Classification Tests for Hydroxyl and Carbonyl Containing Compounds

Classification Tests for Hydroxyl and Carbonyl Compounds University of Santo Tomas Faculty of Pharmacy 2FPH Group 5

Members: Mondoñedo, Samantha; Lopez, Princess; Maternal, Charles; Miranda, Joyce; Nasinopa, Reina; Navata, Jemina

ABSTRACT

The aim of the experiment was to be able to distinguish whether a compound is hydroxyl or carbonyl containing,differentiate the three types of alcohols, differentiate aldehydes from ketones and to explain the mechanisms involved in the differentiating tests. Therefore in this experiment the proponents were able to classify the compounds based on the different tests. The samples were analyzed through tests involving the solubility of alcohols in water, Lucas test, Chromic Acid test, 2,4-Dinitrophenylhydrazone (2,4-DNP) test, Fehling’s test, Tollens’ Silver Mirror test, and Iodoform test. Lucas test differentiates tert-butyl alcohol (primary), sec-butyl alcohol (secondary) and n-butyl alcohol (tertiary) . Chromic test gave a positive result for n butyl alcohol, acetaldehyde, acetaldehyde, acetone, acetone, isopropyl alcohol. 2,4-Dinitrophenylhydra 2,4-Dinitrophenylhydrazone zone test was  performed for aldehydes aldehydes and ketones; all samples gave a positive result. result . Fehling’s Test and Tollens’ Silver Mirror Test are tests for aldehydes.  aldehydes.   These tests prove that acetaldehyde and  benzaldehyde  benzaldehyde are aldehydes aldehydes while acetone and acetophenone acetophenone are ketones. ketones. Iodoform test is a test for methyl carbinol and methylcarbonyl groups.

In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules and examples of functional groups are hydroxyl and carbonyl groups.

classified as primary (1˚), secondary (2˚), or tertiary (3˚), depending on the number of carbon substituents bonded to the hydroxyl-bearing carbon [4]. Hydroxyl groups are especially important in biological chemistry because of their tendency to form hydrogen bonds both as donor and acceptor. This property is also related to their ability to increase hydrophilicity and water solubility[1].

An alcohol is a compound that has a hydroxyl group bonded to a saturated,  sp hybridized carbon atom, R-OH. Alcohols are

Carbonyl group is a family of functional groups composed of a carbon atom double-bonded to an oxygen atom: C=O. The group is a constituent

Introduction

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of carboxylic acids, esters, anhydrides, acylhalides, amides, and quinones, and it is the characteristic functional group of aldehydes and ketones [2]. The functional group of an aldehyde is a carbonyl group bonded to a hydrogen atom (Figure 1). The functional group of a ketone is a carbonyl group bonded to two carbon atoms (Figure 2). Because of the polarity of the carbonyl group, aldehydes and ketones are polar compounds and interact in the liquid state by dipole-dipole interactions. As a result, aldehydes and ketones have higher boiling points than those of nonpolar compounds with comparable molecular weight [3].

Figure 1. Structur e of A ldehyde 

Figure 3. Gener al M echani sm of L ucas test 

The Chromic acid test or also known as Jones oxidation distinguishes primary and secondary alcohols from tertiary. Chromic acid will oxidize a primary alcohol first to an aldehyde and then to a carboxylic acid and it will oxidize a secondary alcohol to a ketone. Tertiary alcohols do not react. The OH-bearing carbon must have a hydrogen atom attached. Since the carbon atom is being oxidized in  primary and secondary, the orange chromium Cr 6+ ion is being reduced to the blue-green Cr 3+ ion[4].

Figure 2. Structur e of Ketone

The Lucas reagent is an aqueous solution of strong acid (HCl) and zinc chloride (ZnCl ₂). The alcohol starting material must be sufficiently soluble in aqueous environments for the reaction to take place. The reaction that occurs in the Lucas test is an S N1 nucleophilic substitution[5]. The acid catalyst activates the OH group of the alcohol by protonating the oxygen atom. The C-OH₂  bond breaks to generate the carbocation, which in turn reacts with the chloride ion (nucleophile) to generate an alkyl halide product. Figure 3 shows the general mechanism for this S N1 reaction [4]. +

Figure 4. General M echanism of Chr omic Acid Test

The 2,4-Dinitrophenylhydrazone test serves as a derivative formation. Both aldehydes and ketones react with 2,4dinitrophenylhydrazine to form a solid 2,4dinitrophenylhydrazone (DNP) derivative. The color of this derivative can also provide useful structural information. If the solid is yellow, this most often means that the carbonyl group in the unknown is unconjugated. A reddish-orange color most likely means that the carbonyl group is conjugated. In a few cases, compounds in

which the carbonyl group is not conjugated  produce orange precipitates. The double bond must be separated from the carbonyl by one single bond only. If the double bond is further away, it is isolated from the carbonyl and not conjugated with the carbonyl[4].

the test is carried out in a clean glass test tube, forms a mirror on the test tube. Ketones are not oxidized by the Tollens’ reagent, so the treatment of a ketone with Tollens’ reagent in a glass test tube does not result in a silver mirror.

Figure 7. General Mechanism of Tollens’ test 

Figure 5. General M echanism of 2,4-DN PH test 

In Fehling’s test, the presence of aldehydes and not ketones is detected by reduction of the deep blue solution of copper (II) to muddy green solution, and then form a brickred precipitate of insoluble cuprous oxide (Cu2O). This test is commonly used for reducing sugars but is known to be not specific for aldehydes[3].

The Iodoform test indicates the presence of an aldehyde or ketone in which one of the groups directly attached to the carbonyl carbon is a methyl group. Such a ketone is called a methyl ketone. In the Iodoform test, the sample is allowed to react with a mixture of iodine and  base. Hydrogens alpha to a carbonyl group are acidic and will react with base to form the anion, which then reacts with iodine in this way to form the triiodo compound, which the reacts with more base to form the carboxylic acid salt plus iodoform, a yellow precipitate. Formation of a yellow precipitate therefore indicates the  presence of a methyl group directly attached to the carbonyl. The mechanism of the iodoform reaction is that of alpha-halogenation of a carbonyl compound under basic conditions, followed by nucleophilic displacement of the resulting triiodomethyl group by hydroxide.

Figure 6. General Mechanism of Fehling’s test 

Tollens’ test, also known as silver mirror test, is a qualitative laboratory test used to distinguish between and aldehyde and a ketone. It exploits the fact that aldehydes are readily oxidized, whereas ketones are not. Tollens’ test uses a reagent known as Tollens’ reagent, which is a colorless, basic, aqueous solution containing silver ions coordinated to ammonia [Ag(NH 3)2+]. Tollens’ reagent oxidizes an aldehyde into the corresponding carboxylic acid. The reaction is accompanied by the reduction of silver ions in Tollens’ reagent into metallic silver, which, if

Figure 8. General M echanism of I odofor m test

Objectives of this experiment: 

To distinguish whether a compound is hydroxyl or carbonyl containing



 

tubes, 2 drops of each of the samples were added separately. The mixtures were shaken and allowed to stand for 10 minutes. When no reaction occurred, the test tubes were placed in a beaker with warm water for 5 minutes. Observations were recorded.

To differentiate the three types of alcohols To differentiate aldehydes from ketones To explain the mechanisms involved in the differentiating tests

Experimental A. Materials  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,   beaker, compounds (ethanol, n-butyl  sample alcohol,  sec-butyl alcohol, tert -butyl alcohol, benzyl alcohol, n butyraldehyde, benzaldehyde, acetone, acetophenone, isopropyl alcohol and acetaldehyde) B. Methods 1. Solubility of Alcohols in water In the five test tubes, 10 drops each of ethanol, n-butyl alcohol, sec butyl alcohol, tert -butyl alcohol and  benzyl alcohol were placed. 1 mL of water was added dropwise in each of the test tubes containing alcohol while shaking the mixture after each addition. When cloudiness resulted, 0.25 mL of water was added at a time, with vigorous shaking, until a homogenous dispersion appeared. The total volume of water added was noted. When no cloudiness resulted after the addition of 2.0 mL water, alcohol was noted down to be soluble in water. 2. Lucas Test

Five test tubes with 1 mL each of freshly prepared Tollens’ reagent were prepared. In each of the test

Three test tubes were prepared and 1 mL of Lucas reagent was dropped in each of the test tubes. On the first test tube, 2-3 drops of n butyl alcohol was added. The test tube was shook vigorously for a few seconds and the mixture was allowed to stand at room temperature. The two other test tubes were placed with 2-3 drops of  sec-butyl alcohol and tert -butyl alcohol, respectively, shook vigorously and the mixtures were allowed to stand. The rate of formation of cloudy suspension or the formation of two layers was observed. 3. Chromic Acid test

Using the samples n-butyl alcohol, acetaldehyde, acetone, isopropyl alchohol one drop of each liquid or small amount of the solid sample was dissolved in 1 mL of acetone in different test tubes. 2 drops of 10% aqueous K 2Cr 2O7 solution and 5 drops of 6 M H2SO4  were added in each of the test tubes. 4.

2,4-Dinitrophenylhydrazone DNP/2,4-DNPH) test

(2,4-

One drop of a liquid sample (acetone, acetaldehyde, benzaldehyde and acetophenone) was placed in a test tube and 5 drops of 95% ethanol were added. After shaking well, 3 drops of 2,4-dinitrophenylhydrazine were added. The solution was allowed to stand for at least 15 minutes when no yellow or orange-red precipitate forms.

5. Fehling’s test

Into each test tube, 1 mL of freshly prepared Fehling’s reagent (made by mixing equal amounts of Fehling’s A and Fehling’s B) was  placed. 3 drops of the sample to be tested were added and the test tubes were placed in a beaker of boiling water. Changes were observed that occurred within 10-15 minutes. The test was performed on acetaldehyde, acetone, benzaldehyde and acetophenone.

ethanol n-butyl alcohol  sec butyl alcohol tert  butyl alcohol  benzyl alcohol

CH3CH2OH

homogenous dispersion 0.25ml 2.00 mL

1.25 mL

Miscible Immiscible miscible

0.35 mL

miscible

2.00 mL

immiscible

6. Tollen’s Test

Five test tubes with 1 mL each of freshly prepared Tollens’ reagent were  prepared. In each of the test tubes, 2 drops of each of the samples (acetaldehyde, acetone) were added separately. The mixtures were shaken and allowed to stand for 10 minutes. When no reaction occurred, the test tubes were placed in a beaker with warm water for 5 minutes. Observations were recorded. 7. Iodoform test

2 drops of each sample (acetone,  benzaldehyde and isopropyl alcohol) were placed in different test tubes. 20 drops of 10% KI solution were added. While shaking, 20 drops of fresh chlorine bleach (5% sodium hypochlorite) were added slowly to each tube and mixed. The formation of a yellow precipitate was noted. Results and Discussion A. Solubility of Alcohols in Water Table 1. Solubili ty of Al cohol in Water  Alcohol

Condensed structural formula

Amount of water (in mL) needed to produce a

Solubility to water

As shown in the table, only benzyl alcohol and n-butyl alcohol were insoluble in water, while ethanol,  sec-butyl alcohol and tert  butyl alcohol were all soluble in water. This follows the principle “like dissolves like”. Thus, it can be said that all of these soluble compounds are polar since water is polar. Of the alcohols that were soluble in water, their required amount of water to be added in order to be considered soluble varies. This indicates that there are other 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 affect solubility. 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[2]. B. Lucas test Table 2. L ucas test Sample n-butyl alcohol  sec-butyl alcohol tert -butyl alcohol

Reaction observed Clear solution slightly turbid solution Turbid solution

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 S N1 reaction. According toMcMurry, depends on the formation of stable carbocations. Reactivity of alcohols in S N1 reaction is 3˚ > 2˚ > 1.

E. Fehling’s Test

C. Chromic Acid

benzaldehyde

Table 5. Fehling’s test  Sample acetaldehyde

Table 3. Chr omic A cid test  Sample n-butyl alcohol Acetaldehyde Acetone Isopropyl alcohol

Reaction observed blue-green solution blue-green solution blue-green solution blue-green solution

Base on the results, n-butyl alcohol, acetaldehyde, acetone, isopropyl alcohol gave a positive result of blue-green solution. Chromic Acid test involved reductionoxidation or redox reaction. 1˚ and 2˚ alcohols and aldehydes underwent oxidation and chromium underwent reduction from Cr 6+  to Cr 3+. 1˚ and 2˚ alcohols and aldehydes reduced the orange-red chromic acid/sulphuric acid reagent to an opaque green or blue suspension of Cr (III) salts.[3] D. 2,4 DNP test Table 4. Resul ts of th e 2,4 DN P test  Sample

Reaction observed

Acetaldehyde Benzaldehyde Acetone Acetaphenone

Yellow orange ppt Yellow orange ppt Yellow orange ppt Yellow orange ppt

Table 4 shows the reaction of the samples to 2,4-DNP test. All the samples exhibited a  positive result because they all formed a yellow orange precipitate. Hence, 2,4-DNP test proved that the samples are carbonyl-containing compounds and are either aldehydes or ketones.

acetone acetophenone

Reaction observed Reddish brown solution with brickred ppt Formed two layers yellow solution and light blue solution with brick-red ppt  blue solution with no  ppt  blue solution

As shown in the given table, only acetone and acetophenone did not react to form a  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 acids while ketones did not undergo oxidation. In here, copper was reduced from Cu 2+ to Cu1+[2]. F. Tollen’s Test Table 6. Tollens’ test  Sample acetaldehyde acetone

Result observed + -

Table 6. Tollens’ Silver Mirror test Sample acetaldehyde n-butyraldehyde  benzaldehyde

acetone acetophenone

Result observed silver mirror flesh solution light yellow solution with globules dark-gray solution turbid gray solution

According to Table 6, only acetaldehyde formed a silver mirror. 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 [3]. G. Iodoform Test Table 7. I odofor m test  Sample  benzaldehyde acetone isopropyl alcohol

Reaction observed Forms an oily layer Forms yellow ppt Forms yellow ppt

Base on the given table, acetone and isopropyl alcohol gave a result of yellow  precipitate. 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 Cl 3-  anion through an addition/elimination pathway[4].

References:

[1] Hydroxyl Group. (2014). In Encyclopædia Britannica. Retrieved October 23, 2010, from Encyclopædia Britannica Online: http://www.daviddarling.info/encyclopedia /P/phenol.html. [2] Shriner, Fuson, Curtin. Systematic Identification of Organic Compound: A Laboratory Manual Fifth Edition. John Wiley & Sons, Inc. New York: Van Hoffmann Press [3] Brown, W., Poon, T. (2014).  Introduction to Organic Chemistry th  International Student Version (7   edition).  NJ, USA: John Wiley & Sons, Inc.

[4] McMurry, J. (2010).  Foundations of Organic Chemistry (Philippine edition). USA: Cengage Learning Asia Pte. Ltd. [5] Zumdahl, S., Zumdahl, S. (2013). Chemistry: An Atoms First Appraoch (International edition). USA: Brooks/Cole, Cengage Learning. [6] Bathan,G., Bayquen,A., Cruz,C., Crisostomo,A., de Guia,R., Farrow,F., Pena,G., Sarile,A., Torres, P. (2014).  Laboratory Manual in Organic Chemistry. Quezon City, Philippines: C & E Publishing.