ORGANIC MOLECULE GEOMETRY

Molecular Geometry

Molecular Geometry Types ||| Types II
 Elmhurst College
Alkanes Alkynes  Optical or Chiral  Chemistry Department
Alkenes ||| Cis / Trans Alkenes All Functional Groups Rings  Virtual ChemBook

RING MOLECULE GEOMETRY

Introduction to Rings:

Many organic compounds contain rings of carbon atoms or other atoms such as oxygen or nitrogen. The simplest ring compound contains 3 carbons as in cyclopropane. The most common ring compounds contain either 5 or 6 carbons. These compounds are also called cyclic.

Cyclopentane:

Although the simplest representation is that of a line drawing of a pentagon as shown on the left. At each angle change is a carbon atom, and each carbon has the correct number of hydrogens, two each, to fulfill the 4 bond rule of carbon.

However, the ring is not flat as implied by the line drawings. As shown in the ball and stick structure, the left most carbon is tipped up from the ring. Within the constraints of a pentagon shape, the carbons still try to maintain a tetrahedral geometry as much as possible.

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Cyclohexane:

Cyclohexane, a ring of 6 carbons with all single bonds, is not a flat ring either. The ring formation attempts to attain the bond angles for the tetrahedral carbon atoms.

More than 2 billion pounds of cyclohexane are produced annually in the U. S., with over 90% being used in the synthesis of nylon.

Conformations:

Free rotation around carbon-carbon single bonds, allows molecules to exist in a variety of forms and different arrangements called conformations. These conformation inter convert very rapidly even at room temperature.

Six membered rings based upon cyclohexane are quite common and important in biological compounds. Among these is glucose, the blood sugar. The chair form of cyclohexane shown on the left is the most stable conformation. Note the tip up on the left side and the tip down on the right side, hence the name "chair" form. All of the hydrogen atoms are perfectly staggered i.e. they are as far apart as possible. In addition, the bond angle between carbons is 109.5 o, exactly the angle expected for the tetrahedral carbon atoms.

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In the boat form, the carbon atoms on both the left and the right are tipped up, while the other four carbons form the bottom of the "boat". This shows the flexibility of the ring.

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Benzene:

The benzene ring consists of six carbon atoms bonded in a flat or planar hexagon ring. Each carbon is bonded to one hydrogen because of the three alternating double bonds. This reveals that each carbon is bonded to 3 others and one double bone. Hence the molecular geometry at each carbon is trigonal planar. Hence the whole ring is flat as compared to the cyclohexane.

As shown in the graphic on the left there are various ways to represent the benzene ring. The left most drawing most clearly shows the alternating double bonds. The flat ring is based seen by rotating the Chime structure.

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The middle structure represents the fact the pi bonding electrons are delocalized around the ring. The p orbitals are perpendicular to the plane of the ring. These orbitals overalap with electron density both above and below the ring. Click on the spacefill button of Chime to see this effect.

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