Alkanes higher, conformations

Higher alkanes having unbranched carbon chains are like butane most stable m then-all anti conformations The energy difference between gauche and anti conformations is similar to that of butane and appreciable quantities of the gauche conformation are pres ent m liquid alkanes at 25°C In depicting the conformations of higher alkanes it is often more helpful to look at them from the side rather than end on as m a Newman projec tion Viewed from this perspective the most stable conformations of pentane and hexane... 

Section 3 3 Higher alkanes adopt a zigzag conformation of the carbon chain m which all the bonds are staggered... 

The same principles just developed for butane apply to pentane, hexane, and all higher alkanes. The most favorable conformation for any alkane has the carbon-carbon bonds in staggered arrangements, with large substituents arranged anti to one another. A generalized alkane structure is shown in Figure 3.10. 

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... 

The higher alkanes resemble butane in their preference for anti and gauche conforma-Conformations tions about the carbon-carbon bonds. The lowest-energy conformation for any straight-Of Hi Cl her Alkanes c la n alkane is the one with all the internal carbon-carbon bonds in their anti a conformations. These anti conformations give the chain a zigzag shape. At room... 

Pitzer strain in any planar cyclic alkane makes that conformation higher in energy and relatively unstable, and the molecule will not exist in that form if pseudorotation leads to a lower energy conformation. In the case of cyclopentane, pseudorotation about the carbon-carbon bonds alleviates the Pitzer strain and leads to a twisted structure such as 47B. This structure is discussed in more detail later, but for the moment note that the eclipsing interactions are largely removed, and the conformation is much lower in energy when compared to 47A. The space-fllling molecular model of this pseudorotated structure is 47D, where the relief of Pitzer strain is apparent when compared to 47C. 

The theory outlined above provides a basis for pre-melting that is attributed to the conformational disorder of terminal sequences. The expectation is that this type of pre-melting should be observed in the higher -alkane homologues that are available. The basic question to be addressed is whether the expectation of this pre-melting mechanism is actually observed experimentally. 

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