Mass Spectrometry. 2000, Paul R. Young University of Illinois at Chicago, All Rights Reserved

Mass Spectrometry 2000, Paul R. Young University of Illinois at Chicago, All Rights Reserved

Mass Spectrometry When a molecule is bombarded with high-energy electrons, one of the process that can occur is the ejection of an electron from the neutral molecule to form a charged species with one unpaired electron. This species is called a radical cation (M + ) M + e 2e + M

Mass Spectrometry This radical cation can fragment, ejecting a radical to form a carbocation, or it can eject an even electron molecule to generate a new radical cation. Carbocations, however, only fragment to form new carbocation species. M + R + a radical M + R + + an even-electron molecule R B + an even-electron molecule

Mass Spectrometry This fragmentation can be simple, such as the loss of methoxy radical from the methyl 2-methylbenzoate radical cation... C C 3 C 3 C 3 C 3 C C 2

Mass Spectrometry This fragmentation can be simple, such as the loss of methoxy radical from the methyl 2-methylbenzoate radical cation, or the concerted loss of methanol. C C 3 C 3 C 3 C 3 C C 2

Mass Spectrometry The charged species formed can be characterized by their mass (m) and their charge (z) as their mass-tocharge ratio (). C = 119 C 3 C 3 C 3 = 150 C 3 C C 2 = 118

Mass Spectrometry In the electron impact mass spectrometer, a gas-phase beam of sample molecules are ionized by an electron beam to produce radical cations and their fragmentation products. Molecular Source Electron Beam Ion Accelerating Array Magnetic Field Bends Path of Charged Particles Collector Exit Slit

Mass Spectrometry These ions are accelerated in an analyzing tube, which uses a magnetic field to focus ions of a given ratio on a exit slit, where they collide with a collector and are detected by the analyzer circuitry. Molecular Source Electron Beam Ion Accelerating Array Magnetic Field Bends Path of Charged Particles Collector Exit Slit

Mass Spectrometry By changing the force of the applied field, the spectrometer can scan through all of the ions produced, detecting the intensity of each ion independently, along with its ratio. Molecular Source Electron Beam Ion Accelerating Array Magnetic Field Bends Path of Charged Particles Collector Exit Slit

Mass Spectrometry The output, a plot of relative intensity vs. ratio, is called a mass spectrum. Molecular Source Electron Beam Ion Accelerating Array Magnetic Field Bends Path of Charged Particles Collector Exit Slit

. Mass Spectrometry = 119 C A representative mass spectrum is shown below for methyl 2- methylbenzoate. C 3 = 150 C 3 C 3 C 3 = 118 C C 2 119 118 150 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

. Mass Spectrometry = 119 C In the mass spectrum, the parent ion is called the molecular ion (M + ) and the most intense peak is called the base peak. C 3 = 150 C 3 C 3 C 3 = 118 C C 2 119 base peak 118 M + 150 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

Mass Spectrometry The relative intensities of peaks in the mass spectrum depend on the ease of the required fragmentation and the stability of the ion which is formed. C C 3 = 150 C 3 C 3 C 3 = 119 a stable oxonium ion C C 2 = 118

. Mass Spectrometry = 119 C The mass spectrum will therefore tell us the molecular weights of the compound and of its most stable cationic fragments. C 3 = 150 C 3 C 3 C 3 = 118 C C 2 119 base peak 118 M + 150 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

The mass spectrum of toluene is shown below. The most stable carbocation which can be readily derived from this molecule is the benzyl carbocation. Toluene (MW = 92) would therefore be expected to lose a hydrogen radical to give the benzyl carbocation ( = 91). 91 = 92 = 91 C 3 C 2 92 65 = 91 = 65 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

The benzyl carbocation ( = 91), however, rapidly rearranges to form the more stable tropylium cation. Loss of ethyne from this ion gives the peak at = 65. In general, the presence of a peak in the mass spectrum at = 91 strongly suggests the presence of a benzyl unit in the parent molecule. 91 = 92 = 91 C 3 C 2 92 loss of C 2 2 65 = 91 = 65 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

Reviewing Carbocations and Carbocation Stability Two views of the 2-propyl carbocation sp 2 hybridization planar, with 120 o bond angles

Alkyl groups are electron releasing and stabilize carbocations by an inductive effect, as shown by electrostatic potential maps for the ethyl, 2-propyl and tert- butyl carbocations.

ne of the most stable carbocations known is the cycloheptatrienyl cation (the tropylium ion) in which the positive charge is evenly distributed over all seven ring carbons by resonance. δ δ δ δ δ δ δ Tropylium cation, showing the uniform distribution of the positive charge.

Summary of Carbocation Stability 1. Most Stable: Carbocations adjacent to heteroatoms having unshared pairs of electrons so that the positive charge is delocalized by resonance. R R R R Cl R Cl R N R N R R R

2. Alkyl or aryl carbocations where the positive charge can be delocalized by resonance C 2 C 2 C 2 C 2

3. Alkyl carbocations are most easily ranked by the nature of the cationic carbon, i.e.: primary, secondary, tertiary, etc. You should also note that carbocations are sp 2 hybridized and therefore must always be planar. C 3 3 C C 3 3 C C 3 C 2 C 3 not planar -- very unstable

. The mass spectrum of 2-methylbutane. 3 C C 3 C 3 MW = 58 43 58 20 30 40 50 60 70 80 90 100

. The mass spectrum of 2-methylbutane. In general an M-15 peak suggests loss of a methyl group. + C 3 3 C C 3 C 3 C 3 C 3 = 43 43 M-15 M + 58 20 30 40 50 60 70 80 90 100

. The mass spectrum of acetone. MW = 58 3 C C 3 43 M-15 M + 58 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

. The mass spectrum of acetone. In general, carbonyl compounds will tend to cleave on either side of the carbonyl group, forming stabilized oxonium ions. 43 3 C C 3 M-15 + C 3 the acylium ion C 3 M + 58 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

. The mass spectrum of acetophenone. 3 C MW = 120 M-43 77 105 M-15 M + 120 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

. The mass spectrum of acetophenone. In general, carbonyl compounds will tend to cleave on either side of the carbonyl group, forming stabilized oxonium ions. 3 C M-43 77 + C 3 = 105 105 M-15 + C = 77 M + 120 3 C 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

. The mass spectrum of hexanal. MW = 100 44 M-56 56 M + 100 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

. The mass spectrum of hexanal. Carbonyl compounds and alkyl benzenes with alkyl chains containing 3 or more carbons will undergo McLafferty rearrangement with loss of an alkene. + 2 C C = 44 44 56 100 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

. The mass spectrum of 2-butanol. MW = 74 45 M-29 59 M + 74 20 30 40 50 60 70 80 90 100

. The mass spectrum of 2-butanol. Alcohols tend to fragment adjacent to the hydroxyl group to give the stable oxonium ion. 3 C C = 45 + C 3 C 2 45 M-29 59 M + 74 20 30 40 50 60 70 80 90 100

. The mass spectrum of benzyl bromide. Br MW = 171 91 M-80 65 M + 172-170 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

The mass spectrum of benzyl bromide. Compounds containing bromine will display two M + ions or equal intensity in the mass spectrum, at M-1 and M+1. Br + Br = 91 91 M-80 65 M + 172-170 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

Effect of isotopic abundance on halogencontaining species in the mass spectrum. Relative Br Br 2 M+2 M M+2 M M+4 Cl Cl 2 Relative M M M+2 M+2 M+4

Fragments commonly lost from M + in the mass spectrum. Mass Group 15 C 3 Mass Group 32 C 3 16 N 2 17 18 2 19 F 20 F 26 C 2 2 29 C 29 C 2 C 3 30 C 2 31 C 3 44 C 3 5 42 C 2 C 42 C 3 6 43 C 3 7 43 C 3 C 44 C 2 44 C 3 8 45 C 2 45 C 2 C 3 46 C 3 C 2

. In-Class Problem: C 5 10 2 43 71 59 87 102 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

. In-Class Problem: C 5 10 2 MW = 102; one degree of unsaturation 43 71 59 87 102 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

. In-Class Problem: C 5 10 2 MW = 102; one degree of unsaturation 43 71 59 87 M + 102 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

. In-Class Problem: C 5 10 2 MW = 102; one degree of unsaturation 43 59 71 M-15; loss of methyl 87 M + 102 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

. In-Class Problem: C 5 10 2 MW = 102; one degree of unsaturation 43 59 M-31; loss of methoxy 71 M-15; loss of methyl 87 M + 102 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

. In-Class Problem: C 5 10 2 MW = 102; one degree of unsaturation 43 M-59; acylium ion or propyl cation 59 M-31; loss of methoxy 71 M-15; loss of methyl 87 M + 102 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

. In-Class Problem: C 5 10 2 MW = 102; one degree of unsaturation 3 C C C 3 3 C 3 43 M-59; acylium ion or propyl cation 59 M-31; loss of methoxy 71 M-15; loss of methyl 87 M + 102 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

. In-Class Problem: C 5 10 2 MW = 102; one degree of unsaturation 3 C C C 3 3 C 3 43 M-59; acylium ion or propyl cation M-31; loss of methoxy 71 M-15; loss of methyl 3 C C 3 59 87 M + 102 C 3 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200