Classification Tests for Hydroxyl- and Carbonyl- Containing Compounds

Janelle Angela O. Siy, Mary Jane S. Sta. Ana, Samantha Hope A. Sy, Kevin G. Turingan, Fraz Joseph Theodore B. Yap, and James Christian J. YuchongcoGroup 10 2B Medical Technology Organic Chemistry Laboratory

ABSTRACTThis very interesting experiment helped students identify the presence and structure of a hydroxyl, aldehyde, or ketone in an unknown compound using seven different tests. The first three tests done, solubility of alcohols in water, Lucas test and Chromic test helped identify the structure of alcohols, whether it is primary (if the alpha carbon atom where the OH group is attached is only attached to one alkyl group), secondary (if the alpha carbon atom is attached to two alkyl groups), or tertiary (if the alpha carbon atom is attached to three alkyl groups). The fourth test done was the 2, 4-Dinitrophenylhydrazone (2,4-DNP) test and this was used to identify the presence of carbonyl groups- aldehydes and ketones. More specifically, as stated in our Laboratory Manual, non-conjugated (aliphatic) carbonyl compounds usually yield yellow color in the reaction while conjugated (aromatic) ones yield orange red color. Hence, the 2, 4-DNP test is used to identify the presence of carbonyls in a compound and also distinguish its aromaticity. The fifth and sixth tests were the Fehlings and the Tollens Silver Mirror tests. Both of these were used to identify the presence of an aldehyde, thus distinguishing an aldehyde from a ketone. Only aldehydes yield a positive result in this test while ketones are non-reactive. Last but not the least, the Iodoform Test was used to see the presence of CH3CO group or methyl carbonyl group in a substance. The samples that were used are n-butyl alcohol, sec-butyl alcohol, tert-butyl alcohol, acetaldehyde, n-butyraldehyde, benzaldehyde, acetone, acetophenone, and isopropyl alchohol.

INTRODUCTION [1] The objectives of this experiment are to be able to distinguish whether a compound is hydroxyl- or carbonyl- containing, to be able to differentiate the three types of alcohols, aldehydes from ketones, and explain the mechanisms involved in the differentiating tests. The samples used were ethanol, n-butyl alcohol, sec-butyl alcohol, tert-butyl alcohol, benzyl alcohol, acetaldehyde, n-butyraldehyde, benzaldehyde, acetone, acetophenone, and isopropyl alcohol.

Figure 1- The structural formula of ethanol Ethanol is a primary two carbon hydroxyl compound.

Figure 2- The structural formula of n-butyl alcohol N-butyl alcohol is a primary hydroxyl compound since the hydroxyl (OH) group is attached to a primary alpha carbon atom. Primary alpha carbon atoms refer to the carbon atoms that are attached to only one alkyl group. In this case, it is attached to a propyl group.

Figure 3- The structural formula of sec-butyl alcohol Sec-butyl alcohol is similar to n-butyl alcohol except that it is a secondary hydroxyl compound since the hydroxyl group (OH) is attached to a secondary alpha carbon atom. It can be seen in the figure that OH is attached to the second carbon atom, having an ethyl group on one side, and a methyl group on the other. Hence, it is surrounded by not one, but two alkyl groups.

Figure 4- The structural formula of tert- butyl alcohol Tert-butyl alcohol is a tertiary hydroxyl compound since the alpha carbon atom at which the hydroxyl group is attached is surrounded by three alkyl groups, one methyl on each of the three sides. Hence, answering one of our objectives, the three types of alcohols are primary, secondary and tertiary alcohols, all of which are part of the hydroxyl group.

Figure 5- The structural formula of benzyl alcohol Benzyl alcohol is an aromatic hydroxyl group.

Figure 6- The structural formula of acetaldehyde Acetaldehyde, also called ethanol, is an aliphatic, non-conjugated, carbonyl (C=O) compound, more specifically a aldehyde, since it is terminal. Aldehydes are reactive to oxidation.

Figure 7- The structural formula of n-butyraldehyde N-butyraldehyde, also called butanal, is a four carbon aliphatic, non-conjugated aldehyde.

Figure 8- The structural formula of benzaldehyde

Benzaldehyde, also known as phenol, is an aromatic, conjugated aldehyde compound.

Figure 9- The structural formula of Acetone Acetone, also known as propanone, is a carbonyl group, more specifically, a ketone. Unlike aldehydes, it isnt terminal. Most ketones dont react in an oxidation reaction.

Figure 10- The structural formula of acetophenone Acetophenone is an aromatic ketone compound.

Figure 11- The structural formula of isopropyl alcohol Isopropyl alcohol is a secondary three carbon hydroxyl compound. In short, ethanol, n-butyl alcohol, sec-butyl alcohol, tert-butyl alcohol, benzyl alcohol, and isopropyl alcohol are all under the hydroxyl group. Acetaldehyde, n-butyraldehyde, and benzaldehyde are all under the aldehydes group while acetone and acetophenone are under the ketone group. All aldehydes and ketones are under the carbonyl group. Benzaldehyde, benzyl alcohol and acetophenone are the only ones that are aromatic because they have a conjugated and cyclic structure, benzene. The first test was to identify the alcohols solubility in water. The fewer the carbon atoms are in a substance, and the lesser the branches, the more miscible it is in water. The trend of solubility from most soluble to least soluble is as follows: primary ROH>secondary ROH>tertiary ROH>aromatic ROH This test was used to identify whether it is a primary, secondary, tertiary hydroxyl compound or an aromatic alcohol. This was also used to compare the quantity of carbons the different samples have in relation with each other. The Lucas test uses the Lucas reagent to identify whether an alcohol is primary, secondary or tertiary. [2] Lucas' reagent is a solution of anhydrous zinc chloride in concentrated hydrochloric acid. This solution is used to classify alcohols of low molecular weight. The reaction is a substitution in which the chloride replaces a hydroxyl group. A positive test is indicated by a change from colorless to turbid, signaling formation of a chloroalkane. It is based on the difference in reactivity of the three classes of alcohols with hydrogen halides. 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. An equimolar mixture of ZnCl2 and HCl is the reagent. The alcohol is protonated by this mixture, and H2O group attached to carbon is replaced by the nucleophile Cl-, which is present in excess. Tertiary alcohols react immediately with Lucas reagent as evidenced by turbidity owing to the low solubility of the organic chloride in the aqueous mixture. Secondary alcohols react within five or so minutes (depending on their solubility). Primary alcohols do not react appreciably with Lucas reagent at room temperature. Hence, 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:no visible reaction at room temperature and cloudy only on heating: primary, such as normal amyl alcohol (1-Pentanol)solution turns cloudy in 35 minutes: secondary, such as sec-amyl alcohol (2-Pentanol)solution turns cloudy immediately, and/or phases separate: tertiary, such as tert-amyl alcohol (2-Methyl-2-butanol) Figure 12- the reaction in Lucas Test. Chromic Acid test is also known as Jones test. [3] The Jones oxidation is an organic reaction used to oxidize alcohols using chromic trioxide and acid in water. A primary alcohol is oxidized to an aldehyde or all the way to a carboxylic acid, while a secondary alcohol to a ketone. The mechanism begins with the reaction of CrO3 with acid (often H2SO4) to form chromic acid or dichromic acid in more concentrated solutions. The alcohol oxidation then occurs with chromic acid which in turn gets reduced in the process. Hence, this is used to check the presence of primary and secondary alcohols, both of which would indicate a positive result, which would be the formation of green color. Figure 13- reaction of (chromic acid test) Jones Test The 2, 4- Dinitrophenylhydrazone (2, 4-DNP) Test is used to determine the presence of carbonyl groups such as aldehydes and ketones in a substance. A positive result would yield a yellow-orange precipitate, due to the characteristic of the formed dinitrophenylhydrazone. This also helps determine whether the carbonyl group is aliphatic or aromatic. An aromatic substance would yield more of the red-orange color while aliphatic ones would yield more of the yellow color.

Figure 14- reaction mechanism of 2, 4-DNP test The Fehlings test is used to determine the presence of an aldehyde in the solution and it doesnt react with ketone. The reagent of this test is cupric ion in an alkaline solution. Aldehydes, when mixed with copper ion, would reduce the copper and create a mossy green solution which will eventually form a brick red precipitate of cuprous oxide, Cu2O.

Figure 15- reaction mechanism of Fehlings test The Tollens Silver Mirror Test also has the same function as the Fehlings test. Its aim is to identify the presence of a carbonyl group and further differentiate them as either an aldehyde or a ketone. The reagent of this test consists of silver ion from mixing silver oxide and ammonia. Aldehydes would reduce the reagent to metallic silver while ketones wont have any reaction. The positive result must look like a formation of a silver mirror.

Figure 16- the reaction of Tollens test The last test, Iodoform test, is used to see the presence of CH3CO in aldehydes and ketones. A positive result would produce yellow precipitate, and if there is no CH3CO group in the sample being tested, then it will get a different result.

Figure 17- the reaction of Iodoform testEXPERIMENTALCompounds Tested (Samples Used)The aliphatic hydroxyls: Ethanol, N-Butyl alcohol, sec-butyl alcohol, tert-butyl alcohol, isopropyl alcohol

The aromatic hydroxyl: benzyl alcoholThe aliphatic aldehydes: acetaldehyde, n-butyraldehydeThe aromatic aldehyde: benzaldehydeThe aliphatic ketone: acetoneThe aromatic ketone: acetophenoneProcedureSolubility of Alcohols in H20 Five test tubes were labeled as ethanol, n-butyl alcohol, sec-butyl alcohol, tert-butyl alcohol, and benzyl alcohol. Using the Pasteur pipet, 10 drops of the corresponding samples were added in each of the five test tubes. One mL of water was added to each of the five test tubes and the test tubes were shaken. A fourth of an mL of water was added when cloudiness occurred. This was done repeatedly until the solution became homogenous. The amount of water that was needed to dissolve each compound was taken note of.

Lucas Test There was no need to prepare the Lucas reagent since it is available in the lab. Three test tubes were prepared and labeled as n-butyl chloride, sec-butyl chloride, and tert-butyl chloride. One mL of Lucas reagent was put on each of the three test tubes, and then two to three drops of each sample were placed in their respective test tubes. The test tubes were shook for a few seconds. The three test tubes were then placed on the test tube rack for the samples to settle in. The rate of formation of the cloudy suspension and the formation of two layers were observed.

Chromic acid test (Jones Oxidation) For this test, seven compounds, as well as seven test tubes were used. The seven test tubes were labeled accordingly as n-butyl alcohol, sec-butyl alcohol, tert-butyl alcohol, n-butyraldehyde, benzaldehyde, acetone and acetophenone. Each contained one mL each of their corresponding samples. Two drops of 10% aqueous K2Cr2O7 solution and 5 drops of 6M H2SO4 were then added to each test tube. Color change and its rate were observed.

2, 4- Dinitrophenylhydrazone test Five test tubes with their respective labels to their corresponding samples, 1 mL each, were utilized in this test. The samples were acetone, acetaldehyde, n-butyraldehyde, benzaldehyde and acetophenone. Five drops of 95% ethanol was added to all five test tubes. These test tubes were shook thoroughly. Three drops of 2, 4-DNP were then added afterwards. The results exhibited were noted. When there was no reaction for some test tubes, they were subjected to 15 mins of settling at room temperature. After that, the observations were recorded.

Fehlings Test Since the Fehlings reagents were already available in the lab, there was no need to prepare it. However, there was a need for the students to mix equal amounts of Fehlings A and B. Six mL of Fehlings A was put inside a small graduated cylinder, then it was mixed with 6 mL of Fehlings B solution. There were five test tubes, with the labels acetaldehyde, n-butyraldehyde, acetone, benzaldehyde and acetophenone. The Fehlings reagent was navy blue in color. One mL of Fehlings reagent was added to each of the test tubes. Three drops of each sample were then dropped on their corresponding test tubes. The tubes were placed in a water bath to boil for 10 -15 minutes. After, the results were observed.

Figure 18 Fehlings reagent (Fehlings A + B)