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ChemWiki: The Dynamic Chemistry E-textbook > Wikitexts > UC Davis > UCD Chem 124A: Kauzlarich > ChemWiki Module Topics > Valence Bond Theory: HCN

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Valence Bond Theory: HCN

HCN, hydrogen cyanide, is a volatile and poisnous compound with distinguished bitter odor. It is linear molecule with a triple bond between C and N atom and has bond angle of 180 degrees. It can be found in fruits that have pits due to the fact that they contain small amounts of cyanohydrins which slowly releases hydrogen cyanide. Also it can be found in exhaust of vehicles and burning nitrogen-containig plastics.

Introduction

The valence bond theory can be explained by overlapping of atomic orbitals which electrons are localized in the reigion to form chemical bonds. However, when you utilize this approach to explain chemical structure of the molecule, you must aware that there are various atomic orbitals for bonding which will significantly influence the structure of the molecule.

Definition of Valence Bond Theory

The Valence Bond thoery simply explains the bond formation just like lewis dot structure, but instead it explains the bonding in terms of covalent bond by quantum mechanics. According to this theory, bond will form when

1) An orbital of one atom occupy another atom's orbital, known as overlap.

2) number of electrons in both orbital is adds up to no more than two.

Just like forming a molecule with lewis dot structure, bonds between atoms complete when two electrons share same orbital together.

Bond strength depends on the the amount of overlap since electrons are attracted to nuclei of both atoms, more electrons will pull more nuceli thus increase bond strength. However, two orbitals can not contian more than two atoms due to the maximum capacity it can hold.

Also, because known atomic geometry can not be able to have effective overlap, atomic orbitals combine with each other and reconfigure themselves into a different configuration. This process is called hybrdization.

This formation of new hybrid orbital is possible by combining several types of orbitals (s,p,d and etc).

 

Describe HCN molecular bond by using Valence Bond Theory

In HCN molecule, the C atom includes sp-hybridized orbital, since it will combine with only two other atoms to form HCN. One of the sp-hybrid orbitals of carbon atom overlaps with the 1s orbital of H atom, while the other sp-hybrid orabital mixes with one of the nitrogen's atom's three atomic  p orbitals which were unhybridized. Because px orbital of C and N will form sigma bond, this leaves with two N atom  p-orbitals which form two mutually perpendicular pi bonds to the two atomic p orbitals on the C atom. HCN thus has one single and one triple bond. The latter consists of a sigma bond from the overlap of a C atom sp hybrid orbital with a N atom p orbital, and two mutually perpendicular pi bonds are formed from parallel atomic p orbitals of carbon and nitrogen atoms.

References

  1. Winter, Mark J. Chemical Bonding. Oxford: Oxford UP, 2005. Print.
  2. Roger L. Dekock and Harry B. Gray. Chemical Structure and Bonding. University Science Books. 1989.

Outside Links

1. Shaik, Sason S., and Philippe C. Hiberty. A Chemist's Guide to Valence Bond Theory. Hoboken, NJ: Wiley-Interscience, 2008. Print.

2. "Bonding and Hybridization." Department of Chemistry & Biochemistry @ Boise State University. Web. 05 Nov. 2010. <http://chemistry.boisestate.edu/peop...rganic/bonding and hybridization/bonding_hybridization.htm>.

Problems

1. What type of bond is present in the HCN molecular orbitals?

2. What theory is necessary to explain the formation of hybridized orbitals?

3. Explain why HCN is linear.

Answers

1. 1 sigma bond between H and C atoms. 1 sigma bond and 2 pi bond is present between C and N atoms. 

2. Valence bond thoery as wells as hybridization. Lewis dot structure can be used to get the basic idea of the structure.

3. Because of the 2 pi bonds and 1 sigma bond formed by the hybridization of 2px, 2py, and 2pz between C and N atoms, this 2p overlap makes the bond stronger and shorter therefore the bond between C and N is linear. Also, based on the property of atoms to be on the position at smallest strichinderance as possible, H atom will be as far away from C atom, which will result in the linear structure.

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Last Modified
10:17, 2 Oct 2013

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