Infrared Spectroscopy Organic Chemistry 4th Edition Paula Yurkanis Bruice Chapter 13 Mass Spectrometry and Infrared Spectroscopy Irene Lee Case Western Reserve University Cleveland, OH ©2004, Prentice Hall

Information Obtained from Mass Spectrometry The molecular mass The molecular formula Certain structural features of the compound

A Mass Spectrometer The mass spectrometer records a mass spectrum

A mass spectrum records only positively charged fragments m/z = mass to charge ratio of the fragment

Nominal molecular mass: the molecular mass to the nearest whole number Each m/z value is the nominal molecular mass of the fragment The peak with the highest m/z value represents the molecular ion (M) Peaks with smaller m/z values__called fragment ion peaks__represent positively charged fragments of the molecule

The base peak is the peak with the greatest intensity, due to its having the greatest abundance Weak bonds break in preference to strong bonds Bonds that break to form more stable fragments break in preference to those that form less stable fragments

The base peak of 43 in the mass spectrum of pentane indicates the preference for C-2 to C-3 fragmentation To identify fragment ions in a spectrum, determine the difference between the m/z value of a given fragment ion and that of the molecular ion

Carbocations can undergo further fragmentation

2-methylbutane has the same m/z as pentane but the peak at m/z = 57 (M – 15) is more intense

Isotopes in Mass Spectrometry peaks that are attributable to isotopes can help identify the compound responsible for a mass spectrum M + 2 peak: a contribution from 18O or from two heavy isotopes in the same molecule a large M + 2 peak suggests a compound containing either chlorine or bromine: a Cl if M + 2 is 1/3 the height of M; a Br if M + 2 is the same height as M In calculating the molecular masses of molecular ions and fragments, the atom mass of a single isotope of an atom must be used

The Mass Spectrum of Bromopropane

The weakest bond is the C–Br bond The base peak is at m/z = 43 [M – 79, or (M + 2) – 81] The propyl cation has the same fragmentation pattern it exhibited when it was formed in the cleavage of pentane

The Mass Spectrum of 2-Chloropropane

The compound contains a chlorine, because M + 2 peak is 1/3 the height of the molecular ion peak The base peak at m/z = 43 results from heterolytic cleavage of the C–Cl bond The peaks at m/z = 63 and m/z = 65 have a 3:1 ratio, indicating the presence of a chlorine atom

a cleavage results from the homolytic cleavage of a C–C bond at the a carbon

The Fragmentation Pattern of Ethers

The Fragmentation of Alcohols

Common Fragmentation Behavior in Alkyl Halides, Ethers, and Alcohols 1. A bond between carbon and a more electronegative atom 2. A bond between carbon and an atom of similar electronegativity break homolytically 3. The bonds most likely to break are those which lead to formation of the most stable cation

Fragmentation Pattern of Ketones An intense molecular ion peak

McLafferty rearrangement may occur

Infrared Spectroscopy An infrared spectrum is obtained by passing infrared radiation through the sample E = hu = hc l ~ wavenumber (u) is another way to describe the frequency of electromagnetic radiation High frequencies, large wavenumbers, and short wavelengths are associated with high energy

The covalent bonds in molecules are constantly vibrating Each stretching and bending vibration of a bond occurs with a characteristic frequency

An Infrared Spectrum The spikes are absorption bands

The functional group region The fingerprint region

It takes more energy to stretch a bond than to bend it

The greater the change in dipole moment, the more intense the absorption When the bond stretches, the increasing distance between the atoms increases the dipole moment The intensity of an absorption band depends on the number of bonds responsible for the absorption

Bond order affects bond strength, so bond order affects the position of absorption bands The approximate wavenumber of an absorption can be calculated from Hooke’s law u = 1 2pc f(m1 + m2) m1m2 1/2 ~ u the wavenumber of the stretching vibration f the force constant m1 and m2 masses of the atoms

The exact position of the absorption band depends on electron delocalization, the electronic effect of neighboring substituents,and hydrogen bonding at 1720 cm–1 at 1680 cm–1

Putting an atom other than carbon next to the carbonyl group causes the position of the carbonyl absorption band to shift The predominant effect of the nitrogen of an amide is electron donation by resonance The predominant effect of the oxygen of an ester is inductive electron withdrawal

The position of a C–O absorption varies because ~1050 cm–1 ~1050 cm–1 ~1250 cm–1 ~1250 cm–1 and 1050 cm–1

The position and the breadth of the O–H absorption band depend on the concentration of the solution It is easier to stretch an O–H bond if it is hydrogen bonded

The strength of a C–H bond depends on the hybridization of the carbon

Examine the absorption bands in the vicinity of 3000 cm–1

If a compound has hydrogen bonded to sp2 carbon The detection of sharp absorption bands at ~1600 cm–1 and 1500–1430 cm–1: benzene The detection of absorption bands at ~1600 cm–1: alkene

The N–H bending vibrations also occur at ~1600 cm–1, but they are broader and more intense They will be accompanied by N–H stretching at 3500–3300 cm–1

Stretch of C–H Bond in an Aldehyde

If a compound has sp3 carbon, absorption at ~1400 cm–1 identifies the methyl group

Analyzing Infrared Spectra The absence of absorption bands can be useful in identifying a compound in IR spectroscopy Bonds in molecules lacking dipole moments will not be detected

wavenumber (cm–1) assignment 3075 2950 1650 and 890 absence of 720 sp2 CH sp3 CH a terminal alkene with two substituents has less than four adjacent CH2 groups

1600 and 1460 wavenumber (cm–1) assignment 3050 sp2 CH 2810 and 2730 1700 sp2 CH an aldehyde benzene ring a partial single-bond character carbonyl

wavenumber (cm–1) assignment 3300 2950 2100 OH group sp3 CH alkyne

wavenumber (cm–1) assignment 3300 2950 1660 1560 N–H sp3 CH amide carbonyl

wavenumber (cm–1) assignment >3000 <3000 1605 and 1500 1720 1380 sp2 CH sp3 CH a benzene ring a ketone carbonyl a methyl group