Final answer:
Cyclopentadiene contains sp3 hybridized carbons and is non-planar, adopting conformations to minimize ring strain and achieve ideal bond angles. Cycloalkanes such as cyclohexane also exhibit puckered conformations like the chair or boat to reduce strain.
Step-by-step explanation:
The student's question mentions cyclopentadiene as having a sp3 carbon and inquires about its planarity. Cyclopentadiene indeed contains sp3 hybridized carbons but is not planar due to its geometry and angle strain.
Cyclopentadiene is originally non-planar but can appear planar in certain reactive intermediates like the cyclopentadienyl anion.
Cycloalkanes,
such as cyclohexane, are not planar because they adopt conformations like the chair or boat to minimize ring strain and achieve bond angles closer to 109.5°, which is the ideal angle for sp³ hybridized carbons. The puckered structure, where alternate carbon atoms are above and below the plane, helps reduce both angle strain and steric strain from eclipsing interactions.