Naming compounds

[shefv]

How would you name this compound? Which alkyl group gets priority?

My chem book said that since they are all alkyl substituents, the priority here is based on alphabetical order so ethyl would get the priority. But the answer key gives 3 ethyl 1 methyl 5 propylcyclooctane as the answer.

Since the priority here based on alphabetical order or size of the substituents (i.e. methyl is smallest so it is on C1)?

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[amy_k]

for cycloalkanes the alkyl substituents should be chosen with the group that comes first in the alphabet as c1, in this case it would be ethyl, which then gives 1-ethyl, 3-methyl, 7-propyl.

BUT Priority is always given first to the lowest combination of substituent numbers, in this case 1-methyl 3-ethyl 5-propyl (1,3,5 is lower than 1,3,7), so the methyl group is chosen as c1.

 

[amy_k]

notice that the propyl group could have also been chosen as c1 to give 1,3,5, but p comes after m in alphabet so propyl is lower priority. The general rule would be, lowest combination of substituents first, then alphabetical order