Difficulty: Intermediate | Prerequisites: Alkane structure, single-bond rotation, basic understanding of steric effects.
Newman projections are a way of visualising the three-dimensional arrangement of groups around a carbon-carbon single bond by looking straight down that bond. This topic covers how to draw them, how to identify anti, gauche, and eclipsed conformations, and how to rank conformations by energy using torsional and steric strain arguments. You will use these ideas again when studying cyclohexane conformations and reaction stereochemistry.
Newman projections let you see how substituents on two adjacent carbons are oriented relative to each other by looking straight down the bond between them. The lowest-energy arrangement puts the largest groups as far apart as possible (anti), and the highest-energy arrangement has them directly overlapping (fully eclipsed). Learning to draw and rank these conformations is essential for understanding cyclohexane chair conformations and stereochemistry later on.
Newman projection
A way of depicting a molecule by looking straight down a carbon-carbon bond. The front carbon is drawn as a dot (or the centre of a circle) with three bonds radiating outward. The back carbon is drawn as a circle with three bonds poking out from behind. Think of it as the view you would get if you held a molecular model up to your eye and stared down one bond.
Staggered conformation
An arrangement where the bonds on the front carbon bisect the angles between the bonds on the back carbon. The substituents are as far apart as possible. This is lower in energy than an eclipsed conformation.
Eclipsed conformation
An arrangement where the bonds on the front carbon line up directly with the bonds on the back carbon. Substituents overlap when viewed in the Newman projection. This is higher in energy.
Anti conformation
A specific staggered conformation where the two largest substituents on the front and back carbons are 180° apart (opposite sides of the projection). This is the lowest-energy conformation for most simple alkanes.
Gauche conformation
A specific staggered conformation where the two largest substituents are 60° apart. Lower in energy than any eclipsed form, but higher than anti because of some steric strain between the nearby groups.
Torsional strain
The increase in energy that arises when bonds on adjacent carbons are eclipsed. It comes from the repulsion between bonding electrons in aligned bonds. Present in every eclipsed conformation.
Steric strain (van der Waals strain)
The increase in energy caused by bulky groups being forced too close together. In a gauche conformation, two large groups at 60° experience steric strain. In a fully eclipsed conformation where two large groups overlap directly, steric strain is at its worst.
Dihedral angle (torsion angle)
The angle between two substituents on adjacent carbons when viewed in a Newman projection. Anti = 180°, gauche = 60°, eclipsed = 0° (or 120° for partially eclipsed arrangements).
Choose which C-C bond you are looking down. The exam will usually specify (e.g. "look down the C2-C3 bond of butane").
Draw the front carbon as a dot in the centre. Attach its three substituents as lines radiating out at 120° angles.
Draw the back carbon as a large circle behind the dot. Attach its three substituents as lines from the circle's edge, offset by 60° from the front carbon's bonds (for a staggered conformation) or aligned with them (for eclipsed).
Label each substituent. For n-butane looking down C2-C3, the front carbon (C2) carries H, H, and CH3 (the C1 methyl). The back carbon (C3) carries H, H, and CH3 (the C4 methyl).
As you rotate the back carbon 360° in 60° increments, you pass through six conformations alternating between staggered and eclipsed:
0° (fully eclipsed): the two methyl groups overlap directly. Highest energy. Both torsional and steric strain are at their maximum.
60° (gauche): staggered, but the two methyls are only 60° apart. A local energy minimum, but higher than anti due to steric strain between the nearby methyls.
120° (partially eclipsed): a methyl eclipses a hydrogen. Higher energy than gauche, lower than fully eclipsed.
180° (anti): staggered, the two methyls are 180° apart. The global energy minimum. No steric strain between the methyls.
240° (partially eclipsed): equivalent to the 120° form by symmetry.
300° (gauche): equivalent to the 60° gauche by symmetry.
Energy ranking from lowest to highest: anti < gauche < partially eclipsed < fully eclipsed.
When looking down the C2-C3 bond of 2-methylbutane, the front carbon (C2) carries two methyl groups and one hydrogen, while the back carbon (C3) carries one methyl (the C4 group), and two hydrogens.
The lowest-energy conformation places the largest groups anti to each other. Here, the ethyl fragment (or the larger substituent on C2) should be anti to the methyl on C3, with as many gauche interactions as possible involving the smaller H atoms. The key reasoning: large groups 180° apart minimises steric strain.
A Newman projection can be rotated by 120° (one-third of a turn) without changing the conformation. This is because rotating the entire projection by 120° simply relabels which bond points "up" on the front carbon, but all the dihedral angles between substituents remain identical. Two projections that look different but are related by a 120° rotation represent the same conformation.
This means there are only three unique staggered conformations (not six) and three unique eclipsed conformations as you rotate 360° around a bond between two carbons that each carry three substituents.
Students often think that gauche and anti are different compounds. They are not. They are conformations (rotational arrangements) of the same molecule, interconverting rapidly at room temperature.
Students sometimes draw Newman projections with the wrong number of substituents. Each carbon in the projection must show exactly three bonds (the fourth bond is the one you are looking down, connecting the front and back carbons).
Students frequently confuse "eclipsed" with "gauche." Eclipsed means bonds are aligned (0° or 120° dihedral); gauche means a staggered arrangement with two large groups 60° apart. Gauche is staggered; eclipsed is not.
Students sometimes forget that a 120° rotation of the entire Newman projection does not create a new conformation. Two projections that can be related by 120° rotation are identical.
Drawing Newman projections of butane (anti, gauche, fully eclipsed) looking down C2-C3 is a classic exam question. Be able to draw all three from memory.
Expect a question asking you to draw the lowest-energy Newman projection for a branched alkane like 2-methylbutane and explain your choice using anti/gauche and steric reasoning.
Questions about whether two Newman projections represent the same or different conformations test your understanding of 120° rotational symmetry.
The energy ranking (anti < gauche < partially eclipsed < fully eclipsed) will appear in multiple-choice and short-answer formats.
These concepts carry forward directly to cyclohexane chair conformations, where axial vs equatorial positions are governed by the same steric logic.
True or false: the anti conformation of butane has the two methyl groups 60° apart. (False. Anti = 180° apart.)
Which conformation of butane has the highest energy: anti, gauche, or fully eclipsed? (Fully eclipsed.)
True or false: rotating a Newman projection by 120° creates a new, distinct conformation. (False. It is the same conformation viewed from a different angle.)
Fill in the blank: the strain that arises specifically from bonds being aligned is called ______ strain. (Torsional.)
In the Newman projection of butane, how many bonds does the front carbon display? (Three.)
Q: Draw the Newman projection of n-butane looking down the C2-C3 bond in the anti conformation. Label all substituents on both the front and back carbons.
A: Front carbon (C2) shows H, H, and CH3 (the C1 methyl) radiating at 120° intervals. Back carbon (C3) shows H, H, and CH3 (the C4 methyl). The two CH3 groups are placed 180° apart (one pointing straight up, the other straight down, or equivalent). All substituents are staggered.
Q: For 2-methylbutane looking down C2-C3, draw the lowest-energy conformation and explain why it is lowest in energy.
A: C2 (front) carries CH3, CH3, and H. C3 (back) carries CH3, H, and H. In the lowest-energy conformation, the two largest groups (a methyl on C2 and the methyl/ethyl on C3) are anti to each other (180° apart). This minimises steric strain. The hydrogens, being small, can tolerate gauche interactions without significant energy cost.
Q: Two Newman projections of the same molecule appear different. One can be converted to the other by a 120° rotation. Are they the same conformation? Explain.
A: Yes, they are the same conformation. A Newman projection has three-fold rotational symmetry because the front carbon has three bonds arranged at 120° intervals. Rotating the entire projection by 120° repositions which bond points up, but every dihedral angle between every pair of substituents remains unchanged. The molecule has not changed its shape.
Q: Rank the following butane conformations from lowest to highest energy: gauche, anti, fully eclipsed, partially eclipsed.
A: Anti (lowest) < gauche < partially eclipsed < fully eclipsed (highest).
Conformational analysis feeds directly into cyclohexane chemistry (Chapter 4 in most textbooks), where you will use the same anti/gauche logic to predict whether substituents prefer axial or equatorial positions on the chair. It also connects to reaction stereochemistry: certain reactions proceed through specific conformations, and understanding which conformation is favoured helps predict the product.
Newman projection, conformational analysis, anti conformation, gauche conformation, eclipsed conformation, staggered conformation, torsional strain, steric strain, van der Waals strain, dihedral angle, torsion angle, butane conformations, 2-methylbutane Newman projection, rotational symmetry Newman, conformational energy diagram, CHEM 237 chapter 2, organic chemistry conformations, lowest energy conformation alkane