6. Cycloalkanes

Question 1

What is the general formula of a cycloalkane?

Answer 1

CnH2n

Question 2

Why are cyclic structures limited to 5 or 6 membered rings?

Answer 2

• The ideal bond angle for a sp3 hybridised C atom is 109.5

- Small sings deviate substantially from this and as a result they suffer ring strain

Question 3

Why are 6 membered rings preferred?

Answer 3

They are strain free with all C-C-C angles close to the optimal 109.5 for tetrahedral geometry and an all staggered arrangement

Question 4

What does ring flipping do?

Answer 4

Ring flipping interchanges axial and equatorial substituents

Question 5

For adding functional groups to a cyclohexane, is axial or equatorial more stable?

Answer 5

• Equatorial substituents result in a more stable conformation due to decreased steric interactions

Question 6

How are isomers created from disubstituted cycloalkanes?

Answer 6

• Rings prevent free rotations around single bonds
• Isomers can exist when two or more substituents are attached
• Isomers do no interconvert (which requires breaking of C-C bonds)
• Called cis/trans isomers, diastereomers