Rotation about Carbon-Carbon Bonds

Carbon-carbon single bonds enable essentially free rotation about their axes. There is actually a small torsional barrier of about 12 kJ/mol (compared to the C-C bond energy of 350 kJ/mol), which slightly favors the staggered over the eclipsed conformation of the two methyl groups in ethane. To a good approximation, each carbon atom forms four bonds from hybrid orbitals directed towards the vertices of a regular tetrahedron.
Each carbon atom involved in a C=C double bond forms three bonds from hybrids, in the same plane, approximately 120° apart. The remaining -orbitals on the two carbon atoms form a -bond, which together with the constitutes a C=C double bond. In contrast to a single bond, a double bond forms a rigid planar structure, with only a small amplitude of torsional motion allowed. If two of the hydrogen atoms in ethylene (ethene) are replaced by chlorine atoms, cis and trans isomers, molecules with distinct physical properties become possible. (These isomeric forms can be interconverted at higher temperatures or by UV radiation.)
A C≡C triple bond, which has two orthogonal -bonds between the two carbon atoms, is, like a single bond, cylindrically symmetrical and allows free rotation. This is most evident in a molecule such as dimethylacetylene.
The central C-C bond in the biphenyl molecule represents an instance in which a single bond exhibits restricted rotation. In this case, the cause is steric hindrance between adjacent hydrogen atoms on the two rings. The most stable dihedral angle between the rings is about 39°. Rotation becomes even more restricted when larger groups are substituted for hydrogens.
The ball-and-stick models of the molecules illustrated in the graphics are intended to optimize display of rotational effects. Some distortions of atomic sizes have been introduced.



  • [Snapshot]
  • [Snapshot]
  • [Snapshot]
  • [Snapshot]


Snapshot 1: ethane in the most stable staggered configuration
Snapshot 2: ethane in eclipsed configuration
Snapshot 3: essentially free rotation about the triple bond in substituted acetylene
Snapshot 4: most stable conformation of biphenyl, responding to steric repulsions of hydrogen atoms
    • Share:

Embed Interactive Demonstration New!

Just copy and paste this snippet of JavaScript code into your website or blog to put the live Demonstration on your site. More details »

Files require Wolfram CDF Player or Mathematica.

Mathematica »
The #1 tool for creating Demonstrations
and anything technical.
Wolfram|Alpha »
Explore anything with the first
computational knowledge engine.
MathWorld »
The web's most extensive
mathematics resource.
Course Assistant Apps »
An app for every course—
right in the palm of your hand.
Wolfram Blog »
Read our views on math,
science, and technology.
Computable Document Format »
The format that makes Demonstrations
(and any information) easy to share and
interact with.
STEM Initiative »
Programs & resources for
educators, schools & students.
Computerbasedmath.org »
Join the initiative for modernizing
math education.
Step-by-Step Solutions »
Walk through homework problems one step at a time, with hints to help along the way.
Wolfram Problem Generator »
Unlimited random practice problems and answers with built-in step-by-step solutions. Practice online or make a printable study sheet.
Wolfram Language »
Knowledge-based programming for everyone.
Powered by Wolfram Mathematica © 2018 Wolfram Demonstrations Project & Contributors  |  Terms of Use  |  Privacy Policy  |  RSS Give us your feedback
Note: To run this Demonstration you need Mathematica 7+ or the free Mathematica Player 7EX
Download or upgrade to Mathematica Player 7EX
I already have Mathematica Player or Mathematica 7+