Question

Why is the IUPAC name of the cyclic compound $3 -$ ethyl $- 1,1 -$ dimethylcyclohexane given instead of $1 -$ ethyl $- 3,3 -$ dimethylcyclohexane?

Answer 1

In cyclic compounds, the ring is usually considered the parent chain, unless it is attached to a longer chain of carbons; indicate a ring with the prefix “cyclo” before the root name. (When there are two longest chains of equal length, use the chain with the greater number of substituents.)

Complete answer:
A cyclic (ring) hydrocarbon is designated by the prefix cyclo- which appears directly in front of the base name. In summary, the name of the compound is written out with the substituents in alphabetical order followed by the base name (derived from the number of carbons in the parent chain). Determine the cycloalkane to use as the parent chain. The parent chain is the one with the highest number of carbon atoms. If there are two cycloalkanes, use the cycloalkane with the higher number of carbons as the parent chain.
In the IUPAC name we named a molecule by the smallest number of branches it had. For example with compound of $3 -$ ethyl $- 1,1 -$ dimethylcyclohexane has total number of branch: $3 + 1 + 1 = 5$ , and the $1 -$ ethyl $- 3,3 -$ dimethylcyclohexane has total number: $1 + 3 + 3 = 7$ . From this we could say that the first name has a smaller number and it will be the name of the compound.

Note:
Hydrocarbons having more than one ring are common, and are referred to as bicyclic (two rings), tricyclic (three rings) and in general, polycyclic compounds. The molecular formulas of such compounds have Hydrocarbons ratios that decrease with the number of rings. In general, for a hydrocarbon composed of $n$ carbon atoms associated with $r$ rings the formula is: ${C_n}{H_{(2n + 2 - 2r)}}$ . The structural relationship of rings in a polycyclic compound can vary. They may be separate and independent, or they may share one or two common atoms.