SolutionsLarger and Polycyclic Cycloalkanes
1. Count the rings
Using the formula CnH2n+2−2r, calculate how many rings cubane (C8H8), bicyclo[1.1.0]butane (C4H6) and dodecahedrane (C20H20) have.
Cubane: 18 − 2r = 8 → r = 5. Bicyclo[1.1.0]butane: 10 − 2r = 6 → r = 2. Dodecahedrane: 42 − 2r = 20 → r = 11.
2. Naming bicycles
What do the numbers in brackets mean in bicyclo[2.2.1]heptane? What is its molecular formula?
They are the number of carbons in each of the three bridges that join the two bridgehead carbons (2, 2 and 1), from largest to smallest. 2 + 2 + 1 + 2 bridgeheads = 7 carbons: C7H12 (norbornane).
3. cis- and trans-Decalin
Why can cis-decalin invert its chairs while trans-decalin cannot?
In cis-decalin one ring-fusion bond is axial and the other equatorial; chair inversion swaps them and gives a conformation of equal energy. In trans-decalin both fusion bonds are equatorial; inversion would make them both axial, which is impossible with such a small ring, so it is locked.
4. Large rings
From what size does strain disappear in cycloalkanes? Why do cycloheptane or cyclodecane still have strain?
From 14 carbons (cyclotetradecane), which can adopt the zigzag conformation of linear alkanes. Intermediate rings cannot stagger all their hydrogens and some of them clash across the ring.