Cyclohexane conformations, cycloalkanes

Summary Rules for Naming Alkanes 94 3-4 Physical Properties of Alkanes 95 3-5 Uses and Sources of Alkanes 97 3-6 Reactions of Alkanes 99 3-7 Structure and Conformations of Alkanes 100 3-8 Conformations of Butane 104 3-9 Conformations of Higher Alkanes 106 3-10 Cycloalkanes 107 3-11 Cis-trans Isomerism in Cycloalkanes 109 3-12 Stabilities of Cycloalkanes Ring Strain 109 3-13 Cyclohexane Conformations 113... 

In this chapter we ll examine the confonnations of various alkanes and cycloalkanes, focusing most of our attention on three of them ethane, butane, and cyclohexane. A detailed study of even these three will take us a long way towar d understanding the main ideas of conformational analysis. 

We JI look in the next chapter at cycloalkanes—saturated cyclic hydrocarbons— and we ll see that the molecules generally adopt puckered, nonplanar conformations. Cyclohexane, for instance, has a puckered shape like a lounge chair rather than a flat shape. Why ... 

A conformational effect was detected for the H-transfer reactions from cycloalkanes to a series of attacking radicals. The data of Table 6 show that cyclopentane is generally a better H-donor than cyclohexane. The rate ratio is generally largest for the least reactive radicals because the change in hybridization at transition state... 

In a similar way the potential constant method as described here allows the simultaneous vibrational analysis of systems which differ in other strain factors. Furthermore, conformations and enthalpies (and other properties see Section 6.5. for examples) may be calculated with the same force field. For instance, vibrational, conformational, and energetic properties of cyclopentane, cyclohexane and cyclodecane can be analysed simultaneously with a single common force field, despite the fact that these cycloalkanes involve different distributions of angle and torsional strain, and of nonbonded interactions 8, 17). This is not possible by means of conventional vibrational spectroscopic calculations. 

Cycloalkanes. Cycloalkanes are conformationally restricted alkanes. Three rings are employed in drug design cyclopropane, cyclopentane, cyclohexane (the latter two are... 

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. 

Cycloalkanes can be oxygenated when irradiated in the presence of nitrobenzene.196 A 50% yield of cyclohexanol and cyclohexanone is achieved from cyclohexane. Since the product ratio is independent of reaction time, the alcohol is not an intermediate in ketone formation. Isomeric 1,2-dimethylcyclohexanes give an identical mixture of the isomeric tertiary alcohols, indicative of conformational equilibration and the presence of a radical intermediate. 

We also discuss the conformations of cycloalkanes, especially cyclohexane, in detail because of their importance to the chemistry of many kinds of naturally occurring organic compounds. Some attention also will be paid to polycyclic compounds, substances with more than one ring, and to cyclo-alkenes and cycloalkynes. 

The prefix cyclo- is used to name cycloalkanes. Cyclopropane is planar, but larger carbon rings are puckered. Cyclohexane exists mainly in a chair conformation with all bonds on adjacent carbons staggered. One bond on each carbon is axial (perpendicular to the mean carbon plane) the other is equatorial (roughly in that plane). The conformations can be interconverted by flipping the ring, which requires only bond rotation and occurs rapidly at room temperature for cyclohexane. Ring substituents usually prefer the less crowded, equatorial position. 

We will cover the conformations of cyclohexane in more detail than other cycloalkanes because cyclohexane ring systems are particularly common. Carbohydrates, steroids, plant products, pesticides, and many other important compounds contain cyclohexanelike rings whose conformations and stereochemistry are critically important to their reactivity. The abundance of cyclohexane rings in nature is probably due to both their stability and the selectivity offered by their predictable conformations. Nature probably forms more six-membered rings than all other ring sizes combined. 

These are not frequently used monomers. There exists a clear connection between the strain of various members in the series of cyclic hydrocarbon molecules and their heats of combustion (see Table 1). The high heats of combustion of the first members are the consequence of the C—C bond angle deviation from 109°28. In cyclohexane, the most stable cycloalkane which can exist in the chair conformation, the C—C bond angle value deviates very little from that observed in unstrained compounds. Cyclopentane exhibits the smallest deviation of the C—C bond angle from the theoretical value. Its higher heat of combustion is due to steric interactions of pairs of neighbouring hydrogen atoms. A similar situation is observed with cycloheptane [12a]. 

Tatlow and his co-workers conducted an extremely comprehensive programme of syntheses and structure derivations of a series of fluorinated cycloalkanes [24], and concluded that the reactivity of the system, as well as the orientation of the cycloalkene produced, are similarly influenced by electronic factors which have been outlined in the preceding sections of this chapter. Anti elimination is generally the more favourable process but conformational effects may make the synjanti rates nearly comparable. Elimination from the cyclohexanes 6.16A and 6.16B illustrates the balance between electronic and conformational effects [25]. Anti elimination is possible from 6.16A, involving removal of fluoride from >CHF rather than >Cp2 since in this case electronic (the carbon-fluorine bond in CFH is weaker than in CF2) and conformational effects (H and F are anf -periplanar) are in concert (Figure 6.16). In contrast, anti elimination from 6.16B can only occur with elimination of fluoride from the more stable >CF2 position and therefore anti and syn eliminations occur together. 

Name an Alkane Using the lUPAC System 122 Name a Cycloalkane Using the lUPAC System 126 Draw a Newman Projection 132 Draw the Chair Form of Cyclohexane 140 Draw the Two Conformations for a Substituted Cyclohexane 143 Draw Two Conformations for a Disubstimted Cyclohexane 146 Stereochemistry... 

Let us look more closely at the matter of puckered rings, starting with cyclohexane, the most important of the cycloalkanes. Let us make a model of the cule, and examine the conformations that are free of angle strain. 

If chair cyclohexane is, conformationally speaking, the perfect specimen of a cycloalkane, planar cyclopentane (Fig. 9.6) must certainly be the poorest there is... 

As has been pointed out by Hart <84CHEC-I(7)185>, thiiranes fused to cyclopentanes adopt a boat conformation with C(3) and the sulfur atom on the same side of the C(1)C(2)C(4)C(5) plane. The angle of deviation from this plane is about 30° for C(3) and 60-70° for the S atom. Cyclohexane rings having fused thiiranes prefer a twisted half-chair conformation. Thiiranes fused to larger cycloalkanes can adopt a wide variety of conformations. 

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