•Consider ethylene (also called ethene): C2H4. Draw a Lewis structure, and use it to
determine the geometry and hybridization of each of the carbon atoms. How can
we describe a double bond in terms of the overlap of orbitals?
Prepare for your exams
Study with the several resources on Docsity
Earn points to download
Earn points by helping other students or get them with a premium plan
Guidelines and tips
Prepare for your exams
Study with the several resources on Docsity
Earn points to download
Earn points by helping other students or get them with a premium plan
Consider ethylene (also called ethene): C2 H4 . Draw a Lewis structure, and use it to determine the geometry and hybridization of each of the carbon atoms. How ...
Typology: Lecture notes
2021/2022
1 / 40
This page cannot be seen from the preview
Don't miss anything!
Related documents
Partial preview of the text
Download Molecular Orbitals and more Lecture notes Chemistry in PDF only on Docsity!
- Consider ethylene (also called ethene): C 2 H 4. Draw a Lewis structure, and use it to determine the geometry and hybridization of each of the carbon atoms. How can we describe a double bond in terms of the overlap of orbitals?
Molecular Orbitals
Molecular Orbital (MO) Theory (continued 1)
- Filling of MOs with electrons is governed by the same rules as for atomic orbitals - Aufbau principle - Fill MOs beginning with the lowest energy unoccupied molecular orbital - Pauli exclusion principle - No more than two electrons can be accommodated in a MO, and their spins must be paired - Hund’s rule - When two or more MOs of equivalent energy are available, add one electron to each before any equivalent orbital is filled with two electrons, and the spins of the single electrons in degenerated orbitals should be aligned.
Molecular Orbital (MO) Theory (continued 2)
- Bonding molecular orbital : MO in which electrons have a lower energy than they would in isolated atomic orbitals
- Sigma ( s ) bonding molecular orbital : MO in which electron density is concentrated between two nuclei, along the axis joining them and is cylindrically symmetric
- Antibonding molecular orbital : MO in which electrons have a higher energy than they would in isolated atomic orbitals - Indicated using an asterisk (*)
Combining VB and MO Theories
- VB theory views bonding as arising from electron pairs localized between adjacent atoms - Pairs create bonds - Organic chemists commonly use atomic orbitals involved in three hybridization states of atoms ( sp^3 , sp^2 , and s p ) to create orbitals that match the experimentally observed geometries
- To create orbitals that are localized between adjacent atoms, atomic orbitals are added and subtracted on the adjacent atoms, which are aligned to overlap each other
Combining VB and MO Theories (continued 1)
- Example - In methane, CH 4 , the sp^3 hybrid orbitals point at the 1 s hydrogen orbitals, and the atomic orbitals are added and subtracted to create molecular orbitals
- The other resulting MO is higher in energy than the two atomic orbitals and is antibonding
- Only the lower-energy orbital is populated with electrons in methane
- Population of the σ bonding orbital results in σ bond between the C and the H
Combining VB and MO Theories (continued 2)
- An identical approach used to create C—H σ bonds is used to create C— C σ bonds - CH 3 CH 3 contains one C—C σ bond and 6 C—H σ bonds
Combining VB and MO Theories (continued 3)
- sp^2 hybridization should be considered wherever there is a double bond
- Consider ethylene, C 2 H (^4)
- σ bond between the carbons is formed by overlapping sp^2 hybrid orbitals along a common axis
- Each carbon also forms σ bonds with two hydrogens
- Consider ethylene, C 2 H (^4)
Combining VB and MO Theories (continued 4)
- sp hybridization is appropriate wherever there is a triple bond
- Example - A carbon-carbon triple bond consists of:
- One σ bond formed by overlapping of sp hybrid orbitals
- Two π bonds formed by the overlap of a pair of parallel 2 p atomic orbitals
- Example - A carbon-carbon triple bond consists of:
Molecular Orbitals: H 2
Two atomic orbitals are combined, two new molecular orbitals must result. How does this work in forming H 2?
Phases and Overlap of Orbitals
- Why do both bonding and antibonding orbitals form from the combination of atomic orbitals?
- What is an antibonding orbital, anyway?
Forming σ -bonds From Hybridized Orbitals
- Construct an MO diagram for the C-C σ -bond in methane (CH 4 ).
- Construct an MO diagram for the C-C σ -bond in ethane (C 2 H 6 ).
Energies of Atomic Orbitals
- The left chart shows the approximate energies of orbitals in several of the second- period elements. Can we make any generalizations about the energies of orbitals based on these observations?
- How would the charge on an atom affect the energies of its orbitals?
- To summarize: What are the three factors that will lead to lower orbital energies?
Forming Molecular Orbitals from Atomic Orbitals with
Different Energies
- Construct an MO diagram for the C–Cl σ-bond in methyl chloride (CH 3 Cl):
- Construct an MO diagram for the C=O π-bond in formaldehyde (H 2 C=O):
- What generalizations can we make about molecular orbitals that are constructed from two atomic orbitals with different energies?