Physical Properties and Identification of Acid-Base Properties of Representative Organic Compounds Using Simple Solubility Tests

ORGANIC CHEMISTRY

Organic Chemistry Laboratory CH 200 L (2014-2015) Experiment 5

Examination of the Physical Properties and Identification of Acid-Base Properties of Representative Organic Compounds Using Simple Solubility Tests Matthew S. Abad*, Ara Patricia DG. Abeleda, Chloe Innah Achumbre, Christine Grace SM. Arce Department of Biological Sciences, College of Science, University of Santo Tomas, España Blvd., Manila 1008 Date submitted: October 2, 2014 Abstract: Organic compounds exhibit distinct individual structural compositions that differentiate them from one another. In this experiment, the acid-base properties of representative organic compounds were identified using solubility tests. Solubility tests for acid-base properties determination involve the mechanism of simple miscibility using four solutions namely, water, sodium hydroxide, hydrochloric acid, and sodium bicarbonate. The results of this experiment would determine and show that different representative organic compounds exhibit different individual physical properties, polarity, acidity, and basicity. Keywords: acid-base properties, dielectric constant, functional groups, polarity,

I.

Introduction

This experiment aims to create a defining set of characteristics for compounds belonging to a particular functional grouping based on their structural features. Also, this experiment intends to differentiate each component in a given mix of organic functionality based on solubility and acid-base properties. By looking at the relationship of the properties exhibited by an organic molecule and its structure, one can be able to predict the function of a given compound. The idea that function is dictated by structure is the central subject that binds the concepts in this experiment. Acidity, basicity, and solubility are a few of the functions that are dictated by the structure of an organic molecule. Knowledge of these functions would be of invaluable aid in the planning and execution of different laboratory operations used in

organic chemistry and in everyday life. A concrete example of this concept would be Dimethyl ether and Ethanol. Both these compounds share the same molecular formula, C 2H6O. Having the same constituents, one would expect that they would also share the same set of characteristics. But based on their structures (Figure 1), they exhibit different structural constitution thus a completely different set of properties.

Dimethyl Ether Boiling point: −24 °C Colorless gas Used as aerosol propellant

Ethanol Boiling point: 78.37 °C Colorless liquid Main constituent of beverages

alcoholic

FIGURE 1. Functional groups are the ones that confer the specific properties of different compounds. These are recognizable group of atoms bound to a parent carbon backbone within a molecule. The chemical reactivity of a given compound is determined by these special groups of atoms, because of these, they are considerably less stable than the parents they are attached to and are more likely to be involved in chemical reactions. When a solvent dissolves a solute completely, the molecules and ions of the solute are more or less randomly distributed among those of the solvent. Solubility measures the equilibrium between the substance in the solid state and the substance in the solution. The Dielectric constant measures the capability of a solvent to separate ionic charges. Polarity and the dielectric constant are related to each other. A polar solvent possess a high dielectric constant and a non-polar solvent possess a low dielectric constant. Water, a known polar solvent, has a high dielectric constant of 80. A generalization for solubility rules would be that “like dissolves like”. Thus, a polar solvent dissolves polar solutes and a non-polar solvent readily dissolves non-polar solutes. However, most organic compounds would possess both a polar and non-polar group, solubility of these compounds would then depend on the balance between these two entities. Many factors affect the solubility of a given compound. Some of these factors are listed as follows: 1.) 2.) 3.) 4.) 5.) 6.)

An increase in molecular weight A pi bond lost upon polymerization A substitution between a hydrogen and a halogen An oxygen’s tendency to form hydrates Branching of compounds Position of the functional group in the carbon chain

Solubility in water would determine the polarity of a compound. Compounds dissolved in water indicates the presence of a polar entity. It follows then that compounds insoluble in water are non-polar. A compound that dissolves in a base like sodium hydroxide (NaOH) indicates the probability of a present acidic functional group, deprotonated by the NaOH. A compound that dissolves in a weaker base like sodium bicarbonate (NaHCO3) further differentiates a given acidic functional group as either a weak or strong acid. Solubility in hydrochloric acid (HCl) indicates the presence of a basic functional group, protonated by the HCl.

II.

Methodology

The experiment made use of twelve known organic compounds namely, cyclohexane, ethanol, acetic acid, ethylamine, acetone, 3-pentanone, ethyl acetate, amyl acetate, benzoic acid, phenol, acetanilide, and benzaldehyde. Prior to reacting the compounds with the four different reagents (Figure 2) – water, sodium hydroxide, hydrochloric acid, and sodium bicarbonate – the physical properties of odor and color of the organic compounds were observed and recorded. For each of the given compounds, a 1mL sample was placed into four separate test tubes, labeled A, B, C, and D. For compounds placed in test tubes labeled A, 1ml water was added. For compounds placed in test tubes labeled B, 1mL 1M HCl was added. For compounds placed in test tubes labeled C, 1 mL 1M NaOH was added. Lastly, 1mL 1M NaHCO3 was added to test tubes labeled D. After the reactions took place, the resulting solutions were observed and the results were recorded as whether the given organic compounds were soluble or insoluble with the reagents introduced, whether there was a change in the color, and whether there was a formation of precipitate. All in all, the experiment made use of 48 test tubes.

FIGURE 2.

III.

Results

TABLE 1. Organic Compound

Molecula r Formula

Cyclohexane

C6H12

Ethanol

C2H6O

Structure

Physical State

Color/Odor

Liquid

Colorless/ detergentlike

Liquid

Colorless/ alcohol smell

Acetic Acid

C2H4O2

Liquid

Ethylamine

C2H7N

Liquid

Acetone

C3H6O

Liquid

3-Pentanone

C5H10O

Liquid

Ethyl Acetate

C4H8O2

Liquid

Amyl Acetate

C7H14O2

Liquid

Benzoic Acid

C7H6O2

Solid

Phenol

C6H6O

Liquid

Acetanilide

C8H9NO

Solid

Benzaldehyd e

C7H6O

Liquid

Colorless/ vinegarlike Colorless/ ammonialike Colorless/ pungent Colorless/ nail polishlike smell Colorless/ nail polishlike smell Colorless/ banana-like White crystals/ faint berrylike smell Clear red/ smells like disinfectant Off-white flakes/ odorless Colorless/ almond or cherry-like smell

Solubility 1M NaOH

H2O

1M HCl

insoluble

insoluble

1M NaHCO3

insoluble

insoluble

soluble

soluble

soluble

soluble, no bubbles, with precipitate

soluble

soluble

soluble

soluble, with bubbles

soluble

soluble

soluble

soluble, no bubbles

soluble

soluble

soluble

soluble, no bubbles, with precipitate

insoluble

insoluble

insoluble

insoluble

insoluble

insoluble

insoluble

insoluble

insoluble

insoluble

insoluble

Insoluble

insoluble

insoluble

soluble

soluble, with bubbles

slightly soluble

insoluble

soluble

Insoluble

insoluble

insoluble

insoluble

Insoluble

insoluble

insoluble

slightly soluble

slightly soluble

Table 1 presents the data gathered for the experiment. The last four columns indicate the solubility of the given organic compounds in water, NaOH, HCl, and NaHCO3, respectively. Given also in the table are the structure, physical state, color, and odor of the organic compounds.

IV.

Discussion

FIGURE 3. Low molecular mass alcohols, ketones, amines, aldehydes, and carboxylic acids are small organic compounds that are soluble in water. If a compound is water-soluble, it is then tested in sodium bicarbonate (NaHCO3). A compound soluble in NaHCO3 indicates the presence of a carboxylic acid. A carboxylic acid reacted with NaHCO3 produces carbon dioxide bubbles. Higher molecular mass alcohols, ketones, amines, aldehydes, and carboxylic acids are insoluble with water. Thus other reagents namely, NaOH and HCl, are used to test the solubility of these compounds Compounds insoluble in water and soluble in sodium hydroxide (NaOH) indicate the presence of an acidic functional group. These compounds are then made to react with sodium bicarbonate (NaHCO 3). NaHCO3 being a weaker base is only capable of deprotonating a functional group with a pKa less than 8. It follows then that NaHCO 3 is only capable of dissolving a weak acid, such as carboxylic acids. Strong acids, such as phenols, are insoluble in NaHCO3. A compound insoluble in water, but soluble in hydrochloric acid (HCl), indicates the presence of a basic functional group. The functional group is protonated by HCl to produce an ionic compound. Amines readily dissolve in HCl solutions. Compound insoluble in all reagents constitute a neutral compound. These reactions are summarized in Figure 3.

V.

Conclusion

Knowledge of the acid-base properties of an unknown compound would enable one to determine the possible functional group present in a mix of organic functionality. The aqueous solubility tests done in the experiment prove to be effective in inferring the presence or absence of an acidic or basic organic

group. It was also seen that compounds sharing similar structures/ functional groups are expected to react in almost a similar manner, as in the case of acetic acid and carboxylic acid that produced carbon dioxide bubbles upon introduction of NaHCO3, attributed to their carboxylic acid functional group. The diversity of organic compounds and their structural features can be attributed to the numerous ways on how functional groups arrange themselves. Knowing these reactions would be of great help in handling these compounds for future laboratory experimentations. For the execution of experiments like this, it is strongly advised that contamination of the reagents be avoided. Use separate pipettes for each reagent so as not to alter the desired results. Also, a keen observation on the reactions that took place is vital, such as in the case of identifying if the solution formed two layers given that both compounds are colorless liquids.

VI.

References