First organic spectral method; rarely used as a primary method for structure determination

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1 Utility First organic spectral method; rarely used as a primary method for structure determination Main contribution is that can readily identify the presence of conjugates π-systems or unique chromophores Can sometimes be used to differentiate double bond isomers In combination with NMR and IR data can use to elucidate unique electronic features not readily apparent from those methods Widely used in other applications Most common detector for HPLC Can be used to moniter reaction kinetics (chemistry, biology, medicine), etc.

2 The Electromagnetic Spectrum x-rays ultraviolet (UV) visible Infrared (IR) microwaves radiowaves nm nm absorbed color violet blue green yellow orange red observed color yellow red violet blue short wavelength high frequency high energy long wavelength short frequency low energy

3 Colors of Different Wavelength Regions Absorbance & Transmittance wavelength absorbed (nm) < > 780 absorbed color ultraviolet violet blue greenish blue bluish green green yellowish green yellow orange red near IR transmitted color (compliment) --- yellowish green yellow orange red purple violet blue greenish blue bluish green red

4 rigin of the Absorption The absorption of UV or visible radiation corresponds to the excitation of valence electrons Valence electrons are typically found in: - σ bonding orbitals (single bonds) - π bonding prbitals (double or triple bonds) - non-bonding orbitals (lone pair electrons) LUM ΔE = [E excited E ground ] = hν HM

5 Electronic Transitions σ σ π σ* π* π* alkanes carbonyls alkenes, carbonyls, alkynes, etc. ΔE = [E excited E ground ] = hν n σ* heteroatoms (, N, S, X, etc.) n π* carbonyls orbitals possible electronic transitions σ* antibonding π* E atomic n E π bonding σ

6 Electronic Transitions Relative Transition Energies σ σ* > n σ* > π π* > n π* > σ π* most useful not all transformations that are possible will be observed some electronic transitions forbidden by certain selection rules even forbidden transitions can be observed, but usually not very intense for example, n π*

7 The Spectrometer

8 Beer-Lambert Law describes relationship between absorbance and concentration A = log ( I 0 / I ) = ε l c l Where: A is absorbance (no units) I 0 = intensity of incident light I = intensity transmitted light ε = molar absorbtivity or extinction coefficient l = path length; length of the sample cell (cm) c = sample concentration (mol/l)

9 Spectrum Features isoprene ε = A / l c values of are termed high intensity absorptions values of are termed low intensity absorptions values of 0 to 10 3 are the absorptions of forbidden transitions

10 Spectrum Features reporting data NH 2 λ max = 206 nm

11 Spectrum Features peak broadening

12 Choice of Solvent Solvent should not absorb UV radiation in the same region as the sample (measuring UV spectra below 200 nm is impractical) solvent cutoff (nm) solvent cutoff (nm) acetonitrile 190 ethanol 205 chloroform 240 hexane 201 cyclohexane 195 methanol 205 diethyl ether 210 isooctane 195 dioxane 215 water 190 sample cell: quartz glass cutoff 210 nm

13 Solvent Effect on Spectra H

14 Chromophores Basic Information All molecules capable of absorbing ultraviolet radiation, though most do so at very high energy (wavelengths < 200 nm). Unsaturated groups which give rise to absorptions involving π or π* orbitals in the in the near-uv/visible region are called chromophores. Saturated groups with non-bonding electrons, which can give rise to transitions involving non-bonding orbitals are called auxochromes. Most useful transitions for analysis are the intense π π * transitions and the weaker, but lower energy, n π* transitions.

15 Chromophores Absorptions of rganic Molecules Alkanes: Saturated molecules that lack lone pairs nly transitions possible are σ σ* high energy; absorb UV radiation at very short wavelengths not accessible using UV spectroscopy Alcohols, Ethers, Amines, & Sulfur Compounds: Satuated molecules with lone pairs of electrons Important transitions are n σ* high energy, most often at wavelengths shorter than 200 nm alcohols and amines: nm thiols and sulfides: nm

16 Chromophores Absorptions of rganic Molecules Alkenes & Alkynes: Important transitions are π π* high energy, but impacted by substitution simple alkenes: 175 nm simple alkynes: 170 nm Carbonyls: Important transitions are π π* (188 nm) n π* also possible ( nm) sensitive to substitution

17 Chromophores

18 Chromophores Terminology for Absorptive Shifts Bathochromic shift: shift of absorption to a longer wavelength Hypsochromic shift: shift of absorption to a shorter wavelength Hyperchromic effect: an increase in absorption intensity Hypochromic effect: a decrease in absorption intensity ε Hypsochromic Hyperchromic Bathochromic Hypochromic 200 nm 700 nm

19 Conjugation Effects compound λ max (nm) ε H 2 C CH , , , ,000 n π* π π* n π* π π* ,100

20 Conjugation Effects Greater the conjugation, the lower the energy required to induce electronic transitions (e.g. the (longer the wavelength) lengthening conjugation also increases band intensity (greater molar absorbitivity) adding substitutents may have same effect, but to a much smaller degree

21 Conjugation Effects Extended π Systems π* H 2 C CH 2 Energy π ethylene butadiene hexatriene octatetraene

22 Auxochromes & Conjugation atoms with lone pairs can extend conjugation by resonance R X R X X = H, R, NH 2, halogen, etc. alkyl substituents can influence wavelength by overlap of C-H bonding orbital with the π system (e.g. by hyperconjugation)

23 Woodward Fieser Rules for Dienes acyclic dienes cyclic dienes s-trans s-cis homoannular (cisoid) heteroannular (transoid) Woodward & Fieser derived a set of empirical rules for the estimation of wavelength for the low energy π π*electronic transition Based on empirical observation of known conjugated structures Can be used to reliably predict absorption wavelength in dienes, enone, and to a lesser extent aromatic systems

24 Woodward Fieser Rules for Dienes s-trans homoannular (cisoid) heteroannular (transoid) base values: 217 nm 253 nm 214 nm Increments: not affected by solvent For each additional conjugated double bond For each exocyclic double bond For each alkyl group For each of the following groups: - R - (C=)R - Cl - Br - SR - NR 2 - Ph + 30 nm + 5 nm + 5 nm + 6 nm + 0 nm + 5 nm + 5 nm + 30 nm + 60 nm + 60 nm Where both types of cyclic dienes are present, the base with the longer λmax is used.

25 Woodward Fieser Rules for Dienes CAUTIN! R This compound had three exocyclic double bonds; the indicated bond is exocyclic to two rings. λ max calc = 284 this is not a heteroannular diene; must use base value for acyclic diene. λ max calc = 232 this is not a homoannular diene; must use base value for acyclic diene λ max calc = 237

26 Woodward Fieser Rules for Dienes examples acyclic diene 3 alkyl subst calculated value observed 217 nm 15 nm 232 nm 234 nm acyclic diene 2 alkyl subst 1 exocyclic db calculated value observed 217 nm 10 nm 5 nm 232 nm 236 nm cisoid diene 4 alkyl subst 1 exocyclic db calculated value observed 253 nm 20 nm 5 nm 278 nm 275 nm

27 Woodward Fieser Rules for Dienes examples Et transoid diene 3 alkyl subst 1 R subst 1 exocyclic db calculated value observed 214 nm 15 nm 6 nm 5 nm 240 nm 241 nm cisoid diene 2 conj db 5 alkyl subst 1 acyl subst 3 exocyclic db calculated value observed 253 nm 60 nm 25 nm 0 nm 15 nm 353 nm 355 nm

28 Woodward Fieser Rules for Dienes double bond regioisomers H H abietic acid levopimaric acid λ max calc: 239 λ max obs: 238 λ max calc: 278 λ max obs: 275 transoid diene 4 alkyl subst 1 exocyclic db calculated value 214 nm 20 nm 5 nm 239 nm cisoid diene 4 alkyl subst 1 exocyclic db calculated value 253 nm 20 nm 5 nm 278 nm

29 Fieser-Kuhn Rules for Extended Polyenes Woodward-Fieser Rules work well up to four conjugated double bonds For more extended conjugation, use the Fieser-Kuhn Rules λ max = (# alkyl substituents) = n(48-1.7n) (# endo) - 10(# exo) lycopene where n = number of conjugated double bonds λ max = (8) = 11( ) (0) - 10(0) = 476 nm observed: 474 nm β-carotene λ max = (10) = 11( ) (2) - 10(0) = 453 nm observed: 452 nm

30 Woodward Fieser Rules for Enones Solvent Correction H 2 EtH CHCl 3 Dioxane Et 2 Hydrocarbon nm R R acyclic enone 6-membered ring enone 5-membered ring enone acyclic dienone base values: 215 nm 215 nm 202 nm 245 nm Increments: For each additional conjugated double bond For each exocyclic double bond For each homodiene component α + 30 nm + 5 nm + 39 nm β γ δ and higher For each alkyl group + 10 nm + 12 nm + 18 nm + 18 nm β β For each of the following groups: δ β R R - H - R - (C=)R - Cl - Br - SR - NR nm + 35 nm + 6 nm + 15 nm + 25 nm + 30 nm + 60 nm + 30 nm + 30 nm + 6 nm + 12 nm + 30 nm + 85 nm + 95 nm + 17 nm + 6 nm + 50 nm + 31 nm + 6 nm α γ α

31 Woodward Fieser Rules for Enones examples acyclic enone 1 α alkyl 2 β alkyl calculated value observed 215 nm nm 249 nm 249 nm Br 5-membere enone 1 α Br 2 β alkyl 1 exocyclic db calculated value observed 202 nm 25 nm 24 nm 5 nm 256 nm 251 nm

32 Woodward Fieser Rules for Enones practice Me Br 2 Br Me or Me H H Br H base base Me or Me H Can you distinguish the two by UV?

33 Woodward Fieser Rules for Enones practice Me vs Me H 6-membered enone β alkyl exocyclic db 215 nm 12 nm 227 nm 215 nm 24 nm 5 nm 244 nm

34 Woodward Fieser Rules for Enones practice absorbance in EtH?

35 Woodward Fieser Rules for ther Conjugated Carbonyls β β β H β R α α (R = H or R') aldehydes carboxylic acid or ester Base Values unsubstituted aldehyde or ester with α or β alkyl groups with α,β or β,β alkyl groups with α,β,β alkyl groups 208 nm 220 nm 230 nm 242 nm 193 nm (not observed) 208 nm 217 nm 225 nm for an exocyclic α,β double bond for an endocyclic α,β double bond in a 5- or 7-membered ring + 5 nm + 5 nm as for enones, solvent correction is also relevant

36 Woodward Fieser Rules for Aromatic Compounds Substitution, auxochromic groups, conjugation and solvent effects can cause shifts in wavelength and intensity of bands for aromatic systems similar to dienes and enones Show multiple bands, often fine structure However, shifts are difficult to predict the formulation of empirical rules is often dificult (there are more exceptions than rules) Can make some useful predictions for benzoyl derivatives We will not worry about predicting UV bands in aromatic compounds

37 Woodward Fieser Rules for Benzoyl Derivatives R H R aryl ketones (R = alkyl) benzaldehydes benzoic acids and esters base values: 246 nm 250 nm 230 nm Increments: ortho meta para For each alkyl group For each H or R (R = alkyl) For each For each of the following groups: - Cl - Br - NH 2 - NH(C=)CH 3 - NHCH 3 - N(CH 3 ) nm + 7 nm + 11 nm + 0 nm + 2 nm + 13 nm + 20 nm + 20 nm + 3 nm + 7 nm + 20 nm + 0 nm + 2 nm + 13 nm + 20 nm + 20 nm + 10 nm + 25 nm + 78 nm + 10 nm + 15 nm + 58 nm + 45 nm + 73 nm + 85 nm