Anti conformation alkanes

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... 

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... 

Table 4.6 shows the effects of a substituent on linear and branched alkanes. The effect on the a-carbon parallels the electronegativity of the substituent except for bromine and iodine.t The effect at the /3-carbon seems fairly constant for all the substituents except for the carbonyl, cyano, and nitro groups. The shift to the right at the y carbon results (as above) from steric compression of a gauche interaction. For Y = N, O, and F, there is also a shift to the right with Y in the anti conformation, attributed to hyperconjugation. 

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... 

Usually, the conformation of a molecule in the crystal is the lowest energy conformation that the molecule can take up in the gas phase. Thus, for example, the n-alkanes tend to crystallize in the all-anti conformation. Occasionally, a crystal is found in which a slightly less stable conformation in the gas phase becomes the only conformation in the crystal phase. For this to happen, the less stable gas-phase molecule must form a crystalline lattice that is more stable than the one formed by the anticipated conformation, and by more than the energy difference between the conformations. Or, it may be an unstable polymorph, which will go over to the stable polymorph (and the stable conformation) under thermodynamic conditions. 

A subtle, but important conformational effect is the g+g- pentane interaction, which gives rise to what is commonly called si/n-pentane strain. Recall that there are two possible gauche forms around an internal bond in a linear alkane, and the + / -designation of Figure 2.7 is used to differentiate them. For ri-pentane the all-anti conformer is the most stable. However, in some situations gauche interactions will develop. A common example is in a reaction transition state, when reactants are held in a cyclic array. 

Derivatives in which the vicinal difluoromethylene motif is extended to include 3,4, 5 and 6 fluorine atoms have been examined to determine conformational preferences [24], The most sophisticated study focused on the hexafluo-romethylene motif, carefully assembled in the diastereomers 3 and 4, that extended earlier studies on tetra- and pentafluoro alkanes [24, 27]. The conformation of these compounds, both in the solid state and in solution, indicates that in order to avoid unfavorable 1,3-dipolar interactions between F atoms, 3 adopts a helical topography that simultaneously allows the vicinal fluorine atoms to adopt the preferred gauche arrangement. The same effect favors an extended anti conformation for 4 that presents no unfavorable 1,3-dipolar interactions and facilitates 3 out 5 possible gauche arrangements of the vicinal fluorine atoms [24, 27],... 

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