Equatorial and axial bonds in cyclohexane

It is no accident that sections of our chair cyclohexane drawings resemble sawhorse projections of staggered conformations of alkanes. The same spatial relationships seen in alkanes carry over to substituents on a six-membered ring. In the stracture 

Given the following partial structure, add a substituent X to C-1 so that it satisfies the indicated stereochemical requirement What is the A—C—C—X torsion (dihedral) angle in each  

Sample Solution (a) In order to be anti to A, substituent X must be axial. The blue lines in the drawing show the A—C—C—X torsion angle to be 180° 

Place equatorial bond at C-1 so that it is parallel to the bonds between C-2 and C-3 and between C-5 and C-6. 

Following this pattern gives the complete set of equatorial bonds. 

A guide to representing the orientations of the bonds in the chair conformation of cyclohexane. 

Chapter 3 Alkanes and Cycloalkanes Conformations and cis-trans Stereoisomers 

STEP 1 Draw two parallel lines, slanted downward and slightly offset from each other. This means that four of the cyclohexane carbon atoms lie in a plane. 

STEP 2 Locate the topmost carbon atom above and to the right of the plane of the other four and connect the bonds. 

When viewing chair cyclohexane, the lower bond is in front and the upper bond is in back. If this convention is not defined, an optical illusion can make the reverse appear true. For clarity, all the cyclohexane rings drawn in this book will have the front (lower) bond heavily shaded to indicate its nearness to the viewer. 

The chair conformation of cyclohexane has many chemical consequences. For example, we ll see in Section 11.12 that the chemical behavior of many substituted cyclohexanes is directly controlled by their conformation. Another consequence of the chair conformation is that there are two kinds of positions for substituents on the ring axial positions and equatorial 

Axial and equatorial hydrogen atoms in chair cyclohexane. The six axial hydrogens (red) are parallel to the ring axis, and the six equatorial hydrogens (blue) are in a band around the ring equator. 

FIGURE 3.12 (a) The boat and (b) skew boat conformations of cyclohexane. A portion of the torsional strain in the boat is relieved by rotation about C—C bonds in the skew boat. Bond rotation is accompanied by movement of flagpole hydrogens away from each other, which reduces the van der Waals strain between them. 

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The chair conformation of cyclohexane has many consequences. We D see in Section 11.9, for instance, that the chemical behavior of many substituted cv clohexanes is influenced bv their conformation. In addition, we h see in Sec-tion 25.5 that simple carbohydrates such as glucose adopt a conformation based on the cvdohexanc chair and that their chemistry is directly affecteci as a result. 

Note that we haven t used the words ds and trans in this di.scussion of cyclohexane conformation. Two hydrogens on the same face of the ring are always cis, regardless of whether they re axial or equatoria. and regardless of whether they re adjacent. Similarly, two hydrogens on opposite faces of the ring are alwavs trans. 

Axial bonds The six axial bonds, one on each carbon, are parallel and alternate up-down. 

Equatorial bonds The six equatorial bonds, one on each carbon, come in three sets of two parallel lines. Each set is also parallel to two ring bonds. Equatorial bonds alternate between sides around the ring. 

Draw 1,1-dimcthylcydohexane in a chair conformation, indicating which methyl group in your drawing is axial and which is equatorial. 

Fig ure 4.10 A procedure for drawing axial and equatorial bonds in chair cyclohexane. 

CHAPTER 4 ORGANIC COMPOUNDS CYCLOALKANES AND THEIR STEREOCHEMISTRY 

FIGURE 4.9 Alternating axial and equatorial positions in chair cyclohexane, as shown in a view looking directly down the ring axis. Each carbon atom has one axial and one equatorial position, and each face has alternating axial and equatorial positions. 

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 methylcydohexane, one bromocyclohexane. 

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Conformations of Cyclobutane and Cyclopentane Conformations of Cyclohexane 127 Axial and Equatorial Bonds in Cyclohexane 129 Conformational Mobility of Cyclohexane 131 Conformations of Monosubstituted Cyclohexanes Conformational Analysis of Disubstituted Cyclohexanes Boat Cyclohexane 140 Conformations of Polycyclic Molecules 141... 

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