The IUPAC name for flouthane is 2-bromo-2-chloro-1,1,1-triflouroethane. i am confused why did not we assign the numbering to the carbon chain the other way possible that would make the name as 1-bromo-1-chloro-2,2,2-triflouroethane? Why would this answer be wrong?
From Section P 14-3.5 of the Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013, IUPAC Blue book, we have,
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; for example, the locant set ‘2,3,5,8’ is lower than ‘3,4,6,8’ and ‘2,4,5,7’.
Because of this rule, comparing the two options of locants we have,
- 1-bromo-1-chloro-2,2,2-trifluoroethane [1,1,2,2,2]
- 2-bromo-2-chloro-1,1,1-trifluoroethane [1,1,1,2,2]
The first point of difference between these two sets of locants is the third locant value. Now, we take the lower value of the two. Therefore, option 2 is the preferred IUPAC name.
Therefore the PIN of the compound becomes 2-bromo-2-chloro-1,1,1-trifluoroethane.