Cyclohexane boat form

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. 

Stereochemistry. Cyclohexane can exist ia two molecular conformations the chair and boat forms. Conversion from one conformation to the other iavolves rotations about carbon—carbon single bonds. Energy barriers associated with this type of rotation are low and transition from one form to the other is rapid. The predominant stereochemistry of cyclohexane has no influence ia its use as a raw material for nylon manufacture or as a solvent. 

For cyclohexane, there are two extreme conformations in which all the angles are tetrahedral. These are called the boat and the chair conformations and in each the ring is said to be puckered. The chair conformation is a rigid structure, but the boat form is flexible and can easily pass over to a somewhat more stable form known as the twist conformation. The twist form is 1.5 kcal mol (6.3 kJ mol )... 

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-... 

Fig. 18. Top transition coordinates (with symmetry species) of conformational transition states of cyclohexane (top and side views). Hydrogen displacements are omitted. The displacement amplitudes given are towards the C2v-symmetric boat form, and towards >2-symmetric twist forms (from left), respectively. Inversion of these displacements leads to the chair and an equivalent T>2-form, respectively. Displacements of obscured atoms are given as open arrows, obscured displacements as an additional top. See Fig. 17 for perspective conformational drawings. Bottom pseudorotational normal coordinates (with symmetry species) of the Cs- and C2-symmetric transition states. The phases of the displacement amplitudes are chosen such that a mutual interconversion of both forms results. The two conformations are viewed down the CC-bonds around which the ring torsion angles - 7.3 and - 13.1° are calculated (Fig. 17). The displacement components perpendicular to the drawing plane are comparatively small. - See text for further details.

Figure 4.14 (a) The boat conformation of cyclohexane is formed by "flipping" one end of the chair form up (or down). This flip requires only rotations about carbon-carbon single bonds, (b) Ball-and-stick model of the boat conformation, (c) A space-filling model. 

The process of assessing the preferred conformation has become of importance of cyclic compounds, at least in six-membered rings because in their formation, almost all the strains are involved. We have seen that cyclohexane exists mostly in the chair conformation and the boat form occurs in negligible proportion because of higher energy which is of the order of >6 K cals/mole. 

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. 

As a result of these unfavourable interactions, i.e., opposition to bond strain i.e., between the pair of hydrogens shown at the bottom) and also due to bowsprit interaction, the potential energy of the boat form becomes high and this is why the boat conformation is not the preferred one. Hassel in 1947 established by means of electron diffraction studies that cyclohexane exists predominantly in the chair form. This has also been confirmed by electron diffraction studies and results obtained from Raman and I.R. spectra. Calculations made on the basis of entropy show that only about one molecule in a thousand will be in the boat form. 

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...

The chair is not the only conformation that cyclohexane might adopt. An alternative boat conformation is attained if the ring flip-type process is confined to just one carbon. The name boat comes from the similarity to boats formed by paper... 

Cineole provides an example of a bridged ring system. The essence of this problem is to spot that we cannot bridge the opposite carbons of a cyclohexane chair conformation it is necessary to have the cyclohexane in a boat form. Then the answer is readily obtained. 

Cyclohexane is the example par excellence. For its boat form one calculates 8c 10.7 for carbons 1 and 4 and 8q 16.5 (ppm from ethane) for the other four... 

Hence A (fra .s) = 4117.24kcal/mol for fra .s-l,4-di- -butylcyclohexane and A /(m) = 4113.72kcal/mol for the cis form. These two results are indicative of ring conformation since cA-l,4-di-t-butylcyclohexane is undoubtedly in a twist-boat form while the other is in chair conformation. The spectra of t-butylcyclohexane (in chair conformation) and of rrani-l,4-di- -butylcyclohexane are indeed very similar, except, of course, for carbon 4, which is the same as carbon 1 in the disubstituted molecule, whereas it is similar to the unsubstimted carbons in the monosubstituted cyclohexane. 

These considerations apply to all cycloalkane derivatives, including steroids. However, the chair form of a ring is inherently more stable than the boat form. Moreover, the fnsed-ring natnre of the system lends it a very considerable rigidity, and cis-trans isomerization wonld necessitate the breaking and formation of covalent bonds. Therefore, steroid snbstitnents maintain their conformation at room temperature, whereas cyclohexane substituents usually do not. Steroids are classified according to their substituents in addition to their occurrence. 

Sinca the energy of the boat form is 23 kJ mor1 above the chair form, the chair conformation is the only one that is considered in these calculations. Moreover, the equatorial-equatorial conformation is the most favoured one in frans-1,4-cyclohexane dlmethanol. 

At this point we should reiterate that the relative positions of atoms in many structures are continuously changing. The term different conformations of a molecule is used if two different three-dimensional arrangements of the atoms in a molecule are rapidly interconvertible, as is the case if free rotations about sigma bonds are possible. If rotation is not possible, we speak of different configurations, which represent isomers that can be separated. Obviously, the conformation(s) with the lowest energy [the most stable form(s)] is(are) the one(s) in which a molecule will preferentially exist. In the case of six-membered rings such as cyclohexane, three stable conformations (i.e., the chair, twist, and boat form) exist (Fig. 2.9). 

Conformations in Six-Membered Rings. The nonplanar ring compound. cyclohexane, which contains six contiguous —CH — units, may exist in chair or boat forms (hydrogens are not shown) ... 

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...

In drawing endo and exo isomers, it is best to represent the actual spatial relationships of the atoms as closely as possible. The cyclohexane ring is shown here in the boat form (Section 12-3 A) because it is held in this configuration by the methylene group that bridges the 1,4 positions. If you do not see this, we strongly advise that you construct models. 

Although cyclohexane is a ring structure it does have free rotation around single bonds Cyclohexane has two main confonnatioi/is. The most stable form is called the chair form, the les stable is cahedtfie boat form ... 

The chair conformer can undergo conformational isomerism to a second chair conformer which is degenerate in energy with the first. Cyclohexane is thus a dynamic molecule which exists largely in one of two chair isomers. These are the lowest energy conformations. Other higher energy conformations of cyclohexane include the boat form, which is 10.1 kcal/mol (42.3 kJ/mol) above the chair form, and the twist boat form, which lies 3.8 kcal/mol (15.9 kJ/mol) above the chair form. 

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. 

Deuterium atom was neatly incorporated at the bridgehead position C-l in ketone 464, the only compound in which the cyclohexane ring is locked in a boat conformation. Examination of molecular models indicates that the cyclohexanone ring can easily adopt a boat form in 460 and 461. It appears to be more difficult with ketone 462 and almost impossible with ketone 463. 

Camphor and its derivatives exist in the boat conformation. Since the gem-dimethyl bridge must be ois, the cyclohexane ring must have the boat form. 

In both Ih and Ic, the hydrogen-bonded systems contain cyclohexane-like buckled hexagonal ring motifs chair form in Ic and boat form in Ih. An important distinction is that in Ic all four hydrogen bonds are equivalent by symmetry. In Ih, the hydrogen bond in the direction of the hexagonal axis can be differentiated from the other three. 

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