Naming Cycloalkanes Homework: Confused on Answer

[cathy]

Homework Statement

On the attachment is the answer to one of my questions written next to it as the instructor provided.
But I am confused. I understand the first common naming answer, however the second, shouldn't it be 1 methyl, rather than 4,
and then 4-(1-methylethyl)? Because with 2 substituents you label in alphabetical order. Am I missing some information that would allow me to label it as my instructor did?
Please advise if you can.

Thank you in advance.

 

[DrClaude]

shouldnt it be 1 methyl, rather than 4,
and then 4-(1-methylethyl)? Because with 2 substituents you label in alphabetical order.

The methylethyl group is a chain of three carbon atoms, so it has precedence over the methyl with one carbon. Therefore, in terms of numbering, it is 1-(1-methylethyl) and 4-methyl. It is for the same reason that it is the isopropyl that gets numbered 1 (the numbering is independent of how you choose to name the group, isopropyl vs 1-methylethyl).

Note that in the full name, alphabetic order is used:

trans-1-isopropyl-4-methylcycloheptane

trans-4-methyl-1-(1-methyethyl)cycloheptane