Boat form, of cyclohexane

From the pseudorotating transition state the inversion process proceeds via an intermediate minimum of D2-symmetry (twist-conformation) and across a symmetry-equivalent second pseudorotational transition state to the inverted chair-conformation. The symmetric boat-form of cyclohexane (symmetry C2v) corresponds to a one dimensional partial maximum, i.e. a transition state (imaginary frequency 101.6 cm-1). It links sym-... 

The energy barrier between the chair and boat form of cyclohexane is about 35 k J/mole and this is not large enough to prevent their rapid interconversion at room temperature. This is why it is not possible to isolate each conformation. 

Among the fused ring systems, the most important compound is decalin. This compound exists as the cis and trans forms and the existence of these two forms on the basis of non-planar structures was predicted by Mohr. He thought that the two decalins were formed by the fusion of two chair forms or two boat forms of cyclohexane and so he assigned, the following structures. 

Fig. 3. Chair and boat forms of cyclohexane and possible transition states for their interconversion energies (kcal/mole) relative to the chair form in parentheses...

Figure 12-4 Extreme boat form of cyclohexane showing interfering and eclipsed hydrogens. Top, space-filling model center, ball-and-stick models bottom, sawhorse representations...

The 1,6-addition to a,s, y, a-dienones is also subject to stereoelectronic effects. Addition on the bottom face of dienone 137 leads to a chair-like intermediate while that on the top face leads to a boat-like intermediate 140 in order to maintain maximum orbital overlap. Also, in 140 the R group encounters an eclipsed 1,2-R/H interaction and more importantly, a 1.4-CH3/R steric interaction which resembles the bowsprit flagpole arrangement of a twist-boat form of cyclohexane. This analysis of Marshall and Roebke (48) predicts that the trans product 139 should prevail over the cis product 141. 

The boat form of cyclohexane Is less stable than the chair forms for two reasons. 

Interconversion between chair and boat forms of cyclohexane derivatives may have x 10 s at room temperature, while rotation about a C-N bond in an amide linkage may be very slow with x 0.1 s. The nonenzymatic hydration of the aldehyde pyridoxal phosphate via Eq. 13-1 occurs with x = 0.01 - 0.1 s, depending upon the pH. ... 

Allinger (p. 305) found the energy difference between cis- and /wwj-l,3-di-/er/-butylcyclohexane to be 5.9 kcal/mole, and considers that this value represents the energy difference between the chair and twist-boat forms of cyclohexane. Defend Allinger s position. 

Dehydration. Johnson and co-workers in a study of the energy difference between the chair and boat forms of cyclohexane required the isomeric lactones (2) and (4) and were able to synthesize the required y-hydroxy acids (1) and (3). Lac-tonization of (1) was accomplished by heating a solution of 1 g. of (1) and 0.4 g. of... 

In an interesting contrast to the chair form of cyclohexane (D3(j symmetry) as the fundamental structural unit of adamantane, which is the prototype of the regular diamond (Fspace group, cubic system), the D2 twist-boat form of cyclohexane may be regarded as the fundamental unit of twistane (134), C2 ditwistane (137), D3 tritwistane (141), and of a twist diamond (P63 space group, hexagonal system). 

Solution, (a) PF5 is nonpolar (trigonal bipyramidal, 031, point group), (b) Co(en)3 + is nonpolar (D3 point group), (c) c/s-Dichloroethene is polar (C2V point group), (d) The boat form of cyclohexane is polar (C2V point group). 

Figure 10.17 The two possible chair forms, and the boat form, of cyclohexane...

FIGURE 5.25 van der Waals strain in the full-boat form of cyclohexane. Don t confuse this strain with attractive van der Waals forces (p. 87). 

A further common example of conformational isomerism is the chair and boat forms of cyclohexane (Figure 20.58). The two forms are able to flip between each other but the boat form is under more strain. The bond angles are close to tetrahedral, so there is little angle strain, but the boat form of cyclohexane does have eclipsed bonds on four of its carbon atoms. This eclipsing produces a significant amount of torsional strain. More importantly, the close contact of the flagpole hydrogens at either end of the molecule destabilizes the boat conformation. 

You will best appreciate the differences between the chair and boat forms of cyclohexane by building and manipulating molecular models of each. 

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