Molecular Orbitals of Alkenes

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Molecular Orbitals of Alkenes
HOMO - highest occupied molecular orbital
LUMO - lowest unoccupied molecular orbital
p
Anti-bonding
p
Bonding
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Electronic Transitions of Alkenes
With increased conjugation, the energy between
the HOMO and LUMO decreases
Visible Light Absorption
Ultra-Violet light absorption
hn
LUMO
HOMO
B -carotene
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UV Absorbance Spectroscopy
the ethene and octatetraene spectra are
simulations
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Color of Azulene?
F,D, C Red No. 40
Lots of conjugation
It absorbs red and green, so it is blue
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Wear Sunscreen.
PABA - para-aminobenzoic acid
1. Electrons are excited from the HOMO to LUMO by
absorption of ultra-violet energy 2. The energy
is emitted at a longer wavelength (not harmful)
and/or dissipated as heat.
The ability of molecules to absorb ultra-violet
and emit longer wavelengths is what allows
certain detergents to make whites brighter than
white. Not only is visible light reflected from
the fabric, but some of the ultra-violet is
converted into additional visible light.
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Reactivity of Conjugated Alkenes
All the addition reaction that alkenes undergo,
dienes also undergo
Resonance allows for the formation of a variety
of products
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Aromaticity
Molecules that are aromatic have an unusual
degree of stability
Aromatic molecules have a cyclic array of
continuous p orbital that contain 4n2 electrons
(2, 6, 10, 14, etc.)
The bonds in aromatic molecules are dynamic, not
localized to specific positions
Common representation of benzene
These are equivalent structures!
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Aromatic Molecules
benzene
pyridine
anthracene
PAH Polyaromatic hydrocarbons
napthalene
furan
adenine
The A in RNA and DNA strands
This also aromatic, the cation is quite stable,
makes the parent compound rather acidic
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Naming of Ketones and Aldehydes (Type of
carbonyl compounds that contain a CO bond)
Ketones are named like ethers or like other
functional group molecules (-one added to end)
Methyl propyl ketone or 2-pentanone
Aldehydes are named like other functional group
molecules (-al added to end)
3-methylbutanal
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Electrostatic Surfaces of Carbonyl and Alkene
No charge separation - no dipole
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Reactivity of Carbonyls
E electrophiles
NADH, LiAlH4, NaBH4
reduction
H2
Fundamental Reactivity
Oxidized
- H2
Reduced
oxidation
Nu nucleophiles
NAD, PCC, Cr(VI), Mn(VII)
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Biological Oxidizing and Reducing Agents
NAD nicotinamide adenine dinucleotide
Oxidizes alcohols
NADH reduced nicotinamide adenine dinucleotide
Reduces carbonyls