Why can't we write the compound 5-ethyl-4-methyloctane as 4-ethyl-5-methyloctane?
Asked
Active
Viewed 311 times
0
-
2Are you sure that you don’t want to ask the other way round? “4-ethyl-5-methyloctane” is a correct systematic name while “5-ethyl-4-methyloctane” is not in accordance with IUPAC nomenclature. – Jun 23 '16 at 17:23
1 Answers
4
As pointed out by Loong, the name you prefer is the correct name: 4-ethyl-5-methyloctane.
Generally we try to do the following:
- Assign the smallest locant possible to the carbon bearing the highest priority functional gorup.
- Assign locants to generate the lowest possible sum of the locants.
Often it is straightforward. In this case, both assignments of generate locants of 4 and 5, which sum to 9. Both place an alkyl substituent at the lower locant.
In this case, the locants are assigned so that the alphabetically first of the two substituents of equivalent priorty receives the smallest locant.
Correct - 4-ethyl-5-methyloxctane: ethyl is alphabetically before methyl and should get a smaller locant
Incorrent - 5-ethyl-4-methyloctane: ethyl is alphabetically before methyl and should get a smaller locant
Ben Norris
- 42,831
- 8
- 123
- 181