Newman formulas

In cis and trans -1, 3 dichloropropenes, the skew form is again more stable (hi the Newman formula for this compound, the CHC1 group would be in place of CH2 group). 

We have indicated how the enantiomers of 2-butanol differ by drawing their structures 5 and 6 (Section 5-ID) in perspective to show the tetrahedral configuration of substituents at the chiral carbon. This configuration also can be represented by the sawhorse or Newman formulas using any one of the several possible staggered conformations such as 5a and 6a or 5b and 6b ... 

Figure 5-13 Translation of a projection formula to a ball-and-stick model and then to a Newman formula. The first step is to assemble the model over the projection formula in the same way as shown in Figure 5-12, except that configurations at each of two atoms are involved. Next, the model is turned and rotated around the center C-C bond to put the methyl and carboxyl groups anti to one another. The Newman formula then is drawn to correspond to the ball-and-stick model.

Therefore, if you are confronted with a particular sawhorse or Newman formula and you have to decide whether it represents a meso compound, the best procedure is to make a ball-and-stick model of the conformation and then rotate around the bonds to see if it can be brought into a conformation (staggered or eclipsed) that has a plane of symmetry (such as 27) or is identical with its mirror image. 

The Newman formula shown in the upper scheme (right) is obtained by looking along the C3-C4 and Ci-Cg bonds. On inspection it becomes clear that the represented conformation has a staggered form and that these are stable conformers. By convention they are called chair conformers. Since all the C-C bonds are in the staggered arrangements the cyclohexane molecule is the most stable cyclic hydrocarbon. There are no C-H bonds that sterically hinder each other. Conformational dynamics of the cyclohexane ring involves limited rotation around the C-C bonds one chair conformer is transformed into another. 

The methyl group is in the axial position in the first conformer and in equatorial position in the second conformer. Both conformers can also be represented by Newman formulas ... 

Sight down the C 2—C 3 bond and draw Newman projection formulas for the... 

In Newman projection formulas (the two figures on the right) the observer looks at the C C bond head on. The three lines emanating from the center of the circle represent the bonds coming from the front carbon, with respect to the observer. 

Figure 4.5 a) The staggered conformation of ethane, b) The Newman projection formula for the staggered conformation. 

A Newman projection formula shows that the P-hydrogen and the chlorine are anti periplanar when they are both axial. 

Fig. 11.— Newman Projection-Formulas, Viewed Along the C-4-C-5 Bond in a, Compound 86, and b, Compound 87 (from Ref. 100, with permission).

Writing the above (a) and (b) forms in Newman projection formula, the picture will be as follows ... 

In order to explain this trend, Nasipuri and co-workers (135) adopted the essentially acyclic models 115 and 116 leading to the (R)- and (S)-carbinols respectively (Scheme 16). Models 115 and 116 are Newman-type formulas viewed along the C—H—C axis. The transition states are assumed to be reactant-like and involve loosely bound opposite dipoles O8- and M8+. 

See Fig. 9-3. The circle represents the C s of any given ring C—C bond. The other C s of the ring bridge these two C s, one to the front C (heavy line) and the other to the back C (ordinary line). This Newman projection formula reveals that the H s on adjacent C s are skewed in the puckered conformation. 

In the transition state of the enantio-determining cyclometalation step, F - G, the Rh, nitrogen, C(l), and one of the hydrogens would be roughly coplanar, which requires either clockwise or counterclockwise rotation about the N—C(l) axis (see the Newman projection formula F in Scheme 16). The transition state models la and lb explain the prevailing clockwise rotation of the conformer Fa with respect to the coun-... 

Programmed cell death. See Apoptosis Progress curve for enzyme-catalyzed reaction 455 Proinsulin 519 Projection formula Fisher 42 Newman 44... 

Figure 1.10 Definitions of the torsional angles fa ip, and w. These are all equal to 180° for a fully extended polypeptide chain (top left). to, defines rotation about the C,—Nl+1 bond. The normal tram planar peptide bond has = i/r = 180°, bottom) left, fa viewed along N —C bond (N >C) right, fa viewed along the C —C(C , — C j).

The sawhorse or Newman representations of 2-butanol, 5a and 5b and 6a and 6b, are excellent for showing the arrangements of the atoms in conformations, but are needlessly complex for representing the stereochemical configuration. Fischer projection formulas are widely used to show configurations and are quite straightforward, once one gets the idea of what they represent. 

What is the relationship between stereoisomers 19-22 This will be clearer if we translate each of the projection formulas into a three-dimensional representation, as shown in Figure 5-13. You will be helped greatly if you work through the sequence yourself with a ball-and-stick model. Drawn as Newman projections, 19-22 come out as shown in 19a-22a ... 

These projection formulas can be translated into the respective Newman representations, 23a-26a. (We highly recommend that you verify this by the procedure of Figure 5-13.)... 

The Newman representation 25a or 26a of meso-tartaric acid does not have a mirror plane. Why is it different from the Fischer projections in this respect The reason is that the projection formulas represent a particular eclipsed conformation 27 of tartaric acid that does have a mirror plane ... 

This is a problem similar to 5-29, except that the structures are written mostly as projection formulas of the Fischer or Newman type. Determine the relationship between the pairs of compounds as one of the following identical, position isomers, enantiomers, diastereomers, conformational isomers, or cis-trans isomers. (D stands for deuterium, the hydrogen isotope of mass 2.)... 

Newman projection Dash-line-wedge formula... 

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

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