Ring-flipping

It makes sense why students think that this would be a flip—after all, this is the common meaning of the word flipping. But we are talking about something very different when we say that rings can flip. Here is what we really mean  

There is one more important feature to recognize. Let s go back to the example above with the chlorine. We said that the chair flip moves the chlorine from an equatorial position into an axial position. But what about the up/down terminology Let s see  

The process for drawing the skeleton of the chair is very similar to how we did it before. The only difference is that we draw our lines in the other direction. When we drew our first chair, we followed these steps  

to draw the other chair, we follow these steps  

Compare the method for drawing the second chair to the method for drawing the first. The key is in step 2. If you compare step 2 for the first and second chair, everything else should flow from there. Use the following space to practice drawing the second chair  

let s make sure you know how to draw the substituents. The rules are the same as before. All axial positions are drawn straight up and down, alternating  

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It is also important to always examine the transition structure geometry to make sure that it is the reaction transition and not the transition in the middle of a ring flip or some other unintended process. If it is not clear from the geometry that the transition structure is correct, displaying an animation of the transition vibrational mode should clarify this. If still unclear, a reaction coordinate can be computed. 

We have seen that alkanes are not locked into a single conformation Rotation around the central carbon-carbon bond m butane occurs rapidly mterconvertmg anti and gauche conformations Cyclohexane too is conformationally mobile Through a process known as ring inversion, chair-chair mterconversion, or more simply ring flipping, one chair conformation is converted to another chair... 

The best way to un derstand ring flip ping in cyclohexane IS to view the animation of Figure 3 18 in Learning By Modeling... 

The most important result of ring inversion is that any substituent that is axial in the original chair conformation becomes equatorial in the ring flipped form and vice versa... 

CIS 1 4 Dimethylcyclohexane can adopt either of two equivalent chair conforma tions each having one axial methyl group and one equatorial methyl group The two are m rapid equilibrium with each other by ring flipping The equatorial methyl group becomes axial and the axial methyl group becomes equatorial... 

The methyl groups are described as cis because both are up relative to the hydrogen present at each carbon If both methyl groups were down they would still be cis to each other Notice that ring flipping does not alter the cis relationship between the methyl groups Nor does it alter their up versus down quality substituents that are up m one conformation remain up m the ring flipped form... 

Conformational inversion (ring flipping) is rapid in cyclohexane and causes all axial bonds to become equatorial and vice versa As a result a monosubstituted derivative of cyclohexane adopts the chair conforma tion in which the substituent is equatorial (see next section) No bonds are made or broken in this process... 

A substituent is less crowded and more stable when it is equatorial than when It IS axial on a cyclohexane ring Ring flipping of a monosubsti tuted cyclohexane allows the substituent to become equatorial... 

Among all the isomers as 1 2 dichlorocycio hexane is unique in that the ring flipping process typ ical of cyclohexane derivatives (Section 3 9) converts it to its enantiomer... 

The answer is related to the very rapid rate of ring flipping m cyclohexane... 

The rate of ring flipping can be slowed down by lowering the temperature At tern peratures on the order of — 100°C separate signals are seen for the axial and equatorial protons of cyclohexane... 

Ring inversion (Section 3 9) Process by which a chair conforma tion of cyclohexane is converted to a mirror image chair All of the equatonal substituents become axial and vice versa Also called ring flipping or chair-chair interconversion... 

The temperature-dependent NMR spectrum of the ion can be analyzed to show that there is a barrier (8.4 kcal/mol) for the ring flip that interchanges the two hydrogens of the methylene group. The C-NMR chemical shift is also compatible with the homoaromatic structure. MO calculations are successful in reproducing the structural and spectroscopic characteristics of the cation and are consistent with a homoaromatic structure. ... 

Fast Tyr ring flips methyl group rotations 2. Slow hinge bending between domains ... 

What effect does a ring flip have on substituent positions Do all of the substituents change position or only certain ones If the latter, then which substituents are affected Obtain the energies of the two conformers. Which conformer is preferred Why ... 

Make a sketch of each decalin isomer, and label the orientation of the bridgehead hydrogens with respect to each ring (equatorial or axial). Build a plastic model of each isomer and determine its conformational flexibility (a flexible molecule can undergo a ring flip, but a locked molecule cannot). Is flexibility responsible for stabihty ... 

H321). Tliese radical cations exist in a nonplanar conformation. With fra s-3,4-dimethyl substituents, the barrier to ring flip was estimated to be >5 kcal/mol. In connection with reactions of thiirane with the thiirane radical cation, ab initio calculations were reported on the 1,2-dithietane radical cation (93JA12510). 

Because chair cyclohexane has two kinds of positions, axial and equatorial, we might expect to find two isomeric forms of a monosubstituted cyclohexane. In fact, we don t. There is only one methylcyclohexane, one bromocydohexane, one cycJohexanol (hydroxycyclohexane), and so on, because cyclohexane rings are confbnnationally mobile at room temperature. Different chair conformations readily interconvert, exchanging axial and equatorial positions. This interconversion, usually called a ring-flip, is shown in Figure 4.11. 

Figure 4.11 A ring-flip in chair cyclohexane interconverts axial and equatorial positions. What is axial (red) in the starting structure becomes equatorial in the r ing-flipped structure, and what is equatorial (blue) in the starting structure is axial after ring-flip.

Identify each of the colored positions—red, blue, and green—as axial or equatorial. Then carry out a ring-flip, and show the new positions occupied by each color. 

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