Organic Pedagogical Electronic Network

Using Molecular Orbital Theory to Explain Bonding in Cyclopropane

Travis Kienholz

Cyclopropane

Freund, A. 1882 J. Prakt. Chem., 26: 367-377http://www.cmbi.ru.nl/molden/manual.html

Following its discovery in 1881 by August Freund cyclopropane fueled the debate of how atoms interact within a molecule. Any bonding model used needed to fit with the following physical properties of cyclopropane.

• C-C-C bond angle of 60⁰

• Electron density found outside the internuclear axis, including electron density found in the center of the ring.

• Cyclopropane has a ring strain of 27.6 kcal/mol, only 1.4 kcal/mol more than cyclobutane despite its significantly reduced bond angles.

Bonding Models

https://en.wikipedia.org/wiki/Valence_bond_theoryhttps://en.wikipedia.org/wiki/Molecular_orbital_theory

Valence Bond Theory

• Two orbitals combine to form a bond between two atoms

• A single pair of electrons can be shared only by two, bonded atoms in the molecule

• Focuses on bonding interactions

• Uses hybridization to explain geometry of molecules

Molecular Orbital Theory

• Orbitals from each atom in the molecule can combine to form molecular orbitals

• A single pair of electrons can be spread across numerous atoms in the molecule

• Atomic orbital interactions produce both bonding and antibonding orbitals

• Uses symmetry of atomic orbitals to explain geometry of molecules.

Valence Bond Theory and the Banana Bonds

Förster, T; Z. Phys. Chem. (Liepzig) B 43, 58 (1939)

To satisfy the fact that the electron density is not on the internuclear axis the banana bond idea was applied. This idea is that the orbitals are angled towards one another and do not meet head on, thus forming a curved, banana shaped bond.

This reduces the overlap of the orbitals but makes the angle between orbitals 104 , far closer to the ⁰ideal angle. This was proposed in conjunction with an sp5 hybridization for the carbon atoms.

This explanation is informative but fails to explain the increased electron density in the center of the molecule or the ring stain anomaly.

Molecular Orbital Theory and the Three Orbital Mixing Problem

Anslyn, E. V.; Dougherty, D. A. Modern Physical Organic Chemistry, 2003

Mixing two orbitals can readily yield both a bonding and an anti bonding orbital, but mixing three orbitals creates a new problem: Where does the third orbital go?

Orbitals of Cyclopropane

To construct the molecular orbitals first assign the orbital orientations on the carbon.

S-orbital

Pz - orbital Py -orbital Px -orbital

Molecular Orbital Diagram for Cyclopropane

Anslyn, E. V.; Dougherty, D. A. Modern Physical Organic Chemistry, 2003

With the exception of the Py orbitals, the three orbital splitting pattern of one down and two up is followed.

S

Pz

Px

Py

A

B

C

D

E

F

Molecular Orbitals of Cyclopropane

A B C

D E

F

Questions1. What physical properties make the valence bond theory a poor model for cyclopropane?

a. Electron density outside the internuclear axis.b. Electron density in the center of the ring.c. Bond angles of 60%.d. SP5 hybridization

2. Which of the following is not a characteristic of molecular orbital theory?

e. Orbitals from multiple atoms can combine to form a bonding interaction.

f. Contains bonding and antibonding interactions.g. Uses hybridization to describe geometry of the

molecule.h. Electrons can be shared by many atoms in the

molecule.

3. Draw the third molecular orbital formed by the Py orbitals and explain why it is higher in energy.

4. Which orbital or orbitals contribute to electron density in the center of the ring?

a. Sb. Px

c. Py

d. Pz

e. Both a and bf. Both a and dg. Both b and d

5. Draw cyclobutane and its Px orbitals. Explain why the ring strain of cyclopropane is only 1.4 kcal/mol more than cyclobutane.

Answers1. B2. C3. It yields only antibonding interactions causing it to be higher in energy

4. E5. In cyclobutane the Px orbitals are too far to interact. Their ability to interact in cyclopropane is a stabilizing factor.

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Contributed by:

Travis Kienholz (Undergraduate)

University of Utah

2015