Qualitative Organic Analysis

Identification of Organic Compounds

1. Spectroscopy (IR, ¹H and ¹³C NMR, MS)

2. Characteristic chemical reactions

3. Derivatives (no longer used to a significant extent)

You will receive a test tube containing a solid or a liquid. You must determine the identity of the substance.

You will be using physical characteristics, spectroscopy, solubility tests, and chemical tests described Experiment 57A.

Your unknown sample will be 1 of the compounds listed on pp. 878-888 of the lab manual (Appendix 1).

Be careful impurities introduced into your samples may lead to confusing results!

Steps for Identifying Unknown

1. Preliminary classification by physical state

2. m.p. and b.p. determination

3. Purification if necessary

4. Determination of solubility behavior

5. Obtain IR, NMR, and MS

6. Simple preliminary tests (only for -NO₂ and -CºN)

7. Application of relevant chemical classification tests

8. Preparation of derivatives (do not perform)

1. Preliminary classification by physical state

- color

- odor

- solid or liquid

2. m.p. or b.p. determination

2 Limitations

- sample must be pure

- several compounds may have similar m.p. or b.p.

For m.p., use melting point apparatus. For b.p., you will be given a b.p. range.

In either m.p. or b.p., make allowance for a ±6^º C error.

3. Purification

If the sample is impure, the spectra and chemical tests will give confusing data.

solid: recrystallization (unnecessary for this experiment)

liquid: distillation (unnecessary for this experiment)

4. Solubility Tests

Ions are more soluble in aqueous solutions than neutral compounds.

"strong" acid water soluble

if R > 4 C, water insoluble

phenol phenoxide anion

weak acid stabilized by delocalization

often water insoluble water soluble

If the unknown is soluble in aqueous NaOH, it is either a carboxylic acid or a phenol.

NaHCO₃: weak base

strong enough weak base soluble

acid

Ph-O-H + HCO₃^- ® NR remains insoluble

too weakly

acidic

If the unknown is insoluble in aqueous NaHCO₃ solutions, but soluble in aqueous NaOH, it is probably a phenol. These properties can be used to purify carboxylic acids and phenols.

RCO₂H + extract with neutral

neutral impurities NaHCO₃ impurities, ether

in ether ----------------------------------------® ___________________

solution aqueous, RCO₂^-

separate and add H⁺ ¯

RCO₂H

If the unknown was insoluble in aqueous NaOH, it is an organic base, a neutral compound, or an inert compound.

1°,2°,3° amines R-NH₂ + H⁺ ® R-NH₃⁺ ammonium ion

water insoluble water soluble

"weak" base

Amines will dissolve in dilute acid (HCl) solutions.

If the unknown is insoluble it is classified either as neutral or inert.

Concentrated H₂SO₄ is such a strong acid, that it will protonate any compound containing a heteroatom with lone pair e^- along with alkenes and alkynes.

In some cases, the protonated species (which will be cation) will just dissolve.

e.g.

water insoluble water soluble

In other cases, such as alkenes, protonation leads to reactions that produce the brown color that is often observed.

Compounds that are soluble with concentrated sulfuric acid are classified as neutral and include:

alkenes, alkynes, alcohols, ketones, aldehydes, esters, ethers, amides, nitro compounds.

Compounds that are insoluble and unreactive with concentrated sulfuric acid are classified as inert and include:

alkanes, alkyl halides, aromatic compounds.

5. IR and ¹HNMR spectra are used to determine the functional groups present and possibly even the complete structure.

It may be necessary to modify the following instructions for your unknown sample.

Preparation of NMR Sample

1. As usual, obtain an NMR tube.

3. Dissolve 100 - 200 mg sample into 1 ml. of solvent.

Solvent must dissolve sample (test nondeuterated solvents first)

Solvent must have no H atoms

CDCl₃, acetone-d6, benzene-d6, DMSO-d6, D₂O

4. Fill tube to depth in the usual manner.

5. Add TMS

Fill pipet with only a drop

Add to tube.

IR Spectra: Use KBr pellet for solid.

GC-MS: You will be told which method to use when you receive you unknown sample. As usual, dissolve 5 mg in 5 ml. of solvent. Once again you may have to test various solvents. Try methanol, methylene chloride, cyclohexane, or acetone.

6. Chemical Tests (see experiment 57 pp. 485 – 530)

Based on the m.p. or b.p., spectra, and solubility tests, you may have a good idea what functional groups might be present.

Perform the specific chemical tests for each functional group that might be present.

e.g. If the spectra and solubility tests indicate a ketone or aldehyde, then perform

2,4-dinitrophenylhydrazine test

chromic acid test

Tollen's test

Iodoform test

If no alcohols have been indicated, do not perform any of the tests prescribed for them.

Most of the tests involve formation of a precipitate or a color change.

e.g.

yellow ppt.

a. Read each procedure for a test carefully.

b. If you are uncertain what the appearance should be for a positive or negative test, use one of the suggested test compounds and compare it to your unknown.

c. Note which compounds will give a false positive test.

Usually, if all tests for a specific functional group are performed, then false positive tests will not pose a problem.

d. Place all waste in the appropriate bottles.

7. Prepare a derivative of your unknown using the procedures given in Appendix 2 (pp. 888 - 892) of the lab manual.

After steps 1 - 6 have been completed, you should be able to narrow down the possibilities of the identity of your unknown to a few compounds in one of the functional group tables.

From Exp 57 and Appendix 1, determine what derivative you should make.

Functional Group Preferred Derivative

Aldehydes and Ketones 2,4-Dinitrophenylhydrazone

Carboxylic Acids Anilide

Phenols a-Naphthylurethane

1° Amines Benzamide

2° Amines Benzamide

3° Amines Picrate

Alcohols 3,5-Dinitrobenzoate

Once you have synthesized, purified (if necessary), and dried your derivative, obtain its m.p. The m.p. of your derivative may correspond to one of the derivative m.p. values in an Appendix 1 functional group table. The derivative m.p. may match one of the likely possibilities based on the m.p. or b.p. of your unknown.

e.g. Your liquid had a b.p. range of 101-104 °C and is definitely an aldehyde.

In Aldehyde Table

Compound BP MP semicarbazone 2,4-dinitrophenyl-

hydrazone

Pentanal 102 - - 106

2-Butenal 104 - 199 190

Compare your spectra and any other results against these possible structures