Lecture 3 Mass Spectrometry and Infrared Spectroscopy
The Fragmentation Pattern of Alkyl Halides Electron sink Simple heterolytic cleavage Cl is more electronegative than C Cl has a positive formal charge
The Fragmentation Pattern of Alkyl Halides -cleavage ● ●+ ● CH3
63 65 m/z
The Fragmentation Pattern of Ethers
Simple heterolytic cleavage
-cleavage
The Fragmentation of Alcohols
Elimination of H 2 O The elimination undergoes a six-member ring transition state
Common Fragmentation Behavior in Alkyl Halides, Ethers, and Alcohols 1. A bond between carbon and a more electronegative atom—heterolytic cleavage 2. A bond between carbon and an atom of similar electronegativity break homolytically, e.g. -cleavage 3. The bonds most likely to break are those which lead to formation of the most stable cation
Stability of Carbocations Tertiary > Secondary > Primary hyperconjugation
Carbocations conjugated with -bonds or lone paired electrons are stable Aromaticity
Fragmentation Pattern of Ketones An intense molecular ion peak
McLafferty rearrangement may occur A six-member ring transition state ● + ●
Molecular Spectroscopy Study of interactions between electromagetic radiation and matter Electreomagnetic Radiation E = h = hc/ Wavenumber = 1/ (cm -1 ) ~ h is called Planck’s constant
Infrared radiation excites covalent bond’s vibration E
The covalent bonds in molecules are constantly vibrating Each stretching and bending vibration of a bond occurs with a characteristic frequency Molecule absorbs infrared radiation at its characteristic vibrational frequency Stretching Vibration Bending Vibration
How an IR Spectrometer Works IR radiation Detector Computer IR Spectrum Sample wavenumber % transmittance
The spikes are absorption bands An Infrared Spectrum 4000 – 625 cm – 16 m
The fingerprint region The functional group region 1400
It takes more energy to stretch a bond than to bend it
The greater the change in dipole moment, the more intense the absorption = 0 C=C stretch is IR inactive r (C=C) = År (C=C) = Å 1.34 Debye 1.51 Debye = 0.17 Debye C=C stretch is IR active