Identification of an Unknown Oxygen-Containing Compound

CH344Bruce A. Hathaway

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Goals of This Lab

• Gain experience with some common functional group tests.

• Determine the functional group in your unknown: 1°, 2° or 3° Alcohol, Aldehyde, Ketone.

• Determine the identity of your unknown, using physical constants, IR and NMR.

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General Procedure

• Determine some physical properties.• Run the chemical tests.• Determine which functional group you have.• Determine the identity of your compound by

making derivatives and interpreting spectra.

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Physical Properties

Boiling Point Range, by simple distillation.• You have about 12 mL of unknown, so distill

from a 25 mL flask.• Collect distillate in clean test-tubes or vials.• Collect the material that boils over a relatively

constant boiling range.• Do not distill to dryness.• I have added a high-boiling impurity!

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Test Procedures

• The following slides, and the handout, summarize the test procedures.

• You need to read the complete procedures in the text, and use them.

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Ignition Test, Page 574

• Place 3-4 drops of unknown on a clean watch-glass, and ignite them.

• Observe the color of the flame. • A blue flame usually indicates oxygen. • A sooty flame usually indicates C=C’s or

benzene rings.

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Water-Solubility

• 4-5 drops of unknown + 3 mL of water, shake: • Does the unknown dissolve?

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2,4-DNPH test, C-11 & D-3, p. 601

• Dissolve 4-5 drops of unknown in 4 mL of 95% ethanol, then add 3 mL of 2,4-DNPH reagent. Cool, filter, wash with water and NaHCO3 solution, then recrystallize from ethanol.

• A yellow to orange precipitate indicates an aldehyde or a ketone.

• You can use the precipitate as a derivative.• Acetone is a ketone!

NHNH2

O2N NO2

R1

O

R2

+

NHN

O2N NO2

R1

R2

2,4-D introphenylhydrazine, 2 ,4-DNPH derivative"DNPH"

H 2 S O 4

E th a n o l

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Chromic acid Test, C-9, p. 585-6

• Add 1-2 drops of unknown to 1 mL of reagent acetone in a small test tube.

• Add 1 drop of chromic acid solution. • For comparison, add 1 drop of chromic acid solution

to 1 mL of acetone in another small test tube.• A relatively quick formation of a green to blue-green

solution usually indicates a 1o or 2o alcohol. • A slow (30-60 seconds) formation of a green to blue-

green solution indicates often indicates an aldehyde.

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Chromic Acid Test

R CH2

OHC rO 3 , H 2 S O 4

H 2 OR C

O

OH + "C r+3 sa lt"b lue-green

1o a lcohol

R1 CH

OH

R2

C rO 3 , H 2 S O 4

H 2 OR1 C

O

R2 + "C r+3 sa lt"b lue-green

2o a lcohol

R CH

OC rO 3 , H 2 S O 4

H 2 OR C

O

OH + "C r+3 sa lt"b lue-green

aldehyde

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Iodoform test, C-16, p. 590-1• Add 3 drops of unknown to 2 mL of water in a large test

tube. • Add 1 mL of 3M sodium hydroxide solution. Add a

dropper-full of iodine-KI solution. • If a yellow ppt. forms, it is a positive test. • See text if no yellow ppt. forms.• 2° methyl alcohols also give a positive test.

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R C

O

CH3 R C

O

O-

3 I 2 + 4 N a O H

H 2 O+ HCI3Na+

"Iodoform"

Semicarbazone and Oxime Derivatives: D-4 and D-5, p. 602

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R1 C

R2

OH 2 N -G

H +R1 C

R2

N G

+ OH2

G = O H O xime

G = SemicarbazoneNH - C

O

NH2

Semicarbazone and Oxime Derivatives: D-4 and D-5, p. 602

• Mix 0.20 g of semicarbazide HCl or hydroxylamine HCl, 0.30 g of sodium acetate, 2 mL of water and 2 mL of 95% ethanol in a large test tube.

• Add 0.20 g of unknown. • Cork and shake for a couple minutes. Cool and see if

crystals form. • If not, heat in a boiling water bath for about 5

minutes, then cool and filter. • Wash with cold water, then recrystallize with ethanol

or ethanol-water mixture.13

3,5-Dinitrobenzoates, D-1, p. 600

• This reaction also works with 4-nitrobenzoyl chloride, to give 4-nitrobenzoate derivatives.

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R OH +

O

ClNO2

NO2

O

RONO2

NO2

h e a t

3,5-Dinitrobenzoates, D-1, p. 600

• Dry the alcohol first with Na2SO4 or MgSO4.• 0.2 g 3,5-dinitrobenzoyl chloride + 4-5 drops of alcohol

in a medium test tube, heated in a small flame just enough to maintain in a liquid state for 5-10 minutes.

• Cool, break up with a spatula, add 4 mL of sodium carbonate solution, heat in a boiling water bath for a few minutes.

• Cool, filter, wash with water. Recrystallize from ethanol or ethanol-water.

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-Naphthylurethanes and Phenylurethanes, D-2, p. 600-1

• We don’t have any -naphthyl isocyanate, so we will use phenyisocyanate instead.

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R OH +

NC

O

h e a t

N

H

CRO

O

-Naphthylurethanes and Phenylurethanes, D-2, p. 600-1

• Dry the alcohol first with Na2SO4 or MgSO4.• Mix 5-drops of alcohol with 5 drops of -naphthyl

isocyanate in a medium test tube. • If no reaction takes place immediately, heat in a boiling

water bath for 5-15 minutes. • Cool, filter, recrystallize from heptane.

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Spectroscopy

• Take an IR spectrum.• Submit a sample of your pure, distilled unknown

for NMR.

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Putting It All Together

• The chemical tests should point you to a particular functional group.

• You can rerun the tests if you have conflicting results with yourself.

• Make at least two derivatives, crystallize them, let them dry, and take melting points.

• The boiling point, derivative melting point data, and the spectral data should point you to a particular unknown.

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Finishing up

• Pour the test solutions/solids into the appropriate waste beakers in the hoods.

• Keep all of the unknown you have left (distilled or undistilled). We will appropriately dispose of it later.

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