A Newman projection is a way of drawing a molecule by looking straight down the bond between two atoms, so you can see exactly how the groups attached to those atoms are positioned relative to each other. It’s one of the most useful tools in organic chemistry for understanding why certain molecular shapes are more stable than others and for predicting how reactions will play out.
How a Newman Projection Works
Imagine holding a molecular model in front of you and peering directly down the bond connecting two carbon atoms, like looking through a telescope. The carbon closest to you (the “front” atom) is drawn as a simple dot. The carbon behind it (the “back” atom) is drawn as a larger circle. Three lines radiate out from the dot, representing the bonds on the front carbon. Three more lines radiate from the edge of the circle, representing the bonds on the back carbon. No wedge or dashed bonds are used, because the projection itself encodes the three-dimensional arrangement.
This view lets you immediately see the angle between any two substituents on the front and back carbons. That angle, measured as you rotate around the central bond, is called the dihedral angle. When substituents on the front carbon line up perfectly with substituents on the back carbon (dihedral angle of 0°), they’re described as eclipsed. When they’re offset by 60°, they’re staggered. These two arrangements have very different energies, which is the whole reason Newman projections matter.
Staggered vs. Eclipsed Conformations
Take ethane, the simplest molecule with a carbon-carbon single bond. In the staggered conformation, each hydrogen on the front carbon sits neatly between two hydrogens on the back carbon, with dihedral angles of 60°, 180°, and 300°. In the eclipsed conformation, the hydrogens on front and back line up directly behind one another at 0°, 120°, and 240°.
The eclipsed form is about 3 kcal/mol higher in energy than the staggered form. That’s the energy barrier to rotation around the bond. It’s a small barrier, so the bond rotates freely at room temperature, but the molecule still spends most of its time near the more comfortable staggered arrangement. The instability of the eclipsed form comes from torsional strain: the electrons in bonds that are forced into alignment repel each other.
Why Butane Gets More Interesting
Ethane has only hydrogens, so all its staggered conformations look the same. Butane, with methyl groups on carbons 2 and 3, introduces variety. When you draw Newman projections looking down the C2-C3 bond, you find several distinct conformations, and their stability differences are large enough to matter.
The most stable arrangement is called “anti.” The two methyl groups sit 180° apart, as far from each other as possible. This is the lowest-energy conformation, and it’s assigned a baseline energy of zero for comparison. About 82% of butane molecules sit in the anti conformation at room temperature.
Rotate 60° from anti, and you reach the “gauche” conformation. Here the methyl groups are only 60° apart. They’re close enough that their electron clouds start to repel each other, a form of steric strain. Each gauche arrangement costs about 0.9 kcal/mol relative to anti. Gauche conformations account for roughly 18% of the mixture at room temperature.
The highest-energy arrangement is “syn,” where the two methyl groups are fully eclipsed at 0° (or equivalently 360°). This piles torsional strain on top of severe steric strain, pushing the energy about 5 kcal/mol above anti. Less than 0.1% of butane molecules occupy this conformation at any given moment.
Torsional Strain vs. Steric Strain
These two types of strain come up constantly when analyzing Newman projections, and they’re distinct concepts. Torsional strain arises when bonds on adjacent atoms are aligned (eclipsed), forcing their electrons into close proximity. It’s present even in ethane, where all the substituents are just hydrogens.
Steric strain is about the size of the groups themselves. It’s the increased energy that results from repulsion between electrons on atoms that aren’t directly bonded to each other but are physically too close. In butane’s gauche conformation, the methyl groups aren’t eclipsed, so there’s minimal torsional strain, but they’re only 60° apart, and that proximity creates steric strain. In the syn conformation, you get both at once.
How to Convert a 3D Structure
If you’re starting with a wedge-dash drawing (the standard way of showing 3D structure on paper), converting to a Newman projection takes a few deliberate steps. First, identify the bond you want to look down and decide which carbon will be the front atom and which will be the back. Mark the front carbon with a dot and the back carbon with a circle.
Next, figure out which substituents go where. The groups on the front carbon radiate from the central dot. The groups on the back carbon radiate from the circle’s edge. A useful rule: dashed-wedge substituents (the ones pointing away from you in the original drawing) end up on the same side of the Newman projection as your viewing direction. Solid-wedge substituents (pointing toward you) go on the opposite side. With practice, these conversions become nearly automatic, but being systematic at first prevents errors that cascade through a problem.
Newman Projections in Reaction Chemistry
Newman projections aren’t just an exercise in drawing molecules. They’re essential for predicting the outcomes of certain reactions, particularly E2 elimination reactions. In an E2 reaction, a base removes a hydrogen while a leaving group departs from the adjacent carbon, forming a double bond. For this to happen efficiently, the hydrogen being removed and the leaving group need to be anti-periplanar: positioned 180° apart, in a staggered arrangement.
Drawing a Newman projection of the reacting bond and rotating until the hydrogen and leaving group are anti to each other reveals which product will form. The remaining substituents on those two carbons end up on specific sides of the new double bond, determining whether you get the E or Z isomer. Without a Newman projection, predicting the stereochemistry of an E2 product is far more error-prone.
This same logic applies whenever a reaction’s outcome depends on the spatial relationship between groups on adjacent carbons. Nucleophilic additions, ring-opening reactions, and even some rearrangements become clearer when you sketch the Newman projection and identify which conformation the molecule needs to adopt for the reaction to proceed.
Reading the Dihedral Angles
Every Newman projection encodes specific dihedral angles, and recognizing them quickly is a practical skill. The standard angles and their labels form a repeating pattern as you rotate 360° around a bond:
- 0° (or 360°): Fully eclipsed. Substituents directly overlap. Highest energy when large groups are involved.
- 60°: Gauche (staggered). Groups are offset but still relatively close. Modest steric strain with bulky substituents.
- 120°: Eclipsed again, though often with a smaller group overlapping a larger one, so less severe than 0°.
- 180°: Anti (staggered). Groups are as far apart as possible. Lowest energy for most molecules.
This pattern repeats symmetrically through 240° and 300°, mirroring the 120° and 60° conformations. In ethane, the staggered and eclipsed forms alternate every 60° of rotation. In substituted molecules like butane, each 60° rotation produces a conformation with a different energy, making the full 360° energy diagram asymmetric and far more informative.