Conformations, anti drawings

Now we decide how to rotate the front carbon atom so as to provide the most stable conformation. The most stable conformation will be a staggered conformation with the two largest groups anti to each other, so in this case, we do not need to rotate at all. The drawing that we just drew is already the most stable conformation, because the methyl and ethyl groups are anti to each other ... 

Next we need to draw rotate to the conformation that puts the Cl and H (on the front) carbons anti to each other ... 

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 Newman projections for the anti and gauche conformations about the C—C bond of these compounds. What other factors, besides steric and torsional strain, influence the stability of these conformations ... 

Both cis and trans alkenes can be formed from this compound by anti elimination. Draw a Newman projection of the conformation required to form each of these products and. on the basis of these projections, predict which of these products would be formed in larger amounts. 

Draw a Newman projection, similar to Figure 3-25, down the Cl —C6 bond in the equatorial conformation of methylcyclohexane. Show that the equatorial methyl group is also anti to C5. (Using your models will help.)... 

Conformational studies on ethane-1,2-diol (HOCH2—CH2OH) have shown the most stable conformation about the central C—C bond to be the gauche conformation, which is 9.6 kJ/mol (2.3 kcal/mol) more stable than the anti conformation. Draw Newman projections of these conformers and explain this curious result. 

You now have a more thorough explanation of the zig-zag arrangement of carbon chains, first introduced in Chapter 2 when we showed you how to draw molecules realistically. This isthe shape you get if you allow all the C-C bonds to take up the anti-periplanar conformation, and will be the most stable conformation for any linear alkane. 

Among the most widespread classes of acyclic compounds to exhibit stereoelectronic control over conformation are acetals. Take the simple acetal of formaldehyde and methanol, for example what is its conformation An obvious suggestion is to draw it fully extended so that every group is fully anti-periplanar to every other—this would be the lowest-energy conformation of pentane, which you get if you just replace the Os with CH2S. 

Convert this drawing into a Newman projection, and draw the conformation having anti periplanar geometry for -H and -Br. 

Draw a Newman projection of the tosylate of (2/ ,35)-3-phenyl-2-butanol. and rotate the projection until the -OTos and the -H on the adjoining carbon atom are anti periplanar. Even though this conformation has several gauche interactions, it is the only conformation in which -OTos and -H are 180° apart. 

Draw (IS,2S)-l,2-dibromo-l,2-diphenylethane so that you can see its stereochemistry and so that the -H and -Br groups to be eliminated are anti periplanar (molecular models are ex( remely lielpful here). Elimination of HBr from this conformation gives (Z)-l-bromo-l,2-diphenylethylene. 

First draw the compound without stereochemistry. Then we will convert it to a Newman projection with the 2R, 3S configuration. This is most easily done by drawing an eclipsed Newman projection and putting the two low priority groups down and placing the others to conform to the stated configuration. Finally, rotate the Newmans to syn and anti elimination and draw the products. 

A word of warning straight away - be careful about how the molecule has been drawn (especially if you have previously suffered from stereochemistry denial ). Redrawing the apparent syn diol 49 reveals the anti relationship between the hydroxyl groups 49b. The plane of symmetry is no longer evident. In fact, the molecule has a centre of symmetry in this conformation. Diol 49 is achiral however we draw it. Normally, our priority should be for planes and axes before we start thinking about centres. 

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