Alkane Newman projections

Higher alkanes having unbranched carbon chains are, like butane, most stable in theh 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 present in 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 in a Newman projection. Viewed from this perspective, the most stable conformations of pentane and hexane... 

This results in a molecule whose energy is comparable to a completely staggered, open-chain alkane. This is easy to see by viewing the molecule in a Newman projection. Viewing the molecule through the ring so that the two side carbon-carbon bonds are seen head on, as in a Newman projection, generates the view in (6.5). 

Draw, using dash-wedge, sawhorse, or Newman projection formulas, the important conformations of ethane, propane, butane, and various halogenated derivatives of these alkanes. 

Propane is the three-carbon alkane, with formula C3H8. Figure 3-8 shows a three-dimensional representation of propane and a Newman projection looking down one of the carbon-carbon bonds. 

Name an Alkane Using the lUPAC System 122 Name a Cycloalkane Using the lUPAC System 126 Draw a Newman Projection 132 Draw the Chair Form of Cyclohexane 140 Draw the Two Conformations for a Substituted Cyclohexane 143 Draw Two Conformations for a Disubstimted Cyclohexane 146 Stereochemistry... 

In alkanes or alkyl systems, an infinite number of conformations is possible as a result of rotation about CC single bonds, and each conformation has a certain potential energy. Two conformational extremes, one of low (minimal steric interaction between bond substituents) and one of high (maximal steric interaction) potential energy, have been described for these systems. These are depicted as Newman projections in Fig. 17. A more recent nomenclature system for conformational isomers arising from energy barriers associated with rotation about a CC single bond is shown in Fig. 18. In this case, the most stable conformer is... 

SAMPLE SOLUTION (a) The Newman projection of this alkane resembles that of ethane except one of the hydrogens has been replaced by a methyl group. The drawing is a Newman projection of propane, CH3CH2CH3. 

Newman projection Representation of an alkane along the carbon axis, so that the conformation is easily seen. 

A portion of an n-alkane is illustrated in Figure 5.10, where the (a) trans(t) and (b) gauche (g) conformations about the central C-C bond are presented in Newman projections. Note that the observed (C ) and y-substituent (C ) carbons are proximal in the gauche conformation (3 A) and distant (4 A) in the trans conformation. It has been demonstrated that when a carbon nucleus is gauche to a substituent carbon in the y position it is shielded by approximately — 5 ppm and therefore resonates 5 ppm upheld from a similar carbon nucleus that is trans to its y-substituent carbon. 

Figure 5.10 Newman projections of a n-alkane chain in the (a) trans ((/> = 0°) and (b) gauche ((j) = 120°) conformations, (c) Derivation of the y-gauche shielding produced by the y-substituents...

Figure 2.1 Potential energy for rotation about a C—C bond in an alkane. The minima correspond togauche ((f> = —120°), trans ((/> = 0°) andgauche (0 = 120°) conformers, as represented by the Newman projections shown for butane...

Use Newman projections to represent the possible conformations of an alkane. 

Alkanes are compounds in which stereochemical differentiation results from their various possible conformations. A number of these compounds, especially with different substituents present in the chain, may exist as enantiomers or diastereoi-somers, that is, compounds that are optically active. Figure 2.10 shows Newman projections of six different conformations for a molecular fragment consisting of two adjacent carbon atoms with substituents. 

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