Organic Spectroscopy 1 Michaelmas Lecture 6 Dr Rob Paton.

Transcription

1 rganic Spectroscopy 1 Michaelmas 2011 Lecture 6 Dr ob Paton robert.paton@chem.ox.ac.uk 1

2 ecap of Lecture 5 UV-vis Spectroscopy Measures the gaps between electronic energy levels Most useful for conjugated double bonds since the M-LUM energy gap is small enough to promote an electron in this spectral region Increasing conjugation leads to greater absorption, and a shift to absorption at longer wavelengths (λ max ) Characteristic absorption for certain classes of organic compounds may be predicted (albeit relatively crudely) using Woodward s rules Steric effects and geometric strain may prevent efficient conjugation, and therefore will affect UV-vis absorption wavelengths Infrared (I) Spectroscopy Measures molecular vibrational energy levels Molecules vibrate in many ways simultaneously, however, fundamental normal modes give rise to characteristic I absorptions. The strength of absorption depends on the change in dipole-moment of the vibrating group. 2

3 The X- region: cm -1 For example: C- bonds: ring C bond: alkene C bond: 3040 terminal alkyne C bond: ethynyl-1-cyclohexene Typical C- stretching frequencies: sp 3 C- sp 2 C- sp C cm -1 usually C 3 and C 2 symmetric and antisymmetric stretches are seen cm -1 exact form depends on the number of alkene substituents cm -1 usually appears as a single sharp peak 3

4 Some C- stretches are diagnostically useful (Bohlmann bands): N- bonds: Slightly higher than C- stretches ( cm -1 ): N Me N symmetric 3400 cm -1 N N antisymmetric 3500 cm -1 N-methylaniline & aniline 4

5 - bonds: Very broad absorption around 3300 cm -1 is characterstic F 4-fluorophenol Carboxylic acids display a characteristic V-shape in the - absoprtion 1-cyclopentene-1-carboxylic acid 5

6 ydrogen bonding affects the strength of an - bond: pka: Steric factors inhibit -bonding: tbu Me 2- t butyl-6-methylphenol 6

7 Example Problem Using NM, UV-vis and I: C 6 6 CCl 3 / - / 2 A + B (C ) A B λ max 285 (ε 16,000) λ max 255 (ε 10,000) δ 7.0 (d, 2), 7.8 (d, 2), 9.8 (s, 1), 10.4 (brs, 1)* δ (m, 4), 9.8 (s, 1), 10.9 (s, 1)* ν 3600 (dil. sol.) (conc. sol.), 1690 ν (no change on dilution.), 1660 *exchanges in D 2 7

8 The triple bond region: cm -1 C N and C C bonds: 1-Pentyne I Spectrum propionitrile 8

9 ethylisocyanoacetate Cumulenes allene C=C 2160 cm -1 3-methyl-1,2-butadiene (an allene) 9

10 The double bond region: cm -1 C=C bonds: Alkenes, aromatics oleyl alcohol E-4-nonene 10

11 11 I Spectroscopy methyl crotonate Nitro groups are also diagnostic: Nitrocyclohexane WAVENUMBES % T A N S M I T T A N C E A B S B A N C E

12 Carbonyl Groups Learn these numbers! I spectroscopy is particularly useful in identifying which of the several different kinds of carbonyl group is present in a molecule. C= double bonds show a strong absorption band since they have a large dipole moment. The position of the absorption is governed by the electronic structure. carbonyl group stretching frequency (approx.) increasing stretching frequency (stronger C= bond) ' anhydrides Cl acid chlorides ' esters aldehydes 1820 & 1760 cm cm cm cm -1 Cl Cl 1.18 Å 1.20 Å ' ketones 1715 cm Å 12

13 ' ketones 1715 cm Å decreasing stretching frequency (weaker C= bond) acids N' 2 amides Si' 3 acylsilanes 1710 cm cm cm -1 N' 2 N' Å 1.25 Å carboxylates 1580 cm -1 Electron Donating Groups weaken the C= bond shifting I frequency down Electron Withdrawing Groups strengthen the C= bond shifting the I frequency up 13

14 Conjugation: Conjugated ketones Non-conjugated ketones C= stretch ca cm -1 C= stretch ca cm -1 Conjugation lowers the stretching frequency, typically by around 30 cm -1. ing Strain: When a carbonyl is part of a ring, the C= stretching frequency depends on ring size: as ring size decreases, the carbonyl stretching frequency increases. X X X = C cm cm cm cm cm -1 X = 1727 cm cm cm cm -1 X = N cm cm cm cm cm cm cm -1 X X Conjugation weakens C= and C=C 14

15 Me N Me Me Me N pk a = -0.5 Stable to hydrolysis pk a = 5.3 ydrolyses in 2 / + in 45s C= stretch 1650 cm -1 C= stretch 1732 cm -1 CN Å C Å CN Å C Å Kirby et al. Angewandte Chemie International Edition 1998, 37,

16 Example: Six derivatives of the steroid cholestane: match up the structures with the I data A B C 2 x C= (conjugated): = 1685 cm -1 (s) 4 1 x C=C (conjugated): = 1620 cm -1 (w) 1 x C= (ketone): 1715 cm -1 (s) 1 x C= (ester): 1730 cm -1 (s) 2 1 x C= (ester): 1730 cm -1 (s) 3 1 x C= (conjugated ketone): = 1685 cm -1 (s) 1 x C=C (conjugated): = 1620 cm -1 (w) D E F Et Et 1 x C= (conj. ketone): = 1670 cm -1 (s) 5 1 x C=C (enol ether): 1630 cm -1 1 x C= (ketone): 1730 cm -1 (s) 1 1 x C= (ester): 1730 cm -1 (s) 1 x C=C: 1650 cm -1 (w) 1 x C=C (enol ether): 1630 cm -1 6 Selected I absorptions (s=strong, w=weak): Spectrum ν (cm -1 ) (s) 1724 (s), 1712 (s) 1730 (s), 1695 (s), 1642 (w) Spectrum ν (cm -1 ) (s), 1686 (s), 1608 (w) 1653 (s), (s), 1658 (w),

17 Example: The molecular mass of X has been determined by high-resolution mass spectrometry as What is the formula and structure of X? (masses : C: : ) I spectrum of X 4 x x x = C conjugated 2100 strength suggests conjugated C 3 or 3 C 3300 strong terminal acetylene C- 17

18 Example: It was envisaged that the condensation of two equivalents of ethyl acetoacetate with formaldehyde would produce agemanns ester. Is the product s I spectrum consistent with this proposal? Et piperidine (cat.) then heat "agemann's ester" (2 equiv) C 2 Et Product I spectrum 1730 (s) 1685 (s) conjugated 1620 (w) conjugated 18

19 Example: Identify the product from the reaction of cyclohexenone and dimethylcopper lithium i) Me 2 CuLi C 7 12 ii) + Nuc Me Nuc Me C= 1690 C=C 1630 C(sp 2 ) eactant I spectrum C= 1710 C=C gone C(sp 2 )- gone no broad! Product I spectrum 19

20 Example: Use the following I and 1 NM spectra to assign the structures of two isomers of C 6 12 both: C=C 1650 C(sp 2 ) verlay of X & Y I spectra 1 NM spectrum of X (ppm) 1 NM spectrum of Y (ppm) 4.7 d 4.7 d 1.8 s C 3 C t 1.6 sextet 2 C C t 5.0 dd 5.0 dd 5.8 m C dd 1.7 nonet C C d C 3 20