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Atomic and Molecular Orbitals. Structure and Properties of Organic Molecules. Electrons act as both particles and waves (duality) An orbital can be described as a 3D standing wave To understand this, we can look at wave properties of guitar strings. Wave Harmonics with Dr. Jimi. - PowerPoint PPT Presentation
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John E. McMurry
www.cengage.com/chemistry/mcmurry
Paul D. Adams • University of Arkansas
Atomic and Molecular Orbitals
Structure and Properties of Organic Molecules
Electrons act as both particles and waves (duality) An orbital can be described as a 3D standing
wave To understand this, we can look at wave
properties of guitar strings
http://www.rollingstone.com/music/lists/100-greatest-guitarists-of-all-time-19691231/jimi-hendrix-19691231; Wade, Organic Chemistry, 2013
Wave Harmonics with Dr. Jimi
http://www.rollingstone.com/music/lists/100-greatest-guitarists-of-all-time-19691231/jimi-hendrix-19691231; Wade, Organic Chemistry, 2013
Wave Harmonics with Dr. Jimi
Atomic orbitals combine and overlap to produce more complex standing waves
Any wave overlap can be constructive or destructive.
Wade, Organic Chemistry, 2013; http://animals.howstuffworks.com/insects/butterfly-colors1.htm
Linear Combination of Atomic Orbitals
When orbitals on different atoms interact (think of covalent bonds), they produce molecular orbitals that lead to bonding or antibonding interactions.
Wade, Organic Chemistry, 2013
Electrostatic potential map of H2
• There is an optimal distance between these nuclei where charge attraction and repulsion are balanced.
• This leads to a somewhat consistently fixed bond length.
• The stability of covalent bond results from increased electron density in the bonding region (i.e., the space between the 2 nuclei where orbitals overlap).
Linear Combination of Atomic Orbitals
Wade, Organic Chemistry, 2013
• Formation of cylindrically symmetrical sigma (σ) bond as a result of constructive addition • e- density in bonding region increases• Forms bonding MO
Sigma Bonding in Hydrogen Molecules
Wade, Organic Chemistry, 2013
• The 2 out of phase 1s hydrogen orbitals overlap destructively to cancel out part of the wave, producing a node
• Forms antibonding MO with (σ*) bond
Sigma Bonding in Hydrogen Molecules
A molecular orbital (MO): where electrons are most likely to be found (specific energy and general shape) in a molecule
Additive combination (bonding) MO is lower in energy Subtractive combination (antibonding) MO is higher energy
Describing Chemical Bonds: Molecular Orbital Theory
The bonding MO is from combining p orbital lobes with the same algebraic sign
The antibonding MO is from combining lobes with opposite signs
Only bonding MO is occupied
Molecular Orbitals in Ethylene
Destructive interference of orbitals
Constructive interference of orbitals
Wade, Organic Chemistry, 2013
Formation of MO’s
Wade, Organic Chemistry, 2013
Formation of the bonding MO requires less E to maintain, i.e., it is more stable
Sigma Bonding in Hydrogen Molecules
Pi (π) bonds formed by overlap of 2 parallel p orbitals. Not cylindrically symmetrical like σ bond Not as strong as σ bond Important for chemical reactions
Pi bonds form double and triple bonds.
Wade, Organic Chemistry, 2013
Pi (π) Bonds
When orbitals on the same atom interact, they produce hybrid atomic orbitals that define bond geometry.
Consider the problem with carbon in the energy diagram below:
Wade, Organic Chemistry, 2013
2p2s1s
EnergyEnergy
Valence Valence shell shell
electronelectronss
C1
2
How does carbon form 4 bonds?
1s22s22p2
Orbital Hybridization
Carbon has 4 valence electrons (2s2 2p2) In CH4, all C–H bonds are identical (tetrahedral) sp3 hybrid orbitals: s orbital and three p orbitals
combine to form four equivalent, unsymmetrical, tetrahedral orbitals (sppp = sp3), Pauling (1931)
Orbital Hybridization
EnergyEnergy
2p2s1s
Valence shell
electrons
1s
4 x sp3
An sp3 orbital has a two-lobed shape, similar to the shape of a p orbital but with different-sized lobes.
Each carbon-hydrogen bond in methane arises from an overlap of a C (sp3) and an H (1s) orbital.
The sharing of two electrons in this overlap region creates a sigma (σ) bond.
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
Orbital Hybridization
Two C’s bond to each other by overlap of an sp3 orbital from each
Three sp3 orbitals on each C overlap with H 1s orbitals to form six C–H bonds
C–H bond strength in ethane 421 kJ/mol C–C bond is 154 pm long and strength is 377 kJ/mol All bond angles of ethane are tetrahedral
sp3 Orbitals and the Structure of Ethane
The tetrahedral geometry can be explained by Valence Shell Electron Pair Repulsion (VSEPR) Theory
(VSEPR) Theory: Electron pairs repulse each other in such a way so that they are as far apart from each other as possible.
The bond angles observed in organic compounds can (currently) only be explained by this repulsion of hybridized orbitals.
Wade, Organic Chemistry, 2013
trigonaltetrahedral linear geometry
VSEPR Theory
In C=C bonds, sp2 hybrid orbitals are formed by the carbon atoms, with one electron left in a 2p orbital. A representation of sp2 hybridization of carbon.
During hybridization, two of the 2p orbitals mix with the single 2s orbital to produce three sp2 hybrid orbitals. One 2p orbital is not hybridized and remains unchanged.
2p2s1s
Energy
2p3 sp2
1s
The Geometry of Alkenes
1. One bond (sigma, σ) is formed by overlap of two sp2 hybrids.
2. The second bond (pi, π) is formed by connecting the unhybridized p orbitals.
2p2s1s
Energy
2p3 sp2
1s
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
The Geometry of Alkenes
The planar geometry of the sp2 hybrid orbitals and the ability of the 2p electron to form a “pi bond” bridge locks the C=C bond firmly in place.
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
The Geometry of Alkenes
Because there is no free rotation about the C=C bond, geometric isomerism is possible.
cis- isomers have two similar or identical groups on the same side of the double bond.
trans- isomers have two similar or identical groups on opposite sides of the double bond.
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
The Geometry of Alkenes
Geometric isomers have different physical properties.
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
The Geometry of Alkenes
2p2s1s
Energy
2p2 sp1s
• Insoluble in water
• Less dense than water
• Low MP, BP
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011
Hybridization/Geometry of Alkynes
Elements other than C can have hybridized orbitals H–N–H bond angle in ammonia (NH3) 107.3°
C-N-H bond angle is 110.3 ° N’s orbitals (sppp) hybridize to form four sp3 orbitals One sp3 orbital is occupied by two nonbonding
electrons, and three sp3 orbitals have one electron each, forming bonds to H and CH3.
Hybridization of Nitrogen and Oxygen
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