2a. UV HO - Syracuse Universitychemistry.syr.edu/totah/che575/support/3a1/6-1.UV.pdf · •...
Transcript of 2a. UV HO - Syracuse Universitychemistry.syr.edu/totah/che575/support/3a1/6-1.UV.pdf · •...
UV Spectroscopy
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.
UV Spectroscopy
The Electromagnetic Spectrum
x-rays ultraviolet(UV) visible Infrared
(IR) microwaves radiowaves
190 - 400 nm 400 - 800 nm
absorbed color
observed
color
violet
blue
green
yellow
orange
red
yellow red violet blue
short wavelengthhigh frequency
high energy
long wavelengthshort frequency
low energy
UV Spectroscopy
Colors of Different Wavelength Regions
Absorbance & Transmittance
< 380380 - 435435 - 480480 - 490490 - 500500 - 560560 - 580580 - 590590 - 625625 - 780
> 780
ultravioletvioletblue
greenish bluebluish green
greenyellowish green
yelloworange
rednear IR
---yellowish green
yelloworange
redpurplevioletblue
greenish bluebluish green
red
wavelengthabsorbed (nm)
absorbedcolor
transmitted color(compliment)
UV Spectroscopy
Origin 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)
ΔE = [Eexcited – Eground] = hν
HOMO
LUMO
UV Spectroscopy
Electronic Transitions
ΔE = [Eexcited – Eground] = hν
σ
σ
π
n
n
σ*
π*
π*
σ*
π*
alkanes
carbonyls
alkenes, carbonyls, alkynes, etc.
heteroatoms (O, N, S, X, etc.)
carbonyls
σ*
π*
n
π
σ
E E
antibonding
bonding
atomic
orbitals possible electronic transitions
UV Spectroscopy
Electronic Transitions
Relative Transition Energies
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*
σ σ* n σ* π π* n π* σ π*> > > >
UV Spectroscopy
The Spectrometer
UV Spectroscopy
Beer-Lambert Law
A = log ( I0 / I ) = ε ℓ c
Where: A is absorbance (no units) I0 = 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)
describes relationship between absorbance and concentration
ℓ
UV Spectroscopy
Spectrum Features
isoprene
ε = A / ℓ c • values of 104-106 are termed high intensity absorptions • values of 103-104 are termed low intensity absorptions • values of 0 to 103 are the absorptions of forbidden transitions
UV Spectroscopy
Spectrum Features
reporting data
λmax = 206 nm 252
317 376
O
NH2
O
UV Spectroscopy
Spectrum Features
peak broadening
UV Spectroscopy
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
UV Spectroscopy
Solvent Effect on Spectra
OH
UV Spectroscopy
Chromophores
• 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.
Basic Information
UV Spectroscopy
Chromophores
Absorptions of Organic Molecules
Alkanes: • Saturated molecules that lack lone pairs • Only 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: 175 – 200 nm • thiols and sulfides: 200 – 220 nm
UV Spectroscopy
Chromophores
Absorptions of Organic 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 (280 – 290 nm) • sensitive to substitution
UV Spectroscopy
Chromophores
UV Spectroscopy
Chromophores
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
Terminology for Absorptive Shifts
200 nm 700 nm
ε
Hypochrom
ic
Hypsochromic
Hyp
erch
rom
ic
Bathochromic
UV Spectroscopy
Conjugation Effects
H2C CH2
O
O
λmax (nm)
175
217
258
465
15,000
21,000
35,000
125,000
280189
280213
12900
277,100
εcompound
n π*
n π*
π π*
π π*
UV Spectroscopy
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
UV Spectroscopy
Conjugation Effects
H2C CH2
Energy
ethylene
butadiene
hexatriene
octatetraene
π
π*
Extended π Systems
UV Spectroscopy
Auxochromes & Conjugation
• atoms with lone pairs can extend conjugation by resonance
RX
RX
X = OH, OR, NH2, halogen, etc.
• alkyl substituents can influence wavelength by overlap of C-H bonding orbital with the π system (e.g. by hyperconjugation)
UV Spectroscopy
Woodward Fieser Rules for Dienes
• 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
s-trans s-cis homoannular(cisoid)
acyclic dienes cyclic dienes
heteroannular(transoid)
UV Spectroscopy
Woodward Fieser Rules for Dienes
not affected by solvent
Where both types of cyclic dienes are present, the base with the longer λmax is used.
s-trans homoannular(cisoid)
heteroannular(transoid)
base values: 217 nm 253 nm 214 nm
Increments:
For each additional conjugated double bondFor each exocyclic double bondFor each alkyl groupFor each of the following groups:
- OR- O(C=O)R- Cl - Br- SR- NR2- Ph
+ 30 nm+ 5 nm+ 5 nm
+ 6 nm+ 0 nm+ 5 nm+ 5 nm+ 30 nm+ 60 nm+ 60 nm
UV Spectroscopy
Woodward Fieser Rules for Dienes
CAUTION!
RThis compound had three exocyclicdouble bonds; the indicated bond isexocyclic to two rings.λmax calc = 284
this is not a heteroannular diene; must use base value for acyclic diene.λmax calc = 232
this is not a homoannular diene; must use base value for acyclic dieneλmax calc = 237
UV Spectroscopy
Woodward Fieser Rules for Dienes
examples acyclic diene3 alkyl substcalculated valueobserved
217 nm15 nm
232 nm234 nm
acyclic diene2 alkyl subst1 exocyclic dbcalculated valueobserved
217 nm10 nm5 nm
232 nm236 nm
cisoid diene4 alkyl subst1 exocyclic dbcalculated valueobserved
253 nm20 nm5 nm
278 nm275 nm
UV Spectroscopy
Woodward Fieser Rules for Dienes
examples
O
O
cisoid diene2 conj db5 alkyl subst1 acyl subst3 exocyclic dbcalculated valueobserved
253 nm60 nm25 nm0 nm
15 nm353 nm355 nm
EtO
transoid diene3 alkyl subst1 OR subst1 exocyclic dbcalculated valueobserved
214 nm15 nm6 nm5 nm
240 nm241 nm
UV Spectroscopy
Woodward Fieser Rules for Dienes
double bond regioisomers
OOH
OOH
abietic acid levopimaric acid
λmax calc: 239λmax obs: 238
λmax calc: 278λmax obs: 275
cisoid diene4 alkyl subst1 exocyclic dbcalculated value
253 nm20 nm5 nm
278 nm
transoid diene4 alkyl subst1 exocyclic dbcalculated value
214 nm20 nm5 nm
239 nm
UV Spectroscopy
Fieser-Kuhn Rules for Extended Polyenes
where n = number of conjugated double bonds
• Woodward-Fieser Rules work well up to four conjugated double bonds • For more extended conjugation, use the Fieser-Kuhn Rules
λmax = 114 + 5(# alkyl substituents) = n(48 - 1.7n) - 16.5(# endo) - 10(# exo)
λmax = 114 + 5(8) = 11(48 - 1.7•11) - 16.5(0) - 10(0) = 476 nmobserved: 474 nm
λmax = 114 + 5(10) = 11(48 - 1.7•11) - 16.5(2) - 10(0) = 453 nmobserved: 452 nm
β-carotene
lycopene
R
acyclic enone 6-membered ringenone
5-membered ringenone
base values: 215 nm 215 nm 202 nm
Increments:
For each additional conjugated double bondFor each exocyclic double bondFor each homodiene component
For each alkyl groupFor each of the following groups: - OH
- OR- O(C=O)R- Cl - Br- SR- NR2
+ 30 nm+ 5 nm+ 39 nm
+ 10 nm+ 35 nm+ 35 nm+ 6 nm+ 15 nm+ 25 nm+ 30 nm+ 60 nm
R
O
α
β
γ
δ
R
O
α
β
β
OO O
R
O
acyclic dienone
245 nm
β γ δ and higherα
+ 12 nm+ 30 nm+ 30 nm+ 6 nm+ 12 nm+ 30 nm+ 85 nm+ 95 nm
+ 18 nm
+ 17 nm+ 6 nm
+ 18 nm+ 50 nm+ 31 nm+ 6 nm
UV Spectroscopy
Woodward Fieser Rules for Enones
Solvent Correction nmH2OEtOHCHCl3DioxaneEt2OHydrocarbon
+ 80
- 1- 5- 7- 11
UV Spectroscopy
Woodward Fieser Rules for Enones
examples
acyclic enone1 α alkyl2 β alkylcalculated valueobserved
215 nm10
24 nm249 nm249 nm
5-membere enone1 α Br2 β alkyl1 exocyclic dbcalculated valueobserved
202 nm25 nm24 nm5 nm
256 nm251 nm
O
O
Br
UV Spectroscopy
Woodward Fieser Rules for Enones
practice
Me
O H
Me
O H
Me
O H
Br
Br
orBr2
base base
Me
O H
Me
O
or
Can you distinguish the two by UV?
UV Spectroscopy
Woodward Fieser Rules for Enones
practice
Me
O H
Me
O
vs
6-membered enoneβ alkylexocyclic db
215 nm12 nm
227 nm
215 nm24 nm5 nm
244 nm
UV Spectroscopy
Woodward Fieser Rules for Enones
practice
O
absorbance in EtOH?
UV Spectroscopy
Woodward Fieser Rules for Other Conjugated Carbonyls
aldehydes carboxylic acid or ester
Base Values
unsubstituted aldehyde or esterwith α or β alkyl groupswith α,β or β,β alkyl groupswith α,β,β alkyl groups
for an exocyclic α,β double bondfor an endocyclic α,β double bond in a 5- or 7-membered ring
208 nm220 nm230 nm242 nm
OR
O
α
β
H
O
α
β
β
193 nm (not observed)208 nm217 nm225 nm
+ 5 nm+ 5 nm
β
(R = H or R')
as for enones, solvent correction is also relevant
UV Spectroscopy
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
UV Spectroscopy
Woodward Fieser Rules for Benzoyl Derivatives
R
aryl ketones(R = alkyl)
benzaldehydes benzoic acidsand esters
base values: 246 nm 250 nm 230 nm
Increments:
For each alkyl groupFor each OH or OR (R = alkyl)For each OFor each of the following groups:
- Cl - Br- NH2- NH(C=O)CH3- NHCH3- N(CH3)2
+ 3 nm+ 7 nm+ 11 nm
+ 0 nm+ 2 nm+ 13 nm+ 20 nm
+ 20 nm
O
+ 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
H
O
OR
O
ortho meta para