Analysis of Oxygen-Bearing Compounds

Macababbad, Rafael; Manguerra, Triccie; Mateo, Daphnee; *Oquendo, Francine RaeDepartment of Psychology, University of Santo Tomas, Manila, Philippines

Abstract

The Unknown sample in the experiment can be tested to yield results such as 1° (primary), 2°

(secondary), 3° (tertiary) alcohols, Aldehydes and Ketones. Tests such as the dichromate test,

Tollen’s test, Lucas test, DNPH test and Iodoform test would be very useful in determining the

type of compound that the unknown sample belongs to. In the experiment, the unknown sample

underwent series of testing to identify what property of alcohol it belonged to or if it is an

Aldehyde or a ketone and the result was that it was an aldehyde. It went through the

Dichromate test, Tollen’s test and the Lucas test.

Introduction

In analyzing the oxygen bearing organic compounds, there are several tests that may be performed to distinguish among the different classes of alcohols, aldehydes, and ketones. All these tests exploit differences in the type and degree of reactivity of each of the functional groups. You will check the reaction characteristics of five different functional groups (primary alcohols, secondary alcohols, tertiary alcohols, aldehydes, and ketones), and an unknown, and, based upon reaction characteristics, determine the functional group of the unknown.

Identifying the structure of each of the functional groups is the first step.

• Primary alcohols (1° alcohol) have the group –CH2OH, where the carbon atom with the alcoholic hydroxyl group has at least two additional hydrogens attached to that carbon. The alcoholic hydroxyl group is attached to a primary carbon. A rimary alcohol (1°) as one where the carbon with the hydroxyl group is attached to one R group (R stands for any other organic chain or group).

• Secondary alcohols (2° alcohol) have the group –CHOH, where the carbon atom with the alcoholic hydroxyl group has only one additional H atom attached to it. There are two R groups (R stands for any other organic chain or group), and the alcoholic hydroxyl group is attached to a secondary carbon. So, a secondary alcohol (2°) is one where the carbon atom with the hydroxyl group is also attached to two R groups.

• Tertiary alcohols (3° alcohol) have the group –COH, where the carbon atom with the alcoholic hydroxyl group has no additional H atoms attached to it. There are three R groups (R stands for any other organic chain or group), and the alcoholic hydroxyl group is attached to a tertiary carbon. So, a tertiary alcohol (3°) is one where the carbon atom with the hydroxyl group is also attached to three R groups.

• Aldehydes and Ketones are characterized by a functional group called a carbonyl group – a carbon atom double-bonded to an oxygen atom.

The difference between aldehydes and ketones is in the groups attached to the carbonyl carbon atom. In the case of an aldehyde, there is always at least one H atom attached to the carbonyl carbon atom. An aldehyde has one R group attached. In the case of ketones, there are no H atoms attached to the carbonyl carbon. The ketone has two R groups attached.

Now that the structures have been identified, there are several chemical tests that are used to distinguish these functional groups.

A. Differences in oxidation: Several different oxidizing agents may be used to cause the oxidation of molecules. Potassium permanganate (KMnO4) or potassium dichromate (K2Cr2O7) or even atmospheric oxygen (O2) are oxidizing agents. The symbol [O] is commonly used to mean any oxidizing agent. It’s the action of the oxidizing agent, not its identity that is important here.

One way to look at oxidation is that oxidation is the reaction that increases the number of C-O bonds and/or decreases the number of C-H bonds. Therefore, reduction is the opposite and is a reaction that decreases the number of C-O bonds and/or increases the number of C-H bonds.

a. A primary alcohol oxidizes readily, first to an aldehyde, then to a carboxylic acid. These two oxidation steps make sense because the primary alcohol functional group has two C-H bonds that can be broken.

b. A secondary alcohol only has only one C-H bond that can be broken, so it can only oxidize once, to a ketone.

c. A tertiary alcohol has no C-H bonds that can be broken, so it is NOT oxidized, no matter how strong the oxidizing agent may be.

d. From this information on alcohols, it is clear that aldehydes will oxidize to carboxylic acids. An aldehyde has one C-H bond that can be broken. Think back to what happens to 1° alcohols. The aldehyde produced by the oxidation of a primary alcohol can be again oxidized to the carboxylic acid, and therefore, any aldehyde can be oxidized to a carboxylic acid.

e. Any ketone, just like the product of the oxidation of a 2° alcohol, has no C-H bonds that can be broken, so ketones cannot be oxidized.

This lab experiment will use a solution of potassium dichromate (K2Cr2O7) as the oxidizing agent. The starting solution of potassium dichromate is yellow-orange, but if any oxidation takes place, the solution will turn blue-green. This color change is due to the Cr+6 ion (orange in K2Cr2O7) being reduced to the Cr+3 ion (blue green).

B. The Lucas test for alcohols: This test distinguishes among the three type of alcohols (1°, 2°, and 3°), by the speed with which they react. The reaction is a replacement reaction, where a Cl from HCl (mixed with ZnCl2) replaces the hydroxyl group on the alcohol. It should be noted that, because aldehydes and ketones do not have a hydroxyl group to replace with Cl, they would both give a negative Lucas test.

Surprisingly enough, 3° alcohols react very fast with the Lucas reagent (HCl mixed with ZnCl2), and will turn cloudy almost immediately. Secondary alcohols do react, but more slowly and must be heated in a water bath in order to react and turn cloudy. Primary alcohols react so slowly (even in a warm water bath) that NO REACTION is observable. A yellow or yellow-orange color without any precipitate is a negative result.

C. 2,4-Dinitrophenylhydrazine test: This test uses an organic reactant, 2,4-dinitrophenylhydrazine, to distinguish the carbonyl compounds (aldehydes and ketones) from the non-carbonyl compounds (the alcohols). The 2,4-dinitrophenylhydrazine reagent is a translucent yellow solution, but in the presence of a carbonyl compound, a yellow-colored precipitate will form. In the presence of an alcohol, the solution will remain translucent yellow, forming no precipitate.

The tests that are included to come up with such results are related with one another like that of the Dichromate test, and if results would be positive it would be followed by the Tollen’s test to yield an aldehyde if positive for mirror coating. The second set of procedure would again start with The Dichromate test, followed by the Tollen’s test and if results would yield negative, the Lucas test is performed to get either primary alcohol if it became turbid or secondary alcohols as its result if it did not turn turbid. The Lucas test in alcohols is a test to differentiate between primary, secondary, and tertiary alcohols. It is based on the difference in reactivity of the three classes of alcohols with hydrogen halides. The last set of interconnected tests will start again with the Dichromate test and if the results would come out negative, it will be followed by the DNPH (2,4-Dinitrophenylhydrazine) can be used to qualitatively detect the carbonyl functionality of a

The iodoform test or iodoform reaction is a qualitative chemical test for the detection of ketones

and aldehydes carrying an alpha methyl group. The reagents are iodine and sodium hydroxide.

Results and Discussion

Chemical

Tests

Primary (1°)

Alcohol

Secondary

(2°) Alcohol

Tertiary

(3°)

alcohol

Aldehydes Ketones Unknown

Dichromat

e

Test

Positive

result. Blue

green

Positive

result. Blue

green

Negative

result.

Orange

Positive

result. Blue

green in

Negative

result.

Clear

Positive

result. Blue

green

precipitate precipitate liquid. color. Orange

liquid.

precipitate

Tollen’s

Test

Negative

result. Clear

transparent

solution.

Negative

result. Gray

solution

Positive

result. Silver

mirror

formed.

Positive

result. Clear

transparent

solution.

Lucas

Test

Cloudy

solution

Clear

transparent

solution.

Cloudy

solution.

The unknown is Primary (1°) Alcohol. It manifested all the reactions of that of a Primary (1°) Alcohol solution. The experiment started off with oxidizing all the samples. In oxidizing, there are a lot of oxidizing agents that can be used. Potassium permanganate (KMnO4) or potassium dichromate (K2Cr2O7) or even atmospheric oxygen (O2) are oxidizing agents. Oxidation is the reaction that increases the number of C-O bonds and/or decreases the number of C-H bonds. Therefore, reduction is the opposite and is a reaction that decreases the number of C-O bonds and/or increases the number of C-H bonds.

Experimental

Preparation of the samples for the dichromate test. Six test tubes were filled with 8 drops of each of the samples (primary, secondary and tertiary alcohols, aldehyde, ketone and the unknown). Next, two drops of 10% K2Cr2O2 (Potassium Dichromate) and five drops of 6M H2SO4

(Sulfuric acid) were added to each of the test tubes.