Structure and Properties of Organic Molecules Organic Chemistry, 8th Edition L. G. Wade, Jr. Chapter 2 Lecture Structure and Properties of Organic Molecules

Lecture 3: Orbital Theory, Molecular Shapes (VSEPR) and Hybridization Wave Properties of Electrons Wave Interactions LCAO Valence Bond Theory Sigma Bonding Pi Bonding Molecular Shapes and Hybridization Bond Rotation in Organic Chemistry

Wave Properties of Electrons Electrons have wave-like properties Orbitals are 3-D standing waves Standing wave vibrates in fixed location (Like a guitar string) Traveling wave is like an ocean wave Chapter 1

Wave Properties of Electrons Orbitals are described by a wave function Wave function, , is a mathematical description of size, shape, and orientation. Amplitude may be positive or negative. Node: Amplitude is zero. Chapter 1

Wave Interactions Linear combination of atomic orbitals on different atoms produce molecular orbitals (MO’s) Lead to Bonding: Covalent Bonds on the same atom give hybrid orbitals (sp, sp2, sp3) Different geometries for molecules

LCAO Linear combination of atomic orbitals Conservation of Orbitals Number of new orbitals = initial number of orbitals Waves that are in phase add together: Amplitude increases Waves that are out of phase cancel: Zero Amplitude

Valence Bond Theory A bond occurs when atomic orbitals overlap. Overlapping orbitals are like overlapping waves. Only constructive interference results in a bond. Chapter 1

Sigma Bonding An MO that is cylindrically symmetrical along the line connecting the nuclei is referred to as a sigma bond (σ ─ bond) A bond may be formed by s-s, p-p, s-p or hybridized orbital overlaps. Every single bond connecting two atoms contains a sigma bond (σ ─ bond)

Bonding Region Electrons are close to both nuclei.

Bonding Molecular Orbital Two Hydrogens, 1s constructive overlap

Anti-Bonding Molecular Orbital Two Hydrogens, destructive overlap

H2: s—s Overlap File Name: AAAKPEK0 Figure: 02-08.jpg Chapter 2 13

Cl2: p-p overlap Constructive Overlap along the same axis forms a sigma bond.

HCl: s-p overlap

Pi Bonding Pi bonds form after sigma bonds. Sideways overlap of parallel p-orbitals.

Pi Bonding Electron density found above and below line connecting two nuclei Overlap is parallel; not linear. Π-bonds are not cylindrically symmetric Π-bonds are not as strong as σ-bonds.

Multiple Bonds A double bond (2 pairs of shared electrons consists of a sigma bond and a pi bond A triple bond (3 pairs of shared electrons consists of a sigma bond and 2 pi bonds)

Molecular Shapes and Hybridization Bond angles cannot be explained with simple s and p orbitals. Valence shell electron-pair repulsion (VSEPR) is used to explain the molecular shape of molecules. Hybridization is LCAO within one atom, just prior to bonding. Hybridized orbitals are lower in energy because electron pairs are farther apart.

sp Hybrid Orbitals 2 VSEPR pairs Linear electron pair geometry. 180° bond angle. File Name: AAAKPER0 Figure: 02-12.jpg Chapter 2 20

The Bonding of BeH2 The bond angle in BeH2 is 180º and the geometry is linear. File Name: AAAKPET0 Figure: 02-13.jpg Chapter 2 21

sp2 Hybrid Orbitals 3 VSEPR pairs Trigonal planar e- pair geometry 120o bond angle Chapter 1

sp3 Hybrid Orbitals 4 VSEPR Pairs Tetrahedral e- pair geometry 109.5o bond angle

Summary of Hybridization and Geometry Hybrid Orbitals Hybridization Geometry Approximate Bond Angle 2 s + p = sp linear 180⁰ 3 s + p + p = sp2 trigonal 120⁰ 4 s + p + p + p = sp3 tetrahedral 109.5⁰ File Name: cannot be found in Wade8e_ch02_artms.pdf Figure: 02_UNT02.jpg Chapter 2 24

Rotation of Single Bonds File Name: AAAKPFL0 Figure: 02-20.jpg Ethane is composed of two methyl groups bonded by the overlap of their sp3 hybrid orbitals. There is free rotation along single bonds. Chapter 2 25

Bonding in Ethylene Ethylene has three sigma bonds formed by its sp2 hybrid orbitals in a trigonal planar geometry. The unhybridized p orbital of one carbon is perpendicular to its sp2 hybrid orbitals, and it is parallel to the unhybridized p orbital of the second carbon. Overlap of these two p orbitals will produce a pi bond (double bond) that is located above and below the sigma bond. File Name: cannot be found in Wade8e_ch02_artms.pdf Figure: 2-17 page 51 in the 8th edition (02-16-03un.jpg in IRC DVD) Chapter 2 26

Rotation Around Double Bonds Double bonds cannot rotate. Compounds that differ in how their substituents are arranged around the double bond can be isolated and separated. File Name:AAAKWGU0 (this image is in chapter 7) Figure: 07_02.jpg Chapter 2 27

Skill Building: Practice Problems Problem 2-1 thru 2-7