Newman projections of butane

Lets look at a Newman projection of butane as it rotates counterclockwise around its axes. 

The staggered-gauche conformation shown in Model 4 is not the most favorable or the least favorable conformation of butane. Draw a Newman Projection of butane in its least favorable (highest potential energy) conformation. 

Increased substitution around a bond leads to increased strain. Take the four substituted butanes listed below, for example. For each compound, sight along the C2-C3 bond and draw Newman projections of the most stable and least stable conformations. Use the data in Table 3.5 to assign strain energy values to each conformation. Which of the eight conformations is most strained Which is least strained ... 

FIGURE 2.7 (a) potential energy profile illustrating the potential energy changes associated with rotation around a C-C bond of ethane (b) Newman projections of designated conformers of n-butane. 

The boat conformation of cyclohexane (18) can be constructed from a molecular model of the chair form by holding the right-hand three carbons C(2), C(3) and C(4) of 15, clamped from the top with the hand and moving the left-hand three carbons upward. A Newman projection of the boat form looking along the C(l)-C(2) bond, and shown in 19, is reminiscent of the highest energy cis conformation of butane. 

Figure 2.4 Newman projections of the eclipsed and staggered conformations of a n-butane molecule.

Fig. 4.3. Newman projections of the confonners of n-butane, together with its potential energy diagram showing the barriers to free rotation.

Torsional strain is the second factor in cyclopropane s large ring strain. The three-membered ring is planar, and all the bonds are eclipsed. A Newman projection of one of the carbon-carbon bonds (Fig. 3-16) shows that the conformation resembles the totally eclipsed conformation of butane. The torsional strain in cyclopropane is not as great as its angle strain, but it helps to account for the large total ring strain. 

Looking along the bond between carbons 2 and 3 of butane, there are two different staggered conformations and two different eclipsed conformations. Draw Newman projections of each, and arrange them in order from the most stable conformation to the least stable conformation. (See Example 3.7)... 

Draw Newman projections of four different conformations of butane. Rank them in stability. Hint Two of the conformations represent the most stable forms and the other two the least stable forms.)... 

FIGURE 2.32 (a) Newman projections for butane. The staggered conformations are shown, (b) A graph of dihedral angle (0) versus energy for hutane as the molecule changes through rotation about the C(2)—C(3) bond. AH three eclipsed conformations are shown. 

WORKED PROBLEM 5.9 Draw the Newman projection of methylcyclohexane looking from the other adjacent methylene group toward the methyl-bearing carbon. Be sure that you see the second gauche methyl—ring interaction. Next, compare this gauche interaction to that ingaett e-butane. Are the two exactly the same ... 

A Newman projection of the conformation of methylcyclohexane with the equatorial methyl group made from the same perspective reveals none of these gauche interactions—this arrangement resembles the anti form of butane (Fig. 5.29). 

The structural representation shown below is a Newman projection of the conformer of butane that is... 

Butane is a straight four-carbon chain. For drawing a Newman projection it was convenient for us to think of the CH3 at either end of the chain as an R group. This same strategy can be used to simplify Newman projections of more complicated molecules. 

Fig. 3.2 Newman projections of the different conformations of the n-butane molecule.

FIGURE 3 6 The gauche and anti conformations of butane shown as ball and spoke mod els left) and as Newman projections right) The gauche conformation is less stable than the anti because of the van der Waals strain between the methyl groups... 

We can view this reaction as the replacement of one or the other of the two methylene protons at C 2 of butane These protons are prochiral atoms and as the red and blue protons m the Newman projection indicate occupy mirror image environments... 

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... 

Gauche conformation (Section 3.7) The conformation of butane in which the two methyl groups lie 60° apart as viewed in a Newman projection. This conformation has 3.8 kj/mol steric strain. 

We have seen that Newman projections are a powerfnl way to show the different conformations of a molecule. We mentioned earlier that there are staggered conformations and eclipsed conformations. In fact, there are three staggered and three eclipsed conformations. Let s draw all three staggered conformations of butane. The best way to do this is to keep the back carbon atom motionless (so the fan in the back is not spinning), and let s slowly turn the groups in the front (only the front fan is spinning) ... 

The many different conformers resulting from rotation around the carbon-carbon bonds in simple molecules like ethane and w-butane may be shown by Newman projections (Figure 2.7). The most stable is the anti or trans projection where the steric hindrance is minimized. There are a number of eclipsed and gauche arrangements of which only one of... 

Other common methods for representing the three-dimensional structures of molecules include Newman projections for showing conformational relationships and sawhorse figures. Newman projections look down a carbon-carbon bond so that the front carbon, designated by a circle, obscures the carbon directly behind it. Valences (bonds) to the front carbon extend to the center of the circle, while bonds to the rear carbon stop at the circle. Sawhorse projections have the carbon-carbon bond at oblique angles, which attempts to represent a perspective drawing of the molecule. Thus for 2-chloro butane, if one chooses to examine the 2,3 bond, then the sawhorse and Newman projections would be... 

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