Came across the following question. The answer only says that option 3 is incorrect which is I agree to but dont you think option 4 is also incorrect. As per IUPAC rules isnt it the Halide that should get lowest possible number (in this case number 3 instead of 4) or is it both are same ? I am confused please help.
Both names, ‘4-bromo-2,4-dimethylhexane’ and ‘3-bromo-3,5-dimethylhexane’ are unambiguous and describe the same compound. However, only ‘4-bromo-2,4-dimethylhexane’ is in accordance with IUPAC recommendations.
First of all, the seniority order of functional classes, which is used to choose the functional class (here: …hexane), does not apply to numbering of locants within the same compound class.
Concerning numbering of locants within the same compound class, the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) reads as follows:
When several structural features appear in cyclic and acyclic compounds, low locants are assigned to them in the following decreasing order of seniority:
(f) detachable alphabetized prefixes, all considered together in a series of increasing numerical order;
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; (…)
Therefore, the example is correctly named as 4-bromo-2,4-dimethylhexane rather than 3-bromo-3,5-dimethylhexane since the locant set ‘2,4,4’ is lower than ‘3,3,5’.
According to the IUPAC organic chemistry nomenclature, alkyl groups take precedence over halide groups so (4) is correct.
Similarly, C=C double bonds also take precedence over halides so (3) is incorrect and should be 2-bromoprop-1-ene.
(2) is also incorrect as there are only five carbons in the chain not six so the correct name is 4-oxopentanoic acid.
A table of functional group priorities can be found here.
Acc. to IUPAC we have to give lowest locant no. to side chain which in this case is 2-Bromo-2,4-dimethyl. Another reason of this ans. being correct is that we take halides as a normal side chain and it is not given priority over methyl, we just only see lowest locant in this case.