spektra ir and ms
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12. Structure Determination: MassSpectrometry and Infrared
Spectroscopy
Based on
McMurrys Organic Chemistry
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Determining the Structure of an
Organic Compound
In the 19thand early 20thcenturies, structures
were determined by synthesis and chemical
degradation that related compounds to eachother
Physical methods now permit structures to be
determined directly. e will e!amine" mass spectrometry #M$%
in&rared #I'% spectroscopy
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12.1 Mass Spectrometry (MS)
$ample (apori)ed and sub*ected to bombardment by
electrons that remo(e an electron
+reates a radical cation
Bonds in cation radicals begin to brea #&ragment%
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High eso!ution Mass Spectrometer
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-uadrupole M$ ne!t to P+.
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"he Mass Spectrum
Plot mass o& ions #m)% #!/a!is% (ersus the intensity o&the signal #roughly corresponding to the number o&
ions%. sually, ) 1
allest pea is base peak#1003%
4ther peas listed as the 3 o& that pea Pea that corresponds to the un&ragmented radical
cation is parent peakor molecular ion #M+%
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Mass Spectra! #ragmentation of
He$ane 5e!ane #m) 67 &or parent% has peas at m) 81, 8,
:;, 29
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12.2 Interpreting Mass Spectra
Double-focusinginstruments pro(ide high/resolution distinguishing speci&ic atoms
?!ample M
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Other Mass Spectra! #eatures
Parent ion may not present due to electron bombardment
causing breadown
Peas abo(e the molecular weight appear as a result o&
naturally occurring hea(ier isotopes in the sample
#M@1% &rom 1;+ that is randomly present, isotope has 1.13abundance.
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12.% Mass&Spectra! 'ehaior of Some
Common #unctiona! roups
Aunctional groups
cause common
patterns o&
clea(age
ong chain
hydrocarbons
cause lie
spectra with 1:mass unit gaps.
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'en*y! groups
;7 :6 71 8; 6 980
1?@:
;?@:
:?@:
?@:
$pectrum 9;
m.)/C
Dbundance/C
;91 7;
91
9;
Aragments include
prominent pea at m)
91, see spectrum o&
toluene"
'
cycloheptatrienyl cationm) 91
aromatic, (ery stable
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Ch!orine and +heny!
0 77 6; 100 117 1;;0
7?@;
1?@:
2?@:
;?@:
;?@:$pectrum 870
m.)/C
D
bundance/C
1
2718
88
111
112
Aragments containing
chlorine show m) (alues
two mass units apart with
ratios ;"1, see spectrum
o& chloroben)ene"
+hlorine contains ;+l and ;8+l, ratio appro!. ;"1 Phenyl groups are Euite stable, m) 88
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Mass Spectra! C!eaage eactions
of ,!coho!s Dlcohols undergo /clea(age #at the bond ne!t to the
+/45% as well as loss o& 5/45 to gi(e ++
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Mass Spectra! C!eaage of ,mines
Dmines undergo /clea(age,
generating radicals
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#ragmentation of -etones and
,!dehydes D +/5 that is three atoms away leads to an internal
trans&er o& a proton to the +4, called the McLafferty
rearrangement
+arbonyl compounds can also undergo clea(age
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Odd m* for mo!ecu!ar ion
4dd m) &or molecular ion usually indicates an odd number o& nitrogens
+aution" doesnt hold &or &ragmentsFF Dlso, e(en m) doesnt indicate
absence o& G.
G
#+5;%;GG
G
GM
.@ 89 M.@ 61 M
.@ 9
H+5;I.@
Aragment, no GF
H52G/G52I.@
2 GJs, e(en m)
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;0 70 90 120 10 1600
@:
@
@
@
@
$pectrum K ;;
m.) /C
'e
sponse
/C
29
:;
8
81
6
961121281:1
180
0 70 80 60 90 100110 1200
1?@:
2?@:
;?@:
:?@:
?@:
$pectrum K 1:82
m.) /C
'
esponse/
C
0
7 7; 8;
88
112
11:
82 61 90 99 106 1180
;?@;
7?@;
9?@;
1?@:
$pectrum K 1;80
m.) /C
'e
sponse
/C
899;
108
L :0 0 70 80 60 90 100110 1200
2?@:
:?@:
7?@:
$pectrum K 90
m.) /C
'
e
sp
o
n
se
/C
08
7;
76861
66
9:
11:
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12./ Spectroscopy of the
0!ectromagnetic Spectrum 'adiant energy is proportional to its &reEuency
#cycless 5)% as a wa(e #Dmplitude is its height%
Li&&erent types are classi&ied by &reEuency or
wa(elength ranges
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12. Infrared Spectroscopy of Organic
Mo!ecu!es I' region lower energy than (isible light #below red >
produces heating as with a heat lamp%
2. 107m to 2. 10m region used by organicchemists &or structural analysis
I' energy in a spectrum is usually measured aswa(enumber #cm/1%, the in(erse o& wa(elength andproportional to &reEuency
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Infrared 0nergy Modes
I' energy absorption corresponds to speci&icmodes, such as bending and stretching o&bonds
?nergy is characteristic o& the atoms in thegroup and their bonding
+orresponds to (ibrations and rotations
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12. Interpreting Infrared Spectra
Most &unctional groups absorb at about the
same energy and intensity independent o& the
molecule they are in
I' spectrum has lower energy regioncharacteristic o& molecule as a whole
#
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egions of the Infrared Spectrum
:000/200 cm/1G/5, +/5, 4/5 #stretching% ;;00/;700 G/5, 4/5 ;000 +/5
200/2000 cm/1++ and +
G #stretching%
2000/100 cm/1doublebonds #stretching% +4 1760/180 ++ 17:0/1760 cm/1
Below 100 cm/1
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12.3 Infrared Spectra of Hydrocar4ons
+/5, +/+, ++, + + ha(e characteristic peas
absence helps rule out ++ or + +
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+yclohe!ane #top%, cyclohe!ene #bottom%
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I: ,romatic Compounds
ea +>5 stretch at ;0;0 cm1
ea absorptions 1770 / 2000 cm1range
Medium/intensity absorptions 1:0 to 1700 cm1
$ee spectrum o& phenylacetylene, Aigure 12.1
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I: ,!coho!s and ,mines
4>5 ;:00 to ;70 cm1
sually broad and intense
G>5 ;;00 to ;00 cm1
$harper and less intense than an 4>5
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+yclohe!anol
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I: Car4ony! Compounds
$trong, sharp +4 pea 1780 to 1860 cm1
?!act absorption characteristic o& type o& carbonyl
compound
18;0 cm1in saturated aldehydes
180 cm1in aldehydes ne!t to double bond or
aromatic ring
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Ben)aldehyde
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C5O in -etones
181 cm1in si!/membered ring and acyclic etones
180 cm1in /membered ring etones
1790 cm1in etones ne!t to a double bond or an aromaticring
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C5O in 0sters
18; cm1in saturated esters
181 cm1in esters ne!t to aromatic ring or a
double bond
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C5O in Car4o$y!ic ,cids
Dlso >45
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diethy!4en*ene
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styrene
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4en*oic acid
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methano!
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acetone
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ani!ine
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pheno!
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