my teacher said that this method of numbering only occurs when dealing with halogens - if there was, for example, carboxyl functional groups, it would be different.

 

[Crisium]

https://imgur.com/rVbvxpi

My teacher says that for haloalkanes, we should number the carbons so that the first alphabetical halogen (in this case, bromine) has priority, rather than numbering them in a way so that the all of the halogens have equal priority in the numbering process. However, I've read some online pages which seem to suggest the opposite. Either way, when it comes to actually naming, we should put the first alphabetical one first (I think?), which means options B and D are out.

So which is the correct way to order the carbons?

Btw, my teacher said that this method of numbering only occurs when dealing with halogens - if there was, for example, carboxyl functional groups, it would be different.

I came across this question when working through PoM in the excel success one HSC past papers book and when I referred to the front of the book (It lists questions from each paper by topic) it wasn't under PoM, so I guess it comes in a different module?

Wait a minute

I just checked and yeah it seems like it comes under chemical monitoring and management (unless you've done that and I derped really hard)

 

[porcupinetree]

I came across this question when working through PoM in the excel success one HSC past papers book and when I referred to the front of the book (It lists questions from each paper by topic) it wasn't under PoM, so I guess it comes in a different module?

Wait a minute

I just checked and yeah it seems like it comes under chemical monitoring and management (unless you've done that and I derped really hard)

Nah haha we haven't done that yet. That's interesting though; I naturally doubt that it would come from CM&M, as it was in a PoM test and is concerned with the naming of Haloalkanes. Either I'm wrong and it is part of CM&M, or your book has a typo (which I also naturally doubt)

 

[Queenroot]

A)

 

[jordankime]

So this will definitely come up in Chemical Monitoring & Management, although you sort of get an introduction to it in Production of Materials. The correct method is as follows:
1. Name the halogens alphabetically
2. Number the halogens alphabetically as well (unless they are symmetrical, then number from most electronegative to least).

For your question, that would mean C is the correct answer.


I strongly suggest you read through (and even print out) this document released by the RACI to NSW HSC chemistry teachers specifically addressing the widespread misunderstanding in the HSC course. There are some good examples in there that I hope will clear up any grey areas.

http://www.raci.org.au/document/item/1012


It is important to note that the BoS have broken these rules in a multiple choice question before where they made you name an isomer (I can't remember the year). The only option that suited did not follow this convention and the question was widely criticised for that. They have never put the correct and incorrect methods of numbering against each other in a multiple choice as what you have shown though. I doubt they ever would as it would be pretty hypocritical to accept the incorrect method one year, then go back on it for the next group.


Hope that clears things up