Newman Projections for Cyclohexanes

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cgarcia
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Joined: Fri Sep 20, 2013 3:00 am

Newman Projections for Cyclohexanes

Postby cgarcia » Sat Mar 08, 2014 12:31 am

So I was experimenting with the molecular structure model kit and was able to build both the boat and chair conformations for cyclohexane. However, I was confused as to why either of these conformations are considered stable since looking down each of the bonds, a gauche configuration appears. Does this fact mean that cyclohexanes aren't the most stable arrangement for hexanes? Also, is it possible to even use Newman projections for cyclic structures, and if so, how are the other four carbons represented since they are all connected?

ch1995
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Joined: Fri Sep 20, 2013 3:00 am

Re: Newman Projections for Cyclohexanes

Postby ch1995 » Sat Mar 08, 2014 7:45 am

We can just look down the carbon-carbon bond of each ring carbon in the hexane, as we do when drawing the Newman projection, but it would be kind of hard to actually draw it out (because of its connected, ring structure). Because of the ring structure, the carbons can only have a gauche configuration (not anti conformation because that would make the hexane chained, not cyclic). The reason that the chair conformation is stable is because each C in the cyclohexane is sp3 hybridized, which means its optimal bond-angles should be 109.5*. According to page 112 of the organic chem txtbook, they are able to achieve a 110.9* angle when in the chair conformation, which is why this structure for cyclohexane is very stable. I'm not exactly sure how cyclohexane compares to chained hexanes in terms of stability and preference, but the chair conformation is definitely considered a stable structure because of the close to ideal bond angles (so less strain as a result). The boat conformation, on the other hand, is not a very stable configuration because it contains eclipsed C-H bonds. They usually only exists briefly as the cyclohexane is changing from one chair conformation to another (see page 113).

irisc23
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Joined: Sat Nov 23, 2013 3:00 am

Re: Newman Projections for Cyclohexanes

Postby irisc23 » Sat Mar 08, 2014 7:48 am

No, cyclic structures are generically not the most stable, this is because the ring structure inhibits free movement around the bonds (severely decreasing entropy). Also remember that boat is much less stable than chair because boat will have four pairs of eclipses Hs and also eclipsed CH2s. Chair conformations are all perfectly staggered.

You are correct though, even chair cyclohexane is less stable because it has gauche conformations (whereas regular chain cyclohexane would be all anti).

You can draw Newman projections for cyclic molecules. For chair cyclohexane, imagine looking down the two bonds that form the seat of the chair (C2-C3 and C5-C6). Both of these have gauche conformations. You can draw them close together, just connect both of them to the top CH2 (the back of the chair) and the bottom CH2 ( bottom of the chair). Make sure you get which carbon is connected to which methylene group, otherwise you may end up twisting the chair. Before you add Hs you should see the two front C-CH2 bonds meet and the two back C-CH2 bonds meet. It looks a bit like a quadrilateral but remember that this is a projection, so you would be counting both the front and back Cs (the dot and the circle) to get six carbons. Now add hydrogens as necessary to complete the structure (this part is easy as they are all perfectly staggered and the hydrogens are not connected. You can generalize this to all cyclic molecules, it's just easier to demonstrate with cyclohexane as it looks better on paper. Sorry for the long description, if image posting becomes available I'll post a picture.

irisc23
Posts: 237
Joined: Sat Nov 23, 2013 3:00 am

Re: Newman Projections for Cyclohexanes

Postby irisc23 » Sat Mar 08, 2014 7:51 am

Here is a website that has a Newman projection for cyclohexane, see scheme 14:

http://openwetware.org/wiki/Todd:Chem3x11_ToddL2

cgarcia
Posts: 33
Joined: Fri Sep 20, 2013 3:00 am

Re: Newman Projections for Cyclohexanes

Postby cgarcia » Sun Mar 09, 2014 7:53 pm

Thank you guys so much! This helped a lot!


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