Naming hydrocarbons (1 Viewer)

[horndog huang 2]

Can someone tell me why the ans is c and not a?
Thank you

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[ProdigyInspired]

I reckon its A.

There was a thread on this and no one really gave a clear answer. AFAIK the rule is to put the lowest number at the front.

If you look at Hurlstone's 2013 Trial you'll see the MC was corrected to be A.

In the thread some people have suggested in order of electronegativity. So since Fluorine has higher electronegativity it's named less.

idk anymore

 

[horndog huang 2]

ah ok, thanks for that! my teacher told us it was in order of weight, and as chlorine is heavier it is first , not sure tho

 

[Fizzy_Cyst]

Can someone tell me why the ans is c and not a?
Thank you

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Using current naming techniques, answer should be (a).

Both (a) and (c) could apply to the molecule and both produce the same value for the set of locants ( 3 + 5 = 5 + 3 ) and when this happens, RACI guidelines state that the lowest number is given to the halogen which is named first. Chloro is named first as naming is now done alphabetically, rather than by electronegativity.

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

Bottom of page 4, points 5 & 6.

 

[BlueGas]

These type of questions scream out "free marks". The way you go about answering these questions is by following two simple rules: Naming is done alphabetically and the numbering/positioning is done from the side closest to the first alphabetical atom. So in this case A B Chloro D E Floro, so you start with 3-chloro, and then you stay consistent and continue reading along to reach 5-fluoro.

 

[Queenroot]

Fluorine is more electronegative than Cl therefore gets higher priority, in IUPAC at least. Then we write in alphabetical order, therefore the answer is C.

 

[Fizzy_Cyst]

Fluorine is more electronegative than Cl therefore gets higher priority, in IUPAC at least. Then we write in alphabetical order, therefore the answer is C.

IUPAC does not use electronegativity for haloalkanes. Functional groups different story, but not haloalkanes.

IUPAC uses alphanumeric.

Please see the RACI guidelines I linked in a previous post.

In red writing at the very beginning

"IUPAC uses the alphanumeric nomenclature system. Older methods using electronegativity are NO LONGER VALID."

I think it's been this way for the last 5 years or so?

 

[Queenroot]

IUPAC does not use electronegativity for haloalkanes. Functional groups different story, but not haloalkanes.

IUPAC uses alphanumeric.

Please see the RACI guidelines I linked in a previous post.

In red writing at the very beginning

"IUPAC uses the alphanumeric nomenclature system. Older methods using electronegativity are NO LONGER VALID."

I think it's been this way for the last 5 years or so?

maybe the question paper is old then?

 

[Fizzy_Cyst]

Yeah, I'd say so! The 2012 HSC has a MC question based on the old naming system and hence there was no correct answer for it! That would have been written towards the end of 2011, so the change probably occurred around 2011/12. Papers before then regarding naming of haloalkanes would have invalid answers according to current naming system