Laboratory Report on Aldehydes and Ketones

• Mark Norly L. Tundag

I. OBJECTIVES

At the end of the experiment, the students are able to identify the functional group present in aldehydes and ketones, also on determining their physical and chemical properties and to perform chemical test to distinguish one from the other.

II. MATERIAL and APPARATUS

The experiment used a bunsen burner, vials, 100 ml beaker, test tube holder, vial brush, clay flame shield, wire gauze, acetaldehyde¹, benzaldehyde², acetone³, 2,4-dinitrophenylhydrozine⁴, ammoniacal silver nitrate solution [ (Ag((NH)₃)₂)NO₃], Fehlings a and b, 6m sulfuric acid [H₂SO₄], very dilute potassium permanganate [KMnO₄], distilled water [H₂O] and Schiff’s reagent⁵.

III. PROCEDURE and OBSERVATIONS

1. The test for the physical characteristics of the carbonyl compounds namely the acetaldehyde (an aliphatic aldehyde), the benzaldehyde (an aromatic aldehyde), and acetone (a ketone) was the first to be investigated. Four drops of each of the carbonyl compounds were mixed with 2 ml of water in three separate vials. Only the aromatic aldehyde did not form a homogeneous mixture it formed two layers instead, wherein benzaldehyde at settled at the bottom.

B1.The reaction between the carbonyl compounds and the 2,4-dinitrophenylhydrozine was performed and the precipitate was identified. After taking five drops from each of the carbonyl compounds that were placed again in three separate vials, yellow-orange precipitate that readily formed was seen after adding into each of the vials another 5 drops of the 2,4-dinitrophenylhydrozine.

2. The reaction of the carbonyl compounds with the Tollen’s reagent was performed next. Only with the aldehydes did a silver mirror on the sides of the vials were observed to exist when the carbonyl compounds mixed with Tollen’s reagent in three separate vials were heated for ten minutes. A black stain was only seen on the ketone vial with the reagent.

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3. The result of the carbonyl compounds with the Fehling’s test was determined. From blue, the only compound that changed its color to green was the aliphatic aldehyde when the mixture of five drops of fehlings a and b with five drops of each of the carbonyl compounds in three separate vials were placed on the water bath. The remaining carbonyl compounds had no observable changes happened.

4. The reaction of potassium permanganate to the carbonyl compounds was observed and the organic products identified. Brown precipitate was formed on both the aliphatic and aromatic aldehyde but not on the ketone, when five drops of the carbonyl compounds was added to the mixture of the five drops pink colored potassium permanganate acidified with 2 drops of 6M sulfuric acid placed into three different vials.

5. The result of the Schiff’s test with the carbonyl compounds was described and interpreted. Only the aldehydes were a changed of color occurred when five drops of the Schiff’s reagent was placed in the three separate vials containing the carbonyl compounds. From cloudy white solution of acetaldehyde to lavender and yellowish solution of benzaldehyde to a colorless solution with pinkish globule that settled at the bottom of the vial.

IV. CONCLUSION

Aldehydes and Ketones are collectively called as carbonyl compounds, referring to their carbonyl [ C=O ] functional group that affects their solubility rendering it relatively higher because of the molecule’s ability to hydrogen bond with water but it is also dependent to the molecular mass and the number of carbon present on the nonpolar ‘R’ group, if the R group is strong enough to cancel out the hydrogen bonding of the functional group with water it will make the entire compound insoluble. Benzaldehyde, for example is insoluble because of the presence of the benzene ring that is nonpolar in nature.

To investigate the chemical properties of carbonyl compounds and to differentiate one from the other some reagents were used in the experiment: 2,4-dinitrophenylhydrozine for example gives off a yellow orange precipitate when it detects the presence of the carbonyl functional group in a solution, the aliphatic aldehyde reacted to the reagent forming acetaldehyde-2,4-dinitrophenylhydrozone¹, the aromatic aldehyde reacted to the reagent forming benzaldehyde-2,4-dinitrophenylhydrozone², the ketone reacted to the reagent forming acetone-2,4-dinitrophenylhydrozone³. Tollen’s that contain ammoniacal silver nitrate on the other hand, differentiates aldehyde from a ketone considering the fact that silver mirror on both the vials were formed due to the reduction of the oxidizing agent forming Ag⁺, only aldehydes can undergo oxidation because of the presence of an oxidizable hydrogen on their structure in which case ketones don’t have. The resulting organic product of the oxidation of aldehydes is carboxylic acid. This result can also be duplicated using another oxidizing agent that is KMnO₄ in an acidic medium giving off brick red precipitate (the oxidizing agent that is reduced) and the corresponding carboxylic acid, obviously still in this reaction there will be no change to be expected with the ketone. The strength of the oxidizing agents can also have a great impact to an impending reaction because if a weak oxidizing agent is used only the aliphatic aldehyde can react, this is evidently observed in the experiment using the Fehling’s test. The reagents contain copper sulfate in five moles of water with two drops of sulfuric acid and potassium tartrate & sodium hydroxide that allowed the formation of the carboxylic acid CH₃COOH and the precipitate that is brick red, the Cu₂O. Although, the stated reactions above can be handful enough evidence to differ an aldehyde and a ketone there is also the Schiff’s test to add the list, the ketone wont still react and the change in color is still on the side of the aldehydes , this ranges from lavender to pink. The more I have journeyed through these experiments, the more I came to be amaze with the organic compounds I once just often paid less attention to other than the comfort room while reading the labels and ingredients at the back of the shampoos and soaps and conditioners I used. Chemistry, my first love.

V. THEORITICAL BACKGROUND

An aldehyde contains at least one hydrogen attached to the C of a C=O (carbonyl group). A ketone contains two alkyl groups attached to the C of the carbonyl group. The carbon in the carbonyl is sp2 hybridized, has a bond angle of 120o, and is trigonal planar. Aldehydes and ketones have dipole-dipole attractions between molecules, and no hydrogen bonding between molecules. These compounds can hydrogen bond with compounds have O-H or N-H bonds. The melting points and boiling points of aldehydes and ketones are between alkanes and alcohols.

The slightly positive carbon atom in the carbonyl group can be attacked by nucleophiles. A nucleophile is a negatively charged ion (for example, a cyanide ion, CN^–), or a slightly negatively charged part of a molecule (for example, the lone pair on a nitrogen atom in ammonia, NH₃). During a reaction, the carbon-oxygen double bond gets broken. The net effect of all this is that the carbonyl group undergoes addition reactions, often followed by the loss of a water molecule. This gives a reaction known as addition-elimination or condensation. An aldehyde differs from a ketone by having a hydrogen atom attached to the carbonyl group. This makes the aldehydes very easy to oxidize. For example, ethanal, CH3CHO, is very easily oxidized to either ethanoic acid, CH3COOH, or ethanoate ions, CH3COO-. Ketones don’t have that hydrogen atom and are resistant to oxidation. They are only oxidized by powerful oxidizing agents which have the ability to break carbon-carbon bonds.

REFERENCE

Stroker, Stephen H., Exploring General, Organic, and Biological Chemistry, Cenage Learning, 2010