|Lecture Notes: 7 June||
Last ime we looked at skeletal or line-angle drawings for common cycloalkanes:
Substituents such as oxygen, are shown. Although not necessary, frequently even carbon substituents are shown for cyclics:
When naming cycloalkanes the ring is named the corresponding open-chain name is used with the prefix cyclo- added, as seen in the figure above.
Substituents as then named as normal. For alkane substituents they are numbered with the lowest number determined alphabetically, e.g. 1-7 in order: butyl-, sec-butyl-, t-butyl, ethyl-, isopropyl-, methyl-, propyl-.
Alkanes can have rotational conformers around any given C-C sigma bond. We will look at ethane as a model for the relationships of adjacent carbons in alkane chains. The two extreme situations occur when (models):
A common way to present these conformers is with Newman projections. In this projection the carbon towards the viewer is represented as the intersection of three lines (line-angle drawing, while the carbon away from the viewer is represented as a circle with the bonds coming off the circumference (models).
The carbons of all alkanes are linked by overlapping sp3 orbitals to give cylindrically symmetrical sigma bonds. Thus we might expect completely free rotation of the two carbons relative to each other around this bond (a sigma bond should act as a perfect "ball bearing" for rotation). But there is a problem - the remaining orbitals, because of their diffuse character, overlap each other to a degree at certain angles of the bond or conformations of the ethane. This is not completely clear from the images in your text, but remember that the surfaces shown enclose a high percentage of electron probability, but not all of it (the orbitals actually extend further into space in all directions). This interaction is such that the potential energy of the eclipsed conformer is 2.9 kcal/mol (12.1 kJ/mol) more energetic than the staggered conformer (1 kcal/mole for each H - H repulsion.). As a result 99% of the molecules in in ethane will be in the staggered conformation at room temperature.
The two most common cyclic structures in nature are cyclopentane and cyclohexane. We will restrict our dicussion to these two structures.
The cyclopentane ring has angles of 108° if drawn in a plane. However, this conformation gives a total of 10 eclipsed hydrogens. As a result cyclopentane takes a strained conformation which reduces the number of eclipsed hydrogen, but gives more bond angle strain. In this conformation, the envelope conformation four carbons are in a plane and the fifth is out of the plane, sort of like th eflap of an envelope. This gives bond angles of 105°, but still preferred, even though it has a straion energy of 5.6 kcal/mol (23.4 kJ/mol).
Chair conformation has about 109.5° angles. Three different images of this molecule are shown below: a wire model, a ball and stick model, and a space-filling model (partially transparent with the wire model inside for comparison).
Boat conformation has about 109.5° angles.
Overall the chair is preferred by 6.5 kcal/mol (27 kJ/mol) so that 99.99% of cyclohexanes will have the chair conformation at room temperature.
Cis-trans Isomers: When cyclic alkanes are created from linear alkanes the new objects have distinct sides, or a top and a bottom. This results in a new variety of isomerism where two substituents can reside on the same side (cis- ) or on opposite sides (trans- ). [models] This is a type of Geometric isomerism. You might notice in the cyclopropane (and cyclobutane) models that the C-C bonds are bent - that is we cannot make good 109.5° angles in a triangle or a square. This means that these molecules will have weaker bonds, they are strained, than normal alkanes or larger cyclic alkanes. We will look at these molecules and especially at cycloalkane in the laboratory.
Chemistry: The alkanes are largely inert (they are sometimes referred to as paraffins because of their lack of affinity or reactivity - polyethylene is a very long chain alkane). They will react with oxygen, chlorine and a few other things under appropriate conditions.
Note that in each case a source of energy is required to initiate the reaction - we must break the C-C or C-H bonds to allow the reactions to take place.
Last modified 7 June 2004