the stepwise procedure for gathering information is...
TRANSCRIPT
The stepwise procedure for gathering information is derived logically so as to avoid performing useless tests; i.e., each datum should lead logically to the next appropriate experiment A: be sure the compound is pure (multiple crystallizations,
distillations, etc)
B: obtain physical constants: melting point, boil ing point, refractive index, etc.;
C: determine solubility characteristics;
D: obtain spectral data when appropriate and/or run the indicated preliminary classification tests;
E: identify the functional group(s) using the data above;
F: make a list of possible compounds based on functional group and melting point/boiling point;
G: narrow (refine) the list as much as possible using additional classification/sub-classification tests and elemental analysis (sodium fusion);
H: prepare a crystalline derivative and determine its melting point to assign the identity of the unknown molecule.
For example:
Your first unknown is a liquid
1. Distill, collect about 90% of the sample
2. Record b.p. (130-132oC)
3. Do the solubility testing
4. Identify the class(es) of compounds
(ketone/aldehyde/alcohol)
A 1
Chemical tests for functional group identification
R OHR OH
O
R O
CrO3 / H2SO4 Cr2(SO4)3
blue to greenor
R' R
ONO2
O2N NHNH2
NO2
O2N NHN
R'
RH2SO4
alcohol
aldehyde/ketone 2,4-dinitrophenylhydrazine 2,4-dinitrophenylhydrazone
Test for the carbonyls
Test for the alcohols (1o and 2o)/aldehydes
O
O
O
O
O
O
O
O
Br
O
O
O O
O
O
O
O
O
O
OH
O
1. 2,4-dimethl-3-pentanone(diisopropyl ketone)
4. 4-methyl-3-pentene-2-one(mesityl oxide)
7. 2,2,4-trimethyl-3-pentanone(t-butyl isopropyl ketone)
10. 4-methyl-3-hexane(sec-Butyl ethyl ketone)
13. 3-methyl-1-pente-4-one
2. 3-hexanone(ethyl propyl ketone)
5. cyclopentanone
8. 1-bromo-2-propoanone(bromoacetone)
15. 4-methyl-2-hexanone(ethyl isobutyl ketone)
14. 2,4-pentadione(acetyl acetone)
3. 2-hexanone(butyl methyl ketone)
6. 5-hexene-2-one(allyl acetone)
9. methyl 2-oxopropanoate(methyl pyruvate)
12. 2-methyl-3-hexanone(isopropyl propyl ketone)
11. 3-hydroxy-3-methyl-2-butanone
KMnO4 KMnO4
KMnO4
I2/NaOH
I2/NaOH
I2/NaOH
Beilstein test
Make the derivatives and measure their melting
points
O N NHO
NH2
semicarbazone: m.p. 113oC
NHN NO2
O2N
b.p. 125oCDNP-derivative: 130oC
O
b.p. 136oC semicarbazone: m.p. 119oC
N NH
NH2O
N
DNP-derivative: 97oC
NH
NO2
NO2
Always compare possible choices - especially if the melting points are close
Try to select the set of derivatives with high melting points - they are easier to crystallize and therefore, m.p. will be more accurate
The more of the reasonable derivatives you make (at least two) - the more accurate assumption of the unknown you will make
General considerations &
1st semester O-Chem reminders
Broken Glass
Broken Hg-thermometers
The top of the thermometerbulb is aligned with the bottomof the side arm of the adapter
When distilling make sure that:
There is no “hand”-held distillations
Compound is in the distilling flask
All the joints are tightly fit
Aluminum foil: don’t wrap hot distillations
Thermometer is properly positioned
How to choose “good” tests for functional group determination?!
Alcohols:
sodium test
acetyl chloride
ceric ammonium nitrate
Jones oxidation
iodoform test
etc…
Alcohols: sodium test for the active hydrogen
2RNH2 + 2Na 2RNH Na + H2
R H
2ROH + 2Na 2RO Na + H2
R2 + 2Na 2 Na + H2
R OH
O
H O H
Alcohols: acetyl chloride test
H3C Cl
OR OH H3C OR
OHCl
R OH R OH
R
R
R
OHR> >
Presence of water?!?!
R OH
R OH
R
R OH
RR
R O R O
OH
R O
R
CrO3 / H2SO4
CrO3 / H2SO4
CrO3 / H2SO4
CrO3 / H2SO4
no reaction
no color change
blue/green color
blue/green colorblue/green color
Alcohols: Jones oxidation
orange color solution
Although color, precipitate are always better than gas evolution, be observant as the color’s intensity will depend on the concentration
color can disappear
precipitate might dissolve back
Although color, precipitate are always better than gas evolution, be observant as the color’s intensity will depend on the concentration
color can disappear
precipitate might dissolve back
The lab conditions are not identical to those descried in the book, so use them mostly as a starting point
Ultraviolet and Visible Spectroscopy
(UV/Vis)
energy absorption transitions that occur are between electronic energy levels of valence electrons
orbitals of lower energy are excited to orbitals of higher energy
Electronic spectra
200 - 700 nm
visibleultraviolet
A
300 400 500wavelength, nm
A = log(Io/I)
A - absorbance
Io - intensity of the reference beam
I - intensity of the sample beam
Beer’s Law: A = εcl
Based on absorption of light by sample
Beer’s Law
(Beer-Lambert-Bouguer Law)
A
c, M
The law ( A = εcl ) only works when the dependence of absorbance ( A ) is directly proportional to the concentration ( c ), i.e., a straight line
A = log(Io/I)
A - absorbance
Io - intensity of the reference beam
I - intensity of the sample beam
Beer’s (Beer-Lambert) Law: A = εcl
ε - molar absoptivity (specific for each molecule)
l - cell length (light path)
c - concentration mol/L
ΔE = hν = hc/λ E - energy absorbed, J
h - Plank constant, 6.6x10-34 J sec
ν - frequency, Hz
c - speed of light, 3x1010 cm/sec
λ - wavelength, cm
λ depends on the ease of electron promotion:
molecules that require more energy for electron promotion absorb at shorter λ, and
vise versa: molecules that require less energy absorb at longer λ
1. Both UV and visible irradiation results in electronic transitions from low-energy ground state orbitals to higher-energy orbitals
2. Transitions require 40-300 kcal/mol
3. This energy is dissipated as heat, light or chemical reaction (isomerization, radical reaction, etc)
4. And the electrons return into the ground state
Uses of UV/Vis spectroscopy
pKa of compounds
Kinetics
Sensors
O3S NN N
HO3S NN N
HO3S NN N
HH
HOH
Methyl orange (yellow in base)
(red in acid)
product
reactant
Quantitative analysis
Isosbestic point
can exclude intermediate state,
exclude light scattering and Beer’s law applies
Indicative of parallel not consecutive reactions
A B + C
A B C
1. Virtually all UV spectra are recorded in solution-phase
2. Cells can be made of plastic, glass or quartz
3. Only quartz is transparent in the full 200-700 nm range; plastic and glass are only suitable for visible spectra
4. Concentration is empirically determined (Beer’s law)
A typical sample cell (commonly called a cuvet):
common solvents and cutoffs
acetonitrile 190
chloroform 240
cyclohexane 195 1,4-dioxane 215
95% ethanol 205
n-hexane 201
methanol 205 isooctane 195
water 190
Compounds that absorb in the visible region (i.e., colored compounds) have more easily excited electrons than compounds that absorb light at shorter UV wavelengths
NN
NH2
NaO3S
NN
H2N
SO3Na
Congo red
λ = 497nm
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NN
NH2
NaO3S
NN
H2N
SO3Na
H3C CH3 σ-bond σ-electrons
H2C CH2 π-bond π-electrons
H3C OH n-electons
ΔE
σ antibonding (σ*)
π antibonding (π*)
nonbonding
σ bonding
π bonding
excited states
ground states
HOMO - highest occupied molecular orbital
LUMO - lowest unoccupied molecular orbital
ΔE
σ antibonding (σ*)
π antibonding (π*)
nonbonding
σ bonding
π bonding
excited states
ground states
HOMO - highest occupied molecular orbital
LUMO - lowest unoccupied molecular orbitaln π* transition
π π* transition
H3C CH3 σ-bond σ-electrons
H2C CH2 π-bond π-electrons
H3C OH n-electons
n π*
<105 kcal>270nm
nπ
σ
σ∗
π∗
n σ*
<150kcal> 185nm
π π*
<170kcal> 165nm σ σ*
>170kcal<165nm
hν hν
HOMO
LUMO
λmax (nm) ε
165 15,000
217 21,000
256 50,000
290 85,000
334 125,000
364 138,000
β-caroteneλmax = 455nm
Signal transduction N
H
NH
11-cis
11-trans
opsin
opsin
UV/vis of ACETONE
hν hν
π
n
π∗
π π* 187nm270nm n π*
H3C
O
CH3
HOMO
LUMO
O
O
O
O
O
λmax, nm
217
270
312
343
370
benzeneλmax = 260nm
naphthaleneλmax = 280nm
anthraceneλmax = 375nm
phenanthreneλmax = 350nm
naphthaceneλmax = 450nm
(yellow)
pentaceneλmax = 575nm
(blue)
coroneneλmax = 400nm
(yellow)
wavelength, nm
Substituent Effects on the chromophore’s λ
Bathochromic shift (red shift) – a shift to longer λ; lower energy
Hypsochromic shift (blue shift) – shift to shorter λ;
higher energy Hyperchromic effect – an increase in intensity Hypochromic effect – a decrease in intensity
A
wavelength, nm
An auxochrome is a substituent in a chromophore that alters the λmax and the intensity of the absorption
OH O NH2 NH3
benzene
255nm
phenol
270nm
phenolate
287nm
aniline
280nm
anilinium ion
254nm
s-cis is less stable
HOMO s-cis > HOMO s-trans
λmax(s-trans) < λmax(s-cis)
217nm 253nm
s-trans s-cis
263nm 256nm
227nm 227nm220nm253nm217nm
Noble Prize in Chemistry 1965
"for his outstanding achievements in the art of organic synthesis”
Woodward rules
(Woodward-Fieser rules)
N
O
HO N
NH
NH3CO
H3COO
H
HH
OCH3
O
O OCH3
OCH3OCH3
reserpine
quinine Vitamin B12
N
O O
N
H
HH
H
H
strychnine
O
O
O
O
O
O
Diels and Alder, 1928
butadiene
maleic anhydride cis-1,2,3,6-tetrahydronaphthalic anhydride
Diels-Alder Reaction
Conjugated dienes can combine with alkenes to form six-membered cyclic compounds
The formation of the ring involves no intermediate (concerted formation of two bonds)
Discovered by Otto Paul Hermann Diels and Kurt Alder, (1928, University of Berlin and University of Cologne)
The Nobel Prize in Chemistry 1950
"for their discovery and development of the diene
synthesis"
The Nobel Prize in Chemistry 1981
“for their theories, developed independently, concerning the course of chemical reactions"
Kenichi Fukui
Kyoto University
Roald Hoffman
Cornell University