CHE 201LLR Lecture Notes - Lecture 2: Alkane, Newman Projection, Lewis Structure
10 Nov 2018
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Chapter 2: Alkanes
Hydrocarbons
- Hydrocarbons are compounds that only contain carbon and hydrogen
Unbranched Alkanes
- Referred to as normal or n-alkanes
- Linear carbon chain
- Lewis structure:
- Condensed formulas
Conformational Isomers
- Conformers are generated from single bond rotations
- Conformations are different arrangements of atoms caused by rotation about a single
bond
o May differ in energy
o Molecules constantly rotate through all possible
conformations
- Newman projections are used to look at conformers
o Viewed down a bond
Newman Projections
• H-C-C-H angle is referred to as the torsional or dihedral angle
Conformations of Butane
- Staggered butane has both anti and gauche arrangement
- Extra energy is required to push two non-bonded atoms within the sum of their van der
Walls radii (van der Waals repulsion)
o van der Waals radius is the measure of an atom’s effective size
- van der Waals repulsion is also referred to as steric hindrance
- Steric strain combines steric hindrance and torsional strain
Conformational Analysis
- Conformational analysis studies how conformational factors affect the structure of a
molecule and its properties
1. Staggered conformations favored over eclipsed
2. Van der Waals repulsions occur when atoms are “squeezed” closer together than their
van der Waals radius
3. Conformations containing van der Waals repulsions are less stable (higher energy)
4. Rotation about C-C single bonds is usually so rapid that it’s hard to imagine separating
conformations
Drawing Conformations
- Newman projections
- Line-and-wedge structures (dash-wedge structures)
- Sawhorse projections
Isomers
- Isomers are different compounds with the same molecular formula
- Constitutional (or Structural) Isomers differ in atom connectivity
- For higher alkanes, there are many isomers and it is not possible to easily predict how
many isomers can be formed
Document Summary
Hydrocarbons are compounds that only contain carbon and hydrogen. Conformers are generated from single bond rotations. Conformations are different arrangements of atoms caused by rotation about a single bond: may differ in energy, molecules constantly rotate through all possible conformations. Newman projections are used to look at conformers: viewed down a bond. Newman projections: h-c-c-h angle is referred to as the torsional or dihedral angle. Staggered butane has both anti and gauche arrangement. Extra energy is required to push two non-bonded atoms within the sum of their van der. Walls radii (van der waals repulsion: van der waals radius is the measure of an atom"s effective size. Van der waals repulsion is also referred to as steric hindrance. Steric strain combines steric hindrance and torsional strain. Isomers are different compounds with the same molecular formula. Constitutional (or structural) isomers differ in atom connectivity.
