structures

Chapter 8 Determining the Structure of Organic Compounds from Spectra

(d)Examine the problem for any additional data concerning composition and determine the molecular formula if possible. From the molecular formula, determine the degree of unsaturation.

(e)Determine the molar absorbance in the UV spectrum, if applicable.

(2) Examine each spectrum (JR, mass spectrum, UV, l3C NMR, 'H NMR) in turn

for obvious structural elements:

(a)Examine the IR spectrum for the presence or absence of groups with diagnostic absorption bands e.g. carbonyl groups, hydroxyl groups, NH groups, C=C or C=N, etc.

(b)Examine the mass spectrum for typical fragments e.g. PhCHz-, CH3CO-,

CH3- , etc.

(c)Examine the UV spectrum for evidence of conjugation, aromatic rings etc.

(d)Examine the lH NMR spectrum for CH 3- groups, CH3CHz- groups, aromatic protons, -C~X, exchangeable protons etc.

(3)Write down all structural elements you have determined. Note that some are

monofunctional (i.e. must be end-groups, such as -CH3, -C=N, -NO z) whereas some are bifunctional (e.g. -CO-, -CHz-, -COO-), or trifunctional (e.g. CH, N).

Add up the atoms of each structural element and compare the total with the molecular formula of the unknown. The difference (if any) may give a clue to the nature of the undetermined structural elements (e.g. an ether oxygen). At this stage, elements of symmetry may become apparent.

(4)Try to assemble the structural elements. Note that there may be more than one way of fitting them together. Spin-spin coupling data or information about conjugation may enable you to make a definite choice between possibilities.

(5)Return to each spectrum (JR, UV, mass spectrum, l3C NMR, lH NMR) in turn and rationalise all major features (especially all major fragments in the mass spectrum and all features of the NMR spectra) in terms of your proposed structure. Ensure that no spectral features are inconsistent with your proposed structure.

Note on the use of data tables. Tabulated data typically give characteristic absorptions or chemical shifts for representative compounds and these may not correlate exactly with those from an unknown compound. The data contained in data tables should always be used indicatively (not mechanically).

87

~

~.'

I

;

Chapter 9 Problems

9

PROBLEMS

9.1

SPECTROSCOPIC IDENTIFICATION OF ORGANIC COMPOUNDS

89

Problem 1

IR Spectrum
(liquid film)	1718

4000

3000

2000

1600

1200

800

0.0

V (ern")

100

43

Mass Spectrum

0.5

CIl

o

80

.>t:.

co

€'"

CIl

'"

0

60

0-

til

CIl

M+"

..c

UV Spectrum

til

..c

'"

40

'"

29

72

1.0

'5

33.3 mg/10 ml

*

20

Y

1.0 cm cell

C4H aO

solvent: ethanol

1.5

120

160

40

80

200

240

280

200

250

300

350

m/e

A. (nm)

I

I

I

I

I

I

I

I

I

I

I

13C NMR Spectrum

(50.0 MHz, CDCI 3 solution)

II

II

solvent

I

--J

--

--'~'__

LlLillL

proton coupled

proton decoupled

I

III

I

I

I

I

I

I

I

I

I

I

I

I

200

160

120

80

40

o

0 (ppm)

1H NMR Spectrum

(200 MHz, CDCI 3 solution)

TMS

1.

90

Problem 2

IR Spectrum
(liquid film)	1715

4000					3000				2000			1600	1200	800
										V (ern")
100
													Mass Spectrum
80	-"
	co
	Q)																No significant uv
60	c.		45
	Q)					M+"
	Vl
	co															absorption above 220 nm
40	.0						74
	'0
					57
	~				j
20			I,											C3 H 602
		40				80			120	160		200	240	280
										m/e
		I			I			I	I		I	I	I	I	I	I	I	I
13C NMR Spectrum

(50.0 MHz. COCI3 solution)

I

DEPT CH2~ CH3t CHt

proton decoupled

solvent

UI I

•

I

I

I

I

I

I

I

I

I

I

200

160

120

80

40

0

o(ppm)

1H NMR Spectrum

(200 MHz. COCI3 solution)

exchanges

with 0 20

~

~

A

r ---

12

11

--1

~

TMS

I

I

I

I

I

I

I

I

I

I

I

I

10

9

8

7

6

5

4

3

2

1

0

o(ppm)

9

r

Problem 3

IR Spectrum

(liquid film)	2984

1741

1243

4000				3000		2000	1600	1200	800	0.0
						V (crn")
100				43				Mass Spectrum		0.5
80	"'"
	'"0-
	Q)
60	(/)
	Q)									1.0
	'"
40	.a	29
	'0					M+"= 88
	Q'!.
20					,I I				C4Ha02
		~								1.5

UV spectrum

solvent: ethanol 15.4 mg /10 mls path length: 1.00 cm

40	80	120	160	200	240	280	200	250	300	350
			m/e						A. (nm)

13C NMR Spectrum

(50.0 MHz, CDCI3 solution)

DEPT CH2 t CH3~ CH~

Ll

proton decoupled			solvent		Il
			III
200	160	120	80	40	0 () (ppm)

1H NMR Spectrum

(200 MHz, CDCI3 solution)

											TMS
10		9	8	7	6	5	4	3	2	1	o
											s (ppm)

92

Problem 4

,			IR Spectrum
c,												1744
				(CCI4	solution)
4000					3000						2000			1600	1200	800
												V (cm'")
100
					29			57							Mass Spectrum
80				Ctl
			.><
				Q)
60				Co															No significant UV
				Q)
				en
				Ctl														absorption above 220 nm
40				J::J						M+" = 88
			'0
20			*'							I
					40				80		120	160		200	240	280
												m/e
				I		I				I	I		I	I	I	I	I	I		I	I

13C NMR Spectrum

(100,0 MHz, COCla solution)

DEPT CH2 t CHat CHt

I I

proton decoupled	solvent

I

•

I

I

I

I

I

I

I

I

I

I

I

I

200

160

120

80

40

o 0 (ppm)

1H NMR Spectrum

(200 MHz, COCla solution)

expansion

	,	ppm	I
2.0	10			TMS

10	9	8	7	6	5	4	3	2	1	o
										o(ppm)

93

Problem 5

	IR Spectrum
		(liquid film)
4000		3000		2000		1600	1200		800
					V	(ern")
100								Mass Spectrum
		27
				107/109

80"'"

'"Q)

60	0-									No significant UV
	Q)
	en
	.c									absorption above 220 nm
40	'"
	'0						M+"
	~
20						186/188/190			C2 H4 Br2
			~	.11			,I,
	40		80	120		160	200	240	280

mle

13C NMR Spectrum

(50.0 MHz, CDCI3 solution)

solvent

proton coupled	I
proton decoupled	•

~

I

200

160

120

80

40

0 o(ppm)

1H NMR Spectrum

(200 MHz, CDCI 3 solution)

										TMS
										l'
I	I	I	I	I	I	I	I	I	I	I
10	9	8	7	6	5	4	3	2	1	0
										s (ppm)

94

Problem 6

IR Spectrum

(liquid film)

1716

4000						3000			2000		1600	1200	800
										V (ern")
100
							43					Mass Spectrum			UV Spectrum
80		""Ql
60		'"0.
		Ql
		CIl														solvent:	methanol
		.0
40		'"						M+'=86
		'0
		0~
20								I
						40		80	120	160	200	240	280
										m/e
			I			I		I	I	I	I	I	I	I	I	I	I
13C NMR Spectrum
(100 MHz, CDCI3 solution)

proton decoupled						,
						solvent
200				160	120	80	40	o s(ppm)
1H NMR Spectrum
(200 MHz, CDCI3 solution)

											TMS
											I
	I	I	I	I	I	I	I	I	I	I	I
10		9	8	7	6	5	4	3	2	1	o
											() (ppm)

95

Problem 7

IR Spectrum	2249
(KBr disc)

4000						3000			2000		1600	1200	800
										V (crn'")
100
												Mass Spectrum
80		"'"
							53
		'"
		Q)
60		o,													No significant UV
		Q)
		Ul			40
		'"					M+" = 80							absorption above 220 nm
40		.I:l
		'0
		'#.	~				1
20
									120	160		240	280
					40	80					200
										m/e
																I
	13C NMR Spectrum
	(100.0 MHz, CDCI					3 solution)
					decoupled								solvent
			proton			--- ' ---L
					200				160		120		80	40		o 0 (ppm)
	1H NMR Spectrum
	(400 MHz, CDCI3 solution)

TMS .

I	I	I	I	I	I	I	I	I	1

10	9	8	7	6	5	4	3	2	1	o
										o(ppm)"

96

Problem 8

IR Spectrum

(liquid film)

4000						3000			2000			1600	1200	800
									V	(em")
100		.,.,					57						Mass Spectrum
80
		'"
		<J)
60		a.															No significant UV
		<J)
		Ul
		'"														absorption above 220 nm
40		..							M+" =114 «
		'0										1%)
20		*-							99
				II
									I
		40				80			120	160		200	240	280
									m/e
			I			I		I	I		I	I	I	I	I	I		I	I
13C NMR Spectrum
(50.0 MHz, CDC'3solution)

proton decoupled	solvent
	I

I	I	I	I
	200		160	120	80	40	o 0 (ppm)

1H NMR Spectrum

(200 MHz, CDC'3solution)

										TMS
					,					I
I	I	I	I	I		I	I	I	I	I
10	9	8	7	6	5	4	3	2	1	0
										o(ppm)

97