Classification Tests for Hydroxyl- & Carbonyl-Containing Compounds

Dion S. Romero, Jonvi I. Romero, Dennis R. Rueda, Monica Lorraine L. Segismundo, and Catrina D. Tan

Group 8 2A Medical Technology Organic Chemistry Laboratory

ABSTRACTThe experiment conducted was aimed to distinguish whether a compound is a hydroxyl- or carbonyl-containing compound; to differentiate the three types of alcohols; and distinguish aldehydes from ketones. Several tests were used for specific objectives on assigned compounds. The tests were: Solubility of Alcohols in Water, Lucas Test, Chromic Acid Test, 2,4-Dinitrophenylhydrazone Test, Fehling's Test, Tollen's Silver Mirror Test and Iodoform Test. Substances that were used for testing are: n-butyl alcohol, sec-butyl alchol, tert-butyl alcohol, acetaldehyde, n-butylraldehyde, benzaldehyde, acetone, acetophenone, and isopropyl alcohol. The members of the group were able to derive data based on varied observations on substances that were subjected on a specific test, as well using the principles described on the specific procedures for the specific test.

INTRODUCTIONHydroxy- or Carbonyl-containing samples

are subject to specific tests for analysis. Hydroxyl group is the functional group containing OH- when it is a substituent in organic compound, whereas the Carbonyl group is a divalent chemical unit consisting of carbon and oxygen atom connected by double bond. Functional group of alcohols and phenols belong to hydroxyl group, and functional group of aldehydes and ketones are under the carbonyl group. 

There are three types of alcohol, namely primary, secondary and tertiary alcohols. The classification of an alcohol would depend on the position of the -OH in the structure. Primary alcohols are alcohols in which the carbon carrying the -OH group is only attached to one alkyl group. Secondary alcohols are alcohols in which the carbon carrying the -OH group is attached to two alkyl groups. Tertiary alcohols are alcohols in which the carbon carrying the -OH group is attached to three alkyl groups.

Figure1. Difference of the structural formulas of the primary, secondary and tertiary alcohols 

Aldehydes and ketones are organic compounds which incorporate a carbonyl functional group. The atom of this group has two remaining bonds that may be occupied by hydrogen, or alkyl or aryl substituents. If at least one of these substituents is hydrogen, the compound is an aldehyde. If neither is hydrogen the compound is a ketone. 

Figure2. Structural formula of Aldehyde

Figure3. Structural formula of Ketone

Classification tests used in the experiment are tests that categorize a substance into classes, and detect functional groups and other features of the substance. Samples were analyzed through tests involving solubility of alcohols in water, Lucas Test, Chromic Acid Test, 2,4-DNP Test, Fehling's Test, Tollen's Test, and Iodoform Test. Each differentiating test has its own mechanism and principle involved. Lucas Tests differentiates primary, secondary, and tertiary alcohols. Chromic Acid Test is a test for compounds that possess reducing property. 2,4-DNP Test is for the  differentiation of aldehydes and ketones. Fehling's and Tollens' Silver Mirror Tests are tests for aldehydes. Iodoform Test is a test for methyl carbonyl groups.  

The hydroxyl-containing compounds in the experiment are ethanol, n-butyl alcohol, sec-butyl alcohol, tert-butyl alcohol, benzyl alcohol and isopropyl alcohol.  

Ethanol is a clear colorless, volatile flammable liquid which is completely miscible with water and organic solvents.

Figure4. Structural formula of Ethanol 

N-butyl alcohol is a four carbon straight chain alcohol. It is a volatile clear liquid with a strong alcoholic odor, and is miscible with water. It is a highly refractive compound which corrodes with some plastics and rubbers. It is miscible with many organic solvents, and incompatible strong oxidizers.

Figure5. Structural formula of N-butyl alcohol

Sec-butyl alcohol is a flammable colorless liquid that is soluble in 12 parts water and completely miscible in polar organic solvents. 

Figure6. Structural formula of Sec-butyl alcohol

Tert-butyl alcohol is a colorless liquid or white solid, depending on the ambient temperature. It is the simplest tertiarty alcohol and one of the four isomers of butanol. Tert-Butanol is a clear liquid with camphor like odor. It is very soluble in water and miscible with ethanol and diethyl ether. It is unique among the isomers of butanol because it tends to be a solid at room temperature. As a tertiary alcohol, tert-butanol is more stable to oxidation and less reactive than other isomers of butanol.

Figure7. Structural formula of Tert-butyl alcohol

Isopropyl alcohol is a colorless, flammable chemical compound with a strong odor. It is the simplest example of a secondary alcohol, where the alcohol carbon is attached to two other carbons. Being a secondary alcohol, isopropyl alcohol can be oxidized to acetone, which is the corresponding ketone. Isopropyl alcohol dissolves a wide range of non-polar compounds. It is also relatively non-toxic and evaporates quickly.

Figure8. Structural formula of Isopropyl alcohol

Benzyl alcohol is a colorless liquid with a mild pleasant aromatic odor. It is a useful solvent due to its polarity, low toxicity and low vaport pressure. Benzyl alcoholis partially soluble in water and completely miscible in alcohols and diethyl ether. Like most alcohols it reacts with carboxylic acids to form esters.

Figure9. Structural formula of Benzyl alcohol

N-butaraldehyde, benzaldehyde, acetone acetaldehyde, and acetophenone are the carbonyl-containing compounds subjected for testing.  

Benzaldehyde is a colorless liquid aldehyde wiith a characteristic almond odor. It is soluble in ethanol but insoluble in water.

Figure10. Structural formula of Benzaldehyde

Acetaldehyde is a colorless liquid aldehyde, sometimes simply called aldehyde. It is soluble in water and ethanol.

Figure11. Structural formula of Acetaldehyde 

N-butyraldehyde is an aldehyde derivative of butane. It is a colorless flammable liquid that smells like sweaty feet. It is miscible with most organic solvents.

Figure12. Structural formula of N-butyraldehyde

Acetone is colorless mobile, flammable liquid with a characteristic sweetish smell is the simplest example of ketones. Acetone is miscible in water.

Figure13. Structural formula of Acetone 

Acetophenone is the simplest aromatic ketone. This colorless viscouse liquid is a precursor to useful resins and fragrances.

Figure14. Structural formula of Acetophenone

In the said experiment, it is important to distinguish whether a compound is hydroxyl- or carbonyl-containing, differentiate the three types of alcohols, differentiate aldehydes from ketones, and explain the mechanisms involved in the differentiating tests.

EXPERIMENTALA. Samples used

N-butyl alcohol, Sec-butyl alcohol, Tert-butyl alcohol, Acetaldehyde, N-butyraldehyde, Benzaldehyde, Acetone, Acetophenone, Isopropyl Alcohol

B. Procedure The following tests were done in the duration of the experiment:

1. Solubility of Alcohols in WaterFive test tubes were labelled accordingly

and ten drops each of ethanol, n-butyl alcohol, sec-butyl alcohol, tert-butyl alcohol, and benzyl alcohol were placed into the test tubes. 1-ml of water was then added dropwise to the tube containing alcohol, and the mixture was shaken thoroughly after each addition. If cloudiness resulted, 0.25-ml of water at a time was added with vigorous shaking until a homogeneous dispersion results. The total volume of water added was noted. If cloudiness resulted after the addition of 2.0-ml of water, the alcohol is said to be soluble in water. The results were noted.

2. Lucas TestThis test was performed on n-butylalcohol,

sec-butyl alcohol, and tert-butylalcohol. Lucas reagent was prepared by dissolving 16g of anhydrous zinc chloride in 10-ml of concentrated hydrochloric acid. The mixture was then allowed to cool. 50-mg or 2-3 drops of the sample was added to 1-ml of the reagent in a small vial and the mixture was shaken vigorously for a few seconds. The mixture was allowed to stand at room temperature. The rate of formation of the cloudy suspension or the formation of two layers was observed.

Figure15. Reaction Mechanism of Lucas Test

3. Chromic Acid Test (Jones Oxidation)This test was performed on n-butyl

alcohol, sec-butyl alcohol, tert-butyl alcohol, n-butyraldehyde, benzaldehyde, acetone, and acetophenone. 1 drop of liquid or a small

amount of the solid sample was dissolved in 1-ml of acetone in a small vial. 2 drops of 10% aqueous Potassium chromate solution and 5 drops of 6M sulphuric acid were added.

Figure16. Reaction Mechanism of Chromic Acid Test

4. 2,4-dinitrophenylhydrazone (2,4-DNP Test)This test was performed on acetone,

acetaldehyde, benzaldehyde, acetophenone, and n-butyraldehyde. The reagent was prepared by slowly adding a solution of 3g of 2,4-dinitrophenylhydrazine in 15-ml of concentrated sulphuric acid, while stirring to a mixture of 20-ml of water and 70-ml of 95% ethanol. The solution was stirred and filtered. A drop of a liquid sample was placed into a small sample. 5 drops of 95% ethanol was added and shaken. Afterwards, 3 drops of 2,4-DNP was added and if no yellow or orange precipitate formed, the solution was allowed to stand for at least 15 minutes.

Figure17. Reaction Mechanism of 2,4-DNP Test

5. Fehling’s TestThis test was performed on acetaldehyde,

n-butyraldehyde, acetone, benzaldehyde, and acetophenone. Fehling’s reagent was prepared by mixing equal amounts of Fehling’s A and Fehling’s B. 1-ml of freshly prepared Fehling’s reagent was placed into each test tube. 3 drops of the sample to be tested was added into the tube. The tubes were then placed in a beaker of boiling water and changes within 10-15 minutes were observed.

Figure18. Reaction Mechanism of Fehling’s Test

6. Tollens’ Silver Mirror TestThis test was performed on acetaldehyde,

benzaldehyde, acetone, n-butyraldehyde, and acetophenone. The reagent was prepared by adding 2 drops of 5% Sodium hydroxide solution to 2-ml of 5% Silver nitrate solution and mixing thoroughly. Next, only enough 2% ammonium hydroxide was added drop by

drop with stirring to dissolve the precipitate. Adding excess ammonia will cause discrepancies. Four test tubes with 1-ml of freshly prepared Tollens’ reagent were prepared. Two drops each of the samples were then added. The mixture was shaken and allowed to stand for 10 minutes. If no reaction has occurred, the test tube was placed in a beaker of warm water (35-50°C) for 5 minutes. Observations were recorded. It was noted that if Tollens’ reagent is left unused for a period of time, it may form explosive silver. This was avoided by neutralizing unused reagent with a little nitric acid and discarded afterwards.

Figure19. Reaction Mechanism of Tollen’s Test

7. Iodoform TestThis test was performed on acetaldehyde,

acetone, acetophenone, benzaldehyde, and isopropyl alcohol. 2 drops of each sample was placed into its own small vial. 20drops of 5% Sodium hypochlorite was added while shaking to each test tube and then mixed. The formation of a yellow participate was noted.

Figure20. Reaction Mechanism of Iodoform Test

RESULTS AND DISCUSSION

Ethanol, n-butyl alcohol, sec-butyl alcohol, and tert-butyl alcohol are all miscible with water with the exception of benzyl alcohol which exhibited insolubility.

Table1: Solubility of Alcohols in WaterAlcohol Amount of water (in

mL) to produce homogenous dispersion

Solubility in water

Ethanol 1mL MiscibleN-butyl alcohol 2mL MiscibleSec-butyl alcohol 1mL MiscibleTert-butyl alcohol 1mL MiscibleBenzyl alcohol n/a Immiscible

There are different factors affecting solubility. One of which is number of carbon atom wherein the higher the number of carbon atoms, the more insoluble the alcohol is in water. Another factor is the branching of carbon chain in which the more branching present, the more soluble it is. Lastly, the presence of polar functional groups also tends to affect alcohol solubility in water. A compound with polar functional group is more soluble in water. As stated, all alcohols are soluble in water except under C6. Ethanol has two carbon atoms, while the other three all have four carbons since they are all derivatives of the alcohol, butanol. Benzyl alcohol is immiscible with water because it is an aromatic alcohol. Ethanol is the most soluble alcohol. Ethanol exhibits fastest solubility because it has only two carbon atoms as compared to the butanol derivatives having four carbon atoms. Tert-butyl alcohol is the most soluble because it has the most branching substituents present.

Table2: Reaction of sample compounds to Lucas Testsubstance reaction InferenceN-butyl alcohol clear solution miscibleSec-butyl alcohol clear solution miscibleTert-butyl alcohol turbid solution;

formation of 2 layers

immiscible

Lucas Test differentiates primary, secondary, and tertiary alcohols. Reagents used include anhydrous ZnCl2 and HCl. Positive result is based on turbidity and the rate of the reaction. Tertiary alcohols form the second layer in less than a minute. Secondary alcohols require 5-10minutes before formation of second layer while primary alcohols are usually unreactive. Based on Table 2, tert-butyl alcohol immediately formed two layers; hence, it is known to be a tertiary alcohol. Sec-butyl alcohol resulted to a clear solution although theoretically, a secondary alcohol dissolves to give a clear solution then form chlorides (cloudy solution) within five minutes. N-butyl alcohol was unreactive and is considered to be the primary alcohol. The order of reactivity of the alcohols toward Lucas reagent is 3°>2°>1° because the greater the alkyl groups present in a compound, the faster its reaction would be with the Lucas solution.

Table3: Reaction of sample compounds to Chromic Acid Testsubstance ReactionN-butyl alcohol Clear blue solution

Sec-butyl alcohol Clear blue solutionTert-butyl alcohol Clear yellow solutionN-butyraldehyde Clear blue solutionBenzaldehyde Clear green solutionAcetone Clear yellow solutionAcetophenone Clear yellow solution

Chromic Acid Test also known as Dichromate Test or Jones Test, is a test for oxidizables or any compounds that possess reducing property. Reagent used includes 10% Potassium chromate and 6 M sulfuric acid. Primary, secondary alcohols and aldehydes give a positive visible result. Positive result exhibits a green or blue-green solution. Chromic acid test involves redox reaction. Primary, secondary alcohols and aldehydes undergo oxidation and chromium undergoes reduction. Primary, secondary alcohols and aldehydes will reduce the orange-red chromic acid/sulfuric acid reagent to an opaque green or blue suspension of Cr(III) salts in 2-5 seconds. A primary alcohol reacts with chromic acid to yield aldehyde, which is further oxidized to carboxylic acid. A secondary alcohol reacts with chromic acid to yield ketone, which does not oxidize further. A tertiary alcohol is usually unreactive.

Table4: Reaction of sample compounds to 2,4-DNP TestSubstance ReactionAcetaldehyde Yellow precipitateN-butyraldehyde Orange solutionBenzaldehyde Yellow PrecipitateAcetone Yellow PrecipitateAcetophenone Orange Precipitate

2,4-Dinitrophenylhydrazone (2,4-DNP) test is a test for carbonyl groups It gives a positive result for aldehydes and ketones. Its mechanism is condensation or addition and elimination. Positive result is the formation of a red-orange or yellow precipitate. The reaction of 2,4-DNPH with aldehydes and ketones in an acidic solution is a dependable and sensitive test. Most aldehydes and ketones yield dinitrophenylhydrazones that are insoluble solids. The precipitate may be oily at first and become crystalline on standing. A number of ketones, however, give dinitrophenyl hydrazones that are oils. A further difficulty with the test is that certain allyl alcohol derivatives may be oxidized by the reagent to aldehydes and ketones, which then give a positive result. The color of a 2,4-dinitrophenylhydrazone may give an indication as to the structure of the aldehyde or ketone. Dinitrophenylhydrazones of aldehydes or ketones in which the carbonyl group is not conjugated with another functional group are yellow. Conjugation with a carbon-carbon double bond or with a benzene ring shifts the absorption maximum towards the visible and is easily

detected by an examination of the ultraviolet spectrum. However, this shift is also responsible for a change from yellow to orange-red. In general, a yellow dinitrophenylhydrazone may be assumed to be unconjugated.

Table5: Reaction of sample compounds to Fehling’s Testsubstance ReactionAcetaldehyde Crude red precipitateN-butyraldehyde Crude red precipitateBenzaldehyde Turbid solution; oily layerAcetone Clear blue solutionAcetophenone Clear blue solution

Fehling’s Test is a test for aldehydes. Reagents include CuSO4 and NaOH. Positive result is the formation of crude-red precipitate (Cu2O/cuprous oxide). In Table 5, acetaldehyde, n-butyraldehyde, and benzaldehyde exhibited positive result. Acetaldehyde, in particular turned from blue to muddy green then formed a crude red precipitate upon heating. Fehling’s test involves redox reaction wherein aldehyde is oxidized to carboxylic acid and ketones do not undergo oxidation.

Table6: Reaction of sample compounds to Tollens’ Silver Mirror TestSubstance reactionAcetaldehyde silver mirrorN-butyraldhyde silver mirrorBenzaldehyde silver mirrorAcetone clear grayish-black solutionAcetophenone Gel-like precipitate

Tollens’ Silver Mirror test is a test for aldehydes. The preparation of Tollens’ reagent is based on the formation of a silver diamine complex that is water soluble in basic solution. As shown in Table 6, acetaldehyde, n-butyraldehyde, and benzaldehyde exhibited positive result of formation of silver mirror. Acetone resulted to a clear grayish-black solution while acetophenone formed a gel-like precipitate. The test proved that acetaldehyde, n-butyraldehyde, and benzaldehyde are aldehydes.

Tollens’ test involves reduction-oxidation reaction wherein aldehyde is oxidized to carboxylic acid and ketones do not undergo oxidation except alpha-hydroxy ketone. Silver is reduced. Formic acid, hydroxylamine, phenols, and other aromatic amines, will give a positive result. The test often results in a smooth deposit of silver metal on the inner surface of the test tube, hence the name “silver mirror” test. In some cases, however, the metal forms merely as agranular gray or black precipitate. The reaction is autocatalyzed by the silver metal and often involves an induction period of a few minutes.

Table7: Reaction of sample compounds to Iodoform Testsubstance reactionAcetaldehyde Yellow precipitateBenzaldehyde White suspended precipitateAcetone Yellow precipitateAcetophenone Yellow precipitateIsopropyl alcohol Yellow precipitate

Iodoform Test is a test for methylcarbinol and methyl carbonyl groups. Reagents include 10% KI and NaClO. Positive result is exhibited by the formation of yellow crystals or precipitate. Table 7 shows that acetaldehyde, acetone, acetophenone, and isopropyl alcohol exhibited positive result. Compounds with a methyl group next to a carbonyl group give a positive result with the iodoform test. Ethanol and secondary alcohols with a methyl group attached to the same carbon as the ±OH group will also give a positive iodoform test. This is because the iodine oxidizes the alcohols to a carbonyl compound with a methyl group next to the carbonyl group. When a - methyl carbonyl compounds react with iodine in the presence of a base, the hydrogen atoms on the carbon adjacent to the carbonyl group are subsituted by iodine to form tri iodo methyl carbonyl compounds which react with OH-to produce iodoform and carboxylic acid.

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