Acid Base Properties

Acid-Base Properties of Cyclohexane, Ethanol, Amyl Acetate, Ethylamine, Phenol, and Benzoic Acid

Rowena Chiang*, Maria Gianna Beatrice Cancio, Joe Mari Isabella Caringal, Patricia Deanne del Valle Department of Biological Sciences, College of Science, University of Santo Tomas, Manila, Philippines

Abstract The organic compounds belonging to different functional groups have different properties affecting their chemical behavior. The experiment results are the following: cyclohexane and amyl acetate immiscible with HCl and NaOH; ethanol and ethylamine miscible with both reagents; phenol immiscible with HCl but miscible with NaOH, also does not form effervescence with NaHCO3; and benzoic acid insoluble in HCl but soluble in NaOH, also form effervescence with NaHCO3.

Introduction Organic compounds are the most abundant constituents of today’s commercially available products. Many of these occur as acids or bases that are utilized by large-scale industries to produce necessary materials for everyday life. Acids and bases each have unique identities by having functional groups attached in their structural compounds. In effect, these functional groups predict the chemical behavior of the compounds to which they are connected [1]. As such, this experiment aims to describe the structural features common to compounds that belong to the same functional group and to differentiate each functional group based on solubility and acid-base properties. All compounds possess physical properties that can be determined without changing the chemical identity of that compound [2]. Examples of these properties are physical state (liquid, solid, gas), color, odor, solubility, and density. As mentioned earlier, acidic and basic compounds occur in nature. According to Bronsted-Lowry, acids are proton donors while bases are proton acceptors. Meanwhile, a Lewis acid serves as an electron pair acceptor and a Lewis base serves as an electron pair donor. [3] Arrhenius, a Swedish chemist, said that acids ionize in aqueous solutions to increase the concentration of hydrogen ions and anions. On the other hand, bases ionize in aqueous solutions to increase the concentration of hydroxide ions and

cations. [4] In general, acids and bases react with each other completely yielding salt and water and this process is called neutralization. However, some acids and bases are weak that they cannot completely ionize in aqueous solutions. One way to determine an organic compound’s behavior is to compare its solubility in aqueous acids and bases. Solubility is the ability of a solvent to dissolve a solute in a solution. It also refers to the maximum amount of solute that dissolves in a solvent at equilibrium. Solubility is slightly different from miscibility, which refers to how easily a liquid dissolves in another liquid when mixed together. Solubility is influenced by the following factors: Temperature, Pressure, and Polarity. First, high temperature is relevant in increasing the solubility of solid substances. The principle behind this is that the heat provides the energy to break the bonds in the molecules of the solid. Second, “the solubility of a gas in a liquid is directly proportional to the pressure of that gas above the surface of that solution,” (Ophardt, 2003) as stated in Henry’s Law [5]. As more pressure is applied to the gas molecules, the more it will dissolve in the solution below it. Nevertheless, changes in pressure do not affect the solubility of liquids and solids. Third, polarity denotes that like dissolves like. Polar solvents like water dissolve polar solutes while nonpolar solvents such as hexane dissolve nonpolar solutes.

Results and Discussion The reagents used in the experiment are hydrochloric acid (HCl), sodium hydroxide (NaOH), and sodium bicarbonate (NaHCO3). Hydrochloric acid completely dissociates in water, therefore it is a strong acid. Sodium hydroxide, a strong metallic base, reacts with strong acids to form water and salts. Sodium bicarbonate, a white crystalline solid, decomposes to water and carbon dioxide when mixed with acids. Effervescence is a characteristic of decomposition giving off bubbles in the solution. As seen in Table 1 on the next page, cyclohexane is an alkane having no functional group. It is also a neutral compound and cannot form hydrogen bonds because of its nonpolar characteristic rendering it immiscible in both HCl and NaOH. Unlike cyclohexane, ethanol can form hydrogen bonds and is considered neutral but can act as either acid or base. When acting as an acid with pKa of 16, it reacts with NaOH and exists in equilibrium with the products as illustrated in the equation. [6] CH3CH2OH + NaOH

CH3CH2ONa + H2O

When acting as a base, it reacts with hydrogen halides such as HCl to yield ethyl halides. This process of halogenation occurs via a substitution reaction. [6] CH3CH2OH + HCl → CH3CH2Cl + H2O Furthermore, the structure of ethanol (with C–H bonds and C–OH bonds) denotes that it dissolves in both polar and nonpolar substances rendering it miscible in NaOH and HCl. Amyl acetate, an ester, participates in hydrogen bonding by accepting but not donating hydrogen bonds. Because of the hydrogen bonding, amyl acetate is more soluble in water than hydrocarbons but not as polar as alcohols or acids due to its limited bonding. Moreover, esters cannot form hydrogen bonds with each other [7]. Because of this property of amyl acetate, it is immiscible in both HCl and NaOH. With the functional group amine, ethylamine acts as a base almost always. As mentioned earlier, Lewis bases donate electron pairs. Thus, basicity of amines depends on the availability of lone electron pairs in the nitrogen atom and on the electronic properties of the compound’s substituents. Since the N atom in the amine group attaches 2 hydrogen atoms and an ethyl group, it is considered as a primary amine (R–NH2). Also, the lone pair on the N atom creates a greater electron density making the amine molecule more basic. [8] Despite the strong basicity of ethylamine, it still has a slight degree of acidity. This is due to the fact that its primary amine group has 2 protic hydrogen atoms unlike tertiary amine (R–N– R), which has no acidic hydrogen. It is also important to note that ethylamine is still more basic than acidic. [8] It can also form hydrogen bonds just like ethanol and amyl acetate. Thus, being basic and slightly acidic, it is miscible in both HCl and NaOH. The balanced equations for these reactions are: CH3CH2NH2 + HCl → CH3CH2NH3 + Cl (reaction with HCl) and CH3CH2NH2 + NaOH → CH3CH2NH + H2O + Na (reaction with NaOH). Phenol or carbolic acid, an aromatic compound, can form hydrogen bonds due to the alcohol group attached to it. It is only slightly acidic because it has a low tendency of losing its hydrogen ion and because it is connected to a hydrocarbon exhibiting resonance. When the hydrogen ion leaves, the delocalized electrons in the resonance make the conjugate base stable and therefore, make phenol a weak acid. [9] Nonetheless, compared to most alcohols, it has a higher tendency of donating the hydrogen ion in its alcohol group. Thus, it reacts with NaOH, a strong base, through the neutralization process.

Figure 1. Reaction of phenol with sodium hydroxide yielding sodium phenoxide and water.

Phenol does not react with HCl considering that they are both acids and cannot take each other’s hydrogen ions. Moreover, phenol also does not react and does not form effervescence with NaHCO3. Sodium bicarbonate is a weak base and can only readily accept protons when a stronger acid is used such as carboxylic acids. Being a weak base, it cannot remove protons easily from phenol. Benzoic acid, an odorless solid, can also form hydrogen bonds and acts as a strong acid due to its carboxylic functional group. It is also soluble in NaOH by the neutralization reaction as illustrated in the figure below.

Figure 2. Reaction of benzoic acid with sodium hydroxide by neutralization.

Acting as acid just like phenol, benzoic acid also does not react with HCl. Conversely, it reacts with sodium bicarbonate by decomposing to water and carbon dioxide. This reaction occurs because of the strong acidity of the benzoic acid. Effervescence, as mentioned before, is a result of mixing sodium bicarbonate with a strong acid.

Figure 3. Decomposition reaction of benzoic acid and sodium bicarbonate.

Experimental Physical Properties. Duplicate test tubes containing 20 drops of the sample were labeled with cyclohexane, ethanol, amyl acetate, ethylamine, and phenol. Test tube for benzoic acid was also prepared. Each of their physical states and colors was observed and recorded. Solubility. The solubility of the compounds was tested in aqueous base by adding 20 drops of 1M NaOH in one duplicate of each test tube. The procedure was repeated using 1M HCl in another duplicate of the test tubes. Effervescence. The test tubes for phenol and benzoic acid were treated with 10 drops of NaHCO3 and resulting effervescence was observed.

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