Introduction

• Strain: a measure of the energy stored in a compound due to a structural distortion; an instability within a structure associated with high internal energy

• Chemicals have an optimal structure - if you disturb the optimal structure you put a strain on the system

• Interconversion between the strained and relaxed structures of alkanes results from collisions and thermal motions

• Some structures are permanently strained and don't readily have a pathway to release that strain

• The opportunity to release permanently stored strain in a chemical structure comes from chemical reactions

Conformations

• Alkanes of 2 or more carbons can be twisted into different 3D arrangements of their atoms by rotating around one or more carbon-carbon bonds

• Conformation: any 3D arrangement of atoms that results from rotation about single bonds

• Staggered conformation: a conformation about a carbon-carbon single bond in which the atoms or groups on one carbon are as far apart as possible from atoms or groups on an adjacent carbon

  • Lowest energy

  • Most stable conformation

• Eclipsed conformation: a conformation about a carbon-carbon single bond in which the atoms or groups on one carbon are as close as possible to the atoms or groups on an adjacent carbon

  • Highest energy

  • Least stable conformation

• Different conformations are called conformational isomers or conformers

• Dihedral angle: the angle created by 2 intersecting planes, each plane defined by 3 atoms

• Since there's a small energy barrier between conformations, rotation isn't completely free

• Anti conformation: a conformation about a single bond in which 2 groups on adjacent carbons lie at a dihedral angle of 180

• Gauche conformation: a conformation about a single bond of an alkane in which 2 groups on adjacent carbons lie at a dihedral angle of 60

Types of Strain

• Angle strain: strain that arises when a bond angle is either compressed or expanded compared to its optimal value

• Steric strain: strain that arises when nonbonded atoms separated by 4 or more bonds are forced closer to each other than their atomic radii would allow

• Strains can also occur when bond lengths are forced to become shorter or longer than normal

• Bond stretching isn't as easy as bond angle bending

• Torsional strain: strain that arises when nonbonded atoms separated by 3 bonds are forced from a staggered conformation to an eclipsed

  • Aka eclipsed-interaction strain

  • Loss of stabilization of staggered conformation