Organic Spectroscopy 1

Lecture 5, 2nd Year Michaelmas 2010

 

Dr Rob Paton

CRL Office 11, 1st floor

 

E-mail: robert.paton@chem.ox.ac.uk http://paton.chem.ox.ac.uk

 

2

Outline of Lectures 5-8

• In lectures 5-6 of this course, the aspects of UV-vis and IR techniques will be introduced that are required in order to assign organic structures. Coverage of the underlying theory and instrumentation associated with each method will be kept to a bare minimum since these aspects are covered elsewhere.

• We will look at a variety of real spectra and learn to correlate distinguishing features in these spectra with functional

groups. • UV-vis and IR spectroscopy provide direct experimental data to support of a number of the underlying concepts in

organic chemistry introduced last year, such as conjugation and the mesomeric effect. We will also take a moment to consider these points.

O OO

N

O

N

• In lectures 7-8 we will show how UV-vis, IR and NMR spectra can be used in combination to assign structures in a

selection of real examples, using a selection of worked examples. The examples will be distributed in lecture 6, to give you a chance to work through them independently before lectures 7/8.

• Handouts, problems and colour slides will also be made available on the web pages in due: http://paton.chem.ox.ac.uk Further Reading

• Chemical Structure and Reactivity: an Integrated Approach – J. Keeler and P. D. Wothers, OUP (Chapter 11) • Introduction to Organic Spectroscopy - L. M. Harwood and T. D.W. Claridge, Oxford Chemistry Primers • Organic Chemistry – Clayden, Greeves, Warren and Wothers, OUP (Chapter 3) • Organic Spectroscopic Analysis – R. J. Anderson. D. J. Bendell and P. W. Groundwater, RSC

For more complete coverage including many more real examples of spectra, tables of spectroscopic data that will be useful in structural elucidation, and worked examples consult the following:

• Experimental Organic Chemistry – L. M. Harwood, C. J. Moody and J. M. Percy • NMR spectroscopy – H Günther • Organic Structure Analysis – P. Crews, J. Rodriguez and M. Jaspers, OUP • Organic Structures from Spectra – L. D. Field, S. Sternhell and J. R. Kalman • Spectroscopic Methods in Organic Chemistry (6th edition) – D. H. Williams and I. Fleming, Mcgraw-Hill • Spectrometric Identification of Organic Compounds – R. M. Silverstein, F. X. Webster and D.

J. Kiemle • Structure Elucidation by NMR in Organic Chemistry – E. Breitmaier

3

On the Web… A wealth of experimental spectra may be found on the internet, in openly accessible repositories. The following may be of interest: NMRshift DB - NMR database for organic structures: http://www.ebi.ac.uk/nmrshiftdb/ The Japanese Spectral Database for Organic Compounds (SDBS) has free access to IR, Raman, 1H and 13C NMR and MS data: http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/cre_index.cgi?lang=eng Sigma-Aldrich (chemical supplier) has IR , Raman and 1H and 13C NMR spectra for many of their commericially available compounds: http://www.sigmaaldrich.com Problems in structure, combining IR with 1H and 13C NMR courtesy of Prof Craig Merlic, UCLA: http://www.chem.ucla.edu/~webspectra/ Past Paper Questions Although the course continues to evolve, the following questions are good practice material (mass spec. is no longer part of the second year course, however): General Paper I: 1993 Q6, 2000 (Q1), 2001 (Q5) and 2004 (Q8) General Paper II: 1991 (Q3, Q5), 1992 (Q8), 1993 (Q3), 1994 (Q1), 1995 (Q3), 1996 (Q7), 1997 (Q5), 1998, Q3), 1999 (Q6), 2000 (Q9), 2002 (Q1) and 2003 (Q3) Part IA: 2004 (Q7), 2005 (Q2), 2006 (Q1), 2007 (Q8), 2008 (Q9), 2009 (Q1) and 2010 (Q1).

4

The Electromagnetic Spectrum

By irradiating molecules at different frequencies, it is possible to gain different types of information about their structure, since these frequencies bring into resonance various modes of molecular motion, or electronic or nuclear excitation. In modern laboratories, NMR spectroscopy is the first choice method for gaining structural information, with Infrared (IR) and mass spectroscopy (MS) techniques acting in a supporting capacity and UV spectra only being required in specialized circumstances (e.g. analysis of specific compound classes such as polymers or porphyrins).

5

Ultraviolet / Visible Spectroscopy

100000

80000

60000

40000

20000

0

-

-

-

-

-

25000

20000

15000

10000

5000

0

-

-

-

-

-

2500

2000

1500

1000

500

0

-

-

-

-

-

Electronic States Vibrational energy levels Rotational energy levels

(energies in wavenumbers, cm-1)

• UV-vis is a form of absorption spectroscopy.

6

• Radiation in the UV-visible region of the EM spectrum is absorbed, causing an electron to be excited to a higher

energy level.

h!

ground state excited state

"E

• UV and visible spectra of organic compounds are associated with excitations of electrons from the ground state to an

excited state higher in energy. The transition occurs from a filled bonding or non-bonding orbital to a formerly empty antibonding orbital.

• The energy gap is proportional to the frequency of absorption, and so this form of spectroscopy is a source of

bonding information • UV spectroscopy is most important in the structural analysis of compounds containing πι-bonds, in particular

conjugated systems.

1!

2!

3!"

4!"

1!

2!"

#

#"

(900 kJ/mol)(750 kJ/mol)

(500 kJ/mol)

7

200wavelength, ! (nm)

800

600 150

400 600

300 200Energy gap (kJ/mol)

mol

ar e

xtin

ctio

n co

effic

ient, "

hypsos = heightbathos = depthhyper = abovehypo = below

Recording UV-vis spectra The ultraviolet or visible spectrum is usually taken using a dilute solution of the sample in a glass or quartz tube, or cuvette. Typically the sides of the cuvette are 1 cm, and the total volume is 2-3 cm3. UV or visible light is passed through the sample and the intensity of the transmitted beam is recorded across the wavelength range of the instrument (I). First the intensity of the light is recorded with pure solvent in the cuvette (I0) the absorbance due to the sample can then be computed as log10 (I0/I).

light source detectorI0 I

l

*

8

• The Beer-Lambert law states that the absorption of light by a given sample is proportional to the number of absorbing molecules, and independent of the source intensity.

I0 and I are the intensities of the incident and transmitted light respectively, l is the path length of the absorbing solution in cm and c is the concentration in moles/litre. ε is the molar extinction coefficient in 1000 cm2 mol-1. log10 (I0/I) is called the absorbance. Example: A 1.12 x 10-4 M solution of paranitroaniline, in a cuvette of path length 1cm, has a measured absorbance maximum of 1.55 at 227 nm. This means the intensity of the transmitted light is 101.55 = 35 times the intensity of the incident light. The ε value for this absorption is: 1.55 / (1 x 1.12 x 10-4) = 13890 This would be quoted as λmax 227 (ε 13890)

9

Choice of solvent: The solvent and vessels must be transparent in the range of interest.

150 170 190 210 230 290 310 330 350wavelength (nm)

chloroform95% ethanolwater

quartzglass

cyclohexane

Absorption of common functional groups:

150 170 190 210 230 290 310 330 350wavelength (nm)

!"!*

n"!*

single bonds

lone pairs (O, N, S)

isolated #"#*double bonds

n"#*

conjugated #"#*

Vacuum UV UV !*

!

"*

"

n (LP)

• The functional groups such as polyenes and poly-ynes that give rise to diagnostic absorptions in the UV-visible region of the EM spectrum are referred to as chromophores

10

Selection Rules and Intensity • The irradiation of organic compounds does not always give rise to excitations of electrons from any filled to unfilled

orbital, because there are rules based on symmetry governing which transitions are allowed. • The intensity of absorption is therefore related to the “allowedness” of a particular transition • A chromophore with two double bonds conjugated together possesses a fully allowed transition, and has associated ε

values of about 10,000 • Forbidden absorptions are in practice observed with weak absorptions, as the symmetry may be broken by a

molecular vibration or by unsymmetrical substitution.

! > 10,000

O

! = 10 - 100 ! = 100 - 1000" - "* n - "* " - "*

allowed "forbidden"

The most important point to be made is that, in general:

Conjugated dienes:

345678

275310342380401411

30,00076,500122,000146,000

--

358384403420435

-

75,00086,50094,000113,000135,000

n ! !"max (nm) "max (nm)

MeMe

n PhPh

n

Values from Nayler, P.; Whiting, M. C. J. Chem. Soc. 1955, 3042.

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The most important point to be made is that, in general:

12

Aromatics: Absorption maxima for substituted benzene rings (Ph-R)

HNHMeIClBrOHOMeSO NHCNCOCO HNHONHAcCOMeCH=CHCHOPhOPhNOCH=CHCO HCH=CHPh

203.5203

206.5207

209.5210

210.5217

217.5224224230230235238

245.5248

249.5251.5

255268.5

273295.5

7,4007,5007,0007,0007,4007,9006,2006,4009,700

13,0008,700

11,6008,6009,400

10,5009,800

14,00011,40018,30011,0007,800

21,00029,000

254254261257

263.5261270269

264.5271268273280287

282

272

204160225700190192

14501480740

1000560970

14302600

750

2000

! !"max (nm) "max (nm)

2 2

222

2

22

3254254261257

263.5261270269

264.5271268273280287

291

278

204160225700190192

14501480740

1000560970

14302600

500

1800

!"max (nm)R

• Acid induced bathochromic shift:

NH2 H NH3

!max 230 nm !max 203 nm • Base induced hypsochromic shift:

OH

-H

O

!max 210.5 nm !max 235 nm

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• Effects of “complementary” EWG/EDG substituents:

NH2 NO2

!max 230 nm !max 269 nm" 7800 " 8600

NH2

O2N

NO2

!max 235 nm!max 229 nm" 16000" 14800

NH2

!max 375 nm" 16000

NO2

H2N

NO2

!max 260 nm" 1300

O2N

• Acid base indicators, e.g phenolphthalein:

OO

HO

HO OO

O

HOpKa 9.4

OH

!max 231 nm (25,800)!max 275 nm (4,200)

!max 230 nm (25,800)!max 553 nm (26,000)

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Carbonyls:

1!

2!

3!"

4!"

1!

2!"

2pO 2pO

O O

1!

2!"

Cyclohexanone vs. 1-cyclohexenone UV-Vis:

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Predicting UV absorptions of conjugated dienes: • Alkyl substitution of butadiene extends the chromophore through hyperconjugative interactions, causing a small red

shift to longer values for λmax. • The effect of alkyl substitution on open chain dienes and dienes in six-membered rings is approximately additive, so a

few rules (first formulated by Nobel Laureate R. B. Woodward in 1941) can be used to predict absorption. Woodward’s rules have since been refined as a result of experience by Fieser.

• Woodward’s rules may be applied to predict the absoroption of a diene that is either homoannular with both double

bonds contained in one ring or heteroannular with two double bonds distributed between two rings.

16

Base value for parent s-trans diene (heteroannular)Base value for parent s-cis diene (homoannular)

Increments for:(a) each alkyl substituent or ring residue(b) exocyclic nature of any double bond(c) additional double bond extending conjugation(d) auxochrome: -OAcyl -OAlkyl -SAlkyl -Cl or -Br -NAlkyl

Woodward's rules for diene and triene absorption214 nm253 nm

+5 nm+5 nm+30 nm

+0 nm -OAcyl+6 nm -OAlkyl+30 nm - -SAlkyl+5 nm -Cl or -Br+60 nm -NAlkyl

Examples:

• More rigourous treatment – particle in a box: En = n2h2/8mL2

17

Parent chromophore:X

alkyl or ring residueHOH or Oalkyl

Increment for each substituent:

Rules for the principal band of substituted benzenes RC6H4OX

246 nm250 nm230 nm

X

OR

o, mpo, mpompo, mp

+3

+10+7

+25+11+20+780+10

-alkyl/ring residue-OH, OMe, OAlkylo, m-Oom-Clo, mp

o, mpo, mpo, mppo, mp

+2+15+13+58+20+45+73+20+85

-Br-OH, OMe, OAlkyl-NHom-NHAco, m-NHMe-NMe

2

2

Examples:

O

MeO

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Steric effects on UV absorptions: • trans-stilbene and cis-stilbene

!max 296 nm (" 29,000) !max 280 nm (" 10,500)

• 2,4,6-trimethylacetophenone and para-methylacetophenone

!max 242 nm (" 3,200) !max 252 nm (" 15,000)

O O

• Strain release in the hydrolysis of a dilactone produced from shelloic acid.

O

H

H

O

OO

HH2O

O

H

H

O H

OOH

OH

no strong absoprtion >210 nm !max 227 nm (" 5,500)

19

Tomatoes are a deeper red than carrots. Given that the conjugated systems of β-carotene and lycopene are both eleven double bonds conjugated together with a similar number of alkyl substituents, why might lycopene absorb at a longer wavelength and with greater intensity?

lycopene

!-carotene

O H

20

Recent applications in organic synthesis: • Dehydration of graphene oxide to graphene

OHH

H+

(Chem. Mater. 2009, 21, 2950) • Expanding the Porphyrin π-system

(Org. Lett. 2008, 10, 3945)