Back to the on-screen lesson ·
A Newman projection looks straight down a C–C bond, the front carbon a point and the back a circle; its bonds appear 120° apart, staggered puts back bonds halfway between front ones, and the dihedral angle can be read off like a clock face.
Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.
You will read the parts of a Newman projection, find angles in it, name conformations from clock-face descriptions, count eclipsed pairs, and use a projection to explain a dipole.
You know single bonds rotate and that the dihedral angle measures a conformation. A Newman projection is the view that shows that angle directly.
A Newman projection is the view along one C–C bond. The front carbon is shown as the point where its three bonds meet; the back carbon as a circle, its three bonds starting at the rim. Here each projection is described as a clock face: which group points to which hour.
Hold ethane so you look straight down its C–C bond. The near carbon hides the far one. Its three C–H bonds, seen end on, spread out 120° apart, like the hands at 12, 4 and 8 o'clock. The far carbon's three bonds also spread 120° apart, starting from the rim of the circle that stands for it.
Staggered: each back bond sits halfway between two front bonds — back bonds at 2, 6 and 10 o'clock. Every front H is 60° from its nearest back H.
Eclipsed: each back bond hides directly behind a front bond — both sets at 12, 4 and 8. Three pairs of bonds are lined up.
Each hour on the clock is 30°, so reading angles is counting hours.
Another way: steps
To read a described Newman projection:
An eclipsed projection lines up three pairs of bonds, and what is in each pair matters. Eclipsed ethane has three H/H pairs. Eclipsed propane, viewed along C1–C2, has one CH3/H pair and two H/H pairs. Butane with its CH3 groups at 0° has one CH3/CH3 pair and two H/H pairs. Counting the pairs is the first step in putting an energy on a conformation, which is the next lesson.
Newman projections also show properties that depend on shape. In anti 1,2-dibromoethane the two polar C–Br bonds point opposite ways and the dipoles cancel; in the gauche form they do not.
The front and back carbons in a Newman projection occupy the same plane. One is behind the other.
The circle is a ring. It stands for the hidden back carbon.
Front bonds are 109.5° apart in the drawing. End on they appear 120° apart.
Eclipsed means the groups touch. It means they line up.
Front carbon (C1): H at 12, 4 and 8 o'clock.
Three hydrogens.
Back carbon (C2): CH3 at 12, H at 4 and 8, directly behind.
Eclipsed.
That makes one CH3/H pair and two H/H pairs lined up.
Count by pair.
How many hours apart are they?
Four hours.
What angle, and is it staggered or eclipsed?
120°: an eclipsed position, with each CH3 lined up with a hydrogen.
In a Newman projection of ethane along its C–C bond, three bonds meet at the centre point and three more stick out from the rim of a circle. What do the point and the circle stand for?
In a Newman projection, what angle separates two neighbouring bonds on the front carbon, as seen end on?
Answer:
Butane is viewed along C2–C3. The front CH3 points to 12 o'clock. Match each position of the back CH3 to the conformation.
| anti | gauche | fully eclipsed | |
|---|---|---|---|
| back CH3 at 6 o'clock | |||
| back CH3 at 2 o'clock | |||
| back CH3 at 10 o'clock | |||
| back CH3 at 12 o'clock |
In the staggered Newman projection of ethane, what is the dihedral angle between a front C–H bond and the nearest back C–H bond?
Answer:
1,2-dibromoethane, BrCH2CH2Br, is viewed along its C–C bond with the front Br at 12 o'clock. In which conformation does the molecule have no overall dipole?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
In the Newman projection of butane with its CH3 groups at 120°, viewed along C2–C3, each front bond sits directly in front of a back bond. How many of those eclipsed pairs are hydrogen in front of hydrogen?
The projection has a H/H eclipsed pairs.
You can read a view down a bond. Tell someone why anti 1,2-dibromoethane has no dipole. Next: why staggered conformations are lower in energy than eclipsed ones.
8. Your turn: butane, front CH3 at 12, back CH3 at 4 o'clock, step 3