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Lab Report-Determining Reactions of Aldehydes and Ketones

Updated on April 17, 2016

Abstract

The aim of this experiment was to identify which functional groups the various chemicals and unknown substances belonged to using the different reaction tests. The main purpose was to determine the reactions of Aldehydes and Ketones. Aldehydes and Ketones are organic compounds consisting of the carbonyl functional group. Aldehydes contain their carbonyl group at the end of the carbon chain and are susceptible to oxidation while Ketones contain theirs in the middle of the carbon chain and are resistant to oxidation. Jones’s Test, Tollen’s Reagent and Iodoform Reaction were the three tests used to determine the reactions of aldehydes and ketones. The Chromic Anhydride test caused Aldehydes to turn blue, and Ketones orange. The Tollen’s Reagent test caused the oxidation of aldehydes thus forming a mirror-like image in the test tube rendering it a positive test and the Iodoform reaction produced a yellow precipitate in the test tube which concluded the presence of an aldehyde.

Introduction

The carbon-oxygen double bond is one of the most important functional groups, due to its ubiquity, which are involved in most important biochemistry processes. Reactivity of this group is ruled by the electron imbalance in the πorbitals of the bond between a more electronegative and a carbon atom. This carbon atom is more likely to undergo a nucleophillic attack, especially if the oxygen is protonated. If the carbonyl group has hydrogen’s in the α-position, it can tautomerise to the enol, thus, Keto tautomer can become Enol tautomer.

Aldehydes and Ketones are organic compounds that consist of the carbonyl functional group, C=O. The carbonyl group that consists of one alkyl substituent and one hydrogen is the Aldehyde and those containing two alkyl substituents are called Ketones. These two organic compounds undergo reactions that are related to the carbonyl group, however,

, they can be distinguished due to their “susceptibility to oxidation” (1)

The carbonyl of an aldehyde is always at the end of the carbon chain whereas the carbonyl of a Ketone can lie anywhere within the carbon chain. These two organic compounds are found abundantly in nature. Since the carbonyl group is polar, aldehydes and ketones have a larger dipole moment. In the carbonyl group, the positive carbon atom will be attacked by nucleophiles. (1)

Addition reactions can be undergone by carbonyl groups. Since Aldehydes have the hydrogen atom attached to it, it makes them more susceptible to oxidation, which is the loss of electrons. However, Ketones lack the hydrogen atom in their carbonyl group and are therefore resistant to oxidation. Small Aldehydes and Ketones are easily dissolved in water but as the chain increases in length, its solubility decreases. (1)

In this experiment, the Chromic Anhydride (Jones’s Test), Tollen’s Reagent and the Iodoform reaction were used to test for the presence of aldehydes and ketones. Jones’s Test is an organic reaction for the oxidation of alcohols to carboxylic acids and ketones. This oxidation is very rapid and exothermic with high yields. Tollen’s Reagent was used to determine whether a substance is an aldehyde or ketone, which shows that aldehydes are easily oxidized while ketones are not. The Iodoform reaction would have a positive result if there is a pale yellow precipitate.

The aim of this experiment was to identify the reactions of Aldehydes and Ketones and which functional groups the unknown substances and known chemicals belonged to, using the different reaction tests.

Materials and Method

Chromic Anhydride (Jones’s Test)

Methanol, Isopropanol, Butanol, Ethanol, Tertiary butanol, Benzaldehyde, Cyclohexane and Secondary butanol, including all unknown samples (A and B) were used. One drop of carbonyl compound was added to 1 ml of acetone in a test tube. A drop of the chromic anhydride reagent was then added, and mixed to observe if the test distinguished aldehydes from ketones

Tollen’s Reagent

Two drops of 10% NaOH was added to 10 ml of 0.3 M aqueous silver nitrate and mixed sufficiently. This was added slowly, together with shaking a dilute solution of ammonia until the precipitate just dissolved. An excess of ammonia should be avoided. This is Tollen’s Reagent.

All the samples including the unknowns were used. A drop of carbonyl compound was added to 1 ml of Tollen’s Reagent in a test tube which was then shaken well, and left aside for 10 minutes. The test was positive if there is a formation of a silver mirror on the tube, or a black precipitate of silver. Heat for 5 minutes in a boiling water bath if the test is negative and re-observe.

Tollen’s reagent forms an explosive precipitate if it stands even for a few hours hence it should be disposed of immediately.

Iodoform Reaction

The samples used were acetone, cyclohexanone, benzaldehyde, ethanol, methanol, isopropanol and an unknown A.

Three drops of sample was added to 1 ml of water in a test tube. Three ml of 10% NaOH was then added using a pipette. Iodine-potassium reagent was then added drop wise until a faint color of iodine persists. It was left to stand for 3 minutes. The tube was heated for 5 minutes at 60ºC if no precipitate formed. More iodine was added if the faint color disappeared. The excess iodine was removed by added NaOH drop wise with an equal volume of water together with shaking and allowing it to stand for 10 minutes.

If a yellow precipitate formed, the test is considered positive.

Results

Chromic Anhydride (Jones’s test)

Jones’s reagent reacts with primary, secondary alcohols and aldehydes. Primary alcohols are oxidized to aldehydes while secondary alcohols are oxidized to ketones.

Table 1: Substances used and the observations that were made

Substance

Observation

Methanol

Black precipitate, blue murky solution

Isopropanol

Dark blue precipitate. Milky blue solution

Butanol

Murky blue solution, black precipitate

Ethanol

Milky blue solution. No precipitate

Unknown A

Milky blue solution. Black precipitate

Unknown B

Grey-blue precipitate. Yellow, oily top layer

Tertiary butanol

Orange-yellow solution. No precipitate.

Benzaldehyde

Clear solution, blue precipitate

Cyclohexane

Murky yellow solution

Secondary butanol

Blue, murky solution. Black precipitate

Unknown A could have been a primary alcohol, Secondary Butanol or an aldehyde since the color changed to blue.

Tollen’s Reagent

Tollen’s reagent shows that aldehydes are more easily oxidized while ketones are not. Tollen’s reagent consists of basic aqueous solution that contains silver ions. The reagent oxidizes and aldehyde into a carboxylic acid by the reduction of silver ions into metallic silver and forms a mirror-like image on the test tube. Tollen’s reagent does not oxidize ketones, therefore, the test tube containing a ketone does not form a mirror-like image.

Table 2: list of positive and negative Tollen’s reagent tests

Chemical

Observation + Test

Methanol

No reaction – Negative

Isopropanol

No reaction – Negative

Butanol

No reaction – Negative

Acetaldehyde

Silver – Positive

Acetone

No reaction – Negative

Propanol

No reaction – Negative

Ethanol

No reaction – Negative

Unknown A

No reaction – Negative

Unknown B

Black precipitate, white liquid - Positive

Tertiary butanol

No reaction – Negative

Benzaldehyde

Grey - Negative

Cyclohexanone

Slightly yellow liquid - Negative

Secondary butanol

No reaction – Negative

Proprion aldehyde

Silver - Positive

Acetaldehyde formed a precipitate which was silver before it was heated. Unknown B formed a mirror-like image 2 minutes after heating. Other chemicals formed no precipitate.

Iodoform reaction

Table 3: The results obtained during the Iodofrom reaction

Chemical

Observation

Methanol

No reaction

Isopropanol

Cloudy yellow. No precipitate

Acetone

No reaction. Should have precipitated

Ethanol

Cloudy yellow. No precipitate

Unknown A

Cloudy yellow. No precipitate

Unknown B

Cloudy yellow. No precipitate

Benzaldehyde

Yellow precipitate

Cyclohexanone

No reaction. Should have precipitated

Discussion

In order to be able to identify an organic compound, it needs to show the same physical and chemical properties as the known compound.

Chromic Anhydride (Jones’s Test)

The Jones reagent is a mixture of chromic anhydride and dilute sulfuric acid (CrO3 + H2SO4 + H2O) in acetone. It is used in the oxidation of secondary alcohols that do not contain acid sensitive groups, to corresponding ketones. Since the oxidation is practically instantaneous, it was encouraged to investigate its usefulness as a qualitative test to distinguish tertiary alcohols from primary or secondary alcohols. It appears to be ideally suited for this purpose.

In this experiment, Aldehydes turned blue in color as they are more susceptible to oxidation under Jones’s Test conditions, and can undergo nucleophillic reactions. They only have one alkyl group that can donate electrons whereas Ketones turned orange in color as they are less reactive and susceptible to nucleophillic reactions as they contain two alkyl substituents.

Unknown A could have been a primary alcohol, Secondary Butanol or an aldehyde since the color changed to blue. Tertiary Butanol changed to orange, thus, rendering it a Ketone.

Tollen’s Reagent

Tollen's reagent is a chemical reagent used to determine the presence of an aldehyde or α-hydroxyl ketone functional groups. The reagent consists of a solution of silver nitrate and ammonia. A positive test with Tollen's reagent is indicated by the precipitation of elemental silver, often producing a characteristic "silver mirror" on the inner surface of the reaction vessel.

Since Aldehydes have the presence of hydrogen atoms, it makes it easier for it to be oxidized, hence forming the mirror-like image on the test tubes. This can be seem in Acetaldehyde as well as Proprion aldehyde as the have formed the silver substance. These chemicals have therefore tested positive to Tollen’s Reagent. The majority of the chemicals that tested negative are said to be Ketones as no reaction had occurred. There was no change to the colorless solution due to Ketones being less susceptible to oxidation.

Iodoform Reaction

A positive result of this test would be indicated by a pale yellow precipitate in the test tube. Chemicals that tested positive to this test include Benzaldehyde. Isopropanol, ethanol and unknown substance A and B produced a cloudy yellow solution with no precipitate and therefore test negative.

The presence of hydroxide ions is important for the reaction to happen - they take part in the mechanism for the reaction. The methyl group of the ketone is then removed from the molecule to produce iodoform (CHI3).


References

  1. Lawrence, N. J. (1937). Journal of the Chemical Society, 59, 760-761
  2. William, E.B, Gabriella, S. Louise, Z. Yang and Huges, D.E. (December 2011). Journal of chemical Research. 55, 675-677

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    • Yaseeny15 profile image
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      Yaseen Essack 5 months ago from South Africa

      I am glad it helped kiokote. I have some new reports that I will be posting. If u need anything, please feel free to contact me.

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      kiokote 5 months ago

      i happy when i get my problems solved here

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      Yaseen Essack 6 months ago from South Africa

      Thank You Nyonzwe

      I'm glad it helped. Please feel free to contact me if you have any questions.

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      NYONZWE 6 months ago

      MATERIAL HAS,REALLY HELPED ME,THANKS ALOT