Spectroscopy Spectroscopy: interaction of light with matter Average Bond energies (kJ/mol) C-H: 413C=C: 610H-F: 565 H-H: 436C  C: 835

Electronic excitation: visible, uv Vibrational excitation: infrared Rotational excitation: microwave

Electronic transitions - visible/ultraviolet Absorption - energy absorbed can excite electrons

Color of compound - complimentary to wavelengths absorbed chlorophyll a chlorophyll b

Absorption

Energy to excite electrons depends on relative position of molecular orbitals If  E is larger - absorption of short wavelength

Absorption spectrum of ozone (O 3 )

Emission - molecules in excited electronic states can loose energy by emitting light Chemiluminiscence in fireflies

Vibrational excitation: infrared IR Tutor Units in IR Wavelength  = m wavenumber Units cm -1 Model the bond between two atoms as a “spring” attached to the two atoms; the spring can vibrate.

Organic Chemistry Chemistry of carbon-containing molecules Carbon:  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

Hydrocarbons 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 naphthalene

Alkanes Saturated hydrocarbons General formula: C n H 2n+2 Methane (CH 4 )

Butane (C 4 H 10 ) and larger alkanes: linear or branched Isomers: same number of atoms, different arrangement iso-butane n-butane

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