Question

Are the cyclohexane conformations not planar? If yes, why?

• A. They are planar.
• B. Because for a planar structure bond angle would be ${120}^o$, too large for $s{p}^3$ hybrid orbitals.
• C. Because for a planar structure bond angle would be ${120}^o$, too small for $s{p}^3$ hybrid orbitals.
• D. Because $s{p}^3$ hybrid orbitals need ${120}^o$ bond angle and cyclohexane planar structure has ${90}^o$

Answer 1

The correct option is B Because for a planar structure bond angle would be ${120}^o$, too large for $s{p}^3$ hybrid orbitals.

Let's have a look at the planar configuration of cyclohexane



It is easy to see that the bond angle between the carbon-carbon bonds is ${120}^o$. But realise that the carbons here are $s{p}^3$ hybridized. So, the natural bond angle would be ${109.5}^o$. This means that this configuration strains the molecule.

To avoid this strain, the molecule conforms itself to a configuration where it can achieve this bond angle. So, some molecules move up and some move down the plane, giving us many possible conformations.

The most stable one of those is the chair conformation.



Here if you imagine a plane passing through carbons 2, 3, 5 and 6, you can see that carbons 4 and 1 are below and above the plane respectively. This would help the molecule achieve the bond angle it needs.