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In a different video, we showed E2 eliminations on cyclohexane rings. Cyclohexanes are great for showing how E2 reactions are more favorable if the leaving group is anti relative to a beta hydrogen. The rings, with their axial and equatorial orientations, allow easier visualization of the anti orientation of adjacent groups. Newman projections allow us to make these visualizations in acyclic alkyl halides.
Here is an alkyl halide – 2-bromobutane. What is the product of the E2 with an unhindered base like NaOH? It is this alkene – 2-butene, the more substituted Saytzeff product. That’s quite simple, but let’s look at it more closely. To really examine this molecule, we need to acknowledge that 2-bromobutane has a stereocenter. I’ll redraw it with the Br “up” for the S-configuration. Now let’s draw this structure in a Newman projection. We are looking down the C2-C3 bond. It is a “staggered” conformation, and it has the correct S-configuration at C2. Our goal is to do an E2. If we look at this Newman projection, then C3 does have a beta hydrogen that is anti (a 180 degree dihedral angle) between the hydrogen and bromine. If this particular conformation did an E2, the molecule would flatten out to give this alkene with carbon 1 and 4 across from each other - trans. So this would be the E alkene isomer. There is another Newman projection we could consider. Let’s hold C2 the same, and just rotate the back carbon. If this structure did an E2, we would get an alkene product like this. And now the two methyl groups off the alkene are cis to one another. This is the Z alkene. Hmm. Both outcomes, whether E or Z, are possible. Both arise from valid, staggered conformations. Both have anti beta hydrogens and leaving groups. Both are Saytzeff products. The E-isomer, however, will be the major product. The Newman projection that leads to the E-isomer is more stable. It has fewer gauche interactions than the Newman projection that leads to the Z-isomer. So, the amount of the top Newman projection will be greater than the lower one in the reaction, and we’ll see more product forming from this top conformation.
OK, so we have good news. Newman projections provide a means for predicting the outcome – the stereochemical outcome, E or Z alkene – from E2 eliminations of alkyl halides. We also have some bad news. When you have an acyclic alkyl halides, you need to use Newman projections to make sure you predict the best product. Newman projections are work. You need to go slowly and be careful, but they will get you to the correct answer for the E2. So, when you encounter acyclic alkyl halides like the ones shown below, be careful. The Saytzeff product from all of these halides could have an E or Z configuration. To be confident in determining the product in each elimination, you’ll very likely need to use Newman projections.

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