CLASSIFICATION TEST FOR CARBOXYLIC ACIDS AND DERIVATIVES
Kathleen Adryon Tan, Vienne Czarina Tongco, Chrystel Marlowe Turla, Carl Jerome Umali, John Henrick Uy, Celina Marie Wong Group 10 2F Medical Technology Organic Chemistry Laboratory ABSTRACT Carboxylic acids are organic compounds which contain at least one carboxyl group (-CO2H). Some of its derivatives which include: acyl halides, acid anhydrides, esters and amides contain the acyl group (C=O). Reagents used were acetyl chloride representing acyl halides, acetic anhydride representing acid anhydrides, ethyl acetate representing esters and acetamide representing amides. Upon hydrolysis of acetyl chloride and ethyl acetate yielded a carboxylic acid, acetic acid. Hydrolysis with aqueous silver nitrate confirmed the presence of acyl chloride by the formation of a silver chloride precipitate. On the other hand, effervescence when sodium bicarbonate was added indicated formation of carboxylic acid. Basic hydrolysis and acidification of ethyl acetate yielded a carboxylic acid, acetic acid, while acetamide turned red litmus paper to blue accompanied an ammonia-like odor indicated the presence of amides. Alcoholysis under acidic condition of acetyl chloride and acetic anhydride yielded an ester, ethyl acetate. Aminolysis of acetyl chloride and acetic anhydride yielded a white precipitate indicating amide formation. Basic Hydroxamic test for ethyl acetate formed hydroxamic acids which was made to react with ferric chloride in acidic environment to form a deep burgundy precipitate indicating an ester.
INTRODUCTION Carboxylic acids are generally weak acids which contain a carbonyl group (C=O) attached to it is a hydroxyl group (-OH) also known as carboxyl group [1]. In the systemic nomenclature of each carboxylic acid, its name ends with the suffix – oic[3]. Carboxylic acid derivatives have a carbonyl group attached to either an electronegative element or a substituent which would determine the reactivity of the compound [2]. The most common carboxylic acid derivatives are acyl halide (RCOX), acid anhydride (RC (O)) 2O), ester (RCOOR’ ) and amide (RCONH2) with acyl halides being the most reactive and amides being the least [3]. Majority are used in industrial and biological processes. Acyl halides are usually used as a starting material for synthesis due to its high reactivity. Acid anhydrides are commonly used in the preparation of acetate esters, aspirin and a precursor to various resins. Esters which are generally found in essential oils and fruits are typically used in perfumes, artificial flavorings, fats, varnishes and paints. Amides are frequently found in nylon, proteins and drugs like: penicillin and LSD [3]. All carboxylic derivatives yield the parent carboxylic acid upon reaction with water [4]. Moreover, one derivative can be converted into another, provided the former is more reactive than the latter [4]. Reaction mechanism is the same for each of its derivatives in which the nucleophile attacks the carbonyl group forming an unstable intermediate. Removal of the weakest base in the compound is a way of stabilizing the newly formed organic compound [1] . Nucleophilic acyl substitution is the mechanism involved in this experiment in which involves the replacement of the leaving group by
a nucleophile resulting to the formation of carboxylic acid and its derivatives. Confirmatory test for carboxylic acid derivatives include: hydrolysis, alcoholysis, aminolysis and hydroxamic acid test. Differentiating the reactivity, distinguishing each and explaining through chemical reaction the mechanisms involved in each test of the carboxylic acid derivatives were achieved by the experiment.
image from Wikipedia
image from Wikipedia
EXPERIMENTAL A. Compounds tested (or samples used) 2% ethanolic AgNO3, saturated NaHCO3, 10%, 20% and 25% NaOH solution, glacial acetic acid, 95% ethanol, test tubes, beaker, parafilm, concentrated H2SO4, aniline, 1M HCl solution, alcoholic NH2OH+HCl, 5% FeCl3 solution, 1M KOH solution, Bunsen burner, test tube holder and rack, litmus paper, sample compounds
(acetyl chloride, acetic anhydride, ethyl acetate, benzamide/acetamide).
B. Procedure 1. Hydrolysis of Acid Derivatives Acyl halides and Acid Anhydrides 1mL of water was added to a clean dry test tube, straightaway carefully add 10 drops of acetyl chloride and acetic anhydride in a dropwise manner. A warming effect of each was noted. The resulting mixtures were divided into two portions. 1mL of 2% AgNO3 was introduced to the first portion of each. Formation of Precipitate was observed. For the second portion, 1mL of saturated NaHCO3 was introduced to each. Evolution of gas was observed. Esters 2mL of 25% NaOH solution was added to 1mL of ethyl acetate. The mouth of the test tube was covered by perforated parafilm and allowed to heat in a boiling water bath for 5 minutes. Afterwards, the mixture was neutralized with 10% HCl solution. The test tube was wafted, so that its odor can be noted. Amides Acetamide was treated with 5mL of 10% NaOH solution and immersed in a boiling water bath. During heating, a moist red litmus paper was placed above the tube.
2. Alcoholysis:Schotten-Baumann Rxn 10 drops of acetic acid with 0.5mL ethanol, 1mL of H2O mixture and 2mL 20% NaOH solution were added to each 0.2mL acetyl chloride and acetic anhydride. The layers and odor formed by the ester was observed.
3. Aminolysis: Anilide Formation A few drops of either acetyl chloride or acetic anhydride were added to 0.5mL aniline. The mixture was transferred to a test tube which contained 5mL of water. Formation of precipitate was observed.
4. Hydroxamic Acid Test Preliminary test was done by mixing 1mL of 95% ethanol and 1M HCl to separate test tubes that contained 2 drops of the samples to be used. If upon addition of 1 drop of 5% FeCl3, a color other than yellow will make the test futile. 2mL of alcoholic NH2OH●HCl and 1mL of 1M KOH was added to separate test tubes of 2 drops ethyl acetate and acetamide. Immersion in a boiling water bath for 2 minutes was done; afterwards the mixture was allowed to cool. 1mL of 5% FeCl3 was introduced in each test tube. Color of the formed precipitate was noted.
Results and Discussion Reaction speed of carboxylic acid derivatives is based on the reactivity of a leaving group.
Leaving group is a/an group of atoms/atom that is displaced as stable species taking with it the bonding electrons [3]. Typically the leaving group is an anion or a neutral compound. A leaving group that tends to be displaced faster is a good leaving group. Good leaving groups are usually conjugate base of a strong acid which means it is a weak base [1]. Carboxylic acid and its derivatives are rank in order of increasing reactivity towards a nucleophile by this arrangement: acid amide< carboxylic acid< ester< acid anhydride< acid halide [2]. Rule of thumb is the more reactive the derivative will be converted to a less reactive derivative. Same kind of reactions happened in hydrolysis, reaction with water to yield carboxylic acids, alcoholysis, reaction with alcohol to yield an ester, and aminolysis, reaction with ammonia/amine to yield an amide. Table 1. Hydrolysis of acyl halide and acid anhydride Sample
+H20
+2% AgNO3
+Saturated NaHCO3
Acetyl Warming White Bubbles Chloride effect ppt. formation Acetic Warming Bubbles Anhydride effect formation Due to acetyl chloride and acetic anhydride’s high reactivity, catalyst was not necessary for the reaction to occur. Hydrolysis of acetyl chloride by the mechanism nucleophilic acyl substitution is initiated by the attack of water on the carbonyl group of the acyl chloride which forms a tetrahedral intermediate that undergoes elimination of Cl - and a loss of H + to give its parent carboxylic acid, acetic acid, and HCl. This reaction is accompanied by a warming effect due to the formation of acetic acid. Same principle also applies to acetic anhydride, but it undergoes the reaction at a slower rate yielding 2 moles of acetic acid due to the fact that Cl - is a weaker base making it a good leaving group compared to ethyl acetate.
During the addition of 2% AgNO3 to the 1 st portion of the original test tube, acetyl chloride yielded a white precipitate known as AgCl(s). Acetic anhydride did not yield a precipitate due to the absence of halide which would react to Ag+ to form silver halide (refer to table 1.). Effervescence was due to the evolution of CO2 as indicated by the formation of bubbles when
saturated NaHCO3 was added to the 2 nd portion of the original test tube (refer to table 1.). Table 2. Hydrolysis of ester Sample
+25% NaOH w/ heating for 5 mins, +10% HCl “Vinegar-like” odor
Ethyl acetate Hydrolysis of an ester in a basic medium converts an ester in to the carboxylate salt of its parent carboxylic acid and an alcohol from which the ester was formed. This process is known as saponification. The resulting solution was acidified with 10% HCl yielding acetic acid, its parent carboxylic acid (refer to table 2.). Most of the time, an acid and an alcohol yielded by hydrolysis are extremely soluble in H2O.
Table 3. Hydrolysis of amide Sample +10% NaOH w/ heating Acetamide Red litmus turned blue Hydrolysis of amides involves the same mechanism which is nucleophilic acyl substitution that yielded ethanoic acid and an amine (NH3). Moist, red litmus paper turned blue due to the presence of NH3 which is a Lewis base accompanied by an ammonia/amine-like odor. Amides which do not react with litmus paper will not also give off an amine-like odor (refer to table 3.). Some amides also yield a precipitate or sometimes a separated liquid phase (a carboxylic acid). The reaction took a lot of time before it occurred due to the fact that amides are strong bases which make it a poor leaving group.
Table 4. Alcoholysis of acyl halide and acid anhydride Sample Acetyl chloride
+Ethanol,+H2O, 20% NaOH “Vinegar-like” odor; 2 layers formed
Acetic acid
“Plastic balloon-like” odor; 2 layers formed Fischer Esterification was used to produce ethyl acetate with the use of glacial acetic acid by nucleophilic acyl substitution by ethanol. Heating the acid and alcohol in the presence of a small quantity of acid catalyst (concentrated H 2SO4) causes ester formation (esterification) along with dehydration. A large excess of H 2O favors the reverse reaction. Bulky (sterically hindered) samples reduce yields. Alcoholysis reactions are usually carried out by the presence of NaOH, so that it would react with HCl to prevent any unnecessary reactions. Alcoholysis of acetyl chloride and acetic anhydride resulted in the formation of an ester. Chemistry between the two is the same; the only difference is the rate at which the reaction occurs. In this case, acetyl chloride is faster due to its good leaving group ability compared to acetic acid. In the case of acetic anhydride, one half of its molecule acted as a nucleophile, while the other half acted as a leaving group making acetyl chloride an ideal introducers of acyl substituents. The appearance of a layer was because of the immiscibility of HCl and the organic layer due to differences in polarity and density of the two. Formation of heat, evolution of HCl gas, ester-like odor and phase separation were signs that the reaction has occurred (refer to table 4.). Esterification of an alcohol using acid chloride is strongly influenced by bulky groups. Steric effect slows down the rate of reaction (reactivity: 1°>2°>3° ROH).
Table 5. Aminolysis of acyl halide and acid anhydride Sample +aniline,+ H2O Acetyl chloride White ppt. Acetic anhydride White ppt. Both samples that underwent aminolysis with aniline, an amine, gave the same result which was a white precipitate indicating the formation of anilide upon addition of water (refer to table 5.). It took more time for the white precipitate to
appear in acetic anhydride, than in acetyl chloride because of the leaving group reactivity. In this reaction two equivalents of aniline were used up by acetyl chloride. One equivalent was used to react with the sample, and the other was used to react with HCl, a minor product of the reaction. Table 6. Hydroxamic Test of ester and amide Sample
+alcoholic NH2OH●HCl, +1M KOH w/ heating, +5% FeCl3 Ethyl acetate Deep burgundy color Acetamide Only ethyl acetate produced a deep burgundy color due to the fact that it is an ester. Esters react with basic hydroxylamine to yield hydroxamic acid which in turn reacts with acidic FeCl3 to form blue-red/magenta colored ferric hydroxamates (refer to table 6.).
References [1] Davis, R. E., Peck, M.L., Stanley, G. G., and Whitten, K. W. (2010). Chemistry . 9th ed. Belmonte, CA: Brooks/ Cole, Cengage Learning. [2] Bettelheim, F.A., Brown, W.H., Campbell, M.K., Farrell, S.O. (2007). Introduction to General, Organic, th Biochemistry . 8 ed. Singapore: Thomson Learning Asia. [3] McMurry, J. (2011 ). Fundamentals of Organic Chemistry . 7th ed. Belmonte, CA: Brooks/Cole, Cengage Learning. [4] Bayquen, A., Cruz, C. et.al. (2009). Laboratory Manual in Organic Chemistry . Philippines: C & E Publishing, Inc. Pgs. 121123.
References [5] Davis, R. E., Peck, M.L., Stanley, G. G., and Whitten, K. W. [2010] Chemistry . 9th ed. Belmonte, CA: Brooks/ Cole, Cengage Learning. [6] Bettelheim, F.A., Brown, W.H., Campbell, M.K., Farrell, S.O. (2007). Introduction to General, Organic, Biochemistry . 8th ed. Singapore: Thomson Learning Asia. [7] Skoog, D.A., West, D.M., Holler, F.J.,Crouch, S.R., Chen, S.C. (2012). Introduction to th Analytical Chemistry . 8 ed. Singapore: Cengage Learning Asia Pte Ltd. [8] Bayquen, A., Cruz, C. et.al. (2009). Laboratory Manual in Organic Chemistry . Philippines: C & E Publishing, Inc. Pgs. 57-59
