Laboratory 2

REACTIONS OF ALDEHYDES AND KETONES

IDENTIFICATION OF UNKNOWN

OBJECTIVE: 

1. To observe some characteristic reactions of aldehydes and ketones
2. To use the observations to identify the unknown aldehyde/ketone

Note that due to the special lab schedule this term, not all tests that are discussed in the theory will be performed but you are responsible for all the theory section.

THEORY

Aldehydes and ketones both contain the carbonyl   functional group. The polarity of this group is important for many of the reaction mechanisms of aldehydes and ketones and is responsible for the similarity of the reactions of these compounds. Most differences in reactions and reaction rates can be attributed to the differences in the groups around the carbonyl group.

Aldehydes have 1 alkyl or aromatic group and a hydrogen atom bonded to the carbonyl group, while ketones have 2 alkyl or aromatic groups bonded to this group. Because a hydrogen atom is very small compared to an alkyl group, aldehydes have less crowding at the carbonyl carbon than ketones and generally react faster than ketones.

OXIDATION REACTIONS

Aldehydes and ketones behave differently towards oxidizing agents. Four common oxidizing agents are chromic acid, Tollens’ reagent, Fehling’s reagent and Benedict’s reagent. Aldehydes can be oxidized relatively quickly to carboxylic acids by these reagents while the oxidation of ketones is much slower if it occurs at all. For most ketones, there is no reaction because C-C bonds must be broken for oxidation to occur.

JONES (CHROMIC ACID) OXIDATION

The Jones Reagent is a solution of chromium trioxide (CrO₃) in diluted sulfuric acid to give chromic acid. The reagent can also be prepared from sodium or potassium dichromate in dilute sulfuric acid. Jones Reagent is especially suitable for the oxidation of primary and secondary alcohols to carboxylic acids and ketones respectively. It also oxidizes aldehydes to the corresponding carboxylic acids. A colour change from the orange-red of chromic acid (Cr⁺⁶) to the green colour of Cr⁺³ indicates that oxidation has occurred. (See the note for cyclohexanone on page 6)

TOLLENS’ REAGENT

Tollens’ reagent is a solution of a silver-ammonia complex [Ag(NH₃)₂]⁺¹ in an ammonium hydroxide solution which acts as a weak oxidizing agent. When this reagent is used as an oxidizing agent, the Ag⁺¹ is reduced to metallic Ag and a silver mirror will be deposited on the walls of a clean test tube, thus this test is sometimes called the

“silver mirror” test. If the conditions are not exact, the silver will precipitate as a black or dark grey solid instead of a silver mirror. Tollens’ reagent is a weak oxidizing agent that will NOT oxidize alcohols or ketones, but will oxidize aldehydes, and can be used to distinguish between aldehydes and ketones. An aldehyde first reacts in basic solution to form a salt, and, if acid is added, the carboxylic acid is produced. (Also, see the cyclohexanone notes on page 6)

 

FEHLING’S REAGENT

Fehling’s reagent is another weak oxidizing agent consisting of the Cu⁺² ion complexed with a tartrate salt in an alkaline solution. Aldehydes reduce the cupric ion (Cu⁺²) to the cuprous ion (Cu⁺¹). The blue solution of the cupric ion first changes to a muddy green and then the red precipitate of cuprous oxide forms. This reagent oxidizes aldehydes but not ketones and can be used to distinguish between the two carbonyl compounds. It can also distinguish between aliphatic and aromatic aldehydes as aliphatic aldehydes react more quickly than aromatic aldehydes. Fehling’s reagent can also oxidize αhydroxyketones. The secondary alcohol group will be oxidized to ketones. Similar to the reaction with Tollens’ reagent, a salt is produced in basic solution which will produce the carboxylic acid if the solution is acidified.

 

Because of the resonance with the ring, aromatic aldehydes are generally less reactive compared to the aliphatic aldehydes.

BISULPHITE ADDITION COMPOUNDS³

All aldehydes and some ketones (those with small R groups), most methyl ketones, and low molecular weight cyclic ketones (up to eight carbons) react with saturated aqueous sodium bisulphite to form white crystals by an addition reaction. Ketones react very slowly. This is an equilibrium reaction and the presence of excess bisulphite shifts the equilibrium to the right, producing more white precipitate.

 

The speed of this reaction depends on the size of the R groups bonded to the carbonyl carbon atom.

Note: The product of the reaction of formaldehyde (methanal) and acetaldehyde (ethanol) is water soluble and therefore the crystals will not be observed if aqueous solution is used. 

This reaction is of great importance as it can be used to separate aldehydes and ketones with small R groups from mixtures containing other substances that do not react with sodium bisulphite. The white precipitate can be further purified by recrystallization and the original aldehyde or ketone can be recovered afterward by hydrolysis in dilute aqueous acid or base.

SCHIFF’S (FUCHSIN ALDEHYDE) TEST

Schiff’s reagent is a solution of the magenta dye, Fuchsin, decolourized by sulphur dioxide. If this reagent is added to an aldehyde, the magenta colour of the dye reappears. This colour is due to various addition products of the aldehyde and Fuchsin and the colour varies from one aldehyde to another. No reaction occurs with ketones. This test is very sensitive and can be used to detect trace amount of aldehydes present.

ENOLIZATION REACTIONS

An enolate anion can be formed in solution by an aldehyde or ketone with a hydrogen on the carbon atom next to the carbonyl carbon (the ? - carbon). This reaction commonly occurs in basic solution and a negatively charged enolate anion is produced. This anion can act as a nucleophile in many reactions such as the iodoform test.

IODOFORM TEST

The iodoform test uses the enolate anion to detect the presence of a methyl group on the ? – carbon. In basic solution, a methyl ketone or acetaldehyde can form an enolate anion which will react with iodine to form a substitution product in which all 3 of the

? − hydrogens are replaced by I. If excess base is present, iodoform (CHI₃) is formed. Iodoform is a bright yellow insoluble crystal that can be easily observed and identified by its colour and medicinal odour. Although all aldehydes and ketones containing an ? − carbon will form an enolate anion in basic solution, only methyl ketones and acetaldehyde will react with excess iodine to produce iodoform. (Methyl secondary alcohols and ethanol also react with excess iodine and are oxidized to methyl ketones, which then react further to form iodoform.).

     

The general equations for the reactions are:

1. Formation of Enolate Anion:

 

 

This anion is stabilized in solution by resonance. 

2. Addition of Iodine:

Iodine can then bond to the negative carbon atom in place of the H that has been removed.

 

If the group next to the carbonyl group is a methyl group, all of the ?-hydrogens will be replaced by the halogen. This is because after the replacement of the first ?-hydrogen by halogen, the acidity of the other two ?-hydrogens increases due to the electronwithdrawing effect of the halogen and therefore they are rapidly replaced by halogens to form a trihalomethyl group.

 

Triiodomethyl group is strongly electon-withdrawing and makes the carbonyl group even more electrophile which can be easily cleaved by base to form iodoform.

 

                                                    Iodoform

CYCLOHEXANONE NOTES

BISULPHITE TEST:³

Sodium bisulphite can form an addition compound with unhindered cyclic ketones, such as cyclohexanone, to form a carbon sulfur bond. In these compounds, the C=O group does not have any steric hindrance from the adjoining CH₂ groups because they are bound into the ring and the C=O group is free to react with the bisulphite.

JONES’ OXIDATION:

Cyclohexanone has an unusual high reactivity due to its angle strain and is oxidized by chromic acid.⁴ The mechanism of this reaction is complicated and beyond the level of this course.

 

BIBLIOGRAPHY: 

1. McMurry J., Organic Chemistry, 6^th ed., Thomson Learning Inc., 2004; Ch. 19
2. Reactions of carbonyl compounds http://authors.library.caltech.edu/25034/17/ BPOCchapter16.pdf  (Accessed on Dec 30, 2019)
3. Bisulfite reaction with aldehydes and ketones http://www.chemguide.co.uk/organicprops/carbonyls/addition.html,  (Accessed on December 30, 2019)
4. Rocek, J. and Riehl, S. A. Chromic acid oxidation of cyclohexanone. Reaction products. J. Org. Chem., 1967, 32 (11), pp 3569–3576

List of Equipment:

Test tubes

Test tube rack

Transfer pipet

250 mL beaker

Buchner funnel

Vacuum flask

Thick-wall rubber tubing

Filter paper

Watch glass

Rubber coated clamps 

Retort stand

 

List of Chemicals:

Acetaldehyde

Acetone

Benzaldehyde

Butanal

Cyclohexanone

Ethanol

2-propanol

2-pentanone

3-pentanone

Ammonium hydroxide (NH₄OH)

Jones’ reagent (CrO3/H2SO4)

Fehling’s reagent

Hydrochloric acid (HCl)

Iodine solution

Nitric acid (dilute) 

Sodium hydroxide (NaOH) (both 6 M and 5% solutions)

Silver nitrate (AgNO₃)

Sodium bisulphite (NaHSO₃) 

Schiff’s reagent  PROCEDURE

Fill a 400 mL beaker about half way with tap water and heat it on a hot plate. Make sure the heater is under your personal vent. Obtain your unknown from your instructor and record your unknown number in your lab notebook.

OXIDATION REACTIONS

A) BORDWELL-WELLMAN TEST (JONES’ REAGENT)

 

           CAUTION:

1. WORK IN THE FUME HOOD.
2. CHROME (VI) IS TOXIC AND A KNOWN CARCINOGEN AND THE

          JONES’ REAGENT IN WHICH IT IS USED IN HIGHLY CORROSIVE.

USE CARE TO AVOID CONTACT WITH SKIN, EYES, AND

 

CLOTHING.

3. THE SOLVENT SULFURIC ACID IS HIGHLY CORROSIVE AND

CAN CAUSE BURNS. USE CAUTION TO AVOID CONTACT WITH     SKIN, EYES, AND CLOTHING.  

 

 

1. Add 1 mL of acetone to each of 5 clean test tubes.

 

2. Add 2 drops of benzaldehyde to test tube 2, acetaldehyde to test tube 3, cyclohexanone to test tube 4 and unknown to test tube 5. 

 

3. Add 1 small drop of Jones’ reagent to each test tube. 

Note that test tube 1 only contains acetone and Jones reagent.

4. Record your observations in your notebook. A positive test is marked by the formation of a green color. For aldehydes, the color usually develops in 5-15 seconds.
5. Dispose the reagents in the appropriate waste container located in the fume hood.

 

B) TOLLENS’ TEST

 

            CAUTION:

1. SILVER NITRATE WILL CAUSE A DARK LONG LASTING

STAIN ON SKIN. (It is unsightly, but not hazardous)

2. TOLLENS’ REAGENT MUST BE PREPARED IMMEDIATELY

                               BEFORE USE BECAUSE IT FORMS SILVER FULMINATE, AN EXPLOSIVE, IF ALLOWED TO STAND AROUND. AFTER

            COMPLETING THE TEST, IMMEDIATELY DISPOSE OF ALL

RESIDUES AS SPECIFIED IN WASTE DISPOSAL.

3. AMMONIUM HYDROXIDE IS A RESPIRATORY IRRITANT.        PERFORM THE TESTS IN A FUME HOOD.

 

Note: For this test to be successful, the glassware must be scrupulously clean. Make sure to rinse the test tubes well with R.O. water. The chloride ions in tap water interfere with mirror formation.

1. Clean 6 test tubes THOROUGHLY with soap and water and rinse with R.O. water. 
2. Add 10 drops (0.5 mL) of 6 M NaOH solution to each test tube and swirl the tube. Allow the NaOH solution to remain in the tube for about 1 min, and then discard the NaOH solution in the sink. DO NOT RINSE THE TEST TUBES!! 
3. Add about 1 mL of 0.1 M silver nitrate solution to each test tube. Brown precipitate of silver hydroxide (AgOH) may form.
4. While stirring vigorously, add enough 6 M ammonium hydroxide (NH₄OH) solution dropwise to just dissolve the precipitate (about 15-20 drops). The test will fail if too much ammonium hydroxide is added. Add a few more drops of NH₄OH solution, if needed, until the precipitate completely dissolves. If all of the precipitate is not dissolved when the test tube is half full, let the precipitate settle and decant the clear solution to use.
5. Add 3 drops of acetone to tube 1; 3 drops of benzaldehyde to tube 2; 3 drops of acetaldehyde to tube 3; 3 drops of cyclohexanone to tube 4 and 3 drops of your unknown to tube The sixth test tube will contain Tollens’ reagent only and is used as a blank.