Classification Tests for Carbonyl- and Hydroxyl- Containing Compounds

La Rosa, J. A., Lee, S. Q., Mangahas, N. K., Manota, G. E., Matuloy, L. S.University of Santo Tomas

Faculty Of Pharmacy2A-Biochemistry

Abstract

Classification of Hydroxyl and Carbonyl containing compounds was done through different tests. The samples were n-butyl alcohol, sec-butyl alcohol, tert-butyl alcohol, acetaldehyde, n-butyraldehyde, benzaldehyde, acetone, acetophenone, and isopropyl alcohol. Each of the samples were tested through solubility test of alcohols in water, Lucas test, Chromic acid test or the Jones oxidation test, 2,4-Dinitrophenylhydrazone (2,4-DNP) test, Fehling’s test, Tollens’ silver mirror test, and Iodoform test. Solubility test of alcohols in water was used to determine the amount of water needed for it to produce a homogenous dispersion and determine whether what kind of alcohol is soluble and insoluble in water. Lucas test was used to differentiate primary, secondary, and tertiary alcohols. Chromic acid test was used to determine if the sample is oxidized and can also be used to differentiate ketones from aldehydes. 2, 4-Dinitrophenylhydrazone test was used to differentiate aldehydes and ketones. Fehling’s test and Tollens’ silver mirror test was used to determine whether the sample is an aldehyde. Lastly, Iodoform test was used for classification of methyl ketones.

Introduction

Alcohols are organic compounds that may be considered derivatives of water in which one of the hydrogen atoms of water molecule (H-O-H) has been replaced by an alkyl or substituted alkyl group (Jaism& Mohammad, 2012). Therefore, properties of alcohols may be related to properties of both water and hydrocarbons (Jaism& Mohammad, 2012). The alkyl group could be primary, secondary, or tertiary, and may be open chain or cyclic. Accordingly, alcohols may be defined as organic compounds that contain hydroxyl groups attached to alkyl, substituted alkyl, or cyclic alkyl group (Vollhardt, 2007).

Figure 1. Structural formula of Alcoholhttp://media-cache-ec0.pinimg.com/736x/39/23/bd/3923bde41273f511199d81a9f0431a6a.jpg

All the three classes of alcohol ( 1°, 2°, and 3°) are capable of hydrogen-bonding in the pure state or to other alcohol molecules because the –OH functional group is always present

(Clayden& Bingham, 2008). Alcohols also have the ability to hydrogen bond with water; however, their solubility decreases as the size of the R group and their hydrocarbon-like character increases (Clayden & Bingham, 2008).

Figure 2. Classes of Alcoholhttp://www.esu.edu/~scady/Experiments/Alcohols(summer).pdf

Aldehydes and ketones are classes of organic compounds in which oxygen is covalently joined to a carbonyl group (Tojo&Fernadez, 2006). Ketones differ from aldehydes by a replacement of hydrogen with an R group. Consequently, aldehyde functional groups are always found at the end of hydrocarbon chains (Tojo & Fernadez, 2006).

Figure 3. Structural formula of Aldehydehttp://media-cache-ec0.pinimg.com/736x/e3/ef/e7/e3efe7ea73142727b810ae434661a396.jpg

Figure 4. Structural formula of Ketonehttp://media-cache-ak0.pinimg.com/736x/0f/dc/d4/0fdcd47ceb12b048fa82898a70099180.jpg

There are different types of differentiating tests to determine whether its alcohol, aldehyde, and ketones; the commonly used tests are Solubility test, Lucas test, Chromic Acid test (Jones Oxidation), 2,4-dinitophenylhydrazine (2,4-DNP) test, Fehling’s test, and Tollens’ Silver Mirror test (Sinton, 2009).

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 (Lehman, 2004). The differing reactivity reflects the differing ease of formation of the corresponding carbocations. Tertiary carbocations are far more stable than secondary carbocations, and primary carbocations are the least stable (Lehman, 2004). The time taken for turbidity to appear is a measure of the reactivity of the class of alcohol, and this time difference is used to differentiate between the three classes of alcohols (Mariappan, 2009). No visible reaction at room temperature and cloudy only on heating means it is a primary alcohol, when the solution turns cloudy in 3–5 minutes means it is a secondary alcohol, and if solution turns cloudy immediately, and/or phases separate it means that it is a tertiary alcohol (Mariappan, 2009).

 Figure 5. Lucas test reactionhttp://www.esu.edu/~scady/Experiments/Alcohols(summer).pdf

The Jones oxidation (also known as Chromic Acid Test) is an organic reaction for the oxidation of primary and secondary alcohols to carboxylic acids and ketones, respectively. Similar to alcohols, aldehydes and ketones differ in their ability to be oxidized. As noted previously, aldehydes can be oxidized to carboxylic acids. Ketones, which lack hydrogen bonded to the carbonyl group, cannot be oxidized. The following reactions below show the expected products aldehydes, and ketones with the Jones reagent (Pellegrini, 2000).

Figure 6. Jones oxidation reactionhttp://upload.wikimedia.org/wikipedia/commons/thumb/a/ab/Jones_Oxidation_Scheme.png/640px-Jones_Oxidation_Scheme.png

Brady’s test or 2,4-dinitophenylhydrazine (2,4-DNP) test is used to qualitatively detect the carbonyl functionality of a ketone or aldehyde functional group. A positive test is signaled by a yellow, orange or red precipitate (known as a dinitrophenylhydrazone.) If the carbonyl compound is aromatic, then the precipitate will be red; if aliphatic, then the precipitate will have a more yellow color.Aldehydes and ketones undergo a condensation reaction with 2,4-dinitrophenylhydrazine to produce yellow to

orange precipitates as products. Alcohols do not undergo this reaction (Simek & Wade, 2003).

Figure 7. 2,4-DNP test reactionhttp://www.esu.edu/~scady/Experiments/Alcohols(summer).pdf

Fehling's test is a chemical test used to differentiate between watersoluble carbohydrate and ketone functional groups, and as a testfor monosaccharides. Fehling's can be used to determine whether a carbonyl-containing compound is an aldehyde or a ketone. The bistartratocuprate (II) complex in Fehling's solution is an oxidizing agent and the active reagent in the test. The compound to be tested is added to the Fehling's solution and the mixture is heated. Aldehydes are oxidized, giving a positive result (precipitation of red copper (I) oxide), but ketones do not react, unless they are alpha-hydroxy-ketones (Jones & Gingrich, 2009).

Figure 8. Fehling’s test reactionhttp://upload.wikimedia.org/wikipedia/commons/thumb/6/65/Fehling_orz.svg/1024px-Fehling_orz.svg.png

Tollens' test is used to determine whether a known carbonyl-containing compound is an aldehyde or alpha-hydroxy ketone. It is usually ammoniacal silver nitrate, but can also be other mixtures, as long as aqueous diammine silver(I) complex is present. A positive test with Tollens' reagent results in elemental silver precipitating out of solution, occasionally onto the inner surface of the reaction vessel, producing a characteristic and memorable "silver mirror" on the inner vessel surface. Aldehydes will be positive in Tollens' test and a mirror-like material will be formed (Jones, 2000).

Iodoform test or haloform reaction is a chemical reaction where a haloform (CHX3, where X is a halogen) is produced by the exhaustive halogenation of a methyl ketone (a molecule containing the R–CO–CH3 group) in the presence of a base.[1] R may be H, alkyl or aryl. The reaction can be used to produce chloroform (CHCl3), bromoform (CHBr3), or iodoform(CHI3). This reaction was traditionally used to determine the presence of a secondary alcohol oxidizable to a methyl ketone. When iodine and sodium hydroxide are used as the reagents, a positive reaction gives iodoform. Iodoform (CHI3) is a pale-yellow substance. Due to its high molar mass caused by the three iodine atoms, it is solid at room temperature (cf. chloroform and bromoform). It is insoluble in water and has an antiseptic smell. A visible precipitate of this compound will form from a sample only when either a methyl ketone, ethanal, ethanol, or a methyl secondary alcohol is present (Martin & Gilbert, 2011).

Figure 9. Iodoform test reactionhttp://upload.wikimedia.org/wikipedia/commons/3/31/Iodoform_test_alcohols.pn

Materials

The materials needed for this experiment were Lucas reagent, chromic acid reagent, 95% ethanol, Fehling’s A and B, Tollens’ reagent, 5% NaOCl solution, iodoform test reagent, 2-4 dinitrophenylhydrazine, Pasteur pipette, test tubes, vials, beaker. The sample compounds used were ethanol, n-butyl alcohol, sec-butyl alcohol, tert-butyl alcohol, benzyl alcohol, n-butyraldehyde, acetone, acetophenone, isopropyl alcohol, and acetaldehyde

Methods

A. Solubility of Alcohols in Water

Five test tubes were labeled A through E (A: ethanol, B: n-butyl alcohol, C: sec-butyl alcohol, D: tert-butyl alcohol, E: benzyl alcohol). With the aid of a Pasteur pipette, 10 drops of each of

the alcohols was placed into the appropriate test tube. One mL of water was added to the test tube containing alcohol, the mixture was thoroughly shakenfor each addition until a homogenous dispersion resulted. The total volume of water was noted. In cases where no cloudiness resulted after the 2.0 mL water was added, the alcohol was considered soluble in water. The results were noted.

B. Lucas Test

The reagent was prepared by dissolving 16 g of anhydrous zinc chloride in a 10 mL of concentrated HCL. Fifty milligrams (2-3 drops) of sample was added to 1 mL of the reagent in a small vial. It was capped and was shaken vigorously for a few seconds. This test was performed on n-butyl alcohol, sec-butyl alcohol, and tert-butyl alcohol as well. The rate of formation of the cloudy suspension or the formation of 2 layers was observed.

C. Chromic Acid Test (Jones Oxidation)

The reagent was prepared by dissolving 20 g of chromium trioxide (CrO3) in 60 mL of cold water in a beaker and adding of 20 mL of concentrated sulfuric acid to the solution. One drop of liquid or a small amount of the solid sample was dissolved in 1 mL of acetone in a small vial. Five drops of chromic acid reagent was added. The test tubes were placed in a 60ºC water bath for 5 minutes. The color of each solution was noted.

D. 2,4-dinitophenylhydrazine (2,4-DNP) Test

The reagent was prepared by adding 3 g if 2,4-dinitophenylhydrazine in 15 mL of concentrated H2SO4 to a mixture of 20 mL of water and 70 mL of 95% ethanol, and then it was stirred and filtered. A drop of liquid sample was placed into a small test tube. Five drops of 95% ethanol was added. Three drops of 2,4-dinitrophenylhydrazine was added. In cases where no yellow or orange-red precipitate formed the solution was allowed to stand for at least 15 minutes. This test was performed on acetone, acetaldehyde, n-butyraldehyde,

benzaldehyde, and acetophenone. The results were described.

E. Fehling’s Test

Fehling’s A was prepared by dissolving 7 g of hydrated copper (II) sulfate in 100 mL of water, while the Fehling’s B was prepared by dissolving 35 g of potassium sodium tartrate and 10 g of NaOH in 100 mL of water. One milliliter of freshly prepared Fehling’s reagent (made by mixing equal amount of Fehling’s A and Fehling’s B) was placed into each test tube. Three drops was added to the sample that was tested. The test tubes were placed in a beaker of boiling water and the changes which occurred within 10-15 minutes were observed. This test was performed on acetaldehyde, n-butyraldehyde, acetone, benzaldehyde, and acetophenone as well.

F. Tollens’ Silver Mirror Test

The reagent was prepared by adding 2 drops of 5% sodium hydroxide solution to 2 mL of 5% silver nitrate solution. Then only enough 2% ammonium hydroxide was added to dissolve the precipitate. No excess ammonia was added because it will jeopardize the results. Four test tubes with 1 mL of freshly prepared Tollens’ reagent were prepared. Two drops each of the sample was added separately: acetaldehyde, benzaldehyde, acetone, n-butyraldehyde, and acetophenone. It was allowed to stand for 10 minutes. In cases where there is no reaction occurred, the test tubes were placed in a beaker of warm water (30-50 ºC) for 5 minutes. The observations were recorded.

G. Iodoform Test

Two drops of each sample (acetaldehyde, acetone, acetophenone, benzaldehyde, and isopropyl alcohol) was placed into its own vial. Twenty drops of 10% KI solution was added. Twenty drops of fresh chlorine bleach (5% sodium hypochlorite) was added to each test tube, and then mixed. Yellow precipitate formation was noted.

Results and Discussion

For the test for solubility of alcohols in water, turbidity of the solution was to be observed. Cloudiness of the solution indicated in solubility of that specific alcohol to water. The amount of water needed to produce homogenous dispersion was also observed. Table 1 shows the data gathered from the test.

Table 1. Solubility of Alcohols in Water

Alcohol Amount of water (mL) needed to produce a homogenous dispersion

Solubility to water

ethanol 1 mL solublen-butyl alcohol 1.50 mL soluble

Sec-butyl alcohol

1 mL soluble

tert –butyl alcohol

1 mL soluble

benzyl alcohol 2 mL insoluble

As indicated in the table, only benzyl alcohol was insoluble in water, while ethanol, n-butyl alcohol, sec-butyl alcohol and tert-butyl alcohol were all soluble in water. This follows the principle “like dissolves like” and therefore, it can be said that the alcohols that were soluble in water are polar compounds since water is polar. Of the alcohols that were soluble in water, ethanol, sec-butyl alcohol and tert-butyl alcohol all required only 1 mL of water to be added in order to be considered soluble. This indicates that there are certain factors affecting solubility. One of these is the presence of number of carbon atoms. The lower the number of carbon atoms present, the more soluble or more miscible a substance is. Branching of carbon chains also affects solubility directly proportional. The more branching present, the more soluble a compound is. This is only true for organic compounds that have the same number of carbon atoms present.

The Lucas test differentiated 1˚, 2˚ and3˚ alcohols. Alkyl chloride formation was observed and caused turbidity or cloudiness. The rate of reaction was also observed. Table 2 presents the results of the Lucas test.

Table 2. Lucas test

Sample Reaction Observedn-butyl alcohol colorless

sec-butyl alcohol slightly turbidtert-butyl alcohol turbid

According to the table above, n-butyl alcohol was soluble in Lucas reagent while sec-butyl alcohol and tert-butyl alcohol were observed to have a formation of cloudy layer. Tert-butyl alcohol took the shortest time to form the layer while sec-butyl alcohol took the longest time. The reaction mechanism involved in the Lucas test is based on SN1 reaction, which depends on the formation of stable carbocations. Reactivity of alcohols in SN1 reaction is 3˚ > 2˚> 1˚. 3˚ alcohols formed the second layer in less than a minute. Secondary alcohols required 5-10 minutes while 1˚alcohols were usually unreactive. The presence of ZnCl2, a good Lewis acid, made the reaction mixture even more acidic; thus, it enhanced the formation of carbocations.

The Chromic Acid test (JonesOxidation) tested for oxidizable or any compounds that possess reducing property (has alpha acidic hydrogen). Table 3 shows the results gathered from the said test.

Table 3. Chromic Acid test (Jones Oxidation)

Sample Reaction Observedn-butyl alcohol blue-green solution

sec-butyl alcohol blue-green solutiontert -butyl alcohol orange solutionn-butyraldehyde blue-green solution

benzaldehyde blue-green solutionacetone orange solution

acetophenone orange solution

According to Table 3, n-butyl alcohol, sec-butyl alcohol, n-butyraldehyde and bezaldehyde gave a positive result of blue-green solution while tert-butyl alcohol, acetone and acetophenone gave a result of orange solution. Chromic Acid test orJones Oxidation involved reduction-oxidation or redox reaction. 1˚ and 2˚ alcohols and aldehydes underwent oxidation and chromium underwent reduction from Cr6+ to Cr3+. 1˚ and 2˚ alcohols and aldehydes reduced the orange-red chromic acid/sulphuric acidreagent to an opaque green or blue suspension of Cr (III) salts in 2-5 seconds. 1˚ alcohols reacted with chromic acid to yield aldehydes, which are further oxidized to carboxylic acids. 2˚ alcohols reacted with chromic acid to yield ketones, which do not oxidize further. 3˚ alcohols were usually unreactive and aldehydes were oxidized to carboxylic acids. The 2,4 Dinitrophenylhydrazone (2,4-DNP) test detected the presence of carbonyl groups and tests positive for aldehydes and ketones. Table 4 shows the results from the test.

Table 4. 2,4-Ditrophenylhydrazone (2,4-DNP) test

Sample Reaction observedacetaldehyde yellow ppt

n-butyraldehyde yellow pptbenzaldehyde yellow ppt

acetone yellow pptacetophenone red-orange ppt

As indicated in the table, only acetophenone gave a result of red-orange precipitate while the rest of the samples gave a result of yellow precipitate. A result of red-orange precipitate indicated the presence of conjugated carbonyl compounds while a result of yellow precipitate indicated the presence of unconjugated carbonyl compounds. The reaction of 2,4-DNPH with aldehydes and ketones in an acidic solution is a

dependable and sensitive test. Its reaction mechanism involved condensation or nucleophilic addition of NH2 to C=O and elimination of H2O. Some high molecular weight ketones may fail to react or may yield oils. Most aromatic aldehydes and ketones produce red dinitrophenylhydrazone while many non aromatic aldehydes and ketones produced yellow products.

Fehling’s test was another differentiating test for aldehydes and ketones. In this test, aldehydes gave a positive result of brick-red precipitate while ketones did not produce any reaction. Table 5 presents the results of the test.

Table 5. Fehling’s testSample Reaction observed

acetaldehyde brick-red pptn-butyraldehyde brick-red ppt

benzaldehyde brick-red pptacetone blue solution

acetophenone blue solution

As shown in the given table, only acetone and acetophenone did not react to forma precipitate while the rest gave a positive result of brick-red precipitate. Fehling’s test involved reduction-oxidation or redox reaction. Aldehydes were oxidized to carboxylic acidswhile ketones did not undergo oxidation. Inhere, copper was reduced from Cu2+ to Cu1+.

Tollens’ Silver Mirror test differentiated aldehydes from ketones wherein aldehydes were expected to be oxidized while ketones did not undergo any oxidation. Table 6 shows the results from the said test.

Table 6. Tollens’ Silver Mirror test

Sample Result observedacetaldehyde silver mirror

n-butyraldehyde flesh solutionbenzaldehyde light yellow solution

with globulesacetone dark-gray solution

acetophenone turbid gray solution

According to Table 6, only acetaldehyde formed a silver mirror. The samples n-butyraldehyde and benzaldehyde, although they are aldehydes, did not form any silver mirror. The ketones acetone and acetophenone formed dark-gray solution and turbid gray solution, respectively. The preparation of Tollens’ reagent was based on the formation of a silver diamine complex that is water soluble in basic solution. The Tollens’ Silver Mirror test involved reduction-oxidation or redox reaction. Aldehydes were oxidized to carboxylic acids while ketones did not undergo oxidation except alpha-hydroxyketone. Silver was reduced from Ag1+ to Ag0

Iodoform test was used to detect the presence of methyl carbinol (2˚ alcohol with adjacent methyl group) and methyl carbonylgroups. Table 7 shows the results from the said test.

Table 7. Iodoform test

Sample Reaction observed acetaldehyde yellow ppt

n-butyraldehyde no reactionbenzaldehyde red ppt with globules

acetone yellow pptacetophenone yellow ppt

isopropyl alcohol yellow crystal ppt

According to the give table, acetaldehyde, acetone and acetophenone gave a result of yellow precipitate. Benzaldehyde gave a result of red precipitate with globules while isopropyl alcohol gave a result of yellow crystal precipitates. No reaction was observed from n- butyraldehyde. In this test, yellow crystals or precipitate gave a positive result. An alkaline solution of sodium hypoiodite, formed from sodium hydroxide and iodine, converted acetaldehyde and aliphatic methyl ketones into iodoform (haloform reaction). Since the reagent was also an oxidizing agent, alcohols which are readily oxidized to acetaldehydes or methyl ketones also gave a positive reaction. The mechanism of iodoform synthesis occurred through a series of enolate anions, which are

iodinated; hydroxide displaced the Cl3- anion through an addition/elimination pathway.

References

Clayden, J., & Bingham, M. J. (2008). Alcohols. Stuttgart: Thieme.

Lehman, J. W. (2004). The student's lab companion: laboratory techniques for organic chemistry. Upper Saddle River, N.J.: Prentice-Hall.

Jaism, A. M., & Mohammad, D. H., (2012). Practical Organic Chemistry. 1st ed. Baghdad: Baghdad University Publishing.

Jones, M. (2000). Organic chemistry (2nd ed.). New York: W.W. Norton.

Jones, M., & Gingrich, H. L. (2005). Study guide/solutions manual for Jones's Organic Chemistry (3rd ed.). New York: W.W. Norton.

Mariappan, M. (2009). Organic Chemistry Lab Manual. Dubuque, IA: Kendall Hunt.

Martin, S. F., & Gilbert, J. C. (2011). Organic Chemistry Lab Experiments: Miniscale and Microscale (5th ed., Int'l ed.). Boston, MA.: Brooks/Cole, Cengage Learning.

Pellegrini, F. (2000). Organic chemistry II. Foster City, CA: IDG Books Worldwide.Simek, J. W., & Wade, L. G. (2003). Organic Chemistry: Solutions Manual (5th ed.). New York: Prentice Hall.

Sinton, M. (2009). Organic chemistry. Dubuque, Iowa: Kendall Hunt Pub. Co..

Tojo, G., & Fernandez, M. (2006). Oxidation of alcohols to aldehydes and ketones: a guide to current common practice. New York, NY: Springer.Vollhardt, K. C., & Neil E. S., (2007). Organic Chemistry. 5th ed. New York: W.H. Freeman.

Appendix

http://www.esu.edu/~scady/Experiments/Alcohols(summer).pdfSchematic diagram of the differentiating tests

http://upload.wikimedia.org/wikipedia/commons/thumb/5/53/Butan-1-ol_Skelett.svg/1280px-Butan-1-ol_Skelett.svg.pngn-butyl alcohol

http://upload.wikimedia.org/wikipedia/commons/thumb/d/d4/Ethanol2.svg/1280px-Ethanol2.svg.pngEthanol

http://

upload.wikimedia.org/wikipedia/commons/thumb/e/e6/2-Butanol_Structural_Formula_V.1.svg/1280px-2-Butanol_Structural_Formula_V.1.svg.pngSec-butyl alcohol

http://

upload.wikimedia.org/wikipedia/commons/thumb/4/4a/Tert-Butanol_Structural_Formula_V.1.svg/785px-Tert-Butanol_Structural_Formula_V.1.svg.pngTert-butyl alcohol

http://upload.wikimedia.org/wikipedia/commons/5/5a/Benzyl-alcohol-2D-skeletal.pngBenzyl alcohol

http://

u

pload.wikimedia.org/wikipedia/commons/thumb/1/1c/Butyraldehyde_200.svg/1280px-Butyraldehyde_200.svg.pngn-butyraldehyde

http:// upload.wikimedia.org/wikipedia/commons/

thumb/2/21/Aceton.svg/1280px-

Aceton.svg.pngAcetone

http:// upload.wikimedia.org/wikipedia/

commons/thumb/8/88/

Acetophenone_200.svg/1280px-Acetophenone_200.svg.png

Acetophenone

http:// upload.wikimedia.org/wikipedia/commons/

thumb/5/57/2-

Propanol2.svg/1280px-2-Propanol2.svg.pngIsopropyl alcohol

http:// upload.wikimedia.org/wikipedia/ commons/thumb/4/47/

Acetaldehyde_200.svg/1280px-

Acetaldehyde_200.svg.pngAcetaldehyde