Chemical Bonding: Valence Bond Theory “in a nutshell” Chapter 10 Section 4 through 6 of Jespersen 6th Ed) Dr. C. Yau Spring 2013 1 1 1

VSEPR Theory You had previously learned how to predict the molecular geometry of a species from examining its Lewis structure. It utilizes the concept of repulsion amongst the charge clouds of the central atoms. This was the Valence Shell Electron Pair Repulsion Theory (VSEPR Theory). It does not explain how a bond is formed and how bonding relates to the s, p, d orbitals. 2 2 2

Valence Bond Theory (VB Theory) The VB Theory explains bonding as an overlap of valence orbitals. H2 bonds are due to the overlap of their 1s valence orbitals. 1s 1s A B 2 separate H atoms H2 molecule with covalent bond due to overlap of the 1s orbitals. 3 3 3

Hybridization in VB Theory Atomic orbitals are mixed to allow formation of bonds that have realistic bond angles The newly mixed orbitals that result are called “hybrid orbitals” with specified shapes: Review: # charge clouds Hybridization Bond Angles 2 sp 180o 3 sp2 120o 4 sp3 109.5o 5 sp3d 120o & 90o 6 sp3d2 90o & 180o 4 4

How are sp3 hybrid orbitals formed? __ __ __ __ p s __ __ __ __ four sp3 hybrid orbitals 109.5o If we take s and all three p we form four sp3 hybrid orbitals. These hybrid orbitals are “degenerate.” That is, they are of the same E, higher than s but lower than p. Note that # orbitals is conserved. 5

How are sp2 hybrid orbitals formed? __ __ __ __ p s __ p __ __ __ three sp2 hybrid orbitals p sp2 If we take s and just two p orbitals, we form three sp2 hybrid orbitals leaving one pure p untouched. These three sp2 hybrid orbitals are planar with angles of 120o. The un-hybridized p (pure p) are used in double and triple bonds. 6

How are sp hybrid orbitals formed? __ __ __ __ p s __ __ two p __ __ two sp hybrid orbitals p If we take s and just one p orbital, we form two sp hybrid orbitals leaving two pure p untouched. p sp sp These two sp hybrid orbitals are linear with angles of 180o. 7

Hybrid Orbitals Involving d-Orbitals Hybrids from one s and three p can only make a maximum of 4 hybrid orbitals. When we run out of p orbitals, we start using d orbitals. Thus we have sp3d hybrid orbitals and sp3d2 hybrid orbitals (but only for elements larger than Ne, beyond Period 2). Remember that elements smaller than Ne do not have d-orbitals and therefore cannot have sp3d and sp3d2 hybrids. 8 8

Bonding Types Two types of bonds result from orbital overlap: sigma () bonds from head-on overlap lie along the bond axis account for the first bond pi ()bonds pi bonds are perpendicular to bond axis account for the second and third bonds in a multiple bond Chem FAQ: Use VB theory to describe double bonds. Use VB theory to describe triple bonds. 9 9

Sigma and Pi Bonding Given the structural formula of a compound, be able to specify the hybridization of each atom, state the bond angles and determine the # of sigma and pi bonds. 10 10

KNOW THIS WELL! X X single bond =  bond X X double bond =  bond +  bond X X triple bond =  bond + two  bonds 11 11

Always start with hybridization. 4 1 2 3 Always start with hybridization. Give the hybridization of each C and each O. Give the bond angles. How many  bonds are there? How many  bonds are there? Ans. C1 = sp C2 = sp C3 =sp2 C4 = sp3 O = sp3 C1-C2-C3 = 180o C2-C3-O = 120o C2-C3=O = 120o O-C4-H = 109.5o 9  bonds 3  bonds 12 12

Chemical Bonding in C2H6 Each C is sp3. The C−C bond is a  bond. It is a “head-to-head” overlap.

Chemical Bonding in C2H4 Each C is sp2, with one pure p. The C=C bond is made of  bond and a  bond. The  bond consists of overlap of parallel p orbitals.

Chemical Bonding in C2H2 Each C is sp, with two pure p. The CC bond is made of one  bond and two  bond. The  bond consists of overlap of parallel p orbitals.