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Questions and Answers
What is the extreme conformation of ethane where hydrogen atoms are as far apart as possible?
Which type of projection is used to view the molecule along the molecular axis in ethane conformations?
What is the term for any conformation of ethane that is between eclipsed and staggered conformations?
What happens to the stability of ethane when it changes from a staggered to an eclipsed conformation?
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In a Newman projection, how are the hydrogen atoms attached to the carbon atom nearer to the viewer represented?
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What factor determines the magnitude of torsional strain in ethane conformations?
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What is the main reason that allows free rotation about a C–C single bond in alkanes?
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What term is used to describe spatial arrangements of atoms in alkanes that can be converted into each other by rotation around a C-C single bond?
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What is the energy barrier associated with hindered rotation around a C-C single bond due to weak repulsive interaction called?
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How many conformations can alkanes have by rotation around C-C single bonds?
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What are the spatial arrangements of hydrogen atoms in ethane called when they can be interconverted by rotation around the C-C axis?
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What type of interaction is responsible for hindering the complete freedom of rotation around a C-C single bond?
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Study Notes
Conformations of Alkanes
- Alkanes contain carbon-carbon sigma (σ) bonds, which permit free rotation about C-C single bonds due to symmetrical electron distribution around the internuclear axis.
- This rotation results in different spatial arrangements of atoms in space, known as conformations or conformers or rotamers.
- Alkanes can have infinite number of conformations by rotation around C-C single bonds.
Conformations of Ethane
- Ethane molecule (C2H6) contains a carbon-carbon single bond with each carbon atom attached to three hydrogen atoms.
- Rotation around the C-C axis results in infinite number of spatial arrangements of hydrogen atoms attached to one carbon atom with respect to the hydrogen atoms attached to the other carbon atom.
- There are two extreme cases: eclipsed conformation (hydrogen atoms attached to two carbons are as close together as possible) and staggered conformation (hydrogen atoms are as far apart as possible).
- Any other intermediate conformation is called a skew conformation.
- Bond angles and bond lengths remain the same in all conformations.
Representations of Conformations
- Sawhorse projections: molecule viewed along the molecular axis, projected on paper by drawing the central C-C bond as a straight line.
- Newman projections: molecule viewed at the C-C bond head-on, with the carbon atom nearer to the eye represented by a point and the rear carbon atom represented by a circle.
Relative Stability of Conformations
- Staggered form of ethane has minimum repulsive forces, minimum energy, and maximum stability due to electron clouds of carbon-hydrogen bonds being as far apart as possible.
- Eclipsed form of ethane has increased electron cloud repulsions, resulting in higher energy and lesser stability.
- Torsional strain, the repulsive interaction between electron clouds, affects stability of a conformation and depends on the angle of rotation about the C-C bond.
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Description
Learn about the spatial arrangements of atoms in alkanes due to free rotation about carbon-carbon single bonds. Understand how electron distribution in the sigma molecular orbital remains symmetrical during rotation, allowing for different conformations. Explore how these conformations can be converted into one another by rotation around a C-C single bond.
