OrganicQualitativeAnalysis-8A (Abala and Asperas, 2008)

-Experiment 8A –

Angela Ray T. Abala

Kurt Roland A. Asperas

University of the Philippines – Manila

College of Arts and Sciences

Department of Physical Sciences and Mathematics

CHEM 31.1 Organic Chemistry

1.1. Physical Properties of compoundsPhysical Properties of compounds2.2. Classification by solubilityClassification by solubility3.3. Elemental analysisElemental analysis4.4. Classification test for functional groupsClassification test for functional groups

This experiment focuses on the fourth area of inquiry.

1.1. Test for UnsaturationTest for Unsaturation2.2. Test for AromaticityTest for Aromaticity3.3. Test for Acidity and BasicityTest for Acidity and Basicity4.4. Test for AlkylhalidesTest for Alkylhalides

The carbon atom has the capability to bond with four other atoms. However, this ability to bond to several atoms (especially to H atoms) becomes reduced because of the presence of multiple bonds. The presence of double or triple bonds is an indication of the unsaturation of the compound.

Alkenes and Alkynes

Test for Unsaturation – Bromine in Methylene Chloride Test

How to prepare Acetylene?


Preparation of Acetylene


- electrophilic addition or halogenation


• Addition reaction of Bromine (Br2), a red liquid, to a compound containing a double or triple bond produces a colorless Dibromide. (A positive Test!)

• The double (or triple bond) must be sufficiently electron-rich to initiate the reaction. Therefore, minimal electron withdrawing groups (Deactivators), such as Carboxyl Groups attached to molecule, would hinder the reaction.

•Unsubstituted Aromatic compounds (Benzene) do not react with the Bromine reagent.

•Even if the ring has substituted activating groups (donate electrons to the ring) the reaction would be a substitution and not an addition.


Test Samples Visible ResultStructure/Formula of

Compound Responsible for the visible result

Acetylene Unpleasant odor, produced smoke, pale yellow-clear solution

Benzene Red solution No visible resultStyrene Presence of smoke,

cloudy-clear solution

Hexane Negative testNo visible result

POSITIVE TEST: clear solution

Test for Unsaturation – Baeyer Test


- Redox reaction


Potassium Permanganate (KMNO4) is an oxidizing agent.

It has a Purple color

Following the oxidation of an unsaturated compound, the Permanganate ion is reduced to Manganese Dioxide (MnO4), a brown precipitate.

The reaction is important because it doesn’t work on alkanes (compounds with carbon-carbon single bonds) or aromatic compounds.


Drawbacks:

Easily oxidized compounds give a positive test: a) Most aldehydes give a positive test. b) Formic acid and its esters give a positive test.

Often, the brown precipitate fails to form and the solution turns reddish-brown.

Alcohols with trace impurities give a positive test.

Phenols and aryl amines give a positive test.

Test for Unsaturation – Ignition Test

POSITIVE TEST: The disappearance of the KMnO4's purple color and the appearance of a brown suspension of MnO2 is a positive test.

Test Samples Visible ResultStructure/Formula of


Acetylene Brown solution, chocolate brown precipitate

MnO2 for the brown precipitate

Benzene Dark purple solution

No visible result

Styrene Dark brown precipitate in clear solution

MnO2 for the brown precipitate

Hexane Violet precipitate No visible result



• Ignition test is used for High Degrees of Unsaturation.

•The ignition test is used to differentiate aliphatic substances from aromatic unsaturated hydrocarbons.

•Multiple bonds of carbon cause the deposition of carbon in the ignition medium → soot → positive test!


•Highly unsaturated compounds such as aromatic compounds, give off a yellow sooty flame.

•The yellow color is due to the presence of carbon and the soot is credited to the deposition of carbon.

•Positive test is indicative of a high degree of Unsaturation and is probably Aromatic


POSITIVE TEST: Positive test is a sooty yellow flame.

Test Samples

Visible ResultStructure/Formula of


**Acetylene Orange-blue flame

C

Toluene Yellow flame, soot

C

Styrene Yellow flame, soot

C

Hexane Yellow flame No visible result for carbon deposition

Test for Aromaticity – Nitration


- electrophilic aromatic substitution


• Benzene derivatives in a general sense react in the same way that benzene does, although there are some interesting differences.

•Nitration of toluene generates a mixture of products. The major products are those with substitution at the ortho and para positions.

•In the case of toluene, ortho (and para) attack result in the positive charge being spread over two secondary carbons and one tertiary carbon atom (the one bearing the CH3 group).

•When reaction of toluene occurs at the meta position, then the resonance forms of the sigma complex put positive charge over 3 secondary carbons - the same as for benzene.


• Appearance of the product (Nitration of toluene) : light yellow to darker yellow-green liquid


Test Samples

Visible Result

Structure/formula of compound

responsible for the visible result

Toluene Yellow color on top of solution; similar to oil-water mixture; smoky; smells like rugby

POSITIVE TEST: Yellow solution

Test for Aromaticity – Le Rosen Test



•In the Le Rosen test, same processes in nitration take place. However, instead of HNO3, formaldehyde is used.


Test Samples

Visible Result

Structure/formula of compound

responsible for the visible result

Toluene Exhibited two layers

Rose pink solution

p - aquinone

Test for Acidity and Basicity – pH of an Aqueous Solution



•pH is a measure of the acidity or alkalinity of a solution.

•Aqueous solutions with a pH less than seven are considered acidic, while those with a pH greater than seven are considered basic.

•When a pH level is 7.0, it is defined as 'neutral' because at this pH the concentration of H3O equals the concentration of OH− in pure water.

•pH is formally dependent upon the activity of hydronium ions (H3O+).


Solution pH

Acetic Acid 2

Phenol 4

Aniline 7

Benzoic Acid 2

*water 5

Test for Acidity and Basicity – NaHCO3 Test



•Solubility will be indicated by the formation of a homogeneous solution, a color change, or the evolution of gas or heat.

•If soluble, then it is a strong organic acid. If not, then it is a weak organic acid.

• The most common weak organic acid are phenols. Typically, only a carboxylic acid will react with NaHCO3.

•Negatively substituted phenols such as nitrophenols, aldehydrophenols, and polyhalophenols are sufficiently acidic to dissolve in 5% sodium bicarbonate.


Sample Observation

Acetic Acid Evolution of gas

Phenol Brown solution

Aniline Distinct 2 layers

Benzoic Acid Dissolved

POSITIVE TEST: Evolution of a carbon dioxide gas is a positive test for the presence of the carboxylic acid

Test for Acidity and Basicity – AgNO3 Test



•The silver nitrate test is designed to detect strong acids especially carboxylic acids.

•The presence of an acid is detected by the presence of replaceable H+ atoms and the formation of precipitates.

•The essence of this test is the reaction between carboxylic acids and silver nitrate. Silver nitrate reacts with carboxylic acids to form salts of the carboxylic acid.

•To verify the presence of the silver salt, dilute nitric acid should be added. Silver salts are soluble in the aforesaid acid.


Sample Observation

Acetic Acid Cloudy solution

Phenol Precipitate appeared

Aniline Brown-yellow precipitate

Benzoic Acid White precipitate

POSITIVE TEST: Appearance of precipitate and dissolution of the precipitate in Nitric acid

Test for Acidity and Basicity – Silver Acetylide Test


- Silver acetylide is formed when acetylene is passed into an ammoniacal solution of silver nitrate.


• These substitution reactions which ethynes undergo to form compounds with metals are not occur with the alkenes. •These reactions can be used as tests to distinguish between acetylene and ethylene.

•When acetylene is passed through an ammonical solution of silver nitrate or cuprous chloride, at room temperature, precipitates of silver acetylide (white) or cuprous acetylide (red) are formed.


Sample Observation

Acetylene Pale yellow precipitate → gray-silver precipitate

Test for Alkylhalides – Beilstein Test


• The Beilstein test is a simple chemical test used in chemistry as a qualitative test for halides.

•In Beilstein test, a green flash is indicative of chlorine, bromine, and iodine.

•Fluorine is not detected because copper fluoride is not volatile.

•This test is very sensitive, thus halogen-containing impurities may give misleading results.


Test SamplesVisible Result

Structure / Formula of Compound

Responsible for the visible

result

Tert-butylchlorideBlue-green

flameCuCl2

Sec-butylchloride Green flame CuCl2

BenzylchlorideRed flame;

sootCuCl2

cholorobenzeneRed-orange flame; soot

CuCl2

ethylchloride Green flame CuCl2ethylbromide Green flame CuBr2

Ethyliodie Green flame CuI2

Test for Alkylhalides – Alcoholic AgNO3 Test



• this test can be used to differentiate aromatic halides from aliphatic halides.

•The reaction of an alkyl halide with silver nitrate in ethanol will result in the formation of a white or yellow silver halide precipitate that is insoluble in nitric acid.

•This reaction quite often proceeds slowly, and occasionally slight warming is necessary.

•Different rate if silver halide precipitation would be expected from halogen in each of these environments, namely, primary<secondary<tertiary.

•Alkyl bromides and iodides react more rapidly than chlorides

•Aryl Halides are unreactive toward the test reagent.


Test Samples Visible Result

Structure / Formula of Compound

Responsible for the visible result

Tert-butylchlorideWhite

precipitateAgCl

Sec-butylchlorideWhite-silver precipitate

AgCl

Benzylchloride Distinct 2 layers AgCl

cholorobenzeneWhite

precipitateAgCl

Ethylchloride White precipitate

AgCl

Ethylbromide White precipitate

AgBr

Ethyliodide White precipitate

AgI

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OrganicQualitativeAnalysis-8A (Abala and Asperas, 2008)