Cyclohexane

From Academic Kids

Missing image
Cyclohexane-structure3.png
A cyclohexane molecule in chair conformation, with hydrogen atoms in axial position in red, equatorial in blue.

Cyclohexane is a molecule with the molecular formula C6H12 (molecular mass=84.18g/mol) consisting of six carbon atoms linked to each other to form a ring, with each carbon atom bearing two hydrogen atoms.

Due to the inherent need of the sp hybrid orbitals (and therefore the carbon-hydrogen bonds) on tetravalent carbons to reach 109.5, cyclohexane is not a planar molecule. Chair and twist cyclohexanes exist with chair being the most stable, virtually strain free (111.5 C-C-C bond angles). Odd Hassel received the Nobel Prize for work on the conformations of cyclohexane.

In the lowest-energy chair conformation, half of the 12 hydrogens are in axial positions, which means their C-H bonds are parallel and appear to stick up and down from the structure, the other half are in equatorial positions; meaning that they are splayed out. Cyclohexane can also exist in the half-chair, twist or boat forms. Only the twist form is isolable as - like the chair form - it represents an energy minima, although of a higher energy than the chair form due to increased torsional strain when compared to the chair form. The boat and half-chair forms are transition states between the twist forms and the twist and chair forms respectively, and are impossible to isolate.

In the chair form, a process known as ring flipping is possible, and leads to the axial hydrogens exchanging positions with the equatorial hydrogens . However, the relative direction of the hydrogens to the ring remains the same, so that an 'up' axial hydrogen, when flipped, remains an 'up' equatorial hydrogen.

In stepping round the ring, it can be seen that the up-axial positions alternate with up-equatorial positions, so that for trans-1,2-cyclohexane, the substituents must either both be axial or both be equatorial to remain on opposite sides of the ring. Similarly, for cis-1,2-cyclohexane, the substituent at 1 must be equatorial and the 2 substituent axial, or vice-versa. Each conformation will differ in its stability depending upon the identity of the functional groups. Generally, substituents are most stable when in equatorial positions, as in this case there are no 1,3-diaxial interaction between the axial substituent group and any other axial groups on the ring. For example, if there is a methyl group on carbon 1 in an axial position, it will interact with the axial hydrogens on carbon 3 and carbon 5.de:Cyclohexan fr:Cyclohexane ja:シクロヘキサン zh-cn:环己烷

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