mass_spec_lecture (minggu 2))edited for print (1)
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Mass
Spectrometry
Parts of ElucidationStructure
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Background
Mass spectrometry (Mass Spec or MS)uses high energy electrons to break a
molecule into fragments.
Separation and analysis of the
fragments provides information about:Molecular weight
Structure
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Background
The impact of a stream of high energy electronscauses the molecule to lose an electron forming aradical cation.
A species with a positive charge and one unpairedelectron
+ e-C H
H
H
H HH
H
HC + 2 e-
Molecular ion (M+)
m/z = 16
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Background
The impact of the stream of high energyelectrons can also break the molecule or theradical cation into fragments.
(not detected by MS)
m/z = 29
molecular ion (M+) m/z = 30
+ C
H
H
H
+ H
HH C
H
H
C
H
H
H C
H
H
C
H
H
H CH
H
+ e-
H C
H
H
C
H
H
H
m/z = 15
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Background
Molecular ion (parent ion):The radical cation corresponding to the mass of
the original molecule
The molecular ion is usually the highest massin the spectrumSome exceptions w/specific isotopes
Some molecular ion peaks are absent.
H
H
H
HC H C
H
H
C
H
H
H
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Background Mass spectrum of ethanol (MW = 46)
SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute ofAdvanced Industrial Science and Technology, 11/1/09)
M+
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Background The cations that are formed are separated by magnetic deflection.
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Background
Only cations are detected.Radicals are invisible in MS.
The amount of deflection observeddepends on the mass to charge ratio(m/z).Most cations formed have a charge of +1 sothe amount of deflection observed isusually dependent on the mass of the ion.
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Background
The resulting mass spectrumis a graph of themass of each cation vs. its relativeabundance.
The peaks are assigned an abundance as apercentage of the base peak. the most intense peak in the spectrum
The base peak is not necessarily the same asthe parent ion peak.
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Background
SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute ofAdvanced Industrial Science and Technology, 11/1/09)
M+base peak
The mass spectrum of ethanol
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Background
Most elements occur naturally as a mixtureof isotopes.The presence of significant amounts of heavier
isotopes leads to small peaks that have masses
that are higher than the parent ion peak.
M+1= a peak that is one mass unithigher than M+
M+2= a peak that is two mass unitshigher than M+
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Molecules withHeteroatoms
Isotopes: present in their usual abundance.
Hydrocarbons contain 1.1% C-13, so there will be asmall 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+. =>
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Easily Recognized Elements in MS
Nitrogen:
Odd number of N = odd MW
CH3CN
M+= 41
SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute ofAdvanced Industrial Science and Technology, 11/2/09)
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Easily Recognized Elements in MS
2-bromopropane
Bromine: M+~ M+2 (50.5% 79Br/49.5% 81Br)
M+~ M+2
SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced IndustrialScience and Technology, 11/1/09)
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Easily Recognized Elements in MS Chlorine:
M+2 is ~ 1/3 as large as M+
Cl
SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute ofAdvanced Industrial Science and Technology, 11/2/09)
M+2
M+
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Easily Recognized Elements in MS
Sulfur:
M+2 larger than usual (4% of M+)
M+
Unusually
large M+2
S
SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute ofAdvanced Industrial Science and Technology, 11/1/09)
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Easily Recognized Elements in MS
Iodine I+at 127
Large gap Large gap
I+
M+
SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute ofAdvanced Industrial Science and Technology, 11/2/09)
ICH2CN
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Fragmentation Patterns
The impact of the stream of high energyelectrons often breaks the molecule intofragments, commonly a cation and a radical.
Bonds break to give the most stable cation.
Stability of the radical is less important.
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Fragmentation Patterns
AlkanesFragmentation often 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 formingthe most stable carbocations.
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Fragmentation Patterns Mass spectrum of 2-methylpentane
Fragmentation Patterns
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Fragmentation Patterns Alkenes:
Fragmentation typically forms resonance stabilized allylic carbocations
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Fragmentation PatternsAromatics:
Fragment at the benzylic carbon, forming a
resonance stabilized benzylic carbocation (whichrearranges to the tropylium ion)
M+
CH
H
CH Br
H
C
H
H
or
F t ti P tt
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Fragmentation Patterns
Aromatics may also have a peak at m/z = 77for thebenzene ring.
NO2
77
M+= 123
77
F t ti P tt
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Fragmentation Patterns
Alcohols
Fragment easily resulting in very small or
missing parent ion peak
May lose hydroxyl radical or water
M+- 17or M+- 18
Commonly lose an alkyl group attached tothe carbinol carbon forming an oxoniumion.
1oalcohol usually has prominent peak at
m/z = 31corresponding to H2C=OH+
ragmen a on a erns
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ragmen a on a erns MS for 1-propanol
M+M+-18
CH3CH2CH2OH
H2C OH
SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute ofAdvanced Industrial Science and Technology, 11/28/09)
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Fragmentation Patterns
Amines
Odd M+(assuming an odd number of nitrogensare present)
a-cleavage dominates forming an iminium ion
CH3CH2 CH2 N
H
CH2 CH2CH2CH3 CH3CH2CH2N CH2
H
m/z =72
iminium ion
86
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Fragmentation Patterns 86
CH3CH2 CH2 N
H
CH2 CH2CH2CH3
72
F t ti P tt
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Fragmentation PatternsEthersa-cleavage forming oxonium ion
Loss of alkyl group forming oxonium ion
Loss of alkyl group forming a carbocation
Fragmentation Patterns
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Fragmentation Patterns
H O CHCH3
MS of diethylether (CH3CH2OCH2CH3)
CH3CH2O CH2H O CH2
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Fragmentation Patterns
Aldehydes (RCHO)Fragmentation may form acylium ion
Common fragments:
M+- 1for
M+- 29for
RC O
R (i.e. RCHO - CHO)
RC O
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Fragmentation Patterns
MS for hydrocinnamaldehyde
M+= 134C C C H
H
H
H
H
O
133
105
91
105
91
SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute ofAdvanced Industrial Science and Technology, 11/28/09)
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Fragmentation Patterns
Ketones
Fragmentation leads to formation ofacylium ion:
Loss of R forming
Loss of R forming RC O
R'C O
RCR'
O
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Fragmentation Patterns
MS for 2-pentanone
CH3CCH2CH2CH3
O
M+
CH3CH2CH2C O
CH3C O
SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute ofAdvanced Industrial Science and Technology, 11/28/09)
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Fragmentation Patterns
Esters (RCO2R)Common fragmentation patterns
include:Loss of OR
peak at M+- OR
Loss of R
peak at M+- R
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Frgamentation Patterns
M+= 136
C
O
O CH3
105
77 105
77
SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute ofAdvanced Industrial Science and Technology, 11/28/09)
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Rule of Thirteen
The Rule of Thirteen can be used to identifypossible molecular formulas for an unknownhydrocarbon, CnHm.
Step 1: n = M+/13 (integer only, use remainder instep 2)
Step 2: m = n + remainder from step 1
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Rule of Thirteen
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Rule of Thirteen
Example: A compound with a molecular ion
peak at m/z = 102 has a strong peak at 1739cm-1in its IR spectrum. Determine itsmolecular formula.
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Aturan-aturan umum untuk meramalkanpuncak-puncak utama dari spektrum
a. Aturan elektron genap
Spesies-spesies elektron genapbiasanya tidak akan pecah menjadi 2spesies yang mengandung elektronganjil (yaitu, tidak akan pecah menjadiradikal dan ion radikal), karena
tenaga total dari hasil campuran iniakan sangat tinggi.
a Aturan elektron genap
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a. Aturan elektron genap
Spesies-spesies elektron genap biasanya tidak akan pecah menjadi 2 spesiesyang mengandung elektron ganjil (yaitu, tidak akan pecah menjadi radikal danion radikal), karena tenaga total dari hasil campuran ini akan sangat tinggi.
A+ B .+ + C
Genap ganjil ganjil
Spesies elektron genap lebih suka pecah menjadi ion lain dan molekul netral
A+ B .+ + C .
Genap genap genap
Ion radikal yaitu spesies elektron ganjil, dapat melepaskan molekul netral dan ion radikal sebagai hasil ikutan.
A .+ B .+ + C .
ganjil ganjil genap
Ion radikal dapat pecah menjadi radikal dan ion
A .+ B+. + C +
Genap ganjil
Atau
A + B . + C +
ganjil genap
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Aturan-aturan umum untuk meramalkanpuncak-puncak utama dari spektrum
b.Aturan Nitrogen :
Suatu senyawa yang hanya mengandung C,H, O memiliki
berat molekul genap, atau jika senyawa tersebutmengandung atom nitrogen maka jumlah atom nitrogenharuslah genap. Sedangkan jika senyawa hanya mengandungC,H,N,O mempunyai berat molekul ganjil jika jumlah atomnitrogennya ganjil. Jika senyawa yang mengandung C,H,O
kehilangan fragmen radikal (CH3, OCH3 dan yang lainnya)maka akan menghasilkan ion fragmen ganjil, tetapi yangdilepaskan molekul netral (H2O, HCN, olefin dan yanglainnya), maka ion fragmen yang dihasilkan adalah genap.
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Aturan-aturan umum untuk meramalkan puncak-puncak utama dari spektrum
1.Tinggi relatif puncak ion molekul terbesar untuk senyawarantai-lurus dan akan menurun dengan bertambahnyapercabangan.
2. Tinggi relatif puncak ion molekul biasanya berkurangdengan bertambahnya berat molekul deret homolog, kecualiuntuk ester lemak.
3. Pemecahan cenderung terjadi pada karbon tersubstitusialkil; semakin tersusbtitusi, semakin mudah putus. Hal ini
merupakan akibat dari terstabilkannya karbation tersierdibandingkan sekunder dan primer
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Aturan-aturan umum untuk meramalkanpuncak-puncak utama dari spektrum
Urutan kestabilan karbo kation :
CH3< RCH2 < R2CH+ < R3C+
Secara umum, gugus ganti terbesar pada suatu
cabang lebih mudah lepas sebagai radikal,kemungkinan karena radikal rantai-panjang dapatmenstabilkan dengan delokalisasi elektron sunyi.
4. Adanya ikatan rangkap dua, struktur siklik khususnyacincin aromatik (heteroaromatik) menstabilkan ionmolekul sehingga meningkatkan keboleh jadianterjadinya.
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Aturan-aturan umum untuk meramalkan puncak-puncak utama dari spektrum
5. Adanya ikatan rangkap dua mendukungpemecahan alil dan memberikan karbokation alilikterstabilkan resonansi. Aturan ini tidak pada
alkalena sederhana disebabkan mudahnya migrasiikatan rangkap dua tetapi terjadi pada sikloalkana.
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Aturan-aturan umum untuk meramalkan puncak-puncak utama dari spektrum
6. Cincin jenuh cenderung melepaskan rantaisamping alkil pada ikatan-. Muatan positifcenderung bertahan pada fragmen cincin.
Aturan aturan umum untuk meramalkan puncak
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Aturan-aturan umum untuk meramalkan puncak-puncak utama dari spektrum
7. Pada senyawa aromatik tersubtitusi alkil,
pemecahan terjadi pada ikatan- terhadap cincindan memberikan ion-benzil terstabilkan resonansiatau sering membentuk ion tropilium.
Aturan-aturan umum untuk meramalkan puncak-
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ppuncak utama dari spektrum
8. Ikatan C-C yang bersebelahan dengan
heteroatom cenderung terpecah meninggalkanmuatan fragmen yang mengandung heteroatomdimana elektron sunyi membantu stabilisasiresonansi
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Aturan-aturan umum untuk meramalkan puncak-puncak utama dari spektrum
9. Pelepasan sering diikuti dengan penghilanganmolekul kecil yang netral seperti karbon monoksida,olefin, air, amonia, hidrogen sulfida, hidrogen
sianida, merkaptan, ketena atau alkohol, juga seringdiikuti dengan penataan ulang.
Aturan-aturan umum untuk meramalkan puncak-
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Aturan aturan umum untuk meramalkan puncakpuncak utama dari spektrum
10. Ion penataan ulang adalah fragmen-fragmenyang asal usulnya tidak dapat dijelaskan denganpemecahan sederhana dari ikatan pada ionmolekul, tetapi hasil dari penataan ulang atomikintramolekular selama fragmentasi. Penataanulang meliputi, migrasi atom hidrogen pada
molekul yang mengandung heteroatom
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Aturan-aturan umum untuk meramalkan puncak-puncak utama dari spektrum
Agar berlangsungnya penataan ulang Mclaffertysuatu molekul harus memiliki heteroatom yangterletak tertentu, sistem-(biasanya ikatan
rangkap) dan atom hidrogen yang terletakterhadap sistem c=0.
Penataan ulang menyebabkan menonjolnya
puncak yang khas.
turan-aturan umum untu merama an
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puncak-puncak utama dari spektrum
Reaksi Retro Diels-Alder (RDA Reaction)
Pola pemecahan yang sering dijumpai pada senyawa takjenuh (ikatan rangkap dua)