Qualitative analysis of common functional group, Chemistry tutorial

Introduction

Qualitative organic analysis, the identification of organic compounds depend on their physical and chemical properties, is analogous in several ways to the identification of plants and animals according to their taxonomy-their structural features and assumed natural relationships. To categorize an organic compound into a given family needs 1st detecting a precise functional group (feature set of atoms) in the molecules of organic compounds.

Since functional groups affect the physical, chemical, and spectral properties of an organic compound, a chemist can recognized a compound's functional groups through determining definite physical properties, observing its chemical behaviour through diverse categorization reagents, and learning other spectral data. In our experiment we will subject a series of organic compounds to specific chemical reactions in order to identify that class of functional group the substance belongs to.

Table 2

1040_Various ordinary organic functional groups.jpg

  

Fig: Various ordinary organic functional groups

Functional group identification tests

Before outlining the common scheme, we should note one or 2 points of practical significance

(a) Quantities of substance for tests. For most tests about 0.1g solid or 0.1 - 0.2mL (2 - 3 drops) of liquid material (not more) should be utilized.

(b) Reagents likely to be met inside organic analysis are on the reagent shelves. You are advised to develop a general knowledge of the physical characteristics of common organic compounds. If in doubt about the expected result of a test between a certain compound and a reagent, carry out a trial test by a known compound and compare through the unknown.

(c) Quantities of substance derivatives. Students have wasted much time and material in the past by taking as well huge a quantity of substance for preparation of a derivative. In common, 0.5 - 1g (or 0.5-1mL) of substance provides the most satisfactory consequences.  If a practical book instructs one to utilize superior quantities (3-4 g or more), the quantities should be scaled down to 1 g or 1 mL of the unknown substance and analogous quantities of reagents should be utilized.

General Scheme of Analysis

A. Preliminary tests 

(a)  Note physical characteristics - solid, liquid, colour and odour.

(b) Perform an ignition test (heat tiny amount on metal spatula) to find out whether the compound is aliphatic or aromatic (for example luminous flame - aliphatic; sooty flame-aromatic).

B. Physical constants

Find out the boiling point or melting point. Distillation is recommended in the case of liquids. It provides the dual purpose of determining the b.p., as well as purification of the liquid for following tests.

C. Analysis for elements present

The elements present will be told to us, but read up the process.

D. Solubility tests

Solubility classification

The solubility of an organic compound in diverse solvents can provide valuable information about the unknown. The common rule of 'like dissolves like' or 'polar compounds dissolve more readily in polar solvents' is helpful. As well, organic acids (such as carboxylic acids and phenols) react with bases to form water soluble salts and organic bases (such as amines) react with acids to form water soluble salts. It should be noted that the polarity of an organic compound is amplified via the type and number of polar functional groups in the molecule and that the polarity diminishes as the size of the non polar aliphatic group (describe aliphatic group in a hyperlink) in the molecule increases. With this background, one begins the solubility classification by adding 3 drops or 3 mg of the unknown to 3 ml of water and shaking the mixture. If the unknown dissolves, it is a polar compound and in placed in solubility group S1.  An unknown in class S1 is then tested as above using ether as the solvent.  If it dissolved in both water and ether it is then placed in class S2.  For unknowns that don't fall into either class S1 or S2, the unknown's solubility in 5% sodium bicarbonate is measured. If it is soluble, the unknown is placed in class A1. If it isn't soluble, the solubility in 5% sodium hydroxide is studied. If it is soluble at this point, the unknown belongs in class A2. If an unknown is insoluble to this point it is next tested for solubility in 5% hydrochloric acid.  

Compounds soluble in 5% hydrochloric acid are situated in solubility class B1. For compounds insoluble to this point the next solvent to try is concentrated sulfuric acid. Unknowns soluble in only this acid are placed in solubility class N1. An additional distinction can be made for compounds soluble in concentrated sulphuric acid via testing their solubility in 85% phosphoric acid. Such compounds that are soluble in 85% phosphoric acid are placed in class N2. Finally, for compounds insoluble to this point are placed in class IN. Such solubility classes and their consequences can be summarized below:

S1 These are extremely polar compounds that consist of salts of carboxylic acids or amines.  It is as well possible the compound is of low molecular weight and has many polar functional groups these as a carbohydrate.

S2 Such compounds are low molecular weight (generally less than 5 carbons) by a polar functional group such as carboxylic acid, amine, alcohol, aldehyde, or ketone.

A1  Higher molecular weight carboxylic acids fall into this class. 

A2  Phenols demonstrate this kind of solubility.

B1 Primary, secondary and tertiary amines fall into this class.  Though, if there are two or more phenyl groups on the nitrogen, the amine will probably not be basic enough to form the salt and will, then, be insoluble. 

N1  These are higher molecular weight compounds (generally more than 9 carbons) enclosing an oxygen atom.

N2  These are medium size molecules (generally containing from 5 to 9 carbons) enclosing an oxygen atom.

IN  These are neutral compounds.  Alkyl halides and alkanes fall into this class.

The consequences of a solubility classification should not be harshly interpreted as there are many overlaps. Utilize the consequences of this classification only as a focus into which classification tests should be done 1st. The solubility of the unknown in the subsequent reagents gives extremely helpful information. In common, about 3 mL of the solvent is utilized through 0.1 g or 0.2 mL (2-3 drops) of the substance. The class of compound might be indicated from the following table:

Table: solubility table

REAGENT AND TEST

CLASS

GROUP OF

COMPOUNDS

Soluble in cold or hot water. (If the unknown

is soluble don't

perform solubility tests

below)

 

Neutral, acidic or basic. (Test with litmus or

universal indicator

paper) 

Lower members of series. Neutral, e.g.

alcohols; Acidic, e.g.

acids, phenols; Basic,

e.g. amines 

Soluble in dil. HCl 

Basic 

Most amines (except

III amines with  only

aromatic groups

Soluble in dil. NaOH

Acidic

Most acids, most

phenols

Soluble in NaHCO3

Strongly acidic

Most carboxylic acids.

Insoluble in water, acid

and alkali 

Neutral

Hydrocarbons, nitrohydro-carbons,  alkyl or aryl halides,

esters and ethers. Higher molecular  weight alcohols,

aldehydes and ketones

E. Group Classification Tests 

From the previous tests it is often possible to deduce the functional groups present in the unknown compound. Individual tests are then performed to identify and confirm the functional groups present.

As we know that:

1. We are strongly advised against carrying out needless tests, because not only are they a waste of time but as well amplify the possibility of error. Therefore it is pointless to 1st test for alcohol or ketone in a basic compound enclosing nitrogen! Instead tests for amines, and so on should be done on these a compound. 

2. A systematic approach can't be overemphasized in group classification tests to avoid confusion and error.

F. Consultation of  Literature

Once the functional group has been recognized, we are to make reference to tables in a book on organic analysis, for assessing possibilities and for the preparation of suitable solid derivatives.

It should be noted that whilst 2 substances through the similar functional group might sometimes have extremely similar b.p. or m.p., solid derivatives can generally be chosen from the literature, through m.p. differences of about 10 (or more), that distinguish between the 2 possibilities.

  Example:

  COMPOUND                                    B.P.                                                      DERIVATIVES (M.P.)

                                                                                                                            2,4-DNPH     

SEMICARBAZONE

  Diethyl ketone                                102                                 156                                 139 

  Methyl n-propyl ketone                    102                                 144                                 112 

G. Preparation of derivatives 

The final characterization of the unknown is made via the preparation of suitable solid derivatives. Select the derivative watchfully and its m.p. should preferably be between 90-150 for ease of crystallization and m.p. determination. Attempt the preparation of 1 derivative. Purify the derivative through recrystallisation, dry and find out the m.p. Submit the derivatives properly labeled for assessment mutually by the record.

Recording of Results

Record the consequences in a systematic manner. Record consequences in the practical book at the time (not written up afterwards). Make a record of every test carried out, no matter whether a negative effect has been attained. Test, observation and inference should be specified.

At the conclusion of the analysis, include a brief summary of consequences giving the name, b.p. or m.p., and formula of the analysed compound. Qualitative Analysis for Elements (for reference only)  In organic compounds the elements commonly occurring along through carbon and hydrogen, are oxygen, nitrogen, sulphur, chlorine, bromine and iodine. The detection of such elements based upon converting them to water-soluble ionic compounds and the application of specific tests.

Lassaigne's Sodium Fusion Test

  C, H, O, N, S, X            NaX

                    NaCN

                 ->   Na

                 NaCN2S S

Procedure 

Situate a piece of clean sodium metal, about the size of a pea into a fusion tube. Add a little of the compound (50 mg or 2-3 drops). Heat the tube gently at 1st, permitting any distillate shaped to drop back onto the molten sodium. When charring begins, heat the bottom of the tube to dull redness for about 3 minutes and lastly plunge the tube, while still hot, into a clean dish containing cold distilled water (6 mL) and cover instantly through clean wire gauze.

For liquids it is better to 1st melt the sodium add the liquid drop via drop.

CAUTION: The tube shatters, and any residual sodium metal reacts by water. Stir the mixture, boil for 1 - 2 minutes, on a tripod and filter hot through a fluted paper.

The 'fusion' filtrate that should be clear and colourless, is utilized for the specific

Tests described below:

1. To a portion (2 mL) of the 'fusion' filtrate add 0.2 g of powdered ferrous sulphate crystals. Boil the mixture for a half a minute, cool and acidify by adding dilute sulphuric acid drop wise. Formation of a bluish-green precipitate (Prussian blue) or a blue solution indicates that the original substance encloses nitrogen. If no precipitate appears, permit to stand for 15 minutes, filter and inspect filter paper.

2. Sulphur (sulphide) 

To the cold 'fusion' filtrate (1 mL) add a few drops of cold, freshly prepared, dilute solution of sodium nitroprusside. The latter might be prepared via adding a tiny crystal of the solid to 2 mL of water. Production of a rich purple colour shows that the original substance encloses sulphur. This test is extremely sensitive. Only strong positive consequences are important.

3. Halogens (halides)

Acidify a portion (1 mL) of the 'fusion' filtrate with 2N nitric acid, and if nitrogen and/or sulphur are present, boil for 1 - 2 minutes. Cool and add aqueous silver nitrate (1 mL), compare through a blank. Formation of a heavy, white or yellow precipitate of silver halide indicates halogen. If a positive consequence is attained: acidify the remaining portion of the 'fusion' filtrate with dilute sulphuric acid, boil and cool. Add carbon tetrachloride (1 mL) and a few drops of freshly prepared chlorine water. Shake the mixture.

(a) If the carbon tetrachloride layer continues colourless - indicates chlorine.

(b) If the carbon tetrachloride layer is brown - indicates bromine.

(c) If the carbon tetrachloride layer is violet - indicates iodine.

If nitrogen and/or sulphur are as well present, the calculation of silver nitrate to the acidified 'fusion' solution will precipitate silver cyanide and/or silver sulphide in addition to the silver halides. The elimination of hydrogen cyanide and/or hydrogen sulphide is resulted via boiling the 'fusion' solution. Group categorization tests.

Tests for unsaturation

1. Cold dilute potassium permanganate solution.

2. Solution of bromine in carbon tetrachloride.

Tests for compounds containing nitrogen

1. Amines

(a) Nitrous acid

(b) Confirmatory tests.

2. Compounds which give amines or ammonia on acid or alkaline hydrolysis:

Amides, substituted  amides, anilides, nitriles.

3. Compounds that provide amines on reduction 

Nitro,  nitroso, azo, hydrazo, nitriles.

Tests for compounds containing C, H and possibly oxygen

1. Carboxylic acids:Na2CO3 or NaHCO3 solution release carbon dioxide.

2. Phenols

(a) Sodium hydroxide solution (soluble). Insoluble in and no CO2 from NaHCO3 (except when electron attracting groups present, for exampel 2, 4-dinitrophenol).

(b) Ferric chloride solution.

(c) Bromine water.

3. Aldehydes and Ketones

(a) 2,4-dinitrophenylhydrazine (as Brady's reagent) for C=O.

(b) Iodoform test for CH3CO-.

4. Aldehydes only (reducing properties)

(a) Fehling's solution.

(b) Tollen's reagent (ammoniacal AgNO3 solution).

(c) Jones reagent.

5. Alcohols

(a) Lucas' reagent to distinguish I, II and III alcohols.

(b) Jones reagent.

(c) Metallic sodium (utilizes dry liquid and dry tube).

6. Sugars

(a) Molisch's test.

7. Esters

(a) Hydroxamic acid test.

(b) Hydrolysis.

Functional group classification tests

Introduction to qualitative tests - The 1st test that should be done is a solubility test to find out the class or classes to that the unknown belongs.  From the consequences of the solubility tests, several thought of the kind of organic compound should be evident. If the solubility test consequences put the unknown substance in the 'Neutral' section, it is suggested that the classification tests be done in this order: aldehydes, ketones, alcohols, esters, amides, nitriles, ethers, alkenes and alkynes. Select a test from the list of tests that would assist verify the presence or absence of the suspected functional group class. Do as many tests that might be needed to completely verify the functional group to that the unknown belongs. Be careful to interpret properly the test consequences for those unknowns that might enclose 2 or more functional groups. At that point, proceed to the preparation of derivatives to identity the exact identity of the unknown.

2,4-dinitrophenylhydrazine test (for aldehydes and ketones) - This test will be positive for an aldehyde or ketone as indicated via the formation of a yellow, orange or red precipitate that is labeled a 2,4-dinitrophenylhydrazone. This precipitate can as well be utilized as a derivative for the unknown if its melting point is measured (see below for derivative utilize). The colour of the precipitate can assist additional identify the extent of conjugation for the carbonyl group. Highly conjugated aromatic aldehydes or ketones usually provide red solids whereas non conjugated carbonyl compounds provide yellow products.

Acetyl chloride (for acidic hydrogen compounds) - This test will assist identify carboxylic acids, phenols and alcohols.  A positive test will be reminded via the evolution of heat that might be hard to detect. So, this test may provide false positive or negative tests depending on the expertise of the person doing the test.  In several cases, a solid (usually white) might shape.  If this occurs, the solid, if isolated and its melting point is measured, could be utilized as a derivative for the unknown.  If water is present in the unknown, the test will probably provide a false positive test as acetyl chloride reacts energetically through water.

Basic hydrolysis (for amides, esters and nitriles) - Amides and esters can be hydrolyzed through heating in a sodium hydroxide solution. This reaction pH provides the acid as a water soluble carboxylate salt. Acidifying this solution by concentrated hydrochloric acid would consequence in a precipitate if the carboxylic acid is water insoluble. If this precipitate is shaped, it should be filtered and utilized as a derivative for the unknown.

Beilstein test (for halogenated compounds) - Placing a small amount of an organic compound on the end of a copper wire and heating it in the open flame of a Bunsen burner results in a transient green colour in the flame if the compound contains a halogen atom.  If the unknown is volatile, it may evaporate before it burns resulting in a negative test.  

Benedict test (for aldehydes and sugars) - Whenever easily oxidized organic compound (these as aldehydes and reducing sugars) is heated in Benedict's solution (that is a blue solution enclosing a complexed copper (II) ion) a brick red precipitate of cuprous oxide forms. If the unknown isn't soluble in the reagent a negative test might be examined due to the lack of a reaction. 

Bromine in carbon tetrachloride (for alkenes and alkynes) - When a solution bromine in carbon tetrachloride is added dropwise to an unknown compound, the brownish colour of elemental bromine vanishes as the bromine adds to the unsaturated organic compound. 

Ceric nitrate (for alcohols and phenols) - Alcohol with 10 carbons or less will give a red colour with ceric nitrate solution whereas phenols will give a green-brown to brown precipitate.  Easily oxidized compounds may destroy the ceric nitrate solution before the test may be observed. 

Chromic acid (for aldehydes, primary and secondary alcohols) - Easily oxidized compounds convert the red chromium (VI) ion to a green chromium (III) precipitate.

Combustion (for flammable or combustible organic compounds) - When a few milligrams of an organic liquid or solid are placed directly into a Bunsen burner flame they often burn.  Note that highly halogenated organic compounds may not burn. Very volatile compounds may evapourate before burning or burn very rapidly.  The manner in which a compound burns can give some information about its nature.  Highly oxygenated compounds burn with a blue flame, aliphatic compounds give a yellow flame and aromatic compounds give a sooty flame.

Ferric chloride (for phenols) - Some (but not all) phenols give a colour when ferric chloride solution is added.  This test is not a definitive one and the results should be carefully evaluated.

Ferric hydroxamate (for esters, acid chloride and anhydrides) - Esters of carboxylic acids give a magenta colour with this reagent.  Acid chloride and anhydrides give a magenta or burgundy colour with the test reagent.

Ferrous hydroxide (for nitro compounds) - Most compounds that contain a nitro group will give a brown to red-brown precipitate of ferric hydroxide by oxidation of ferrous hydroxide.

Hinsberg test (to distinguish primary, secondary and tertiary amines) - Benzenesulfonyl chloride can be used to distinguish primary, secondary and tertiary amines.  The amine functional group must be confirmed before this test can be performed as the test will give very confusing results with any other functional group.  Primary amines give a solid benzenesulfonamide product that is soluble in 5% sodium hydroxide.  Secondary amines give a solid benzenesulfonamide product that is insoluble in 5% sodium hydroxide.  Tertiary amines do not react with benzenesulfonyl chloride. 

Hydroxylamine hydrochloride (for aldehydes and ketones) - Aldehydes and most ketones give a red colour when added to a solution of hydroxylamine hydrochloride in ethanol-water that has a universal indicator added.  

Iodoform test (for methyl carbonyl compounds) - This test is mainly used to identify methyl ketones. The iodoform regent iodinates the methyl group which they cleaves in the basic solution. One should confirm the presence of a carbonyl group in the unknown before this test is done as misleading results could occur with other compounds.  For example, acetaldehyde and alcohols that have a methyl group bonded to the C-OH group can also give a positive test since such an alcohol can be oxidized to a methyl ketone by the iodoform reagent.

Lucas test (to distinguish primary, secondary and tertiary alcohols of six carbons or less) - A solution of zinc chloride in aqueous hydrochloric acid can be used to distinguish primary, secondary and tertiary alcohols.  The unknown compound must be soluble in the reagent in order for the test to be valid.  When a tertiary alcohol is added dropwise to the reagent, an immediate second layer or a liquid alkyl chloride is formed.  Secondary alcohols form a second layer of the insoluble alkyl chloride in three to 5 minutes.  Primary alcohols are unreactive with the Lucas reagent.  

Nitrous acid (to distinguish primary, secondary and tertiary amines) - Primary aromatic amines give nitrogen gas evolution with the nitrous acid reagent.  Other aromatic amines can undergo coupling reactions to form coloured products.   

Ph in ethanol (to distinguish low molecular weight acidic or basic compounds) - The pH of compounds that are soluble in water or aqueous alcohol can be measured.  If the pH is in the acid range the compound can be a carboxylic acid, acid chloride or anhydride.  If the pH is in the basic range, the compound may be an amine.  Organic salts may hydrolyze in water which can lead to acidic or basic solutions. 

Potassium permanganate (for compounds that can be oxidized) - Organic compounds that can be readily oxidized will convert the purple of the permanganate ion to a brown precipitate of manganese dioxide.  Such organic compounds include: aldehydes, reducing sugars, primary or secondary alcohols and some alkenes and alkynes.

Silver nitrate in ethanol (for alkyl halides that can undergo Sn1 reactions) - Tertiary alkyl halides will give a white to yellow silver halide precipitate with this reagent.  Some secondary halides will react more slowly.  Aryl and vinyl halides do not react. 

Sodium fusion (for compounds that contain halogen, nitrogen or sulfur) - When an organic compound is placed in molten elemental sodium the molecules are violently destroyed.  Any halogen, nitrogen or sulfur in the original molecule is converted to ionic materials which are then identified.  The halide is identified by precipitation with silver ions.  The sulfide ion is identified by precipitation with lead ions.  The cyanide ion formed the nitrogen in the molecule is converted into Prussian blue by ferrous sulfate.

Sodium iodide in acetone (for alkyl halides that can undergo Sn2 reactions) - Primary and some secondary alkyl chlorides or bromides will give a precipitate of sodium iodide in the reagent.  Alkyl iodides will not give the precipitate.  Aryl or vinyl halides do not react.

Solubility (for general classification of organic compounds) See solubility

Classification section above.

Tollens test (for aldehydes and reducing sugars) - Water soluble aldehydes and reducing sugars give a silver mirror or black precipitate of elemental silver through the Tollens reagent.

General notes: Conditions and techniques utilized in qualitative analysis

1. Mixing solutions

After addition of any reagent to a solution, one must make certain proper mixing. To mix the reagent and the solution in a centrifuge tube, tap the bottom of the centrifuge tube against the table or snap it by our fingers while holding the upper part in the other hand.

2. Centrifuging

Precipitates are divided from the supernatant solution through centrifuging. This is the procedure of dividing more dense solid particles from less dense liquid (solution) through spinning (separation via means of centrifugal force). The apparatus utilized here is termed a centrifuge. It must be equilibrium to properly function. Balancing is done through putting the centrifuge tube enclosing the reaction mixture  (tightly capped)  in a sleeved  centrifuge slot,  then placing a centrifuge  tube with an  equal volume of    tap water  in a slot across from the first tube. Make sure that the tubes are tightly capped before they are place d in the centrifuge.

3. Decanting

The supernatant solution is shifted from above the precipitate to another tube through decanting. When a 2- phase system (solid-liquid) is considered: after the solid settled to the bottom upon centrifuging, decanting is pouring the supernatant liquid out of the tube, leaving the solid behind.

4. Rinsing precipitates

All precipitates, after they  have  been separated  from  the  supernatant solution, must be rinsed by distilled water  before  proceeding to  the  classification of  the  cation present. This procedure must be done to eliminate any cations present in the supernatant solution adhering to the solid. The presence of these ions may cause confusing consequences in the procedure of additional recognition or division. The solid remaining after the supernatant solution has been eliminated is mixed through 10 drops of distilled water and the tube is tapped to systematically mix the contents. The tube is centrifuged, rinse water decanted, and the procedure replicated 1 to 2 more times.

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