Spectroscopy: interaction of light with matter
Average Bond energies (kJ/mol)
C-H: 413
C=C: 610
H-H: 436
CC: 835
H-F: 565
Electronic excitation: visible, uv
Vibrational excitation: infrared
Rotational excitation: microwave
Electronic transitions - visible/ultraviolet
Absorption - energy absorbed can excite electrons
chlorophyll a
chlorophyll b
Color of compound - complimentary to wavelengths
Energy to excite electrons depends on relative position of
molecular orbitals
If DE is larger - absorption of short wavelength
Absorption spectrum of ozone (O3)
Emission - molecules in excited electronic states can
loose energy by emitting light
Chemiluminiscence in fireflies
Vibrational excitation: infrared
Model the bond between two atoms as a “spring” attached
to the two atoms; the spring can vibrate.
IR Tutor
Units in IR
Wavelength m = 10-6 m
wavenumber  1
Units cm-1
Organic Chemistry
Chemistry of carbon-containing molecules
 has four electrons and four valence orbitals, and so its
compounds have neither too few electrons, requiring
electron deficient structures, nor too many electrons
resulting in excessive lone pair-lone pair repulsions
 can form compounds containing stable C-C bonds
 can form stable bonds with a number of elements
Result: almost an endless range of compounds with
elements bonded to C in straight chains, rings, branched
chains and with single, double and triple bonds
Saturated hydrocarbons: all C - C bonds are single bonds
Unsaturated hydrocarbons: C=C or CC bonds
Aliphatic hydrocarbons - no benzene ring
Aromatic hydrocarbon - contains benzene ring
Saturated hydrocarbons
General formula: CnH2n+2
Methane (CH4)
Butane (C4H10) and larger alkanes: linear or branched
Isomers: same number of atoms, different arrangement
Alkanes are considered to be non-polar since the
electronegativities of C and H are similar
Interaction between hydrocarbon molecules are due to
London dispersion forces
Strength of these interactions
increase with the number of
electrons; determines melting
and boiling points