Article pubs.acs.org/jchemeduc
The Five Senses of Christmas Chemistry Derek A. Jackson and Andrew P. Dicks* Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6 ABSTRACT: This article describes the organic chemistry of five compounds that are directly associated with the Christmas season. These substances and related materials are presented within the framework of the five senses: silver fulminate (sound), α-pinene (sight), sodium acetate (touch), tryptophan (taste), and gingerol (smell). Connections with the introductory organic curriculum are emphasized throughout to illustrate how the following lecture topics can be reviewed or introduced each November and December: Lewis structures, resonance theory, constitutional isomers, stereoisomers, cycloalkane ring conformations, thermodynamics, acids and bases, nucleophiles and electrophiles, carbonyl condensation reaction mechanisms, and functional group transformations.
KEYWORDS: Second-Year Undergraduate, Organic Chemistry, Analogies/Transfer, Lewis Structures, Resonance Theory, Constitutional Isomers, Stereochemistry, Thermodynamics, Reactions, Mechanisms of Reactions
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SILVER FULMINATE: SOUNDS GOOD The satisfying “snap” heard from a pulled Christmas cracker (Figure 1) comes from the rapid breakdown of silver fulminate
icture the scene: a table is being set in preparation for a family Christmas dinner. Crackers are placed next to plates while a turkey roasts in a nearby oven. A familiar aroma of gingerbread fills the air as a Christmas pine tree stands decorated in the corner of the room. Then, children arrive home after playing outside in the snow, with frostbite averted by the hand warmers received as gifts that day. Now consider the wealth of organic chemistry contained in that scene, which impacts all five senses of sound, taste, smell, sight, and touch. The theme of Christmas “presents” a fascinating opportunity to review a number of concepts taught during the first semester of an introductory organic class. These include Lewis structures, resonance theory, constitutional isomers, stereoisomers, cycloalkane ring conformations, thermodynamics, acids and bases, and nucleophiles and electrophiles. In addition, discussion of carbonyl condensation reaction mechanisms and functional group transformations serves as a prelude to topics typically covered during the second semester. This article briefly highlights the chemistry of five “Christmas compounds” in the context of the five senses: silver fulminate (sound), α-pinene (sight), sodium acetate (touch), tryptophan (taste), and gingerol (smell), along with relevant associated substances. Links are specifically made with the introductory organic curriculum to illustrate how material can be integrated into classes toward the end of the fall semester. A related approach has recently been reported by Mannschreck and von Angerer, where compounds found in roses form the basis of a second-year undergraduate lecture about fragrance.1 The substances are discussed in the order in which related theoretical material is routinely presented in a two-semester introductory organic course. © 2012 American Chemical Society and Division of Chemical Education, Inc.
Figure 1. Two christmas crackers.
(AgCNO), an unstable substance present in small quantities in the paper strip. Behind the formation and decomposition of AgCNO lies a very important milestone in the history of organic chemistry, as well as multistep reactions involving fundamental functional group transformations. The chemistry related to silver fulminate provides a stimulating context for students to draw Lewis structures of the fulminate anion (CNO−) and isoelectronic cyanate anion (−OCN), along with significant resonance forms. In addition, a discussion of relative bond strengths related to AgCNO decomposition is facilitated. The story of how silver fulminate was discovered and characterized is well documented in this Journal2,3 and others.4 To summarize, Justus Liebig correctly determined the molecular formula of AgCNO in 1824 with the help of GayLussac. At approximately the same time, Friedrich Wöhler concluded the identical molecular formula existed for silver cyanate, yet the two compounds were clearly different (a Published: August 10, 2012 1267
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contradiction of the law of definite proportions). After Wöhler’s results were confirmed by Liebig, the two scientists collaborated and formed a strong personal and professional relationship. Eventually, Berzelius rationalized these results by introducing the notion of isomerism. Fulminates and cyanates were subsequently concluded to have the same molecular formula but different structures. The electronic structures of the fulminate and cyanate anions afford organic students the chance to review resonance theory and its connection with molecular stability. The pedagogy of electronic resonance forms has previously been described in this Journal.5−7 The fulminate and cyanate anions are isoelectronic with each having 16 valence electrons. For both species students will be able to draw three resonance forms as Lewis structures where every atom obeys the octet rule. These structures have varying degrees of significance in terms of contribution toward the appropriate resonance hybrid (Figure 2). The fulminate anion has only one important resonance
HONCHCOOH + HNO3 → HONC(NO2 )COOH (nitrosylation)
HONC(NO2 )COOH → CO2 + HONC(NO2 )H (decarboxylation)
HONC + Ag + + NO−3 → AgONC(s) + HNO3 (salt precipitation)
2AgCNO → 2Ag + (CNO)2
(CNO)2 → 2CO + N2
(redox reaction)
(release of gas)
(8) (9)
The sudden production of the two gases produces the distinct noise of a Christmas cracker, the only other byproduct being silver metal. In such crackers, two thin strips of cardboard are glued together. One strip contains a small amount of silver fulminate and the other contains a rough surface such as sandpaper.8 When the two strips are pulled apart, the friction of sandpaper rubbing against silver fulminate facilitates the rapid breakdown. Students therefore have the opportunity to revise underlying concepts regarding Lewis structures, resonance, and thermodynamics and to look toward functional group reactivity in the backdrop of the humble Christmas cracker.
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PINENES AND ISOMERISM A fully decorated and lit Christmas pine tree can be an impressive sight. Assuming the tree is not artificial, it is emitting measurable quantities of diverse hydrocarbons into the surrounding air. The release of terpene hydrocarbons from conifer forests is considered indirectly responsible for the blue haze of the Great Smoky Mountains in North Carolina and Tennessee.15,16 One class of biogenic hydrocarbons derived from pine trees are the monoterpenoids, which have the molecular formula C10H16 and are composed of two isoprene building blocks. Two monoterpenoid compounds found in pine oil are α-pinene and β-pinene, which jointly comprise almost 30% of its chemical composition.17 These substances offer an excellent opportunity for undergraduate students to review concepts of isomerism and ring structures of cycloalkanes and cycloalkenes. α-Pinene and β-pinene are constitutional (structural) isomers that are both liquids at room temperature. α-Pinene is the most common monoterpenoid in the natural
(1)
(nitrosylation) (2)
(oxidation)
(7)
In this mechanism, oxidation of ethanol to acetaldehyde is followed by nitrosylation, oxidation, and decarboxylation steps to produce fulminic acid and nitrous acid. In the presence of silver nitrate, silver fulminate precipitates out of solution as a solid.10 The decomposition of silver fulminate was investigated in detail by Boddington and Iqbal.13 In its solid form, silver fulminate is capable of dimerization followed by release of nitrogen gas and carbon monoxide as proposed by eqs 8 and 9. The reaction is presumably forced to completion by the formation of the very strong N−N triple bond, which reinforces thermodynamic principles in a real-world scenario. The Wolff− Kishner reduction of an aldehyde or ketone to an alkane via a hydrazone intermediate is also driven by the thermodynamic stability of N2 and commonly taught in the introductory organic curricula.14
CH3CH 2OH + 2HNO3 → CH3CHO + 2NO2 + 2H 2O
ONCH 2CHO → HONCHCOOH
(decomposition) (6)
form, with the remaining two having multiple negative charges on carbon (which is energetically unfavorable). In contrast, the cyanate anion has two resonance forms that are clearly more significant contributors to the hybrid than the third. In the first two cases, a formal negative charge is placed on an electronegative atom. Therefore, the cyanate anion possesses a higher degree of electron delocalization than the fulminate anion. Silver fulminate is a highly unstable compound and even a tiny amount of friction can lead to its violent decomposition. It therefore has practical use in explosive snaps such as Christmas crackers, 5 million of which are produced every week.8 Workers in factories that produce crackers are reported to experience a skin complaint known as “fulminate itch”, which is thought to have both an irritant and an allergic basis.8 Mercury(II) fulminate, Hg(CNO)2, is significantly more stable and previously saw important use as a detonator in early explosive devices.2 Silver fulminate can be synthesized by reacting ethanol with nitric acid to produce fulminic acid, which is then transformed into its silver salt. The mechanism described below by eqs 1−7 was proposed by Wieland9 in 1907 (where fulminic acid is represented as HONC and silver fulminate as AgONC) and described in this Journal.2,10 It is now established that the correct representations of these compounds are HCNO and AgCNO, respectively.11,12
CH3CHO + HNO2 → ONCH 2CHO
(5)
HONC(NO2 )H → HNO2 + HONC
Figure 2. Resonance structures of (a) fulminate anion and (b) cyanate anion.
(oxidation)
(4)
(3) 1268
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world.18 As constitutional isomers, each has the molecular formula C10H16 but their structures do not share identical atomic connectivity: α-pinene is a trisubstituted alkene and βpinene is a disubstituted alkene (Figure 3). As expected, the
property. Because enantiomers are mirror images of each other, they possess identical physical and chemical properties. It is also apparent that the unstable enantiomeric pair of (1R,5S) and (1S,5R) are both diastereomers of the (1R,5R) and (1S,5S) pair, as they each have a different configuration at only one stereocenter. The (1R,5R) and (1S,5S) stereoisomers are described as “syn” and the (1R,5S) and (1S,5R) stereoisomers are described as “anti”. These terms clarify the relative positions of the two C−C bonds originating from each stereocenter. If all four stereoisomers were to exist as stable compounds, the syn and anti diastereomers of α-pinene would not have identical physical and chemical properties. It is intriguing to note that both enantiomers of α-pinene are found in nature, although in certain species and areas of the world one enantiomer dominates the profile.18 For β-pinene, its (−)-enantiomer is usually the most common one found.18 The fact that both constitutional isomers of pinene and their enantiomeric forms all have a highly similar odor is a property not often shared with other terpenoid compounds. For example, (R)-limonene is responsible for the smell of oranges but the (S)-enantiomer (not produced in natural systems) has the aroma of lemons.20 The isomeric hierarchy diagram for αpinene allows students to appreciate different types of molecular isomerism while providing a direct connection to Christmas pine trees. They will observe that the four stereoisomers drawn for α-pinene (or any other compound with two stereocenters) are enantiomeric pairs of two diastereomers. In addition, there are several undergraduate experiments published in this Journal that utilize α-pinene21,22 and β-pinene23 as starting materials for functional group transformations.
Figure 3. Constitutional isomerism and stereoisomerism in α-pinene and β-pinene.
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SODIUM ACETATE HAND WARMERS Personal hand warmers are popular stocking gifts during the cold northern hemisphere Christmas season. Many hand warmers are based on crystallization of a supersaturated sodium acetate solution, which releases heat. Sodium acetate crystallization is well established as a teaching tool, with several articles and videos published in this Journal.24−29 A typical hand warmer consists of a sealed enclosure filled with sodium acetate trihydrate and water (Figure 4).28 The
two pinenes do not exhibit identical physical and chemical properties. For example, the boiling points of α- and β-pinene are 155−156 °C and 164−166 °C, respectively.19 Using α- and β-pinene as examples, organic students will appreciate that a hierarchy of isomerism exists for certain chiral compounds. Both pinenes contain two stereocenters and students should be able to identify them without much difficulty. Consequently there are a maximum of four possible stereoisomers for each of α- and β-pinene. As shown in Figure 3, if the four possible stereoisomers of α-pinene are drawn out, two important details are revealed. First, the four structures are composed of two pairs of enantiomers, and second, that two of these structures cannot exist as a stable compound. The latter may only be apparent after redrawing each structure in its actual chairlike conformation. Every step taken in this learning process can be repeated for β-pinene. Figure 3 indicates that one pair of enantiomers can form a structure in which the strained four-membered cyclobutane ring is present. The other pair of enantiomers simply cannot form this geometry. A plastic molecular model demonstration will easily illustrate this concept as the latter two structures are too strained to exist. Students can use the Cahn−Ingold−Prelog rules to identify each stereocenter as (R) or (S) in each stereoisomer of αpinene. The two stable enantiomeric structures are (1R,5R) and (1S,5S). (1R,5R)-α-Pinene is also referred to as (+)-α-pinene, and (1S,5S)-α-pinene as (−)-α-pinene. The “+” and “−” nomenclature is an older convention referring to the direction each enantiomer rotates plane polarized light when in solution. It must be emphasized that this notation does not correspond to an (R) or (S) configuration at a specific stereocenter, which is dictated by an actual physical structure rather than a bulk
Figure 4. Crystallization of supersaturated sodium acetate in a Christmas hand warmer: (a) t = 0 s; (b) t = 10 s; (c) t = 90 s.
mass of the sodium acetate dissolved exceeds its aqueous solubility at room temperature. When the hand warmer is heated to 100 °C using boiling water, all the sodium acetate dissolves due to the increased temperature. Upon cooling, a metastable supersaturated solution is formed that persists until the hand warmer is activated. When warmth is desired, a nucleation site is created causing rapid crystallization. This highly exothermic process releases the liberated energy to the surroundings as heat. The entire process is dependent on the supersaturated solution remaining undisturbed until deliber1269
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Scheme 1. (A) In Vivo Metabolism of L-Tryptophan To Form Melatonin and (B) the Challenges of Converting L-Tryptophan to Melatonin in the Laboratory
ately “triggered” by the user. The exact nucleation mechanism at work has been determined by Sandnes.30 Most modern hand warmers contain a metal disk that induces nucleation in the supersaturated solution when flexed by the user (Figure 4). Small sodium acetate seed crystals preserved on the disk are introduced into the solution upon flexing. Introductory organic students will appreciate the similarity of this crystallization process to the solid recrystallization technique they learn in the laboratory. During a recrystallization, an impure solid is dissolved in a hot solvent that forms a supersaturated solution when allowed to cool to ambient temperature. If spontaneous crystallization does not occur, a nucleation site is introduced by adding a seed crystal to the solution or by scratching the glassware to initiate the solidification of pure product crystals. The overall hand warmer concept of solid dissolution at high temperatures followed by cooling and nucleation is very similar.28 The solubility of sodium acetate in water is extremely high at room temperature (408 g/L)31 so relatively large quantities are needed for a typical hand warmer. Ahmad has reported that 30 g of solid is required per 10 mL of water for a classroom demonstration.26 The dissolution process occurs in a purely liquid phase as the melting point of sodium acetate is 58 °C.19 When crystallization occurs, the entropy of the system decreases, which is more than offset by the enthalpy change. This leads to the subsequent warming of the solution that
accompanies crystallization. Students will be able to use the equation for Gibbs energy change (ΔG° = ΔH° − TΔS°) to rationalize these observations by noting the signs of ΔH° (highly negative) and ΔS° (negative). A ΔH° value of −855 kJ/mol has been stated for the sodium acetate crystallization process.28 User safety is ensured once the temperature of the hand warmer reaches 58 °C, as additional heat produced will melt any recently formed solid sodium acetate. As this is its latent heat of fusion, the hand warmer temperature will not exceed 58 °C.28 As the hand warmer becomes opaque during the release of heat (Figure 4C), sodium acetate hand warmers are sometimes called “hot ice”. Although sodium acetate products are extremely popular, hand warmers based on other compounds also exist. One approach is linked to the exothermic oxidation of iron when exposed to air.32 Such hand warmers contain activated charcoal to catalyze the reaction, along with vermiculite and salt as additives. These items have recently found instructional use as a demonstration aid for Avogadro’s Law.33 This design is not reusable because the iron is chemically oxidized to produce iron(III) oxide (rust). In contrast, sodium acetate hand warmers can be easily reused since their mode of action is based solely on a physical process (crystallization of a supersaturated solution). The original state in Figure 4A can be restored by vigorously heating the hand warmer in boiling water. 1270
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group to produce N-acetylserotonin48 (Scheme 1A). Laboratory synthesis of an amide from an amine requires an electrophilic acetyl source such as acetic anhydride (Scheme 1B). This is a common reagent for preparation of acetylsalicylic acid (aspirin) in undergraduate teaching laboratories.49 The body uses acetyl coenzyme A (acetyl-coA) as the electrophilic reagent, with the process mediated by N-acetyltransferase.48 This enzyme acts as an important linker between serotonin (whose levels are high during the day) and melatonin (whose are elevated at night).48 Acetyl-coA contains a thiolate leaving group that accepts another acetyl moiety during pyruvate metabolism.50 The final metabolic step is methylation of the phenolic OH group in N-acetylserotonin to form melatonin (Scheme 1A). This can be achieved in the laboratory via a Williamson ether synthesis, where a phenol is deprotonated under basic conditions. The phenoxide ion then reacts with an alkyl halide to form the ether product (Scheme 1B).51 Under biological conditions, the body uses S-adenosylmethionine (SAM) as an electrophilic methyl source with the enzyme 5-hydroxyindoleO-methyltransferase catalyzing the reaction.52 SAM is an extremely significant metabolite that serves as the most important methylating agent in vivo.53 Melatonin (the “hormone of darkness”) is a neurohormone secreted by the pineal gland in mammals.54 It is involved in regulating circadian rhythms48 and may also have strong antioxidant effects.55 As it is important for maintaining the “biological clock”, melatonin is also often purchased over the counter as a sleep aid.
TRYPTOPHAN METABOLISM One of the most popular meals to have at Christmas is a traditional turkey dinner. Consumption of turkey has led to one of the most enduring misconceptions surrounding human metabolism, namely, that ingestion of tryptophan leads to sleepiness.34 Turkey actually contains relatively low levels of tryptophan (0.3 g/100 g),35 and the drowsiness felt after Christmas dinner is probably related to the quantity of food eaten (or alcohol consumed!), rather than the turkey.34 Tryptophan is an essential aromatic amino acid and one of the 20 commonly found in proteins. Only the L-enantiomer exists in nature, which has a pronounced bitter taste. In contrast, synthetic D-tryptophan has a very sweet taste.36 Despite research casting doubt on the link between turkey and drowsiness, many people purchase tryptophan supplements as a sleep aid. Studies have shown that although tryptophan may facilitate sleep, its effects are only felt at very high doses and usually by people already suffering from insomnia.37 Once tryptophan is consumed, whether from a turkey dinner or via a supplement pill, it undergoes a series of metabolic reactions in the body. One pathway produces melatonin as the final compound (Scheme 1A). This series of biological reactions offers the chance for organic chemistry and biochemistry students to appreciate the power of enzymes in catalyzing in vivo transformations under mild conditions. Significantly, the fundamental functional group conversions observed are all found in introductory organic courses. The initial step in tryptophan metabolism is aromatic hydroxylation to form 5-hydroxytryptophan (5-HT) (Scheme 1A). This reaction is catalyzed by tryptophan hydroxylase and is the rate-limiting process in serotonin production.38 Tryptophan hydroxylation utilizes tetrahydrobiopterin as a cofactor which has an Fe2+ active site and molecular oxygen serves as the OH source.38 In contrast, substitution of an OH group onto an aromatic ring is not usually a simple one-step reaction in the laboratory (Scheme 1B). One potential approach is to sulfonate an aromatic ring with SO3 in sulfuric acid followed by basic hydrolysis at a high temperature.39 However, this strategy is generally limited to the preparation of simple alkyl-substituted phenols. This comparison emphasizes to students the efficiency and ability of enzymes to effect reactions under physiological conditions that are very challenging using traditional reagents. In a second step, 5-HT is decarboxylated in vivo by the aromatic L-amino acid decarboxylase enzyme (AAAD) to generate CO2 and 5-hydroxytryptamine (serotonin).40 In the laboratory, decarboxylation reactions often only proceed if the carboxyl group is in the β-position to a second carbonyl group.41 Amino acids are known to be exceptionally stable to decarboxylation,42 and therefore, decarboxylases possess some of the highest rate enhancements known for enzymatic reactivity.43 AAAD accomplishes this by using pyridoxal phosphate (vitamin B6) as a cofactor. This decarboxylase is also responsible for the biosynthesis of dopamine from Ltyrosine.44 At this stage of L-tryptophan metabolism, students will appreciate that the chirality of the substrate is lost. Serotonin is a neurotransmitter found in the central nervous system that performs several functions in mood regulation.45 For this reason, selective serotonin reuptake inhibitors (SSRIs) are a popular class of antidepressant drugs.46 Synthesis of an SSRI precursor has been described in this Journal as an undergraduate laboratory experiment.47 During the third step of tryptophan metabolism, serotonin is acetylated at its amino
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GINGEROL: THE ALDOL CHEMISTRY OF GINGER Ginger cookies and gingerbread houses are commonly associated with Christmas time and emit a powerful aroma. All ginger products including powders and flavorings come from the rhizomes (underground stems) of the plant Zingiber of f icinale.56 This species presents a variety of organic compounds that give ginger its characteristic odor and taste. One example is gingerol, an aromatic vanilloid substance containing a β-hydroxyketone functionality, which can be considered as the product of an aldol reaction (Scheme 2). The formation of aldol products is taught in second year organic lectures and during many accompanying laboratories.57 The aldol chemistry of gingerol is a good example of how these reactions have applications in the food that students cook and eat. Gingerol and its related derivatives are additionally known to have medicinal benefits.58 The precursors for the laboratory synthesis of gingerol are zingerone and hexanal (Scheme 2), and preparation of racemic gingerol from these compounds has been reported.59 Students can be introduced to the mechanism of this nucleophilic addition reaction and explain the role of lithium diisopropylamide (LDA). Once gingerol is synthesized, a dehydration reaction can occur to give the aldol condensation product (an enone known as shogaol, Scheme 2). The extent to which an aldol dehydrates to give an enone depends on many factors such as temperature and pH and is substrate-specific.60 In addition, both acids and bases can catalyze aldol dehydration reactions.61 Shogaol (derived from the Japanese word for “ginger”) is more pungent than gingerol.62 This can be verified by drying fresh ginger and noting the change in flavor. Provided that water is constantly removed during this process, shogaol should be completely formed from gingerol. Early attempts to isolate the flavorful components of ginger often used conditions favoring gingerol decomposition.63 A later study using a more 1271
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zingerone exhibits an antidiarrheal effect.67 Shogaol has recently been shown to have some promising anticancer effects against human lung cancers.68 Previous work indicates that under conditions similar to the human stomach, the aldol reactions will highly favor gingerol over shogaol at equilibrium.65 If shogaol is the desired active compound, the hydration reaction to gingerol will proceed slowly at 37 °C and not significantly affect shogaol bioavailability.65 Zingerone will not undergo a condensation reaction in the absence of an appropriate aldehyde, thus it can be administered and bioavailable without aldol chemistry being a concern.
Scheme 2. Chemical Transformations between Zingerone, Gingerol, and Shogaol
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SUMMARY This article underscores the connection between fundamental organic principles and the types of substances regularly encountered throughout the Christmas season. The compounds profiled impinge on each of the five senses of sound, taste, smell, sight, and touch. Instructors may wish to use the compounds silver fulminate, α-pinene, β-pinene, and sodium acetate as a way to review some elementary concepts, potentially as 15-min lecture introductions during late November and December. Tryptophan and gingerol additionally serve as starting points to introduce or reinforce some functional group reactivity and mechanistic considerations, depending on curricular structure.
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AUTHOR INFORMATION
Corresponding Author
gentle extraction approach showed that only gingerol contributes significantly to the pungency of fresh ginger.62 Contrary to this study, an article in this Journal states that zingerone is the substance mainly responsible for the pungency of ginger.64 Shogaol is capable of undergoing a retro-aldol reaction to reform racemic gingerol. As water is required for the hydration reaction, cooking dehydrated ginger in the presence of water may lead to a decrease in pungency. In one report, gingerol and shogaol reached a near 50:50 equilibrium when heated to 80 °C and pH 1, whereas maximum stability of shogaol was observed at pH 4.65 Performing the reaction at alkaline pH was not investigated, although an analogous result might be predicted as both acids and bases catalyze aldol reactions. As expected, lower temperatures were found to favor gingerol at equilibrium whereas higher temperatures favored the dehydration reaction to shogaol.65 It is also possible to convert gingerol into zingerone and hexanal using more vigorous conditions (Scheme 2). Assuming the volatile aldehyde is lost to the atmosphere in this process, zingerone and hexanal will be formed until the reaction is complete. Gingerol was found to completely break down to zingerone after refluxing in strong aqueous base.62,63 In contrast, refluxing gingerol with concentrated sulfuric acid gave shogaol as the major product.62 This is consistent with the experiment mentioned previously where shogaol was favored under conditions of high temperature and low pH.65 Because zingerone is the least pungent of the three vanilloids,62 cooking ginger extensively (particularly in the absence of acid) should lead to the mildest taste. Under reaction conditions found in the typical kitchen, a mixture of all three compounds might be expected. Aside from culinary applications at Christmas, compounds found in ginger root exhibit important medicinal properties. Indeed, ginger has long been used to treat a wide variety of symptoms.58 Gingerol is thought to alleviate motion sickness and relieve the symptoms of migraine headaches,66 whereas
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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