Organic Chemistry: the Chemical Tests

The chemical tests in alphabetical order.

Aromatics Test The Aromatics test confirms the presence of an aromtic ring. Ring size can be judge by color: benzene derivatives give a yellow-orange color, naphthalene a blue-purple and anthracene a green. This is a Friedel-Crafts alkylation reaction that produces a colored product. The left test tube is benzene, the middle test tube, with a pale blue color, is naphthalene and the right test tube is 9-methylanthracene. Note: electron donating or withdrawing groups will affect the color of the triarylcarbenium cation.
Baeyer (Permanganate) Test The Baeyer test confirms the presence of an alkene, alkyne or aldehyde. The permanganate can oxide the alkene to a diol, an alkyne to a dione and an aldehyde to a carboxylic acid. The Mn7+ (a deep purple solution) is reduced to MnO2 a brown precipitate. The left test tube is a control and remains purple (negative), the middle test tube is an alkene (positive) while the right test tube is an alkyne (positive).
Beilstein Test The Beilstein test confirms the presence of a halide (chlorine, bromine or iodine) in the compound. A copper wire is dipped into the compound and heated in an open flame. A blue-green to green colored flame is a postive test.
Benedict's Test The Benedict's test confirms the presence of a reducing sugar, a compound in the hemi-acetal form that can be oxidized to the carboxylic acid. The Cu2+ in the reagent is reduced and forms Cu2O (s ) as a solid (usually) red-brown precipitate. The solution usually requires heating in a water bath to complete the reaction. The red colored test tube on the left is a positive test, the blue on the right a negative test.
Bicarbonate Test The Bicarbonate test confirms the presence of a carboxylic acid or sulfonic acid. The sodium bicarbonate reagent reacts as a base with the carboxylic acid in a classic acid-base neutralization reaction, resulting in the formation of carbon dioxide. The test tube on the right with the bubbles is a positive test, the tube on the left a negative test.
Biuret's Test The Biuret's test confirms the presence of a peptide or protein. The Biuret's reagent, containing Cu2+, forms a complex with the amide group of the peptide or protein. The complex is pink for peptides and purple for proteins. The test tube on the left is a negative test, the tube on the right a positive test.
Brady's (2,4-DNPH) Test The Brady's test confirms the presence of an aldehyde or ketone. Ester and other carbonyl compounds are usually not reactive enough to give a positive test. The 2,4-DNPH reacts with the carbonyl of the aldehyde or ketone to form a highly colored hydrazone precipitate. The color of the precipitate may give an indication of the amount of conjugation in the product. Increasing conjugation shifts the color from yellow - orange - red. The left test tube is an ester (negative), the middle test tube a ketone (positive) while the right test test is an aldehyde (positve).
Bromine Test The Bromine test confirms the presence of unstaturated hydrocarbons, specifically alkenes or alkynes, not aromatics. The bromine reagent adds across the carbon-carbon π-bond in an electrophilic addition reaction. The bromine water reagent is a dark orange color while the products are colorless. A positive test is the loss of color as the reagent is added. The test tube on the left is a negative test, the tube on the right a positive test.
Chromic Acid (Jone's) Test The Chromic acid test confirms the presence of a functional group that can be oxidized. This includes aldehydes, primary and secondary alcohols. The Cr6+ in the orange colored reagent is reduced to Cr3+ and forms a blue/green solution (or precipitate in acetone). The left test tube is a 1° alcohol (positive), the middle test tube a 2° alcohol (positive) while the right test test is a 3° alcohol (negative).
Dehydration of Alcohols Test The dehydration of alcohols using sulfuric acid can be used to indicate which type of alcohol is present. The dehydration of an alcohol using sulfuric acid is an E1 elimination reaction. As a consequence the 3° reacts rapidly and exothermically, 2° react slower and may need heat while 1° do not react. The reaction produces a separate yellow aqueous layer and a layer of the alkene. The left test tube is a 3° alcohol (positive (rapid with heat)), the middle test tube a 2° alcohol (positive (slow)) while the right test test is a 1° alcohol (negative).
Fehling's Test The Fehling's test confirms the presence of an aldehyde, or a reducing sugar. The aldehyde group is oxidized to a carboxylic acid while the Cu2+ is reduced to Cu2O which forms a red precipitate. The left test tube is water, a negative control, the middle test tube is a ketone (negative) while the right test test is glucose (positive).
Ferric Hydroxamate Test The Ferric Hydroxamate test confirms the presence of an ester. The feric hydroxamate, when heated with an ester, forms an intensely colored complex with Fe3+, usually dark maroon. The left test tube is a ketone (negative) the right test test is an ester (positive).
Finkelstein's (Sodium Iodide) Test The Finkelstein's test confirms the presence of alkyl chlorides or bromides. The NaI reagent reacts in an SN2 mechanism so this can be used to determine the nature of the alkyl halide. 1° alkyl halides react rapidly, 2° slower while 3° do not react. The formation of a white - yellow precipitate is a positive test. The left test tube is a 1° alkyl halide (positive), the middle test tube is a 2° alkyl halide (positive) while the right test test is a 3° alkyl halide (negative).
Hinsberg Test The Hinsberg test confirms the presence of amines, 1°, 2° or 3°. The benzenesulfonyl chloride reacts with 1° and 2° amines to produce a sulfonamide. The 1° amine produces a sulfonamide that is soluble in the basic solution, but precipiates on addition of HCl. The 2° amines forms an insoluble sulfonamide since it doesn't have an acidic proton on the N atom. The 3° amine doesn't react. The bottom row is the reaction with benzenesulfonyl chloride in NaOH. The top row is after addition of HCl. The left test tube is a 1° amine (positive), the middle test tube is a 2° almine (positive) while the right test test is a 3° amine (negative).
Iodoform Test The Iodoform test confirms the presence of a methyl ketone or 2° alcohol next to a methyl group, i.e. a R(C=O)CH3 or RCH(OH)CH3. The reagent first replaces the hydrogens on the terminal methyl group with I, then the methyl triiodide is lost as iofoform which forms a yellow precipitate. The left test tube is a ketone (negative) the right test test is an acetophenone (positive).
Lucas Test The Lucas test confirms the presence of a 2° or 3° alcohol. The reagent reacts in an SN1 mechanism to produce an alkyl chloride which shows as a precipitate or cloudiness. The test does not work for water insoluble alcohols (greater than 5 carbons) as they usually produce a cloudiness anyways. Since it is SN1, 3° reacts quickly, 2° slowly and 1° do not react. The left test tube is a 1° (negative), the middle test tube is a 2° (positive after heating) while the right test test is a 3° alcohol (positive).
pH Test - Acids The pH test confirms the presence of a carboxylic acid The reaction is a classic acid-base reaction with an indicator. If the pH or blue litmus paper turns red it is a positive test. The left is an acid (positive), the right is a non-acid (negative).
pH Test - Bases The pH test confirms the presence of a carboxylic acid The reaction is a classic acid-base reaction with an indicator. If the pH or red litmus paper turns blue it is a positive test. The left is a non-base (negative), the right is a base (positive).
Phenol (Ferric chloride) Test The Phenol test confirms the presence of a phenol. The reagent reacts with a phenol to produce a complex that is usually blue to purple, or red for bidentate ligands. The left test tube is a control (negative), the middle test tube is a 1° alcohol (negative) while the right test test is a phenol (positive).
Schiff's Test The Schiff's test confirms the presence of an aldehyde. The reagent reacts with an aldehyde in a nucleophilic acyl addition reaction. The addition of the aldehyde to the colorless reagent produces a magenta colored solution. The left test tube is a ketone (negative), while the right test test is an aldehyde (positive).
Silver Nitrate Test The Silver nitrate test confirms the presence of an alkyl halide. The reagent reacts with an alkyl halide in an SN1 mechanism to produce an ether and AgX which will appear as a white - yellow precipitate. Since it an SN1 mechanism 3° alkyl halides react rapidly, 2° slowly (usually only with heating) while 1° (or aromatic halides) don't react. The left test tube is a 1° alkyl halide (negative), the middle test tube is a 2° alkyl halide (positive on heating) while the right test test is a 3° alkyl halide (positive).
Tollen's Test The Tollen's test confirms the presence of an aldehyde. The reagent, ([Ag(NH3)2]+) a mild oxidizing reagent, reacts with aldehydes, but not alcohols or other carbonyl compounds, and oxides it to the carboxylic acid. The silver is reduced to solid silver which precipitates as a "mirror" on the inner surface of the test tube, or as black collodial particles. The left test tube is a ketone (negative), the right test test is an aldehyde (positive).