Cyclohexane, boat

The boat conformation of cyclohexane. There is steric strain and torsionai strain in this conformation but no angle strain. 

Problem 4.20 trans-l,3-Di-fert-batylcyclohexane is one of the few molecules that e.xists largely in a twist-boat conformation. Draw both a chair conformation and the likely twist-boat conformation, and then explain why the twist-boat form is favored. 

Boat and twist-boat conformations of cyclohexane. The twist-boat conformation is lower in energy than the boat conformation by 6 kj/mol. Both conformations are much more strained than chair cyclohexane. 

---

Identify the lowest-energy conformer from among those provided cyclopropane, planar and puckered cyclobutane, planar and puckered cyclopentane and chair, half-chair, boat and twist-boat cyclohexane. (If... 

Norbornane has a conformationally locked boat cyclohexane ring (Section 4.5) in which carbons 1 and 4 are joined by an additional CH group. Note how, in drawing this structure, a break in the rear bond indicates that the vertical bond crosses in front of it. Making a molecular mode) is particularly helpful when trying to see the three-dimensionality of norbornane. 

Boat cyclohexane (Section 4.5) A conformation of cyclohexane that bears a slight resemblance to a boat. Boat cyclohexane has no angle strain but has a large number of... 

Boat cyclohexane has torsional strain and flagpole interaction. 

This reaction is endothermic by ca 18 kJmol-1. It is tempting to identify the source of the destabilization as the three boat cyclohexanes (or, more properly, piperazines 29) on the right49. 

The comparison of the genuine theoretical results with those predicted by this approximation shows a root-mean-square (rms) deviation of 0.2kcal/mol with those obtained in the HF/6-31G(if) calculations reported in Table 9.1. This result is all the more remarkable as it includes polycychc molecules (15-21), boat-cyclohexane stmctures (15, 21), as well as a twist-boat structure (19, twistane = tricyclo [4.4.00 ]decane). The use of this approximation for ZPE + — Hq in problems... 

The analogy between S-exo transition states and the chair/boat cyclohexane is simple to apply and has useful predictive and interpretive value, provided that one recalls the differences as well as the similarities. 

Guella G, Chiasera G, Pietra F (1992) Conformational Studies of Marine Polyhalogenated a-Chamigrenes Using Temperature-Dependent NMR Spectra. Inverted Chair and Twist-Boat Cyclohexane Moieties in the Presence of an Axial Halogen Atom at C(8). Helv Chim Acta 75 2026... 

According to the foregoing analysis, conformers such as gauche- and anri-butane or chair and twist-boat cyclohexane would be considered to be diastereomers of each other. However, under most conditions these conformers interconvert so rapidly that butane and cyclohexane are considered to be single species and not mixtures of stereoisomers. When we have to write chemistry books for people living on the outer planets of the solar system, we might have to modify these concepts. 

In addition to the chair confortnation of cyclohexane, a cond pos ibiht]r called the lN>at conformation ts also free of angle strain. Wc haven t paid it any attention thus far, however, because boat cyclohexane is less staWe than chair cyclohexane (Figwre 4.S3). 

In contrast to the results for monocyclic compounds, in bicyclic compounds the rigidity of bridged structure frequently permits more substantial effects than in acyclic or monocyclic compounds. The bridging locks the cyclohexane ring into the boat and/or twist-bond conformation. For example, bicyclo [2.2.1] heptane and bicyclo [2.2.2] octane have one and two boat cyclohexane rings respectively. The enthalpies of the boat and twist-boat conformers of cyclohexane are 27 and 23 kJ/mol higher than that of the chair conformer. Therefore, bicyclo [2.2.1] heptane and bicyclo [2.2.2] octane have ring strains of 64 and 54 kJ/mol, respectively, as shown in Table 4. 

The absolute configuration of the product may be predicted by using a simple six-membered ring transition state model (structures 1 and 2). In this model the predicted transition state resembles a boat cyclohexane with the small group occupying an axial-like position. 

Carbons 2, 3, 5, and 6 in boat cyclohexane lie in a plane, with carbons 1 and 4 above the plane. The inside hydrogen atoms on carbons 1 and 4 approach each other closely enough to produce considerable steric strain, and the four eclipsed pairs of hydrogens on carbons 2, 3, 5, and 6 produce torsional strain. The Newman projection in Figure 4.23, obtained by sighting along the C2-C3 and C5-C6 bonds, shows this eclipsing clearly. 

The bridged bicyclic systems bicyclo[2.2.1]heptane, also called norbornane, and bicyclo[2.2.2]octane contain locked boat cyclohexane rings. Because of their rigidity, these systems have played an important role in the development of theories of structure-reactivity relationships. 

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... 

Boat cyclohexane is approximately 29 kJ/mol (7.0 kcal/mol) less stable than chair cyclohexane, although this value is reduced to about 23 kJ/mol (5.5 kcal/mol) by twisting slightly, thereby relieving some torsional strain (Figure 4.24). Even this twist-boat conformation is still much more strained than the chair conformation, though, and molecules adopt this geometry only under special circumstances. 

---