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Química Orgânica I

Ciências Farmacêuticas

Bioquímica

Química

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Análise estrutural

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Adaptado de:

� Organic Chemistry, 6th Edition; L. G. Wade, Jr.

� Organic Chemistry, William H. BrownWilliam H. Brown

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methods determine the molecular structure

� Crystallography

� Mass Spectrometry

� Elemental Analysis

� Infrared Spectroscopy

� UV/VIS Spectroscopy

� Nuclear Magnetic Resonance

� Coupled methods: GC-MS; HPLC-NMR...

� (comparative methods for know structures)

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methods� Crystallography: diffraction patterns relates to atomic

positions � Mass Spectrometry: molecular weight and fragmentation

pattern � Elemental Analysis: determination of elemental and isotopic

composition. � Infrared Spectroscopy: determine the presence or absence

of functional groups. � UV/VIS Spectroscopy: Spectrophotometry uses a

spectrophotometer to measure how much light is absorbed by the sample (Gives information about conjugated πelectron systems).

� Nuclear Magnetic Resonance (NMR): NMR spectroscopyworks by studying the magnetism of a nucleus by placing it in alignment with a magnetic field, and then using an electromagnetic field to disrupt this alignment.

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E = hν and spectroscopy

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Electromagnetic Radiation

� Common units used to express wavelength λλλλ

Angstrom (Å) 1 Å = 10-10 m

Relationto MeterUnit

1 mm = 10-3 m

1 nm = 10-9 m1 µµµµm = 10-6 m

Nanometer (nm)Micrometer (µµµµm)

Millimeter (mm)Meter (m) ----

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Electromagnetic Radiation

�� Electromagnetic radiation:Electromagnetic radiation: light and other forms of radiant energy

�� Wavelength (Wavelength (λλ):): the distance between consecutive peaks on a wave

�� Frequency (Frequency (νν):): the number of full cycles of a wave that pass a given point in a second

�� Hertz (Hz):Hertz (Hz): the unit in which radiation frequency is reported; s-1 (read “per second”)

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�electronic

�vibrational

�rotational

�nuclear spin

UVUV--VisVis

infraredinfrared

microwavemicrowave

radiofrequencyradiofrequency

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UV-vis spectroscopy

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Electromagnetic radiation is absorbed when the energy of photon corresponds to difference in energy between two states.

∆∆∆∆E = hνννν

UV-vis

ν = c / λ

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Why UV-vis?

� Many organic molecules have chromophores that absorb UV

� UV absorbance is about 1000 x easier to detect per mole than NMR

� Still used in following reactions where the chromophore changes. Useful because timescale is so fast, and sensitivity so high. Kinetics, esp. in biochemistry, enzymology.

� Most quantitative Analytical chemistry in organic chemistry is conducted using HPLC with UV detectors

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…

� One of the best ways for identifying the presence of acidic or basic groups, due to big shifts in λ for a chromophore containing a phenol, carboxylic acid, etc.

λ

“bathochromic” shift“hypsochromic” shift

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UV-vis - range

� Conjugated compounds can absorb light in the ultraviolet region of the spectrum

� The region from 2 x 10-7m to 4 x 10-7m (200 to 400 nm) is most useful in organic chemistry

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UV-vis – range in detail

•σ → σ* and σ → π* transitions:

•high-energy, accessible in vacuum UV (λmax <150 nm).

•n → σ* and π → σ* transitions: non-bonding electrons

(lone pairs), wavelength (λmax) in the 150-250 nm region.

•n → π* and π → π* transitions: most common

transitions observed in organic molecular UV-Vis,

observed in compounds with lone pairs and multiple

bonds with λmax = 200-600 nm.

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Energy

σ∗σ∗σ∗σ∗

ππππ

σσσσ

π∗π∗π∗π∗

n

σσσσ

σσσσ

ππππ

n

n

σσσσ∗∗∗∗

ππππ∗∗∗∗

ππππ∗∗∗∗

σσσσ∗∗∗∗

ππππ∗∗∗∗

alkanes

carbonyls

unsaturated cmpds.

O, N, S, halogens

carbonyls

Electronic transitions

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Example: π → π* transitions responsible for ethylene UV absorption at ~170 nm calculated with ZINDO semi-empirical excited-states methods (Gaussian 03W):

HOMO πu bonding molecular orbitalLUMO πg antibonding molecular orbital

hν 170nm photon

calculated π → π* transitions

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1,3-butadiene

� The electrons in the highest occupied molecular orbital (HOMO) undergo a transition to the lowest unoccupied molecular orbital (LUMO)

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Other electronic transitions

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How Do UV spectrometers work?

Two photomultiplier

inputs, differential

voltage drives amplifier.

Matched quartz cuvettes

Sample in solution at ca. 10-5 M.

System protects PM tube from

stray light

D2 lamp-UV

Tungsten lamp-Vis

Double Beam makes it a

difference technique

Rotates, to

achieve scan

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Transmittance vs Absorbance

� % Transmittance = (I / I0) * 100%

where:

I = intensity of transmitted light

I0 = intensity of incident light

� Absorbance = -log(I / I0)

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Diode Array DetectorsDiode array

alternative puts

grating, array of

photosens.

Semiconductors after

the light goes through

the sample.

Advantage, speed,

sensitivity,

The Multiplex

advantage

Disadvantage,

resolution is 1 nm, vs

0.1 nm for normal

UV

Model from Agilent literature. Imagine

replacing “cell” with a microflow cell for

HPLC!

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UV vis SpectrumA

bso

rban

ce

Wavelength, λλλλ, generally in nanometers (nm)

0.0400 800

1.0

200

UV Visible

λλλλmaxwith certain

extinction εεεε

Make solution of concentration low enough that A≤ 1

Even though a dual beam goes through a solvent blank, choose solvents that are UV transparent.

Can extract the εεεε value if conc. (M) and b (cm) are known

UV bands are much broader than the photonic transition event. This is because vibration levels

are superimposed on UV.

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Electronic Transitions and UV-visible Spectra in Molecules

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Quantitative Use of UV Spectra

� Beers’ law:

A = εεεεcl� “A” absorbance

� “εεεε” is molar absorptivity (extinction coefficient

� “c” is concentration in mol/L

� “l” is path of light through sample in cm

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meaning

� λmax: wavelength where UV absorbance for a compound is greatest

� Energy difference between HOMO and LUMO decreases as the extent of conjugation increases

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Effect of Conjugation

� λmax increases as conjugation increases (lower energy)

� 1,3-butadiene: 217 nm

� 1,3,5-hexatriene: 258 nm

�Substituents on π system increase λmax

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ΨΨΨΨ2222∗∗∗∗

ππππΨΨΨΨ1111ΨΨΨΨ1111

ΨΨΨΨ2222

ΨΨΨΨ3333∗∗∗∗

ΨΨΨΨ4444∗∗∗∗

∆E for the HOMO � LUMO transition is reduced

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Energy

ethylene

butadiene

hexatriene

octatetraene

Lower energy =Longer wavelengths

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Conjugation, Color and the Chemistry of Vision

� Visible region is about 400 to 800 nm� Extended systems of conjugation absorb in

visible region� β-Carotene, 11 double bonds in conjugation

� λmax = 455 nm

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Conjugation, Color and the Chemistry of Vision

� β-Carotene is converted to Vitamin A, which is converted to 11-cis-retinal:

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Conjugation, Color and the Chemistry of Vision

� 11-cis-retinal is converted to rhodopsin in the rod cells of the retina.

� Visual pigments are responsible for absorbing light in eye and triggering nerves to send signal to brain

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Transmission and Color

The human eye sees the complementary color to that which is

absorbed

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Absorbance and Complementary Colors

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Orange Common Food Uses

Cereals, baked goods, snack foods, ice-cream, beverages, dessert powders, confections

Lemon-yellow Common Food Uses

Custards, beverages, ice-cream, confections, preserves, cereals.

Orange-red Common Food Uses

Gelatins, puddings, dairy products, confections, beverages, condiments.

Royal BlueCommon Food Uses

Baked goods, cereals, snack foods, ice-cream, confections, cherries.

Bright BlueCommon Food Uses

Beverages, dairy products, powders, jellies, confections, condiments, icing.

The colors of M&M’s

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NNO3S

SO3

N N

HO

• In the biological sciences these compounds are used as dyes to selectively stain different tissues or cell structures

• Biebrich Scarlet - Used with picric acid/aniline blue for staining collagen, recticulum, muscle, and plasma. Luna's method for erythrocytes & eosinophil granules. Guard's method for sex chromatin and nuclear chromatin.

Biological applications

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NNO3S N

CH3

CH3

NHNO3S N

CH3

CH3

Yellow, pH > 4.4 Red, pH < 3.2

Methyl Orange

• In the chemical sciences these are the acid-base indicators used for the various pH ranges:

• Remember the effects of pH on aromatic substituents

pH paper

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designing colours

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UV spectra and structure

From Skoog and West et al. Ch 14

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Empirical understandind of structural effects

R.B. Woodward, L.F. Fieser and others have predicted λmax for

π⇒π* in extended conjugation systems to within ca. 2-3 nm.

Homoannular, base 253 nm

heteroannular, base 214 nm

Acyclic, base 217 nm

Attached group increment, nm

Extend conjugation +30

Addn exocyclic DB +5

Alkyl +5

O-Acyl 0

S-alkyl +30

O-alkyl +6

NR2 +60

Cl, Br +5

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Expected UV (Woodward)

O

Base value 217

2 x alkyl subst. 10

exo DB 5

total 232

Obs. 237

Base value 214

3 x alkyl subst. 30

exo DB 5

total 234

Obs. 235

Base value 215

2 ß alkyl subst. 24

total 239

Obs. 237

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Distinguish Isomers!

HO2C

HO2C

Base value 214

4 x alkyl subst. 20

exo DB 5

total 239

Obs. 238

Base value 253

4 x alkyl subst. 20

total 273

Obs. 273

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i. Bathochromic shift (red shift) – a shift to longer λ; lower energyii. Hypsochromic shift (blue shift) – shift to shorter λ; higher energyiii. Hyperchromic effect – an increase in intensityiv. Hypochromic effect – a decrease in intensity

200 nm 700 nm

εεεε

Hyp

oc

hro

mic

Hypsochromic

Hyp

erc

hro

mic

Bathochromic

Substituent effects

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Quantitative analysis

Great for non-

aqueous titrations

Example here

gives detn of

endpoint for

bromcresol green

Binding studies

Form I to form II

Isosbestic points

Single clear point, can exclude

intermediate state, exclude light

scattering and Beer’s law applies

Binding of a lanthanide complex to

an oligonucleotide

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Other Electronic Processes

� Fluorescence: absorption of radiation to an excited state, followed by emission of radiation to a lower state of the same multiplicity

� Phosphorescence: absorption of radiation to an excited state, followed by emission of radiation to a lower state of different multiplicity

� Singlet state: spins are paired, no net angular momentum (and no net magnetic field)

� Triplet state: spins are unpaired, net angular momentum (and net magnetic field)

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� http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/UV-Vis/spectrum.htm#uv3b

� http://www.chemistry.ccsu.edu/glagovich/teaching/316/index.html