Structure and Conformations of Alkanes

Although alkanes are not as reactive as other classes of organic compounds, they have many of the same structural characteristics. We will use simple alkanes as examples to study some of the properties of organic compounds, including the structure of sp hybridized carbon atoms and properties of C—C and C—H single bonds. 

The simplest alkane is methane, CH4. Methane is perfectly tetrahedral, with the 109.5° bond angles predicted for an sp hybrid carbon. Four hydrogen atoms are covalently bonded to the central carbon atom, with bond lengths of 1.09 A. 

Ethane, the two-carbon alkane, is composed of two methyl groups with overlapping sp hybrid orbitals forming a sigma bond between them. 

In drawing conformations, we often use Newman projections, a way of drawing a molecule looking straight down the bond connecting two carbon atoms (Fig. 3-5). The front carbon atom is represented by three lines (three bonds) coming together in a 

The Newman projection looks straight down the carbon-carbon bond. 

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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... 

A. Warshel and S. Lifson, ]. Chem. Phys., 53, 582 (1970). Consistent Force Field Calculations. II. Crystal Structures Sublimation Energies, Molecular and Lattice Vibrations, Molecular Conformations and Enthalpies of Alkanes. 

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. 

CONSISTENT FORCE FIELD CALCULATIONS. II. CRYSTAL STRUCTURES, SUBLIMATION ENERGIES, MOLECULAR AND LATTICE VIBRATIONS, MOLECULAR CONFORMATIONS, AND ENTHALPIES OF ALKANES. 

Warshel A., Lifson, S. (1970). Consistent force field calculations. II. Crystal structures, sublimation energies, molecular and lattice vibrations, molecular conformations, and enthalpies of alkanes. Journal of Chemical Physics, 53, 582. 

This paper represents the most penetrating analysis yet performed of structures and spectra of small alkanes, and it is imperative for serious students of potential energy functions employed in conformational analysis. Here is a set of functions, with parameters, which reproduce very well observations on methane, cyclopentane, normal alkanes, and, mirabile dictu, tris-(tert-butyl)-methane. 

Typical carbon-oxygen bond distances m ethers are similar to those of alcohols (—142 pm) and are shorter than carbon-carbon bond distances m alkanes (—153 pm) An ether oxygen affects the conformation of a molecule m much the same way that a CH2 unit does The most stable conformation of diethyl ether is the all staggered anti conformation Tetrahydropyran is most stable m the chair conformation—a fact that has an important bearing on the structures of many carbohydrates... 

The number (nj) of the cross ct conjugations of the trios of a C-C bond and two antiperiplanar C-H bonds is important for the stabilities of alkanes. The cross conjugation number (nj) of an alkane is defined as that of the conformer where the longest C-C chain has trans a zigzag structure. For example, there are three cross conjugations (n = 3) in isobutene and none in n-butane (n = 0) (Scheme 27). Isobutane is more stable than n-butane [34, 35]. 

A short presentation of the Consistent Force Field is given, with emphasis on parametrization and optimization of energy function parameters. For best possible calculation of structure, potential energy functions with parameter values optimized on both structural and other properties must be used. Results from optimization with the Consistent Force Field on alkanes and ethers are applied to glucose, gentiobiose, maltose and cellobiose. Comparison is made with earlier and with parallel work. The meaning and use of conformational maps is discussed shortly. 

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