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CA12. Rings with Heteroatoms

CA12.  Rings Containing Heteroatoms

Rings often contain atoms other than carbon.  In many cases, the same basic shape of the carbocyclic compound is still adopted.  For example, replacing a carbon of cyclohexane with an oxygen, as in tetrahydropyran, preserves the same basic shape.

 

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Animation CA12.1.  Model of tetrahydropyran.  Note the similarity to a cyclohexane structure.

The tetrahydropyran structure is common in nature, showing up most frequently in the form of sugars such as glucose.  Glucose has a number of different forms that exist in equilibrium.  The predominant form in aqueous solution is b-D-glucopyranose.

 

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Animation CA12.2.  Model of b-D-glucopyranose.  Note the tetrahydropyran substructure.

Problem CA12.1. 

Draw b-D-glucopyranose in a chair.  Label each substituent axial or equatorial.  Perform a ring flip and compare steric strain in the two conformers.

Problem CA12.2. 

One of the other isomers in equilibrium with b-D-glucopyranose in solution is a-D-glucopyranose. 

a) Compare steric strain in these isomers.  Which isomer should be the major one?

 

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Animation CA12.3.  Model of a-D-glucopyranose. 

Problem CA12.3. 

An unusual effect is seen in cases in which an oxygen in a ring is attached to a carbon that also holds an OH group (see animation CA12.4).  This O-C-OH group is called a hemiacetal.  In a six membered ring, the hemiacetal could be found in two different isomeric positions.  These are called alpha and beta positions.

 

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Animation CA12.4.  Axial (top) and equatorial (bottom) conformers of 1-hydroxytetrahydropyran.

a)  Based on steric considerations, which of these two isomers should be more stable?

b)  These two isomers exist in equilibrium with each other.  Which isomer should be the major species present?

c)  In solvents of modest polarity, such as dichloromethane, a factor other than sterics actually reverses this equilibrium; the sterically disfavoured compound becomes more stable.  What factor do you think is responsible for this phenomenon?

d)  In solvents of high polarity, such as water, the factor in question (c) becomes less important, and sterics dominate the equilibrium.  Why would a highly polar environment diminish the effect discussed in part (c)?

Inflexible Rings

Many rings do not undergo conformational changes because they are not very flexible.  Frequently these rings contain a number of double bonds; remember that rotation is not allowed around a double bond because the p bond would break.

  • Benzene does not undergo conformational changes because of delocalized bonding around the ring. 

  • This aromatic ring is completely flat, maximizing interaction between the p bonding orbitals.

 

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Animation CA12.5.  Model of benzene.

Other rings also display aromatic delocalization in which heteroatoms also participate.  In some cases, the heteroatom simply participates in a regular double bond.  In others, the heteroatom donates a lone pair into the p system.

  • Furan is an example in which a lone pair from the oxygen is delocalized in order to reach aromatic stability.

 

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Animation CA12.6.  Model of furan.

  • Pyridine is an example in which the ring has aromatic delocalization, but the lone pair on the nitrogen does not participate.  Because the nitrogen already has a p bond, the lone pair is orthogonal to that bond, and lies in the plane of the ring.

 

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Animation CA12.7.  Model of pyridine.