Bruger:Sasha/Kladde1

Cyclohexans opbygning er et meget studeret emne i organisk kemi på grund af det komplekse samspil mellem de forskellige opbygninger af cyclohexan og derivater. Forskellige opbygninger kan have forskellige egenskaber, herunder stabilitet og kemisk reaktivitet.

Historisk baggrund

The very first suggestion that cyclohexane may not be a flat molecule goes back a surprisingly long time. In 1890, Hermann Sachse, a 28-year-old assistant in Berlin, published instructions for folding a piece of paper to represent two forms of cyclohexane he called symmetrical and unsymmetrical (what we would now call chair and boat). He clearly understood that these forms had two positions for the hydrogens (again, to use modern terminology, axial and equatorial), that two chairs would probably interconvert, and even how certain substituents might favor one of the chair forms. Because he expressed all this in mathematical language, few chemists of the time understood his arguments. He had several attempts at publishing these ideas, but none succeeded in capturing the imagination of chemists.

Stolopbygningen

Due to the inherent tendency of the sp³ hybrid orbitals (and therefore the carbon-hydrogen bonds) on tetravalent carbons to form bond angles of 109.5 °, cyclohexane does not form a planar hexagonal arrangement with interior bond angles of 120 °. The chair conformation is a term used for the most stable chemical conformation of a six membered single bonded carbon ring like cyclohexane. Derek Barton and Odd Hassel both shared the Nobel Prize for work on the conformations of cyclohexane and various other molecules.

In the lowest-energy chair conformation, 6 of the 12 hydrogens are in axial positions (colored red)—their C-H bonds are parallel to each other and appear to stick up and down from the ring structure, the other 6 are in equatorial positions (colored blue)—they are splayed out around the perimeter of the ring. Note that in addition, one hydrogen at each position is "up" relative to the other being "down" at that position.

Bådopbygningen

In addition to the chair conformation (1) with D_3d symmetry cyclohexane can also exist in the half-chair or envelope (2), twist or twist-boat (3,5) with D₂ symmetry and boat (4) conformers. Only the twist form is isolable as - like the chair form - it represents an energy minimum. The boat conformation does not suffer from angle strain but has a higher energy than the chair form due to steric strain resulting from the two axial 1,4-hydrogen atoms, in what is called the flagpole interaction. The torsional strain in the boat conformation has a maximum value because two of the carbon bonds are eclipsed. Compare this to the chair with all bonds staggered and complete absence of torsional strain and the twist-boat with 4 out 6 bonds partially eclipsed. In the half-chair conformation 4 carbon atoms are located on a plane in which two bonds are fully eclipsed.

The boat and envelope forms are transition states between the twist forms and the twist and chair forms respectively, and are impossible to isolate. The twist-boat conformation is 5.5 kcal/mol (23 kJ/mol) less stable than the chair conformation. The energies of the two transition states are 6.6 kcal/mol (28 kJ/mol) (boat) and 10.8 kcal/mol (45 kJ/mol) (half chair) higher than that of the chair. The ring flipping process can now be described with more precision as taking place through a twist-boat conformation and through two half-chair transition states.

cyclohexane universe: 1 chair, 2 half-chair, 3 twist-boat, 4 boat

The difference in energy between the chair and the twist-boat conformation of cyclohexane can be measured indirectly by taking the difference in activation energy for the conversion of the chair to the twist-boat conformation and that of the reverse isomerization. The concentration of the twist-boat conformation at room temperature is very low (less than 0.1%) but at 1073 kelvins this concentration can reach 30%. The reverse reaction is measured by IR spectroscopy after rapidly cooling cyclohexane from 1073 K to 40 K, freezing in the large concentration of twist-boat conformation.

Twist-boat chair interconversion

[6.6]Chiralane is a point group T molecule wholly composed of identical fused twist-boat cyclohexanes. Twistane is another compound with a forced twist-boat conformation.