SolutionsStructure and Nomenclature
1. General formula
Write the molecular formulas of cyclopentane, methylcyclohexane and decalin (two fused six-membered rings). How many hydrogens does the formula lose for each ring?
Cyclopentane: C5H10; methylcyclohexane: C7H14; decalin: C10H18. Each ring removes two hydrogens from the formula of the acyclic alkane: CnH2n+2−2r, with r rings.
2. Name these cycloalkanes
- A cyclohexane bearing an ethyl group
- A cyclopentane with two methyl groups on adjacent carbons
- A cyclohexane with an ethyl and a methyl group at positions 1 and 3
- A cyclobutane bearing an isopropyl group
- Ethylcyclohexane
- 1,2-Dimethylcyclopentane
- 1-Ethyl-3-methylcyclohexane (substituents are cited in alphabetical order and the first one gets number 1)
- Isopropylcyclobutane (propan-2-ylcyclobutane)
3. Cis/trans isomerism
Which of these compounds show geometrical isomerism?
- 1,1-Dimethylcyclohexane
- 1,2-Dimethylcyclopropane
- 1,4-Dimethylcyclohexane
- Methylcyclopentane
- No: both methyl groups are on the same carbon.
- Yes: cis and trans.
- Yes: cis and trans.
- No: it has only one substituent.
4. Stereoisomers of 1,2-dimethylcyclopropane
How many stereoisomers does 1,2-dimethylcyclopropane have? Is any of them chiral?
Three: the cis isomer, which has a plane of symmetry and is a meso (achiral) form, and the pair of trans enantiomers (1R,2R) and (1S,2S), which are chiral.