Benzene and cyclohexane molecules both contain six-membered rings. Benzene is a planar molecule, and cyclohexane is nonplanar. Explain.
What is there to explain? A planar molecule has all of the atoms in the same plane. A nonplanar one does not.
Explanations for why benzene is one type and cyclohexane the other can be found at
http://in.answers.yahoo.com/question/index?qid=20090817022032AAT8Zax
It has to do with what kind of carbon bonds are formed.
To understand why benzene is planar while cyclohexane is nonplanar, we need to consider the bond angles and hybridization of the carbon atoms in each molecule.
In benzene, each carbon atom is sp2 hybridized, meaning that it forms three sigma bonds (one to each of its neighboring carbon atoms) and has one unhybridized p orbital. The p orbitals from each carbon atom form a continuous system of overlapping π electrons above and below the plane of the molecule. This delocalized π bond system gives benzene its characteristic stability and planarity. The six-membered ring in benzene remains flat to maximize the overlap of the π electrons, resulting in a planar molecule.
On the other hand, cyclohexane adopts a nonplanar conformation due to the unfavorable eclipsing interactions between the hydrogen atoms attached to adjacent carbon atoms. If cyclohexane were planar, it would result in the eclipsing of most of these hydrogen atoms, leading to significant steric strain and instability. To alleviate this strain, the molecule adopts a nonplanar conformation known as the chair conformation, where the ring adopts a three-dimensional shape resembling a chair. In this conformation, the hydrogens attached to the carbon atoms alternate between pointing up and down, reducing steric strain and increasing stability.
In summary, the difference in planarity between benzene and cyclohexane is due to the presence of the delocalized π bond system in benzene, which allows for the molecule to remain flat, while cyclohexane adopts a nonplanar conformation to minimize steric strain.