Mass$Spectroscopy$ - Los Angeles Mission College · PDF filenumber$of$ions$hing$it.$ •...

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

=>

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



• Chlorine: – M+2 is ~ 1/3 as large as M+


M+2

M+

• Sulfur: – M+2 larger than usual (4% of M+)


M+

Unusually large M+2


Easily Recognized Elements in MS • Iodine

– I+ at 127 – Large gap

Large gap

I+

M+


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.


• 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.


• 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


• 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+


• MS for 1-‐propanol

M+ M+-18


α-‐Cleavage in an Alcohol

Loss of a Hydrogen from a γ-‐Carbon


• Ethers – α-‐cleavage forming oxonium ion

– Loss of alkyl group forming oxonium ion

– Loss of alkyl group forming a carboca=on


MS of diethylether (CH3CH2OCH2CH3)

The Carbon—Oxygen Bond Breaks Heteroly=cally

α-‐Cleavage in an Ether


• Amines – Odd M+ (assuming an odd number of nitrogens are present)

– α-‐cleavage dominates forming an iminium ion



• 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



• Ketones – Fragmenta=on leads to forma=on of acylium ion:

• Loss of R forming

• Loss of R’ forming


• MS for 2-‐pentanone

M+


α-‐Cleavage in a Ketone


• 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


Mass Spectrum with Sulfur

=>

Mass Spectrum with Chlorine

=>

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

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