Conformations of cyclohexane

All of the C—C—C bond angles are 109.5°, so this comformation has no angle strain. In addition, it has no torsional strain because all of the C—H bonds are perfectly staggered. This can best be seen by examining a Newman projection down C—C bonds on opposite sides of the ring  

The staggered arrangement of all the bonds can be seen clearly in the Newman projection. This same picture is seen when the projection is viewed down any C—C bond. All the C—C bonds in the molecule are in conformations in which the hydrogens are perfectly staggered. 

In the chair conformation cyclohexane has two different types of hydrogens. The bonds to one type are parallel to the axis of the ring. These are called axial hydrogens. The axial bonds alternate up and down around the ring. 

Like the chair conformation, all of the C—C—C bond angles of the boat conformation are 109.5°, so it has no angle strain. However, it does have other types of strain. The two red hydrogens, called flagpole hydrogens, approach each other too closely and cause some sceric strain. In addition, the conformations about the green bonds are eclipsed. This can be seen more easily in the Newman projection down these bonds  

In the twist boat conformation the bow and the stern of the boat have been twisted slightly. Although this decreases the flagpole interaction and relieves some of the torsional strain, angle strain is introduced. Overall, the twist boat conformation is a little more stable than the boat conformation but not nearly as stable as the chair conformation. 

Chair cyclohexane bears some resemblance to a chaise lounge. 

Hassel shared the 1969 Nobel Prize in Chemistry with Sir Derek Barton of Imperial College (London). Barton demonstrated how Hassel s structural results could be extended to an analysis of conformational effects on chemical reactivity. 

Staggered arrangement of bonds in chair conformation of cyclohexane 

Chapter 3 Alkanes and Cycloalkanes Conformations and cis-trans Stereoisomers 

The various conformations of cyclohexane are in rapid equilibrium with one another, but at any moment almost all of the molecules exist in the chair conformation. Less than five molecules per 100,000 are present in the skew boat conformation at 25°C. Thus, the discussion of cyclohexane conformational analysis that follows focuses exclusively on the chair conformation. 

Look down both of these bonds simultaneously 

The conformation of cyclopentane. Carbons 1, 2. 3, and 4 are nearly planar, but carbon 5 is out of the plane. Part (c) is a Newman pro ectlon along the C1 -C2 bond, showing that neighboring C-H bonds are nearly staggered. 

Problem 4.9 How many hydrogen-hydrogen eclipsing interactions would be present if cyclopentane were planar Assuming an energy cost of 4.0 kJ/mol for each eclipsing interaction, how much torsional strain would planar cyclopentane have How much of this strain is relieved by puckering if the measured total strain of cyclopentane is 26.0 kJ/mol  

Problem 4.10 Draw the most stable conformation of cis-l,3-dimcthylcyclobutane. Draw the least stable conformation. 

Substituted cyclohexanes are the most common cycloalkanes because of their wide occurrence in nature. A vast number of compounds, including many important pharmaceutical agents, contain cyclohexane rings. 

Hermann Sach e (1862-1893) was born in 6er(in, Germany, where he also received his Ph.D. (1889) and taught at the Technische Hochschule Charlottenburg-Berlin. 

Two conformations of cis-l,3-dimethylcyclobutane are shown. What is the difference between them, and which do you think is likely to be more stable  

CHAPTER 4 ORGANIC COMPOUNDS CYCLOALKANES AND THEIR STEREOCHEMISTRY 

CHAPTER 4 Organic Compounds Cycloalkanes and Their Stereochemistry 

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

Place the topmost carbon atom above and to the right of the plane of the other four, and connect the bonds. 

Step 3 Place the bottommost carbon atom below and to the left of tlie plane of the middle four, and connect the bonds. Note that the bonds to the bottommost carbon atom are parallel to the bonds to the topmost carbon. 

When viewing cyclohexane, it s helpful to remember that the lower bond is in front and the upper bond is in back. If this convention is not defined, an optical illusion can make it appear that the reverse is true. For clarity, all c t1o-hexane rings drawn in this book will have the front (lower) bono heavily shaded to indicate nearness to the viewer. 

Cyclohe.xane adopts a strain-free, three-dimensional shape, called a chair conformation because of its similarity to a lounge chair, with a back, a seat, and a footrest (l-igure 4.7). Chair cyclohexane has neither angle strain nor torsional strain—all C-C-C bond angles are near 109°, and all neighboring C-H bonds are staggered. 

Make a molecular model of the chair conformation of cyclohexane, and turn it so that you can look down one of the C—C bonds. 

Recall from Section 3.2 that the sum of the van der Waals radii of two hydrogen atoms is 240 pm. 

Ernst Mohr (1873-1926) was born in Dresden, Germany, and received his Ph.D. at the University of Kiel (1897). He was then professor of chemistry at the University of Heidelberg. 

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Figure 8-6. Comparison of the radial distribution function of the ctiair, boat, and twist conformations of cyclohexane (hydrogen atoms are not considered).

Section 3 7 Three conformations of cyclohexane have approximately tetrahedral angles at carbon the chair the boat and the skew boat The chair is by far the most stable it is free of torsional strain but the boat and skew boat are not When a cyclohexane ring is present m a compound it almost always adopts a chair conformation... 

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... 

One property of NMR spectroscopy is that it is too slow a technique to see the mdi vidual conformations of cyclohexane What NMR sees is the average environment of the protons Because chair-chair mterconversion m cyclohexane converts each axial pro ton to an equatorial one and vice versa the average environments of all the protons are the same A single peak is observed that has a chemical shift midway between the true chemical shifts of the axial and the equatorial protons... 

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... 

Birch reduction (Section 11 11) Reduction of an aromatic nng to a 1 4 cyclohexadiene on treatment with a group I metal (Li Na K) and an alcohol in liquid ammonia Boat conformation (Section 3 7) An unstable conformation of cyclohexane depicted as... 

FIGURE 1.6 The two chair conformations of cyclohexane a = axial hydrogen atom and e = equatorial hydrogen atom. 

A modified boat conformation of cyclohexane, known as the twist boat (Fig. 1.8), or skew boat, has been suggested to minimize torsional and nonbounded interactions. This particular conformation is estimated to be about 1.5 kcal moE (6 kJ moE ) lower in energy than the boat form at room temperature. 

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°. ... 

FIGURE 3.14 (a) A ball-and-spoke model and (h) a space-filling model of the boat conformation of cyclohexane. Torsional strain from eclipsed bonds and van der Waals strain involving the "flagpole" hydrogens (red) make the boat less stable than the chair. 

FIGURE 3.18 Energy diagram showing the interconversion of various conformations of cyclohexane. 

Boat conformation (Section 3.7) An unstable conformation of cyclohexane, depicted as... 

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