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ChemWiki: The Dynamic Chemistry E-textbook > Theoretical Chemistry > Chemical Bonding > Valence Bond Theory > Valence Bond Theory: Ethylene

Valence Bond Theory: Ethylene

C2H4, also known as ethylene or ethene, is a gaseous material created synthetically through steam cracking. In nature, it is released in trace amounts by plants to signal their fruits to ripen. Ethene consists of two sp​2-hybridized carbon atoms, which are sigma bonded to each other and to two hydrogen atoms each. The remaining unhybridized p orbitals on the carbon form a pi bond, which gives ethene its reactivity.  

Introduction

In 1916 G. N. Lewis proposed that a chemical bond is formed by the valence electrons of atoms which would prove fundamental in the valence bond theory. We all remember our first alkene discussed in Organic Chemistry and it was usually C2H4. This is becuase ethene is the smallest alkene and how it is also planar. The Valence Bond theory will show us how the bonds in C2H4 come together. Two things that will help us utilize the Valence Bond Theory effectively are familiarity with Lewis Structures and VSEPR theory.

Lewis Structures

A key component of using Valence Bond Theory correctly is being able to use the Lewis dot diagram correctly. Ethene has a double bond between the carbons and single bonds between each hydrogen and carbon: each bond is represented by a pair of dots, which represent electrons. Each carbon requires a full octet and each hydrogen requires a pair of electrons. The correct Lewis structure for ethene is shown below:

 

For more information on how to use Lewis Dot Structures refer to http://chemwiki.ucdavis.edu/Wikitexts/UCD_Chem_124A%3a_Kauzlarich/ChemWiki_Module_Topics/Lewis_Structures.

VSEPR Theory

Valence Shell Electron Pair Repulsion (VSEPR) Theory is used to predict the bond angles and spatial positions of the carbon and hydrogen atoms of ethene and to determine the bond order of the carbon atoms (the number of bonds formed between them). Each carbon atom is of the general arrangement AX3, where A is the central atom surrounded by three other atoms (denoted by X); compounds of this form adopt trigonal planar geometry, forming 120 degree bond angles. In order for the unhybridized p orbitals to successfully overlap, the CH​2 must be coplanar: therefore, C2H4 is a planar molecule and each bond angle is about 120 degrees. The diagram below shows the bond lengths and hydrogen-carbon-carbon bond angles of ethene:

According to valence bond theory, two atoms form a covalent bond through the overlap of individual half-filled valence atomic orbitals, each containing one unpaired electron. In ethene, each hydrogen atom has one unpaired electron and each carbon is sp2 hybridized with one electron each sp​2 orbital. The fourth electron is in the p orbital that will form the pi bond. The bond order for ethene is simply the number of bonds between each atom: the carbon-carbon bond has a bond order of two, and each carbon-hydrogen bond has a bond order of one. For more information see  http://chemwiki.ucdavis.edu/Wikitexts/UCD_Chem_124A%3a_Kauzlarich/ChemWiki_Module_Topics/VSEPR

References

  1. Murrel, J. N.; S. F. Tedder (1985). The Chemical Bond. John Wiley & Sons.      
  2. Shaik, Sason S.; Phillipe C. Hiberty (2008). A Chemist's Guide to Valence Bond Theory. New Jersey: Wiley-Interscience

Problems

1) What is the hybridization of carbon in C2H4

(sp2)

2) What is the hybridization of hydrogen in C2H4?

(s)

3) What is the bond order of the C to C bond in C2H4?

(2)

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Last Modified
22:07, 4 Dec 2013

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