Spektra IR and MS

7/26/2019 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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