Newman Projections & Cycloalkane Structure

Newman Projections Newman Projections are a way of viewing a carbon carbon bond from the end…. This is an example of the simplest projection from ethane.

Lowest Energy: Anti The view shown is the “lowest energy” conformation of the structure, which results because there is a minimal amount of strain (a.k.a. the most space between the groups as possible)…

Highest Energy: Eclipsed The single bonds are able to rotate . When they have rotated 60 degrees, then the atoms get closer to each other. This is called an eclipsed position. Note that the groups on the “3D” structure are basically lined up…

Comparing Anti & Eclipsed… Anti Eclipsed Low energy Higher energy

Staggered Projection of Propane When you look at Propane, there is an extra “methyl” group drawn on the projection since on of the carbons is not included in the C-C bond of the Newman Projection (here it is Carbon “3” in blue).

Eclipsed Projection of Propane Now when propane rotates 60 degrees, the hydrogens of carbon three are closer to the hydrogens of carbon one than they are in the staggered position. (Another view is on the following slide…)

Staggered vs. eclipsed 3D views… Eclipsed has Two carbons touching, Which is higher Energy… Staggered has a hydrogen touching a carbon

With butane there are 4 typical forms… When you look at butane, it is normally down the C2-C3 bond. I have renumbered the carbons to help us see this a little better. Carbon 1 is on the end, and is purple…

Staggered form of butane This is the lowest energy conformation of butane. Note the placement of carbons… C1 = methyl on the front C2 = front of circle C3 = hidden at back of circle C4 = methyl on the top back

Note that you can rotate butane the other way, but the first is more common (to have the group in the front down)… C1 = methyl on the back bottom C2 = front of circle C3 = hidden at back of circle C4 = methyl on front top

Eclipsed form of butane (rotated 60 o) Rotating around the C2-C3 bond 60 degrees gives an eclipsed form… C1 = methyl on the front, now “eclipsing” a hydrogen C2 = front of circle C3 = hidden at back of circle C4 = methyl on the top back, now “behind” a hydrogen

Eclipsed form of butane

Gauche form of butane (rotated 120 o) Rotating another 60 degrees around the C2-C3 bond gives the gauche form… C1 = methyl on the front, now C2 = front of circle, note that a hydrogen is straight across from carbon four C3 = hidden at back of circle C4 = methyl on the top back

Anti form of butane (rotated 180 o) Rotating another 60 degrees around the C2-C3 bond gives the highest energy “anti” form… C1 = methyl on the front, now “eclipsing” group 4 C2 = front of circle C3 = hidden at back of circle C4 = methyl on the top back, behind carbon 1

Anti form of butane (rotated 180 o) This is the highest energy form of butane since the Hydrogens of the two “methyl” groups can actually touch each other…

Energy Diagram & the forms… From anti to eclipsed, the forms are lower in energy in staggered positions, and higher in eclipsed…(see diagram)…

When all of the carbon-carbon bonds are “anti” the shape should look familiar…

This is why we draw the “zig zag” shape This is why we draw the “zig zag” shape!… If you click on butane, it opens a chem 3D file so you can rotate it.

Note that if the carbon points down, its other bonds point down & vice versa for pointing up…

Typical Questions you might see… Which of these Newman Projections is highest in energy?

The eclipsed form with both groups pointing in the same direction is highest in energy…

Typical Questions you might see… Which of these Newman Projections is lowest in energy?

The anti position is lowest in energy…

Using the template below, draw the lowest energy form of 1,2 dibromo ethane… Please panel submit when you are done! 

Using the template below, draw the lowest energy form of 1,2 dibromo ethane…(hint…here is the structure…)

Pick the staggered form, and add bromines to the top and bottom positions… It should look something like this… (you don’t need to cross off the high energy form, though!) 

There are other uses of the Newman Projections which we will discuss later in the semester… For now, let’s look at the structures of cycloalkanes…

Cycloalkane Structures… We normally draw the rings as triangles, squares, pentagons & hexagons, but they do have a 3D shape…as shown…

Cyclopentane & cyclohexane are the most common Click on the structures for a Chem 3D view… You can see that cyclopentane is slightly puckered, while cyclohexane is not (recall that cyclohexane has the optimal 109.5 degree angles…

Cyclohexane shown in its “chair” conformation… Either way is okay. Note that they look like “beach” chairs, hence the name.

How to recognize the positions of the groups… There are two types of positions. Axial – white hydrogens, which point straight up or straight down Equatorial – green hydrogens, which point out to the side, or “equator”

The ring can “flip” and therefore reverse the groups… Note that the groups have reversed. What was axial is now equitorial, etc. Axial – green hydrogens, which point straight up or straight down Equatorial – white hydrogens, which point out to the side, or “equator”

When you have groups on the ring the axial or equatorial position can make a big difference in Energy… Equatorial positions are typically favored, so we will look at why…

When you have groups on the ring the axial or equatorial position can make a big difference in Energy…most groups are best in the equatorial position because of the strain in the axial positions. 1-bromocyclohexane with the bromine in an equatorial position.

Strain in the axial position 1-bromocyclohexane with the bromine in an axial position has 1,3 diaxial interactions (causing an increase in Energy)…

Two groups 1,2 … trans 1 2 Recall to have the lowest possible energy, you need to have both groups in equatorial positions. For compounds that are substituted 1,2, the lowest energy conformation is trans (bromine 1 is on the top of the ring, bromine 2 is on the bottom). Both however, are in equatorial positions.

Two groups 1,2 …cis In order to have the groups be “cis” they both have to be on the top or the bottom of the ring. Either way, if they are 1,2, then one of the groups has to be in an axial position therefore making the molecule less stable than its trans counterpart.

Two groups 1,3 …cis a b The pattern changes when you go to 1,3. you still want both groups equatorial, but this time the coformer that allows this position is the trans one. Note that in the cis form, both groups (in “a” ) can be equitorial.….”b” can always ring flip back to “a”…

Two groups 1,3 …trans a b Now one of the groups always has to be axial, making this the higher energy form.

Two groups 1,4 …cis In 1,4 conformations, the cis form requires that one of the groups be axial again, so the trans conformation (next slide) is the more stable one…

Two groups 1,4 …trans Both groups are equatorial, so this is the lowest energy form of 1,4-dibromocyclohexane…

Which is lowest in energy of the compounds shown? Typical questions… Which is lowest in energy of the compounds shown? a b

Typical questions… The first, a, would be lower in energy since both groups are equatorial. It is the trans conformation… a b

For more “tips” go to the following page… Typical questions… Draw the most stable form of 1,3-dichloro cyclohexane using the template below & panel submit… Note 1: carbon 1 is typically the one on the top right, but does not have to be. Note 2: you only need to draw the two chlorine atoms in (not any hydrogens) For more “tips” go to the following page…

Typical questions… Tips for approaching this type of problem… Draw the first group (on C1) in the equatorial position. Go to carbon 3, with the A & B notations. Which is in the equatorial position (where you want to put the other cl)? Draw the form, then determine if it is cis or trans…

Typical questions… Once you have drawn in the second chlorine, you can see that they are both on the bottom of the structure, so the correct answer is cis…

Typical questions… Which is more stable? a b

Typical questions… If you have two different groups to pick from (like chlorine and tertbutyl) the larger group will preferentially be in the equatorial position… b a

If you need to draw everything out… A stepwise approach makes drawing the rings relatively easy… 1) Draw the middle lines 2) Add two dots 3) Add lines to connect the carbons…

1) 3 carbons point up, so 3 axial bonds point up Adding Axial Bonds… Remember that the axial bonds are either up or down. The way to tell is that they match the direction of the carbons. 1) 3 carbons point up, so 3 axial bonds point up 2) The other 3 point down, as do the other axial bonds (I normally draw the one in back …c6…behind the C2-C3 bond).

Adding Equatorial Bonds… If you’ve drawn the axial bonds first, it is easy to tell where to put the equatorial bonds. On the carbons where the axial bond is up, the Equatorial bond goes down. If it’s on the right of the line, It goes to the right, if on the left, it goes to the left…

Adding Equatorial Bonds… If you’ve drawn the axial bonds first, it is easy to tell where to put the equatorial bonds. Carbon 1 is on the right of the middle line, and the axial position is up. Where does the equatorial group go? a) up b) down

Adding Equatorial Bonds… The equatorial bond on C1 goes down Carbon 2 is on the right of the middle line, and the axial position is down. Where does the equatorial group go? a) up b) down

Adding Equatorial Bonds… The equatorial bond on C2 goes up since the axial bond is down… We’ll do one more together, and then you can try to draw the rest…

Adding Equatorial Bonds… Where does the equatorial bond go on C4?… Note the position of the axial bond…

Adding Equatorial Bonds… Since the axial bond is down, the equatorial bond is up and to the left (since it is on the left of the structure). The next slide has a template starting from here, but without the arrow and line…try to draw the rest of the Equatorial bonds…

Adding 3 more Equatorial Bonds…

The three additional bonds are shown in blue.