Cyclohexane, axial bonds ring-flip

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... 

In the trans isomer, one methyl is written down (dotted bond) whilst the other is written up (wedged bond). If we transform this to a chair conformation, as shown in the left-hand structure, the down methyl will be equatorial and the up methyl will also be equatorial. With ring flip, both of these substituents then become axial as in the right-hand conformer. From what we have learned about monosubstituted cyclohexanes, it is now easily predicted that the diequatorial conformer will be very much favoured over the diaxial conformer. 

At this point, it probably will be helpful to construct models of cis- and trans-decalins to appreciate the following (a) The two compounds cannot interconvert unless C-C or C-H bonds first are broken, (b) traw -Decalin is a relatively rigid system and, unlike cyclohexane, the two rings cannot flip from one chair form to another. Accordingly, the orientation of the substituent is fixed in the chair-chair conformation of trans-decalm, (c) The chair-chair forms of cw-decalin are relatively flexible, and inversion of both rings at once occurs fairly easily (the barrier to inversion is about 14 kcal mole-1). A substituent therefore can interconvert between axial and equatorial conformations (Figure 12-24). 

When ring flipping occurs from one chair to another, all the axial bonds become equatorial bonds and all the equatorial bonds become axial bonds. This does not matter for cyclohexane itself, but it becomes important when there is a substituent present in... 

Cyclohexane rapidly interconverts between two stable chair conformations because of the ease of rotation about its carbon-carbon bonds. This interconversion is known as ring flip (Figure 2.8). When the two chair conformers interconvert, bonds that are equatorial in one chair conformer become axial in the other chair conformer and vice versa. 

Ril flip (Section 4.12) The change in a cyclohexane ring (resulting from partial bond rotations) that converts one ring conformation to another. A chair-chair ring flip converts any equatorial substitutent to an axial substituent and vice versa. 

When a cyclohexane ring undergoes a chair-chair conformational change (a ring flip), all of the bonds that were axial become equatorial, and all bonds that were equatorial become axial. 

We have seen that when a cyclohexane ring flips, all equatorial bonds become axial and all axial bonds become equatorial. Now lets consider the consequences of flipping a substituted cyclohexane ring. The chair—chair interconversion of monosubstituted cyclohexanes occurs very rapidly. However, the two conformations of monosubstituted cyclohexanes, unlike those of cyclohexane, are not equally stable. 

The prefix cyclo- is used to name cycloalkanes. Cyclopropane is planar, but larger carbon rings are puckered. Cyclohexane exists mainly in a chair conformation with all bonds on adjacent carbons staggered. One bond on each carbon is axial (perpendicular to the mean carbon plane) the other is equatorial (roughly in that plane). The conformations can be interconverted by flipping the ring, which requires only bond rotation and occurs rapidly at room temperature for cyclohexane. Ring substituents usually prefer the less crowded, equatorial position. 

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