Mass Spectroscopy
Mass Spectrometry (MS) • Molecular weight can be obtained from a very small sample.
• It does not involve the absorp=on or emission of light.
• A beam of high-‐energy electrons breaks the molecule apart.
• The masses of the fragments and their rela=ve abundance reveal informa=on about the structure of the molecule.
Mass Spectrometry (MS)
THE MAIN USE OF MASS SPECTROMETRY IN ORGANIC CHEM IS:
• DETERMINE THE MOLECULAR MASS OF ORGANIC COMPOUND
• DETERMINE THE MOLECULAR FORMULA OF ORGANIC COMPOUND
• IDENTIFY THE PRESENCE OF ISOTOPES PATTERNS FOR Cl or Br
Mass Spectrometry (MS) A small amount of sample is introduced into the mass spectrometer, where is bombarded by a stream of high-‐energy electrons.
The energy of an electron volt is around 70 eV (1600 kcal/mol or 6700 kJ/mol).
Mass Spectrometry (MS) When a high-‐energy electron strikes an organic molecule, it dislodges a valence electron from the molecule, producing a ca=on-‐radical.
Electron bombardment transfers such a large amount of energy to the sample molecule that ca=on-‐radical fragments aYer ioniza=on they fly apart into numerous smaller pieces; some of which retain a posi=ve charge, and some of which are neutral.
The fragments then pass through a strong magne=c field, where they are deflected through a curved pipe according to their mass to charge ra=o (m/z).
Neutral fragments are not deflected by the magne=c field and are lost on the walls of the pipe, but posi=vely charged are sorted by the mass spectrometer onto a detector, which records the peaks at the proper m/z ra=os.
Since the number of charges, z is usually 1, the peaks of ra=o m/z are simply m, the masses of the ions.
Mass Spectrometer
How Does it Work?
A:B$$$$$$$$$$$$$$$$$$$$$$ABElectron$Impact
Molecular$ionor
Cation5radical
Molecular)ionor
Cation.radical
AB A+))+)Bcation)))))))))))))radical
A))+)B+
radical))))))))))cation
Dissociates Dissociates
Dissocia=on depends on the type of func=onal group, weakest bond breaks. Most stable fragments are formed.
Mass Spectrometry (MS) The mass spectrum of a compound is usually presented as a bar graph with unit masses of m/z values on the x-‐axis and intensity (number of ions of a given m/z striking the detector) on the y-‐axis.
The tallest peak, called the base peak, is arbitrarily assigned as intensity of 100%.
Methane:
ELECTRON IMPACT
H-C:HH
H+ e H-C
H
HH + 2e
RADICAL CATION
CH4
H-CH
HH
H-CH
H+ + H
ONLY CATIONS ARE CARRIED TO DETECTOR
H-CH
HH++
CATION RADICAL BOND-‐BREAKING
Typical Mass Spectrum
100%
50%
43
58
71114
m/e
base peak given 100% abundance
molecular ion
Isotope peaks -‐ P+1, P+2, etc
Alarm Pheromone of Honey Bee
NOTE: 114-‐71 = 43
NOTE: 43 = mass of radical
Separa=on of Ions
• Only the ca=ons are deflected by the magne=c field.
• Amount of deflec=on depends on m/z.
• The detector signal is propor=onal to the number of ions hihng it.
• By varying the magne=c field, ions of all masses are collected and counted.
Mass Spectrum of Alkane, Pentane
The Molecular Ion
Loss of H2 From a Fragment
More Stable Fragments are More Abundant
The peak at m/z = 57 is more abundant for isopentane than for pentane because a secondary carboca=on is more stable than a primary carboca=on.
The GC-‐MS
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A mixture of compounds is separated by gas chromatography, then identified by mass spectrometry.
High Resolu=on MS
• Masses measured to 1 part in 20,000. • A molecule with mass of 44 could be C3H8, C2H4O, CO2, or CN2H4.
• If a more exact mass is 44.029, pick the correct structure from the table:
C3H8 C2H4O CO2 CN2H4
44.06260 44.02620 43.98983 44.03740
Natural Abundance of Isotopes
High Resolu=on Mass Spectrometry Can Dis=nguish Between Compound with the Same Molecular Mass
Exact Masses of Isotopes
Molecules with Heteroatoms
• Isotopes: present in their usual abundance. • Hydrocarbons contain 1.1% C-‐13, so there will be a small M+1 peak.
• If Br is present, M+2 is equal to M+. • If Cl is present, M+2 is one-‐third of M+. • If iodine is present, peak at 127, large gap. • If N is present, M+ will be an odd number. • If S is present, M+2 will be 4% of M+.
Use of Natural Abundance of Common Isotopes
From intensi=es of molecular ion and molecular ion plus one (M + 1), can calculate the number of carbon atoms in a
molecule.
#Carbon-‐atoms = rela=ve intensity of M+1 peak 0.01107 X rela=ve intensity of M+
Example: Determine the molecular formula of a molecule with a M peak at (m/z = 100), and with rela=ve intensity of 27,32 %, and a M+1 peak at a rela=ve intensity of 2.10%.
Answer: # of carbons in the molecule is 7
Use of Natural Abundance of Common Isotopes
• Can calculate intensity of M to M+1
M+1 = 0.01107 c + 0.00015 h + 0.00367 n + 0.00037 o + 0.0080 s
M 0.98893
Where M = intensity of the molecular ion, M+1 = intensity of the molecular ion + 1 peak (ions containing one 13C, 2H, 15N, 17O, or 33S) and c, h, n, o, s = the number of carbons, hydrogens, nitrogens, oxygens, sulfurs.
Isotopic Abundance
Easily Recognized Elements in MS
• Nitrogen: – Odd number of N = odd MW
SDBSWeb : hop://riodb01.ibase.aist.go.jp/sdbs/ (Na=onal Ins=tute of Advanced Industrial Science and Technology, 11/2/09)
Easily Recognized Elements in MS Bromine:
M+ ~ M+2 (50.5% 79Br/49.5% 81Br)
2-‐bromopropane
M+ ~ M+2
SDBSWeb : hop://riodb01.ibase.aist.go.jp/sdbs/ (Na=onal Ins=tute of Advanced Industrial Science and Technology, 11/1/09)
Easily Recognized Elements in MS
• Chlorine: – M+2 is ~ 1/3 as large as M+
SDBSWeb : hop://riodb01.ibase.aist.go.jp/sdbs/ (Na=onal Ins=tute of Advanced Industrial Science and Technology, 11/2/09)
M+2
M+
• Sulfur: – M+2 larger than usual (4% of M+)
Easily Recognized Elements in MS
M+
Unusually large M+2
SDBSWeb : hop://riodb01.ibase.aist.go.jp/sdbs/ (Na=onal Ins=tute of Advanced Industrial Science and Technology, 11/1/09)
Easily Recognized Elements in MS • Iodine
– I+ at 127 – Large gap
Large gap
I+
M+
SDBSWeb : hop://riodb01.ibase.aist.go.jp/sdbs/ (Na=onal Ins=tute of Advanced Industrial Science and Technology, 11/2/09)
Fragmenta=on Paoerns
• The impact of the stream of high energy electrons oYen breaks the molecule into fragments, commonly a ca=on and a radical. – Bonds break to give the most stable ca=on.
– Stability of the radical is less important.
Fragmenta=on Paoerns
• Alkanes – Fragmenta=on oYen splits off simple alkyl groups:
• Loss of methyl M+ -‐ 15 • Loss of ethyl M+ -‐ 29 • Loss of propyl M+ -‐ 43 • Loss of butyl M+ -‐ 57
– Branched alkanes tend to fragment forming the most stable carboca=ons.
Fragmenta=on Paoerns
• Mass spectrum of 2-‐methylpentane
The Mass Spectrum of Alkane, C7H16
Fragmenta=on Paoerns • Alkenes:
– Fragmenta=on typically forms resonance stabilized allylic carboca=ons
Fragmenta=on Paoerns • Aroma=cs:
– Fragment at the benzylic carbon, forming a resonance stabilized benzylic carboca=on (which rearranges to the tropylium ion)
M+
Fragmenta=on Paoerns Aroma=cs may also have a peak at m/z = 77 for the benzene ring.
M+ = 123
77
Fragmenta=on Paoerns
• Alcohols – Fragment easily resul=ng in very small or missing parent ion peak
– May lose hydroxyl radical or water • M+ -‐ 17 or M+ -‐ 18
– Commonly lose an alkyl group aoached to the carbinol carbon forming an oxonium ion.
• 1o alcohol usually has prominent peak at m/z = 31 corresponding to H2C=OH+
Fragmenta=on Paoerns
• MS for 1-‐propanol
M+ M+-18
SDBSWeb : hop://riodb01.ibase.aist.go.jp/sdbs/ (Na=onal Ins=tute of Advanced Industrial Science and Technology, 11/28/09)
α-‐Cleavage in an Alcohol
Loss of a Hydrogen from a γ-‐Carbon
Fragmenta=on Paoerns
• Ethers – α-‐cleavage forming oxonium ion
– Loss of alkyl group forming oxonium ion
– Loss of alkyl group forming a carboca=on
Fragmenta=on Paoerns
MS of diethylether (CH3CH2OCH2CH3)
The Carbon—Oxygen Bond Breaks Heteroly=cally
α-‐Cleavage in an Ether
Fragmenta=on Paoerns
• Amines – Odd M+ (assuming an odd number of nitrogens are present)
– α-‐cleavage dominates forming an iminium ion
Fragmenta=on Paoerns
Fragmenta=on Paoerns
• Aldehydes (RCHO) – Fragmenta=on may form acylium ion
– Common fragments:
• M+ -‐ 1 for
• M+ -‐ 29 for
Fragmenta=on Paoerns • MS for hydrocinnamaldehyde
M+ = 134 105
91
SDBSWeb : hop://riodb01.ibase.aist.go.jp/sdbs/ (Na=onal Ins=tute of Advanced Industrial Science and Technology, 11/28/09)
Fragmenta=on Paoerns
• Ketones – Fragmenta=on leads to forma=on of acylium ion:
• Loss of R forming
• Loss of R’ forming
Fragmenta=on Paoerns
• MS for 2-‐pentanone
M+
SDBSWeb : hop://riodb01.ibase.aist.go.jp/sdbs/ (Na=onal Ins=tute of Advanced Industrial Science and Technology, 11/28/09)
α-‐Cleavage in a Ketone
Fragmenta=on Paoerns
• Esters (RCO2R’) – Common fragmenta=on paoerns include:
• Loss of OR’ – peak at M+ -‐ OR’
• Loss of R’ – peak at M+ -‐ R’
Frgamenta=on Paoerns
M+ = 136
105
77
SDBSWeb : hop://riodb01.ibase.aist.go.jp/sdbs/ (Na=onal Ins=tute of Advanced Industrial Science and Technology, 11/28/09)
Mass Spectrum with Sulfur
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Mass Spectrum with Chlorine
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The Carbon—Chlorine Bond Breaks Heteroly=cally The Carbon—Carbon Bond Breaks Homoly=cally
α-‐Cleavage in an Alkyl Chloride
The homoly=c cleavage of the carbon—carbon bond is called α-‐cleavage.
The bonds that break are • the weakest bonds, and • the bonds that form the most stable fragments.
The Mass Spectrum of 2-‐Chloropentane
Mass Spectrum with Bromine
The Carbon—Bromine Bond Breaks Heteroly=cally
α-‐Cleavage Occurs in Alkyl Chlorides but is Less Likely to Occur in Alkyl Bromides
The carbon—carbon bond and the carbon—chlorine bond have similar strengths.
The carbon—carbon bond is much stronger than the carbon—bromine bond.
Rule of Thirteen
• The “Rule of Thirteen” can be used to iden=fy possible molecular formulas for an unknown hydrocarbon, CnHm.
– Step 1: n = M+/13 (integer only, use remainder in step 2)
– Step 2: m = n + remainder from step 1
Rule of Thirteen
• Example: The formula for a hydrocarbon with M+ =106 can be found:
– Step 1: n = 106/13 = 8 (R = 2)
– Step 2: m = 8 + 2 = 10
– Formula: C8H10
Rule of Thirteen
• If a heteroatom is present, – Subtract the mass of each heteroatom from the MW
– Calculate the formula for the corresponding hydrocarbon
– Add the heteroatoms to the formula
Rule of Thirteen
Example: A compound with a molecular ion peak at m/z = 102 has a strong peak at 1739 cm-‐1 in its IR spectrum. Determine its molecular formula.
1. A bond between carbon and a more electronega=ve atom breaks heteroly=cally.
2. A bond between carbon and an atom of similar electronega=vity breaks homoly=cally.
3. The bonds most likely to break are the weakest bonds and those that lead to forma=on of the most stable ca=on.
Common Fragmenta=on Behavior in Alkyl Halides, Ethers, Alcohols, and Ketones