Chlorocyclohexane, conformation

Draw the most stable conformation of ds-l-te/t-butyM-chlorocyclohexane. By how much is it favored ... 

Anti periplanar geometry for E2 reactions is particularly important in cyclohexane rings, where chair geometry forces a rigid relationship between the substituents on neighboring carbon atoms (Section 4.8). As pointed out by Derek Barton in a landmark 1950 paper, much of the chemical reactivity of substituted cyclohexanes is controlled by their conformation. Let s look at the E2 dehydro-halogenation of chlorocyclohexanes to see an example. 

Even though we define the atropisomerism as above for present purposes, there remain some ambiguities. sym-Tetrabromoethane was obtained in different modifications according to the method of crystallization at low temperature (13). These were found by spectroscopy to correspond to retainers. Similar situations occur in other alkyl halides and acetates (14,15). Such cases will not be included in the discussion, mainly because crystalline atropisomers are isolated at far lower temperatures than die ambient, and their barriers to rotation have not been determined by equilibration. Also excluded is the isolation of chlorocyclohexane (16). The isolation of the equatorial and axial conformational isomers was possible only by crystallization of the former at - 150°C, although it was possible to observe equilibration between the equatorial and the axial forms at higher temperatures. 

Except for fluorocyclohexane and chlorocyclohexane, the lower-energy conformer for all molecules examined has the smaller dipole moment. This is in accord with Coulomb s law (increased separation of charge leads to increased energy). 

From many studies it is known that the interconversion of conformations with the substituent in the equatorial and the axial positions occurs about 100,000 times per second, which corresponds to a transition-state energy (activation energy) of about 11 kcal mole-1 above the ground-state energy. The rate decreases as the temperature is lowered. If one cools chlorocyclohexane to its melting point (—44°), the substance crystallizes to give the pure equatorial isomer. The crystals then can be cooled to —150° and dissolved at this temperature in a suitable solvent. At —150° it would take about 130 days for half of the equatorial form to be converted to the axial form. However, when the solution is warmed to —60° the equatorial conformation is converted to the equilibrium mixture in a few tenths of a second. 

Exercise 12-4 Using the sawhorse convention, draw the possible conformations of chlorocyclohexane with the ring carbons in the planar, in the chair, and in the extreme boat forms. Arrange these in order of expected stability. Show your reasoning. 

In the two cis/trans isomers of 4-chlorocyclohexane carboxylic acid the conformers with axial chloro atoms have conformational energies ca. 30% below those with equatorial chloro atoms (99MI7) in this connection the proportion of the trans-di-axial conformer is greater than expected. 

Recall from Section 4.13 that cyclohexane exists as two chair conformations that rapidly interconvert, and that substituted cyclohexanes are more stable with substituents in the roomier equatorial position. Thus, chlorocyclohexane exists as two chair conformations, but A is preferred because the Cl group is equatorial. 

On each carbon, one bond is directed up or down and the other more or less in the plane of the ring. The up or down bonds are called axial and the others equatorial. The axial bonds point alternately up and down. If a molecule were frozen into a chair form, there would be isomerism in mono-substituted cyclohexanes. For example, there would be an equatorial methylcyclohexane and an axial isomer. However, it has never been possible to isolate isomers of this type at room temperature. This proves the transient existence of the boat or twist form, since in order for the two types of methylcyclohexane to be nonseparable, there must be rapid interconversion of one chair form to another (in which aU axial bonds become equatorial and vice versa) and this is possible only through a boat or twist conformation. Conversion of one chair form to another requires an activation energy of lOkcalmoP (42kJmol ) and is very rapid at room temperature. However, by working at low temperatures, Jensen and Bushweller were able to obtain the pure equatorial conformers of chlorocyclohexane and trideuteriomethoxycyclo-hexane as solids and in solution. Equatorial chlorocyclohexane has a half-life of 22 years in solution at — 160°C. 

The same kind of conformational analysis just carried out for cis- and fra/i.s-1,2-dimethylcyclohexane can be done for any substituted cyclohexane, such as c7.s-l-ftT(-butyl-4-chlorocyclohexane (see Worked Fixample 4.3). As you might imagine, though, the situation becomes more complex as the number of... 

Draw the most stable conformation of as-l-fc/M)utyl-4-chlorocyclohexane. hy how... 

Half-Life for Conformation Inversion for Chlorocyclohexane at Various Temperatures... 

Crystallization of chlorocyclohexane at low temperature provided crystals containing only the equatorial isomer. When the solid is dissolved at — 150°C, the NMR spectrum of the solution exhibits only the signal characteristic of the equatorial conformer. When the solution is warmed to -115°, the conformation equilibrium is reestablished. The appearance of the 60-MHz spectrum of the H-C—C hydrogen is shown in Figure 2.15. 

The more stable conformer of chlorocyclohexane does not undergo an E2 reaction, because the chloro substituent is in an equatorial position. (Recall from Section 2.13 that the more stable conformer of a monosubstituted cyclohexane is the one in which the substituent is in an equatorial position because there is more room... 

Ans. Structures I and II are conformational isomers of chlorocyclohexane but the two are not of equal stability. There are two types of bonds at each carbon of the ring—axial (a) and equitorial (e). 

Axial bonds are those in the vertical plane. Equitorial bonds are those at an angle to the horizontal plane. Substituents larger than hydrogen are more stable in equitorial positions. This makes conformation II more stable than I since II has the large Cl in the less stable axial position while I has the smaller H in the axial position. Chlorocyclohexane consists primarily of conformation II. 

Fig. 15. a) Stereoview, along the c axis, of the encaged conformations of chlorocyclohexane. Dotted line crystallographic twofold axis. For clarity the guest molecules are shown single-positioned. b) Endocyclic torsion angles of chlorocyclohexane in various conformations (a) major observed (b) major calculated, symmetry (c) minor observed (d) minor calculated. The axial Cl atom. is linked to atom C(l). Reproduced with permission from J. Inch Phenom. 3, 335 (1985)... 

A number of experimental techniques have also been used to show that certain disubstituted cyclohexanes can adopt uncharacteristic conformations inside the thiourea tunnel structure. For rra z.y-l-bromo-2-chlorocyclohexane, intramolecular Br-. -Cl and Br-. -C(3) distances of 4.5 A and 3.27 A determined from bromine K-edge EXAFS spectra demonstrate that the diaxial conformation is preferred. In contrast, the diequatorial conformation is preferred in dispersed phases. 

Shannon, I.J. Jones, M.J. Harris, K.D.M. Siddiqui. M.R.H. Joyner. R.W. Probing the conformational properties of guest molecules in solid inclusion compounds via EXAFS spectroscopy Bromine K-edge EXAFS studies of the bromocyclohexane/thiourea and frans-l-bromo-2-chlorocyclohexane/thiourea inclusion compounds. J. Chem. Soc., Faraday Trans. 1995, 91. 1497. 

At much lower temperatures, conformational change is slowed. For example, Jensen, F. R. Bushweller, C. H.. Am. Chem. Soc. 1966,88,4279 studied pure equatorial chlorocyclohexane by NMR at -151°C. 

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