Conformations of acyclic alkanes

Despite these structural features, the adamantane ring system is not strain-free 142). Comparison of an estimated heat of formation for adamantane based on group increments derived from acyclic alkanes in completely skew-free conformations 142> with the experimentally determined value 143 indicates that adamantane is strained to the extent of 6.48 kcal/mole. 

The source of the strain in adamantane is not readily apparent but appears to be due to features present in the rigid, cage structure of the molecule. Less rigid molecules, e.g. acyclic alkanes, cyclohexane and /nms-decalin, are free to relax and to adopt conformations in which the best balance between angle, nonbonded and torsional strain is achieved. Thus, C-C-C bond angles of 112.4... 

It is easy to list the various chemical and biological events influenced by flexibility, but unfortunately efforts to quantitate this structural attribute have been few. Mann analyzed the conformation of alkanes by modifying the number of gauche arrangements with Pitzer s steric partition function. Luisiranked alkanes on a. scale of conformational rigidity based on three-states rotational isomerism. Unfortunately, these schemes are designed for acyclic hydrocarbons and have no inherent capability to be adapted to heteroatomic molecules. 

The effect of introducing /j -hybridized atoms into acyclic molecules was discussed in Section 2.2.1, and it was noted that torsional barriers in 1-alkenes and aldehydes are somewhat smaller than in alkanes. Similar effects are seen when sp centers are incorporated into six-membered rings. Whereas the energy barrier for ring inversion in cyclohexane is 10.3 kcal/mol, it is reduced to 7.7 kcal/mol in methylenecy-clohexane ° and to 4.9 kcal/mol in cyclohexanone. The conformation of cyclohexene is described as a half-chair. Structural parameters determined on the basis of electron diffraction and microwave spectroscopy reveal that the double bond can be accommodated into the ring without serious distortion. The C(l)—C(2) bond length is 1.335 A, and the C(l)-C(2)-C(3) bond angle is 123°. The substituents at C(3) and C(6) are tilted from the usual axial and equatorial directions and are referred to as pseudoaxial and pseudoequatorial. 

Kolossvary I and W C Guida 1996 Low Mode Search An Efficient, Automated Computational Method for Conformational Analysis- Application to Cyclic and Acyclic Alkanes and Cyclic Peptides Journal of the American Chemical Society 118 5011-5019. 

Saturated aliphatic amines are similar to the alkanes, the main differences being the short C—N bond (C—C 1.523 A, C—N 1.450 A) and the replacement of one substituent by a lone pair of electrons (LP). The pyramidal structure of acyclic amines is quite labile, with barriers to inversion in the range 4-8 kcal mol (Rauk et al, 1971). The problem of the steric requirements of the lone pair has been much discussed in relation to conformational equilibria in saturated six-membered N-heterocycIes (Lambert and Feather-man, 1975 Blackburne et al, 1975). 

Hz, if 2,4-dimethylpentane can be taken as an example. In the tri-tm-butylethane, which is a good model for the eclipsed conformation, ZJ = 7.2 Hz. Table 1 shows these values and several more for acyclic alkanes which seem to span the range of conformations intermediate between staggered and eclipsed, with the sum of the coupling constants quantifying the intermediacy. 

Continuing interest in rotation barriers about single bonds in acyclic radicals, as determined by the interpretation of e.s.r. data, is illustrated in a study of R CHSR radicals and alkane-, arene-, and alkoxy-sulphonyl radicals (obtained by high-intensity u.v. irradiation of sulphinic acids in the presence of di-t-butyl peroxide at low temperatures). Further evidence for hindered rotation about the C—S bond in these species is obtained, - and an unusual order of proton hyperfine splittings a 0-H)> a (a-H) a (y-H) is reported for propanesulphonyl radicals. The potential of the e.s.r. method in conformational analysis is shown in studies of radical cations (3) from 2,5-bis(alkylthio)thiophens, where the S-cis-cis conformer (3) is identified as more stable than other rotamers this is assumed to be the case too for the neutral molecule. 

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