Propane Newman projection

Vicinal CH coupling constants Hqh resemble vicinal HH coupling constants in the way that they depend on the cosine of the dihedral angle 9 between the CC bond to the coupled C atom and the C//bond to the coupled proton (cf Fig. 2.16), as illustrated by the Newman projections of the conformers 20a-c of a propane fragment. 

Figure 3.8 Newman projections of propane showing staggered and eclipsed conformations. The staggered conformer is lower in energy by 14 kJ/mol.

FIGURE 3. The conformation of 1 -isopropyl-1-methylcyclo-propane (19). Newman projection down the C4—Cl bond... 

A-l. Draw Newman projections for both the gauche and the anti conformations of 1-chloro-propane, CH3CH2CH2C1. Sight along the C-l, C-2 bond (the chlorine is attached to C-l). 

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. 

Follow this procedure for any C-C bond. With a Newman projection, always consider one C-C bond only and draw the atoms bonded to the carbon atoms, not the carbon atoms in the bond itself. Newman projections for the staggered and eclipsed conformations of propane are drawn in Figure 4.7. 

Newman projections for the staggered and eclipsed conformations of propane... 

In a Newman projection it doesn t matter which C you pick to be in the front or the back. All of the Newman projections shown here represent the staggered conformation of propane. 

Consideration of the Newman projection of propane (9) suggests that the energy profile associated with rotation around the CH3-CH2 bond contains both three identical maxima and three identical minima per revolution. In eclipsed conformations there is now a non-bonded repulsion between a methyl and a hydrogen accordingly, the torsional barrier, at 14.2 kJ mol1, is a little higher than in ethane. 

SAMPLE SOLUTION (a) The Newman projection of this alkane resembles that of ethane except one of the hydrogens has been replaced by a methyl group.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

Draw perspective formulas and Newman projections for the eclipsed and staggered forms of propane. 

In the eclipsed form of propane there is more steric hindrance than in the staggered form. There is also a greater torsional strain in the eclipsed form. As a result, the eclipsed form is expected to be higher in energy than the staggered form. Therefore the staggered conformation is more stable. The perspective formulas and Newman projections are sketched below ... 

Draw Newman projections for one staggered conformation and one eclipsed conformation of propane. 

Draw the line-angle formula of propane and choose a bond along which to view for the Newman projection. Keep track of the carbons in the line-angle formula and in the Newman projection (numbering them helps). Draw the staggered and eclipsed Newman projections and complete them by adding in the carbons and hydrogens. 

Problem 4.L Sketch a potential energy diagram, such as that in Figures 4.7 and 4.9, for propane (CH3CH2CH3). You may use the numbers for H-H and H-CFI3 interactions given in the text. Clearly indicate bond angles and, with sawhorse and Newman projections, show which rotational (or conformational) isomer is most stable and which is least stable. 

All of the Newman projections shown here represent the staggered conformation of propane. 

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