Cyclohexane, axial bonds chair conformation

Section 3.9 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 conformation in which the substituent is equatorial (see next section). No bonds are made or broken in this process. 

If we could freeze cyclohexane in a chair conformation, we would see that there are two different kinds of carbon-hydrogen bonds. Six of the bonds (one on each carbon atom) are directed up and down, parallel to the axis of the ring. These are called axial bonds. The other six bonds point out from the ring, along the equator of the ring. These are... 

Cycloalkanes C H2 hydrocarbons n = 3, 4 Cyclopropane, cyclobutane (strained bond angles) n = 6 Cyclohexane (most stable, chair conformation) axial (less stable) equatorial (more stable) substituent positions (4-2, 3)... 

Cyclohexane adopts a chair conformation. C-H bonds are described as being axial, perpendicular to the mean plane of the ring, or equatorial, close to the mean plane of the ring. The chair can invert, rendering all axial substituents equatorial and all equatorial substituents axial. 

The C—H bonds in the chair conformation of cyclohexane are not all equivalent but are divided into two sets of six each called axial and equatorial... 

Axial bond (Section 3 8) A bond to a carbon in the chair conformation of cyclohexane oriented like the six up and down bonds in the following... 

The 12 hydrogen atoms of cyclohexane do not occupy equivalent positions. In the chair conformation six hydrogen atoms are perpendicular to the average plane of the molecule and six are directed outward from the ring, slightly above or below the molecular plane (see Fig. 1.6). Bonds which are perpendicular to the molecular plane are known as axial bonds, and those which extend outward... 

The most stable conformation of cyclohexane is the chair. Electron diffraction studies in the gas phase reveal a slight flattening of the chair compared with the geometry obtained when tetrahedral molecular models are used. The torsion angles are 55.9°, compared with 60° for the ideal chair conformation, and the axial C—H bonds are not perfectly parallel but are oriented outward by about 7°. The length of the C—C bonds is 1.528 A, the length of the C—H bonds is 1.119 A, and the C—C—C angles are 111.05°. ... 

Cyclohexane is strain-free because it adopts a puckered chair conformation, in which all bond angles are near 109° and all neighboring C—H bonds are staggered. Chair cyclohexane has two kinds of positions axial and equatorial. Axial positions are oriented up and down, parallel to the ring axis, whereas equatorial positions lie in a belt around the equator of the ring. Each carbon atom has one axial and one equatorial position. 

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. 

We saw early in Section 3.3.2 that, if we draw cyclohexane in typical two-dimensional form, the bonds to the ring could be described as up or down , according to whether they are wedged or dotted. This is how we would see the molecule if we viewed it from the top. When we look at the molecule from the side, we now see the chair conformation the ring is not planar as the two-dimensional form suggests. Bonds still maintain their up and down relationship, but this means bonds shown as up alternate axial-equatorial around the ring they are... 

On each carbon, one bond is directed up or down and the other more or less in the plane of the ring. The up or down bonds are called axial and the others equatorial. The axial bonds point alternately up and down. If a molecule were frozen into a chair form, there would be isomerism in monosubstituted cyclohexanes. For example, there would be an equatorial methylcyclohexane and an axial isomer. However, it has never been possible to isolate isomers of this type at room temperature.219 This proves the transient existence of the boat or twist form, since in order for the two types of methylcyclohexane to be non-separable, there must be rapid interconversion of one chair form to another (in which all axial bonds become equatorial and vice versa) and this is possible only through a boat or twist conformation. Conversion of one chair form to another requires an activation energy of about 10 kcal/mol (42 kJ/mol)220 and is very rapid at room temperature.221 However, by... 

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). 

First write a chair conformation of cyclohexane, then add two methyl groups at C-1, and draw in the axial and equatorial bonds at C-3 and C-4. Next, add methyl groups to C-3 and C-4 so that they are cis to each other. There are two different ways that this can be accomplished either the C-3 and C-4 methyl groups are both up or they are both down. 

Interestingly, trimer was formed through secondary bonds between the iodine(III) and the two oxygen atoms (01 and 02) in the solid state of 1-alkynyl-lA3,2-benziodoxol-3(lH)-one 142 (Fig. 8) [225]. In the solid state the ethynyl substituent occupies an axial position of the cyclohexane chair conformer. 

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. 

Draw the chair conformation of cyclohexane and show clearly the distinction between axial and equatorial bonds. 

A torsion angle of 60 between C-C bonds represents a gauche interaction and so an axial methyl substituents experiences two gauche interactions with the cyclohexane ring whereas the equatorial methyl substituents experiences none. Due to this, the latter chair conformation is preferred and about 95 per cent of methylcyclohexane molecules are in this conformation at any point of time, compared to 5 per cent in the other conformation. 

What is the reason for this apparent discrepancy It is a solvent effect. In aqueous solution (Figure 9.6), the OH group at the anomeric C atom of the glucose becomes so voluminous due to hydration that it strives for the position in which the steric interactions are as weak as possible. Thus, it moves into the equatorial position—with a AG° value of approximately -1.6 kcal/mol—to avoid a gauche interaction with the six-membered ring skeleton. (Remember that axially oriented substituents on the chair conformer of cyclohexane are subject to two gauche interactions with the two next-to-nearest C. —C bonds. They therefore have a... 

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