Background:

Qualitative Analysis of Organic Compounds

The analysis and identification of unknown organic compounds constitutes a very important aspect of experimental organic chemistry. There is no definite set procedure that can be applied overall to organic qualitative analysis.

Some basic experimental tests and physical constants are necessary for identifying an unknown compound. It is important that you understand just what information can and cannot be obtained from each of them.

The identification and characterization of an unknown organic compound normally refers to those processes or test that need to be preformed in order to answer the following important questions:

  1. What kind of elements (carbon, hydrogen, nitrogen, ….. ) are in the compound?
  2. What kind of functional groups are in the compound?
  3. Where in the molecule are these functional groups located?

Identification of organic compounds consists in a systematic procedure, this procedure may be carried out in steps are:

  1. Preliminary examination or investigation: The preliminary investigation consists in the study of physical properties of organic compound. of steps. The various physical properties include condition (state), color, odor, and solubility.
  2. Determination of physical properties such as melting point, melting point.
  3. Classification by solubility
  4. Functional-group classifications
  5. Elemental analysis (detection of Carbon, nitrogen, sulfur, halogen elements using sodium fusion test).
  6. Confirmation through the preparation of derivatives.
  7. Special tests.

Most organic liquids are less dense than water, but alkyl halides, polyhalogenated compounds, and many aryl halides are denser. If an organic liquid is denser than water, it probably contains halogen.

Note physical characteristics: solid, liquid, color, and odor. Compounds that are yellow to red in color are often highly conjugated.

1) Condition:

The simple observation as to whether the material is solid or liquid is an important first guide for the unknown substance

The solid material should be examined to notice whether it is crystalline, amorphous or fine powder.

In case of the liquid state, the constancy of the liquid material should be observed (mobile, oily or viscous). Also, determine if the unknown is true liquid or solution by adding anhydrous copper sulphate placed in a watch glass. The color will not be changed with true liquids while with solutions it turns blue color.

2) Color:

The color of the original sample is noted, some compounds show color due to impurities, while many others show colors due to the presence of chromophoric groups (Conjugation System) in them Such as diene group. The color observation may help to expect the unknown compound. Many chemical compounds are coloured because they absorb visible light

3) Odor:

Some organic compounds have characteristic odors, others have disagreeable odors. Any advantage from the minimal information obtained in this manner is heavily offset by the risks involved; therefore strong smelling of the sample must be avoided.

The characteristic odors of some compounds are:

  1. Alcoholic odor: methanol and ethanol.
  2. Bitter almond odor: benzaldehyde and nitrobenzene
  3. Mice like odor: Acetamide.
  4. Pleasant fruity or floral smell odor: Esters
  5. Vinegar like odor; acetic acid.
  6. Fishy odor: Amines.
  7. Phenolic odor: Many phenols

4) Solubility:

The solubility of the unknown in the following reagents provides very useful information. In general

The solubility of organic compounds can be divided into two major categories:

·  Solubility in which a chemical reaction is the driving force

Example:

·  Solubility in acidic solution (such as carboxylic acids or phenols)

·  Solubility in basic solution (such as basic organic compounds, Amines)

·  Solubility in which simple solubility or miscibility due to Dipole-dipole Interaction, Hydrogen bonding- special case of dipole-dipole when there is a hydrogen bonded to a N, O, or F. (or Ion-dipole- interaction of an ion with a polar molecule)
examples:dissolving any ionic compound in water), such as

o  miscibility of Methanol (CH3OH) with water (H2O)

o  dissolving of oxalic acid in water

After analysis of the previous tests (preliminary test), further information can be deduced by performing carefully selected functional group classification tests.

Qualitative tests for Alkanes and Alkenes

I. Unsaturated compounds:

Two common types of unsaturated compounds are alkenes (Example cyclohexene) and alkynes characterized by the carbon-carbon double and triple bond, respectively, as the functional group. The two common qualitative tests for unsaturation are the reactions of the compounds with

1.  Bromine in carbon tetrachloride.

2.  Potassium permanganate.

(a) Bromine in carbon tetrachloride test:

Dissolve the compound and add the bromine and shake. rapid disappearance of the bromine color to give a colorless solution is a positive test for unsaturation.

(b) Aqueous potassium permanganate test (Bayer test):

The reaction is based on the ability of potassium permanganate to oxidize double bond to diols.
Dissolve the compound and add the potassium permanganate solution dropwise and observe the result.

Qualitative tests for Alkyl halides (Halogenated hydrocarbons)

n-Butyl bromide / Sec. Butyl bromide / Tertiary Butyl bromide

Qualitative tests for alkyl halides are useful in deciding whether the compound in question is a primary, secondary, or tertiary halide.

Alcoholic Silver Nitrate The overall reaction is shown in the following equation:

Such a reaction will be of the SN1 type. Tertiary halides are more reactive in an SN1 reaction than secondary halides, which are in turn more reactive than primary halides. Differing rates of silver halide precipitation would be expected from halogen in each of these environments, namely, primary < secondary < tertiary. These differences are best determined by testing with silver nitrate and observing the results.

Add the alkyl halide to solution of silver nitrate, If no reaction is observed within five minutes at room temperature, warm the mixture and observe any change.

Qualitative tests for Alcohols (R-OH)

The identifying of alcohol is the hydroxyl functional group, -OH bonded to carbon. Alcohols are neutral and are classified into:

1.  Monohydric alcohols
They are almost colourless neutral liquids, alcohols less than six carbon atoms are miscible with water, but more than six carbon atoms are sparingly soluble
e.g., methyl alcohol, Ethyl alcohol and Tertiary butanol / 2.  Polyhydric alcohols
e.g. glycerol. The poly-hydric alcohols are colourless, neutral, viscous liquids having sweet taste and are soluble in water. / 3.  Aromatic alcohols
e.g. benzyl alcohol, aromatic alcohols are oily colourless liquids, insoluble in water and
The monohydric alcohols are divided to…
1.  Primary alcohols / 2.  Secondary alcohols / 3.  tertiary alcohols

General class reactions:

The tests for the hydroxyl group not only detect the presence of the group, but may also indicate whether it is primary, secondary or tertiary.

1-  Jones Reagent (CrO3-H2SO4 in H2O)
This reagent distinguishes primary and secondary alcohols from tertiary alcohols; the test is based on the much greater resistance to oxidation of tertiary alcohols compared to the other two types.

Dissolve one drop of your alcohol in acetone and add one drop of the jones reagent, and shake the tube to mix the contents. Primary and secondary alcohols react within two seconds as indicated by the disappearance of the orange color of the jones reagent and the formation of a green or blue-green precipitate or emulsion. Tertiary alcohols do not react even after 3 minutes.

(I) RCH2OH RCHO RCOOH

(II) R2CHOH R2C=O

(III) R3COH no visible reaction.

2-  Esterification The esters are flavoring or odor concentrates.

The esters may be formed by the direct reaction of a carboxylic acid with an alcohol. Such a reaction is called an esterification reaction.

Methyl alcohol react with Salicylic Acid yield wintergreen odour of methyl Salicylate
Ethyl alcohol react with Acetic Acid yield fruity odour of ethyl Acetate

3-  Iodoform test: (Presence of three alpha hydrogens)

This experiment shows how methanol and ethanol behave differently when treated with iodine solution and sodium hydroxide solution. Ethanol reacts to form a yellow solid, triiodomethane, often called iodoform.

Dissolve your compound in water; Add NaOH solution and warm, then introduce the potassium iodide - iodine reagent dropwise with shaking, the test is positive if a yellow precipitate of iodoform is deposited.

Qualitative tests for Phenols (Ar-OH)

Phenols are a class of organic compounds consisting of a hydroxyl group (-OH) bonded directly to benzene ring.

The identification of phenols by

(1)  FeCl3 test:

Phenols react with FeCl3 to form a colored complex with the Fe3+ ion. The color varies from purple to orange depending on the structure of the phenol tested.

Ø  Phenol + FeCl3 give violet

Ø  Cresol + FeCl3 give blue

Ø  Hydroquinine (quinol) + FeCl3 give blue

Ø  α-naphthol +FeCl3 give green

Ø  β-naphthol + FeCl3 give green

(2) With Chloroform and Sodium hydroxide reaction (formylation)(Reimann-Timann reaction)

When phenol is react with chloroform and sodium hydroxide and heat gently, a formyl group –CHO is introduced, and colored polymer formation.


Qualitative tests for Carbonyl compounds (Aldehyde and Ketone)

Identification of carbonyl functional groups:

2,4-Dinitrophenyl hydrazine test (DNP)

(as Brady's reagent),

Transfer two drops of unknown to a small test tube and add 4 or 7 drops of 2,4 DNP reagent and shake. Formation of a red - yellow precipitate (a positive test) indicates the presence of an aldehyde or ketone functional group.

Distinguishing between aldehyde and ketone functional group:

Fehling's solution.

Aldehydes reduce Fehling's solution to yellow or red copper (I) oxide, but ketone do not react.

Identification of presence of CH3CO- group in aldehyde and ketone

Iodoform reaction:

The iodoform test indicates the presence of an aldehyde or ketone in which one of the groups directly attached to the carbonyl carbon is a methyl group (presence of three alpha hydrogens).

The nitroprusside ion forms a coloured complex with methyl ketones. In presence of alkali

Example, acetone is converted to CH3COCH2-ion which reacts with nitroprusside ion [Fe(CN)5NO]2-to give highly coloured ion [Fe(CN)5NOCH2COCH3]2-.

The reaction of salicylic acid with formaldehyde in the presence of catalytic amounts of strong mineral acid (such as Sulphuric acid) results in the condensation product, and polymer formation. The final polymer is inevitably highly colored (the crimson red color are produced).

Qualitative tests for carboxylic Acids

General Reactions of Carboxylic Acids:

(1) Na2CO3 test:

They decompose Na2CO3 to CO2 and sodium salt of the corresponding acid (distinction from most phenols).

R-COOH + Na2CO3 → RCOONa + CO2 + H2O

The warm aqueous Na2CO3 is treated with few crystals (or few drops of acid). Effervescence due the evolution of CO2 indicates the presence of COOH group.

(2) Ferric Chloride Reaction (fe3+ complex carboxylic acid):

FeCl3 will indicate the presence of aliphatic acids turning the solution a yellow color. Aromatic acids will test as a beige-tan color.

For the success of this reaction it is important that the solution should be-neutral. Excess of acid usually inhibits the production of color or precipitate, and excess of alkali gives gelatinous reddish-brown precipitate of ferric hydroxide.

To the cold neutral solution of the acid add a few drops of FeCl3, solution and observe the colour or the ppt. formed.

Examples

Formic acid is the only one, which possesses both carboxyl and aldehyde groups activities

Formic acid is the only carboxylic acid that reduce Fehling reagent, mercuric chloride and KMnO4

Mercuric chloride test (HgCl2):

Add mercuric chloride solution to formic acid or a solution of formate and warm. A white ppt. of mercurous chloride Hg2Cl2, insoluble in dil. HCl, is produced.

Iodoform Test:

Lactic acid gives positive iodoform test because it contain the CH3CH(OH) group. This test is done by adding exess NaOH till alkali and blue litmus to the liquid lactic acid. the add excess iodine solution and warm gently to give yellow ppt of iodoform.

Action of CaCl2;

When a neutral solution of a tartarate is treated with CaC12 solution a white ppt. of calcium tartarate is separated on the cold after scratching and stirring

2- Fenton's Reagent: (Oxidation of tartaric acid in the presence of iron)

To a solution of tartaric acid or a tartarate add 1 drop of freshly prepared ferrous sulphate solution, 1 drop of hydrogen peroxide solution and excess of NaOH solution. An intense violet colour is produced due to the ferrous salt of dihydroxyfumaric acid, (HOOC-C(OH)=C(OH)-COOH). This test is positive to tartaric and tartarates only.

Qualitative tests for salts of carboxylic Acids

Carboxylic acid salt is the salt (RCOO-M+) formed when acarboxylic acidreacts with abase. carboxylic acid salts are ionic compounds and soluble in water

Qualitative tests for salts of carboxylic Acids Ester (R-CO-OR-)

Hydroxamic acid test

R-CO-OR' + H2N-OH R-CO-NH-OH + R'-OH

Esters react with hydroxylamine in the presence of sodium hydroxide to form the sodium salt of the corresponding hydroxamic acid. On acidification and addition of ferric chloride the magenta-coloured iron (III) complex of the hydroxamic acid is formed.

It is always advisable to ensure that an unknown compound does not give a colour with iron (III) chloride before carrying out the hydroxamic acid test.

Qualitative tests for salts of Amines

Reaction with benzenesulphonyl chloride
Benzenesulphonyl chloride reacts with primary and secondary but not with

tertiary amines to yield substituted sulphonamides.

a)  C6H5SO2Cl + H-NHR + NaOH C6H5SO2NHR + NaCl + H2O

b)  C6H5SO2Cl + H-NR2 + NaOH C6H5SO2NR2 + NaCl + H2O

The substituted sulphonamide formed from a primary amine dissolves in the alkali medium whilst that produced from a secondary amine is insoluble in alkali.

Qualitative tests for salts of Amides (R-CO-NH2)

Example: Urea

Action of nitric acid:

When nitric acid is presented, it ionizes. A hydrogen ion [proton] contributed by the acid is attracted to the oxygen and forms a covalent bond [electrophile H+]. The electronegative NO3-ion then is attracted to the positive hydrogen ion. This forms an ionic bond and hence the compound urea nitrate (white ppt).

Acidity test:
(1)  When the Unknown is Liquid
In your test tube place 2 ml of the unknown, and then add ½ spatula of Na2CO3 powder.
(2)  When the Unknown is Solid
In your test tube place 2 ml Na2CO3 Solution and heat it in the water bath, and then add ½ spatula of unknown. / Preparation of the Neutral Solution:
(1)  When the unknown is liquid:
In small beaker, 5 ml Water
+ 3 ml of Unknown + Add Ammonia dropwise until neutralization occur, using the Litmus Paper
(2)  When the unknown is Solid (soluble or insoluble in water):