The stepwise procedure for gathering information is...

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