Cycloalkanes & Aromaticity Flashcards
(26 cards)
What is the general formula for cycloalkanes?
CₙH₂ₙ
Why is cyclopropane the most strained cycloalkane?
It has 60° bond angles causing severe angle strain and all hydrogens are eclipsed, causing high torsional strain.
What are “banana bonds” in cyclopropane?
Bent σ-bonds that avoid direct orbital overlap to reduce strain.
What is the bond angle in cyclobutane?
Approximately 90°.
What bond angle does cyclohexane have in its flat form?
120°, which is not the actual bond angle in its stable conformations.
Which cycloalkane is the most stable?
Cyclohexane, due to relief of angle and torsional strain in its conformations.
What are the three types of ring strain?
Angle strain, torsional strain, and steric strain.
Which cyclohexane conformation is the most stable?
The chair conformation.
Why is the boat conformation less stable than the chair?
Due to eclipsed hydrogens and flagpole interactions causing steric and torsional strain.
What is a ring flip in cyclohexane?
An interconversion between chair conformations that switches axial and equatorial positions.
What is the difference between axial and equatorial hydrogens in cyclohexane?
Axial are perpendicular (vertical) to the ring plane; equatorial are roughly in the plane, sticking outward.
What is 1,3-diaxial strain?
Steric interactions between axial substituents at C1 and the axial hydrogens on C3 and C5 causing strain.
Which conformation does methylcyclohexane prefer?
Equatorial methyl group (~95.8%) because axial causes 1,3-diaxial strain.
How much energy penalty is associated with axial methyl in methylcyclohexane?
About +7.6 kJ/mol.
How does tert-butylcyclohexane prefer to orient?
Almost exclusively equatorial (~99%) due to larger steric strain when axial (+22.8 kJ/mol).
What does the term “cis” mean in cycloalkanes?
Substituents are on the same face of the ring.
What does “trans” mean in cycloalkanes?
Substituents are on opposite faces of the ring.
How do the chiral centers differ in cis and trans disubstituted cyclohexanes?
Cis: both R,R or S,S; Trans: one R and one S.
For 1,2-disubstituted cyclohexanes, which trans conformation is more stable?
Equatorial–equatorial is more stable than axial–axial.
For 1,3-disubstituted cyclohexanes, which cis conformation is more stable?
Equatorial–equatorial is more stable than axial–axial.
What is trans-decalin and why is it unique?
Two fused cyclohexane rings in parallel, no ring flip possible, trans hydrogens are axial.
What is cis-decalin and how does it differ from trans?
Two fused cyclohexane rings perpendicular to each other; ring flip is possible.
What are the four requirements for aromaticity?
Planar, cyclic, fully conjugated with continuous p-orbitals, and follows Hückel’s rule (4n + 2 π electrons).
What is Hückel’s rule?
Aromatic compounds have (4n + 2) π electrons where n = 0, 1, 2,…
