11 CHEM 344 Organic Chemistry Lab September 9 th and 10 th 2008 Spectroscopy of Organic Compounds Lecture 3 –Infrared and Mass Spec

Infrared radiation excites vibrational energy levels of a molecule Bond stretching, bending, twisting, rocking etc. IR spectroscopy most useful for functional group assignment IR bands reported in wavenumbers (cm -1 )

Wavenumber (cm -1 ) N – H O – H C – H (sp > sp 2 > sp 3 ) C≡CC≡NC≡CC≡N C=C C=N C=O Fingerprint region Transmittance ( %) Alkynes Nitriles Alkenes Imines Carbonyls C-C C-O C-X C-N IR correlation tables in lab manual, textbook Not used in CHEM 344

N-H 3281 cm cm -1 sp 3 C-H 2 o amine R 2 NH

asym N-H 3369 cm cm -1 sym N-H cm -1 sp 3 C-H 1 o amine RNH 2 Sym and asym N-H bands typical of 1 o amine RNH 2

C≡CC≡C 2120 cm -1 sp 3 C-H sp C-H 3315 cm -1 Terminal alkyne

2260 cm -1 Nitrile C≡NC≡N

O-H 3639 cm -1 C=O 1698 cm -1 C-O band in fingerprint region

1685 cm -1 C=O 1619 cm -1 C=C sp 2 sp 3 C-H C=O usually most intense band in IR spectrum C=O and C-C in same region (1700 – 1600 cm -1 ) – which is which?

1716 cm cm -1 Broad band typical of RCO 2 H

Mass Spectrometry Uses high energy electron beam (70 eV), sample in gas phase Ionization potential for most organic molecules 8-15 eV Gives info on molecular weight of compound by m/z Gives info on connectivity of molecule by fragmentation pattern

Molecular Fragments Molecule Molecular Ion [M].+ (also called the parent peak) gives the mass of the molecule Fragments give info on connectivity (i.e. structure) of the molecule

Mass Spectrum of methanol CH 3 OH Base peak [M] +. m/z = mass/charge ratio In CHEM 344, we only consider singly charged species (i.e. z=1) The most intense peak is referred to as the base peak All other peaks measured relative to base peak

Cation stability more important than radical stability Molecular Ion Cation radical A + B CationRadical A + B CationRadical Radicals are not detected by MS

Mass Spectrum of methanol CH 3 OH [CH 2 OH] + Base peak [CHO] + [CH 3 ] + [CH 3 OH] +. [M].+

Mass Spectrum of 2,2,4-trimethylpentane o carbocation 2 o carbocation No [M].+ detected

Mass Spectrum of 1-butanol via loss of H 2 O No [M].+ detected

Mass Spectrum of 2-methyl-2-butanol

Fragmentation pattern of 2-methyl-2-butanol

Mass Spectrum of 2-octanone via  - cleavage McLafferty rearragement [M].+

m/z = 43 m/z = 113 Molecular ion m/z = 128  -cleavage at a C=O group Resonance stabilized acylium cation

McLafferty rearrangement of 1-octanone 1-pentene Carbonyl compound must possess gamma H-atoms to undergo McLafferty rearrangement Process always eliminates a neutral alkene, hence McLafferty product is a radical cation α β γ McLafferty rearrangement of a carboxylic acid ethene

Mass Spectrum of Benzamide via  - cleavage [M].+

Mass Spectrum of Toluene Loss of H. Benzyl cation???

Tropylium cation m/z = 91 Toluene radical cation m/z = 92 m/z = 91 common to all alkyl benzenes