Newman projections are drawings that show the arrangement of atoms around a single sigma bond by looking straight down that bond. They help chemists compare different conformations, which are shapes made by rotation around single bonds. This matters because molecules are not rigid, and some conformations are more stable than others.
Understanding these shapes helps explain reactivity, physical properties, and the preferred structures of organic molecules.
In a Newman projection, the front carbon is shown as a point or center of a circle, while the back carbon is shown as a circle behind it. Staggered conformations place bonds as far apart as possible and usually have lower energy, while eclipsed conformations line bonds up and have higher energy. In butane, the anti staggered conformation is lowest in energy because the two methyl groups are 180 degrees apart, reducing steric strain.
Cyclohexane avoids angle strain and torsional strain by adopting a chair conformation, where substituents can occupy axial or equatorial positions.
Key Facts
- A Newman projection views a molecule directly along a C-C single bond.
- Staggered conformations have a dihedral angle of 60 degrees between front and back bonds.
- Eclipsed conformations have a dihedral angle of 0 degrees between front and back bonds.
- For ethane, the eclipsed conformation is about 12 kJ/mol higher in energy than the staggered conformation.
- For butane, the anti conformation has the methyl groups 180 degrees apart and is more stable than the gauche conformation.
- Cyclohexane chair conformations reduce strain by keeping bond angles near 109.5 degrees and avoiding most eclipsing interactions.
Vocabulary
- Newman projection
- A Newman projection is a drawing that shows the positions of atoms on two bonded carbons by looking directly down the bond between them.
- Conformation
- A conformation is a temporary three-dimensional shape of a molecule formed by rotation around single bonds.
- Staggered conformation
- A staggered conformation is an arrangement in which bonds on the back carbon are positioned between the bonds on the front carbon.
- Eclipsed conformation
- An eclipsed conformation is an arrangement in which bonds on the back carbon line up with bonds on the front carbon.
- Chair conformation
- A chair conformation is the most stable common shape of cyclohexane, with alternating carbons above and below the ring plane.
Common Mistakes to Avoid
- Drawing the back carbon bonds from the center of the circle, which is wrong because the center represents the front carbon and the circle represents the back carbon.
- Calling every staggered butane conformation anti, which is wrong because anti requires the two methyl groups to be 180 degrees apart while gauche places them 60 degrees apart.
- Assuming eclipsed and staggered conformations have the same energy, which is wrong because eclipsed conformations have greater torsional strain from aligned bonds.
- Confusing axial and equatorial positions in cyclohexane, which is wrong because axial bonds point roughly up or down while equatorial bonds point outward around the ring.
Practice Questions
- 1 Ethane has an energy barrier of about 12 kJ/mol between staggered and eclipsed conformations. If a staggered conformation is assigned 0 kJ/mol, what is the approximate energy of the eclipsed conformation?
- 2 In butane, draw Newman projections looking down the C2-C3 bond for the anti and gauche staggered conformations. What is the dihedral angle between the two methyl groups in each case?
- 3 Explain why a cyclohexane chair conformation is more stable than a flat hexagonal drawing of cyclohexane, using angle strain and torsional strain in your answer.