Hexane, conformational analysis

In this chapter we 11 examine the conformations of various alkanes and cyclo alkanes focusing most of our attention on three of them ethane butane and cyclo hexane A detailed study of even these three will take us a long way toward understanding the mam ideas of conformational analysis... 

It is not dificult to imagine that there is considerable scope for conformational analysis of dialkylcyclohexanes and polyalkylcyclohexanes, many of which have been prepared in a state of high stereochemical purityIn the case of 1,3- and 1,4-disubstituted and 1,3,5-trisubstituted cyclohexanes, conformational effects are expected to be nearly additive but there may be some interest in the extent to which they are not exactly so. Dialkylcyclohexanes with t rt-butyl as alkyl groups have received detailed consideration as discussed in Section V.E below. I am unaware of systematic work on 1,2-dialkycyclo-hexanes, although the interaction between substituents in such cases should be interesting. 

If the steric bulk is further increased, stable rotamers can be isolated, when two adamantyl groups and two tert-butyl groups are attached to the ethane skeleton (X, X = tert-butyl, Y, Y = adamantyl) The sym.-tetra-tert-butylethane (X,X, Y,Y = tert-butyl) shows non-equivalent methyl groups which is attributed to an equilibrium of distorted conformers. The conformational analysis of tri-tert-butylethane has just recently been published Other examples are triisopropylmethane, and tetramethyl-pentane and -hexane With modern low-temperature NMR techniques even more complicated conformational equilibria can be assessed. As an example, the spectrum of 3-isopropyl-2,3,4-trimethylpentane (1,1,1-triisopropylethane) 30 is shown in Figure 8 for which a detailed analysis of the conformational equilibria was given. ... 

The synthesis and conformational analysis of c(5,C(5-l,3,S-trimethylcyclo-hexane-l,3,5-tricarboxylic acid has been reported. ... 

Increasing the chain length of the alkanes, conformational analysis of a series of methyl substituted hexanes indicates that the important potential energy profiles can be identified with those bonds which contain the methyl substituent. This has important consequences for polymers. The theoretical rotational isomeric potential profile for 2-methylhexane is shown in Figure 3.8. 

Absolute asymmetric synthesis was observed in the solid-state photoreaction of benzoylbenzamide 59 to phthalide 60 however, the reaction mechanism was completely different from that of thioester 57. [35] Recrystallization of these amides 59a-c from the chloroform-hexane solution afforded colorless prisms in all cases. X-ray crystallographic analysis revealed that all prochiral amides 59a-c adopted orthorhombic chiral space group P2 2 2 and were frozen in chiral and helical conformation in the crystal lattice. 

Analysis of the conformations of other alkanes can be done in a similar manner. For any linear alkane the most stable conformation is the so-called zigzag conformation, which is anti about all of the carbon-carbon bonds. The zigzag confonnation for hexane follows ... 

Fully protonated hexane (C6H14) has 14 protons in the spin system and Drobny9 reported the MQ spectra of //-hexane-terminal methyl groups) dissolved in a nematic solvent. Analysis of the 6Q and 7Q spectra of hexane-c/6, (6) was used to establish the relative signs of the dipolar couplings. The 6-quantum proton spectrum contains 36 transitions and the 7 quantum spectrum contains 4 transitions, as expected for an 8-proton spin system with C2h symmetry. The best fit of theoretically generated MQNMR spectra to the experimental spectra were obtained with a model where only the all-trans conformations of the alkane chain were appreciably populated. 

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