91Ó°ÊÓ

Cis configuration is when two groups are on the same face of the ring, and trans configuration is when the groups are on opposite faces.Both isomers are stable, and neither can be converted into the other without breaking and reforming chemical bonds.

Count the number of carbon atoms in the ring and the number in the largest substituent. Whichever has the highest carbon atom will be considered the parent. If the number of carbon atoms in the ring is equal to the number in the substituent, in that case, the ring is taken as the parent.

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For an alkyl- or halo-substituted cycloalkane, choose a point of attachment as carbon one and number the substituents on the ring so that the secondsubstituent has as low a number as possible. The same procedure for the third or fourth substituent has a low number as much as possible until a point of difference is found.

• The ring has five carbon atoms.
• Two methyl groups are present.
• Two methyl groups are cis here as both are in the same plane.
• Hence, the name of the compound is$\mathrm{cis}-1,2-\mathrm{Dimethylcyclopentane}$