Molecular Formula & Mass
Chem 325
• C o mbus tio n analys is : ratio o f e le ments present in
co mpo und: the EM PIRIC AL fo rmula
• M o lecular fo rmula fro m e mpirical fo rmula an d the
Mass Spectrometry
mo le cular we ig ht (mo lar mass)
• Gas law, free zing po int de pressio n, bo iling po int e lev atio n,
os mo tic press ure, etc.
• M ass s pectro metry: dire ct mass analys is o f mo lecule
Mass Spectrometry
•Info rmatio n dire ctly av ailable fro m M S
– M ass o f mo lecular catio n (mo lecular we ig ht)
– M asses o f frag me nts
– Re lative amo unt o f the io ns
• Uses o f M S
– ID by finge rprint (M W plus frag me ntatio n patte rn)
– C o mbine d with GC , the ultimate separatio n -ide ntificatio n
tool due to extre me sens itivity
– Frag me ntatio n (mass patte rn, re lative amo unts ) prov ides
s tructural info rmatio n
Mass Spectrometry
Ionization
• Io nizatio n o f mo lecules
– L oss o f e, frag me ntatio n
M + e ne rg y ® M +· + e-
m1 +·, m2 +·, m3 +·, …
frag me nt io ns
•Pro ductio n o f (radical) catio ns
•
Put thro ug h a mass analyzer : ‘mass analys is ’
•
M ass o f mo lecular catio n plus any res ulting frag me nts
•
Frag me nts will be g ove rne d by mo lecular
arrang e me nts and re lative stabilities of fragme nt
catio ns
Ionization
T hree things happe n to mo lecules ins ide a mass s pectro mete r:
1 . T he mo lecules are e ne rg ize d in so me way to cre ate io ns .
2 . T he io ns are se parate d base d o n the ir mass /charge ratio.
3 . T he io ns are de tected.
• H ard ve rs us so ft me tho ds
– M inimum: re mov al o f an e lectro n
• Pho to n po we r (lase rs ): pho to io nizatio n us ually use d for
pro ductio n o f mo lecular (radical) catio ns but not e nough
e ne rg y to pro duce frag me ntatio n which pro duces the rich
s tructural info rmatio n
• E le ctro n Impact (EI)
- mos t co mmo n and ‘s tandard ’
- che ap, eas y to co ntro l, re place
< 10-6 mm Hg
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Electron Impact Ionizaton
• Ge ne rate e lectro ns: he ate d filame nt
• A ccele rate electro ns thro ug h a po te ntial diffe re nce
(v o ltage ). C o mmo nly: electro n k ine tic ene rg y is 80 e V.
(co nve rs io n 1 e V = 9 6.5 k J /mo le )
• E no ug h ene rg y to io nize and cause exte nsive fragme ntation.
MS Instrumentation
• M us t be able to vo latilize s ample to a ce rtain deg ree (can he at).
• E x tractio n o f pos itively charge d catio ns and frag me nts
• A ccele ratio n to seve ral ke V
• M ass analys is –
– M ag ne tic fie ld
– E le ctric fie ld
– Q uadrupo le fie ld
– T ime -o f-flight
– Io n cyclotro n reso nance
• H ig h v acuum re quire me nt: av o id co llis io ns, anihilatio n by air
mo le cules!
Magnetic Sector Mass Spectrometer
Single Focusing Spectrometer
Fo r a g ive n fie ld stre ng th, o nly io ns with a s pecific mass will
have the co rrect mass to have the rig ht traje cto ry and make it
to the de tecto r. D iffe re nt io ns can be bro ug ht ‘into fo cus ’ by
adjus ting the mag netic fie ld stre ng th.
Double Focusing Instrument
So me times and additio nal e lectros tatic analyze r is use d in
seque nce with the mag ne tic analyze r. A do uble focus ing
ins trume nt will s ho w impro ved reso lutio n, but lo we r
Double -Focusing Mass Spectrometer
M ag ne tic Fie ld
E le ctric Fie ld
GC
sens itivity.
De tecto r
Io n So urce
Vacuum Pumps
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Mass Spectrun
Mass Spectrum - EI
If only one electron is los t, the M + pe ak is detecte d, and o fte n
• Grap h o f re lative amo unts as a functio n o f mass
labe le d.
The res t of the pe aks are fragments. O fte n, an e ffo rt is made
to g uess what these frag me nts mig ht be base d o n their mass
and the structure o f the initial co mpo und.
Neutral frag me nts are NO T detecte d.
or Da ltons (Da)
Peak Clusters
Important Isotopes
Inte nse pe aks tend to be acco mpanie d by s malle r pe aks
within o ne o r two mass units , us ually caused by the prese nce
o f diffe re nt iso to pes o f ato ms within the io n o f inte res t.
Fo r ex ample, the most abundant iso to pe o f carbo n is 12 C ,
ho weve r aro und 1 % o f carbo n ex is ts as 1 3C . Eve n tho ug h
this iso to pe is aro und 1 00 times less abundant, M S is se nsitive
e no ug h to de tect it.
T he de tectio n o f o the r iso to pes prov ides crucial data fo r
de te rmining the s tructure o f the co mpo und.
Isotopes in MS
Formulas from Masses
• M S prov ides mass o f pare nt and frag me nt io ns
• A ll co mpo unds co ntaining o nly C, H , and O will have a
mo le cular we ig ht that is an even number.
• If the M + is an odd number mass, it like ly contains an odd
numbe r o f N IT RO GE N ato ms . T H E N IT RO GE N R U L E !
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Isotopes of Carbon – M+1
Molecular Formula from MS
F or c o m po un ds c o nt aining C, H , O , c an c alc ula te , bas e d o n n at ur al
a bun da nc e s , h o w lar g e we wo ul d e x pe c t t he M +1 pe ak t o be r e la ti ve t o
the M + pe ak .
T he he ig ht o f the M + and M +1 and M +2 are v aluable in
o btaining the mo lecular fo rmula.
% (M+1 ) = n u m be r of c ar bo ns ´ 1 .1%
Fo r ex ample, an unk no wn was run with M + = 58 m/z and the
M +1 pe ak is 3.3% o f the M + pe ak.
If nitrogen is present, this is changed to:
% (M+1 ) = (# C ´ 1 .1 ) + (# N ´ 0 .3 6 )
F or na ph th ale ne , pr e dic t t he M +1 pe ak to be 1 1 % of the M + pe ak .
Firs t, s ince M + is even, if nitro ge n is prese nt the re mus t be at
le ast two o f the m. Ass uming the re is no nitroge n, can calculate
the numbe r o f carbo ns to be:
M+1
3 .3 % = # o f carbo ns ´ 1.1%
T he re are three carbo ns in this co mpo und.
Molecular Formula from MS
T hree carbo ns we ig h 36 mass units , le av ing 22 units yet
unex plaine d.
C arbo n us ually has hydro ge ns , but the re ce rtainly are not
g o ing to be 2 2 o f the m. Need so me o the r ato m that is no t N
o r C that we ig hs less than 22 units .
O x yge n (16 u) is a g oo d cho ice !
3 C + 1 O = 52 mass units . 6 re main, and mus t be 6
hydro ge ns .
Initial g uess fo r the mo lecular fo rmula is C 3H 6 O, which co uld
be ace to ne, pro panal , 2-pro pe n -1-ol, cyclo pro pano ne, etc.
High Resolution MS (HRMS)
T o av o id so me o f the ambig uity associate d with us ing the
M +1 pe aks to assig n the mo lecular fo rmul a in so me cases, we
can turn to H RM S.
In H RM S, we can de te rmine what is calle d an exact mass ,
which is the mass o f the M + pe ak to 4 places afte r the de cimal.
While C 3H 6O , C H 2 N 2O , C 2 H 2 O 2, C 2H 6N 2 , and C 4H 1 0 have the
s ame unit mass (58) the y all have diffe re nt exact masses which
can be dis ting uis he d by H RM S.
C 3 H 6O – exact mass = 58 .0 419
C H 2 N 2 O - exact mass = 5 8.016 7
C 2 H 2O 2 – exact mass = 58 .0 054
C 2 H 6N 2 – exact mass = 58 .0 532
Molecular Formula from Mass
T he M + 1 pe ak allo we d us to de te rmine the mo lecular fo rmula o f
the m/z = 58 io n to be C 3H 6 O.
O the r mo lecular fo rmulae with the s ame m/z = 58 are :
C H 2 N 2 O, C 2H 2O 2 , C 2H 6 N 2, and C 4H 10 .
T he calculatio n ass ume d the re we re no nitroge ns , but it is
poss ible to arrive at the s ame even m/z if 2 nitroge ns are present.
T he y mig ht be rule d o ut us ing the mo difie d fo rm o f the M + 1
calculatio n.
Fo r CH 2 N 2O , M +1 s ho uld be (1 ´ 1 .1 % ) + (2 ´ 0.36%) = 1 .8 %
Fo r C 2H 6 N 2, M + 1 s ho uld be (2 ´ 1.1% ) + (2 ´ 0.36%) = 2.9%
With 2 .9% be ing so close to 3.3% , canno t disco unt.
Calculating Exact Mass
E x act mass is calculate d by
adding the exact masses of the
ato ms in the mo lecular
fo rmula. N o te the ex act mass
for the atom is for only one
iso to pe and is different from the
atomic weight that appears in
the periodic table .
T he ato mic we ig ht is the
average mass o f the ato m tak ing
into acco unt the re lative
natural a bunda nces o f e ach
iso to pe .
C 4 H 1 0 – ex act mass = 58 .07 83
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Formulas from Masses
Exact Masses
• T ables o f ex act masses fo r fo rmula co mbinatio ns
• (lo w reso lutio n) mass : M What fo rmula po ssibilities?
M
r
=n+
13
13
R ule o f T hirteen
B ase fo rmula is the n C nH n+r
E x ample : M = 58 ,
58
13
=4 +
6
13
B ase fo rmula is the n C 4H 10
Halogenated Compounds
Rule of Thirteen
M = 58 , base formula C 4 H 10
A n io n with o ne C l will have an M +2 abo ut 1 /3 the s ize o f the M + .
If we s us pect o ne O (1 6 u), s ubtract mass equiv ale nt C 1H 4
è C 3H 6O
Cl
If we s us pect o ne N (14 u), s ubtract mass equiv ale nt C 1 H 2
è C 3H 8 N
If we s us pect two N (2 8 u), s ubtract mass equiv ale nt C 2H 4
è C 2H 6 N 2
If we s us pect o ne S (32 u), s ubtract mass e quiv ale nt C 2H 8
è C 2H 2 S
O ur s us picio ns are g uide d by M + 1, M + 2 inte ns ities, othe r
s pectral and che mical data, and chemical sense !
Halogenated Compounds
A n io n with o ne B r will have an M + 2 abo ut e qual to the M + .
Halogenated Compounds
A co mpo und with 2 chlo rines o r 2 bro mines will s ho w an
inte nse M +2 and a dis tinct M + 4 pe ak.
CH 2Br 2
79
Br
81Br
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Multiple Halogen Atoms
Calculated Patterns
Example: hexachlorobenzene
Mass Spec - Fragmentation
An e x tr e me ly us e ful r e s ult o f EI i o niz a tio n in pa r tic ul ar is a
phe n o me n on k n o wn as fragmentation.
The r a dic al c a tio n t h at is pr o duc e d whe n a n e le c tr o n is k noc k e d ou t
Cl
Cl
Cl
Cl
Cl
of a ne utr al c los e d-s he ll m ole c ule in EI MS i nitiall y pos s e ss es a lot o f
e ne r g y.
Ene r g y s uf fic ie nt t o br e ak c he mic al bo n ds : r a dic al c ati on will us ually
br e ak in to a ne utr al r a dic al an d a c ati on. It is als o pos s ible for a
ne utr al c l os e d s he ll fr ag me nt (s uc h as wa te r ) t o fall of f.
Cl
H3C OH
+ 1 electron
+
H3C OH
+
H3C OH
Predicting Fragmentation
A co ns ide rable v o lume o f lite rature reg arding the
frag me ntatio n re actio ns o f ce rtain mo lecules is av ailable.
Us ing this k no wle dge , we can pre dict ho w a g iven mo lecule
mig ht frag me nt.
+
H3C OH
+
2 electrons
CH2
OH+
+
H
+
CH3
+
OH
Conventions
It is always best to localize the catio n and radical, o be ying the
rules o f Le wis structures (i.e. do n ’t put more than 8
e lectro ns o n carbo n).
B ut firs t a fe w co nve ntio ns ….
Fo r ex ample, fo r the three structures above, A is disco urage d
in fav o ur o f B o r C. T his co nve ntio n is no t always poss ible
to o be y (alk anes ).
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More on Conventions…
Fragmentation Notes
Whe n a mo lecule is io nize d by E IM S, mus t decide which
e lectro n to re move. A lways best to re mo ve an e lectro n fro m
a p -bond , or from a heteroatom lone pair. It will be come
mo re cle ar which is the bette r cho ice whe n we look at
indiv idual ex amples .
T he bo nd bre ak ing can be do ne e ithe r by homolytic or
heterolytic cleav age .
H3 C CH2 O
R
H3C CH2 Br
R
+
CH3 CH2 Br
EI
1e
CH3
2e
CH3 + H2C
+
O
CH3 CH2+ + Br
R
Homolytic
- T he pro bability o f cle av ing a g ive n bo nd is re late d to the
bo nd s tre ng th, and to the s tability o f the frag me nts fo rme d.
In particular, catio ns like to re arrang e o r decay into mo re
s table catio ns .
- Re me mbe r that fo r a ring s ys te m, at le as t 2 bo nds mus t
EI
+
CH2 O
- M ost frag me nts are eve n-electron catio ns. T hese s plit to
make mo re eve n e lectro n catio ns .
bre ak fo r the ring to frag me nt.
In additio n, the re are 10 ge ne ral rules to keep in mind whe n
pre dicting the most like ly io ns to be fo rme d fo r a g ive n
mo le cule.
Heterolytic
Fragmentation Guidelines
Fragmentation Guidelines
1 . T he re lative he ig ht o f the M + pe ak is g re atest fo r s traig htchain mo lecules and decre ases as the branching incre ases.
4 . Do uble bo nds , cyclic s tructures , and es pecially aro matic rings
will s tabilize the mo lecular io n and incre ase its pro bability o f
appe arance .
2 . T he re lative he ig ht o f the M + pe ak decre ases with chain le ng th
fo r a ho mo logo us se ries.
5 . Do uble bo nds fav o r allylic cle avage to give a resonance
s tabilize d allylic carbocatio n, especially fo r cyclo alke nes .
3 . Cle avage is fav o ure d at alkyl-s ubs tituted carbo ns, with the
pro bability o f cle av age incre as ing as the s ubs titutio n incre ases .
These rules mostly arise from the fact that carbocation and radical
stability show the following trend:
Mos t Sta ble Be nz ylic
> Allylic > Tertiary > S econdary >> Primary Le a st S ta ble
EI
6 . Fo r s aturate d rings (like cyclo hex anes ), the side chains te nd
to cle ave firs t le av ing the positive charge with the ring .
+
R
+
Fragmentation Guidelines
+
+ R
Fragmentation Guidelines
7 . U ns aturate d rings can also unde rg o retro-Die ls-A lde r
re actio ns to eliminate a ne utral alkene .
+
+
+
A t the po int o f bre ak age, the large r frag me nt us ually takes the
radical to le ave the s malle r catio n.
EI
+
9 . C -C bo nds nex t to hete roatoms o fte n bre ak le av ing the
pos itive charge o n the carbo n with the he te ro ato m.
+
O
+
+
O
O
+
8 . A ro matic co mpo unds tend to cle ave to g ive be nzylic catio ns,
or more like ly tro pylium catio ns .
R
+
R
EI
+
+
+
+
1 0. Cle avage is ofte n fav o ure d if it can expe l small stable
molecules like wate r, CO , N H 3, H 2 S, etc.
In additio n to bo nd frag me ntatio n, v ario us intramolecular
rearrangements can take place to give some times unexpecte d
io ns .
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Rearrangements
O ne co mmo n type o f re arrange me nt in M S is the McLafferty
rearrangement which takes place in co mpo unds that
co ntain a carbo nyl g ro up.
+
O
H
+
OH
+
Y
M any o the r re arrang e me nts are possible , eve n so me that are
no t we ll unde rs to o d and are co ns ide re d ‘rando m ’.
+
CH3
H3 C
CH3
+
CH3 CH2
CH3
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