CHM 2210C    Chapter 4 Key Terms


  1. Angle strain: the strain induced in a molecule when a bond angle deviates from the ideal tetrahedral value.


  1. Anti conformation: the lowest-energy arrangement, is the one in which the two large methyl groups are as far apart as possible-180 away from each other.


  1. Axial position: chair cyclohexane has six axial hydrogens that are perpendicular to the ring (parallel to the ring axis).


  1. Boat conformation: a second possibility in addition to the chair conformation of cyclohexane, is also free of angle strain and less stable than chair cyclohexane, having both steric strain and torsional strain. Carbons 2, 3, 5, and 6 lie in a plane, with carbons 1 and 4 above the plane.


  1. Chair conformation: cyclohexane is not flat; instead, it is puckered into a three-dimentional conformation that relieves all strain. The C-C-C angles of cyclohexane can reach the strain-free tetrahedral value if the ring adopts a chair conformation, so called because of its similarity to a lounge chair, with a back, a seat, and footrest. Furthermore, sighting along any one of the carbon-carbon bonds in a newman projection shows that chair cyclohexane has no torsional strain; all neighboring C-H bonds are staggered.


  1. Conformation: the different arrangements of atoms that result from the rotation around the carbon-carbon single bond.


  1. Conformational analysis: learn to draw and access the stability of the substituted cyclohexans.


  1. Conformer: a specific conformation is called a conformer.


  1. 1,3-diaxial interaction: the energy difference between axial and equatorial conformers is due to steric strain caused by so-called 1,3-diaxial interactions.


  1. Eclipsed conformation: the six C-H bonds are as close as possible, is the highest-energy, least stable conformation.


  1. Equatorial position: six equatorial hydrogens that are in the rough plane of the ring (around the ring equator).


  1. Gauche conformation: as bond rotation continues, an energy minimum is reached at the staggered conformation where the methyl groups are 60 degree apart.


  1. Newman projection: view the carbon-carbon bond directly end-on and represent the two carbon atoms by a circle. Bonds attached to the front carbon are represent by lines going to the center if the circle, and bonds attached to the rear carbon are represented by lines going to the edge of the circle.


  1. Polycyclic compound: When two or more cycloalkane rings are fused together along a common bond to construct a polycyclic molecule.


  1. Ring-flip (cyclohexane): Because chair cyclohexane has two kinds of positions, axial and equatorial, we might expect to find two isomeric forms of a monosubstituted cyclohexane. In fact, there is only one methylcyclohexane, one bromo-cyclohexane, and so on, because cyclohexane rings are conformationally mobile at room temperature. Different chair conformations readily interconvert, resulting in the exchange of axial and equatorial positions. This interconversion of chair conformations, usually referred to as s ring-flip.


  1. Sawhorse representation: view the carbon-carbon bond from an oblique angle and indicate spatial orientation by showing all C-H bonds


  1. Staggered conformation: all six C-H bonds are as far away from one another as possible, is the lowest-energy, most stable conformation.


  1. Stereochemistry: is the branch of chemistry concerned with the three-dimensional aspects of molecules.


  1. Steric strain: is the repulsive interaction that occurs when atoms are forced closer together than their atomic radii allow.


  1. Torshional strain: the extra 12 kJ/mol of energy present in the eclipsed conformation of ethane.


  1. Twist-boat conformation: boat clyclohexane is approximately 29 kJ/mol less stable than chair cyclohexane, although this value is reduced to about 23 kJ/mol by twisting slightly, thereby relieving some torsinal strain. Even this twist-boat conformation is still much more strained than the chair conformation, though, and molecules adopt this geometry only under special circumstances.