Newman projection chair conformation

There are two very important drawing styles that show conformations and give us the power to predict what conformations are available to different types of molecules Newman projections and chair conformations ... 

Figure 4.13 (a) A Newman projection of the chair conformation of cyclohexane. 

Newman projection of chair conformation looking along two opposite C —C bonds all bonds are staggered... 

The equivalence of the predictions of ALPH and of PLNM are best illustrated by consideration of the reactions of axial and equatorial tetrahydro-pyranyl or pyranosyl derivatives3 (Scheme 13). The preferred conformation of the cationic intermediate is assumed to be the half-chair, and the nature of the atomic motions is best seen if one takes as a plane of reference the plane defined by C(l), C(2), 0(5) and C(5) of the oxocarbonium ion intermediate, and considers motions about the C(l)—0(5) bond, set out in Newman projections in Scheme 13. 

The two main conformational shapes for cyclohexane are called the chair and the boat (Fig.M). The chair form is more stable than the boat form since the latter has eclipsed C-C and C-H bonds. This can be observed in the Newman projections (Fig.N) that have been drawn in such a way that we are looking along two bonds at the same time-bonds 2-3 and 6-5. In the chair conformation, there are no eclipsed C-C bonds. But, in the boat conformation, bond 1-2 is eclipse with bond 3-4, and bond 1-6 is eclipsed with bond 5-4. This indicates that the boat conformation is less stable than the chair conformation and the majority of... 

Fig.N. Newman projections of the chair and boat conformations of cyclohexane.

Figure 16. Chair conformations of frans-(a-axial)-methyl-4-rm-butylcyclohexanone (a) and (a-axial)-methylbicyclo[3.2.1]octanone (c). Newman projections showing relative orientations of methyl and C=0 groups (b) and (d), with torsion angles indicated.

Draw the structural formula of 2,2-dichloro-l,l-difluoro-4,4-dimethylcyclo-hexane as a Newman projection formula viewed along the C1-C2 and C5-C4 bonds. Assume that the compound adopts a chair conformation. 

Like the chair conformation, all of the C—C—C bond angles of the boat conformation are 109.5°, so it has no angle strain. However, it does have other types of strain. The two red hydrogens, called flagpole hydrogens, approach each other too closely and cause some sceric strain. In addition, the conformations about the green bonds are eclipsed. This can be seen more easily in the Newman projection down these bonds ... 

Conformations of methylcyclohexane. Plat ring perspective, 0 CHAIR CONFORMATIONS, AND Q NEWMAN PROJECTION. 

Viewed from the side, the chair conformation of cyclohexane appears to have one methylene group puckered upward and another puckered downward. Viewed from the Newman projection, the chair has no eclipsing of the carbon-carbon bonds. The bond angles are 109.5°. 

The two chair conformations of methylcyclohexane interconvert at room temperature, so the one that is lower in energy predominates. Careful measurements have shown that the chair with the methyl group in an equatorial position is the most stable conformation. It is about 7.6 kJ/mol (1.8 kcal/mol) lower in energy than the conformation with the methyl group in an axial position. Both of these chair conformations are lower in energy than any boat conformation. We can show how the 7.6 kJ energy difference between the axial and equatorial positions arises by examining molecular models and Newman projections of the two conformations. First, make a model of methylcyclohexane and use it to follow this discussion. 

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... 

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. 

The doublet is from the C-H at the radical centre, with a coupling constant similar to other alkyl radicals. The cyclohexyl radical will be expected to have the normal chair conformation, though with an sp hybridized radical centre. A Newman projection below) looking from Cl to either C2 or C6 will show an axial hydrogen nearly eclipsing the p orbital with the unpaired electron. 

The chair conformer of cyclohexane, a Newman projection of the chair conformer, and a ball-and-stick model showing that all the bonds are staggered. 

One conformation appears to be lowest in energy. This conformation is shown as molecular model 48D in the same Newman-type projection as 48C. This conformation is thought to look a little like an easy chair, and it is called a chair conformation. The chair shape can be seen in 48B or in molecrdar model 48E, which views the conformation from the side. Note that 48D is the molecular model of48B, and that 48D and 48E are identical, but simply viewed from a different perspective. 

Aspects of cyclohexane conformational analysis. A. Interconverting chair forms of cyclohexane, with axial and equatorial locations labeled. In the left structure letters x are axial while letters y are equatorial. Note that the chair flip moves axial substituents to the equatorial position and vice versa. B. Newman projections down the C1-C2/ C5-C4 bonds of methylcyclohexane. In the axial form, there is a gauche butane interaction between the methyl and C3. C. Views of cyclohexane, equatorial methylcyclohexane, and axial methylcyclohexane. Note that chair cyclohexane is a relatively disk-shaped molecule, and an equatorial methyl does little to disrupt this shape. In contrast, an axial substituent puts a "kink" into the structure. Also evident is the steric interaction between one methyl hydrogen and the two axial hydrogens on C3 and C5. 

In one chair conformation, the leaving group occupies an axial position. In the other chair conformation, the leaving group occupies an equatorial position. The requirement for an periplanar conformation demands that an E2 reaction can only occur from the chair conformation in which the leaving group occupies an axial position. To see this more clearly, consider the Newman projections for each chair conformation ... 

Fig. 37 (a) Fisher projection of D-glucose. (b) Haworth projections of a and p anomers. (c) Ci chair conformations, (d) Newman projections of the plausible conformations of the hydroxymethyl group around C5-C6 and Cg-O bonds. (From [225])... 

---