Properties of Ethanol & Ethanoic Acid - Tenth Grade Science
Ethanol and ethanoic acid represent two of the most industrially significant and commercially manufactured organic compounds derived from the carbon family. Ethanol, commonly known as ethyl alcohol, serves as a universal solvent, chemical intermediate, pharmaceutical antiseptic, and alternative renewable biofuel. Ethanoic acid, commonly known as acetic acid, is the defining organic acid of domestic vinegar and an essential industrial raw material for manufacturing cellulose acetate textiles, polyvinyl acetate adhesives, and synthetic esters. Because both compounds share a two-carbon aliphatic skeleton while bearing distinct functional groups—a neutral hydroxyl group in ethanol and an acidic carboxyl group in ethanoic acid—they provide a clear comparison of organic structure, hydrogen bonding, and chemical reactivity. In this chapter, you will investigate the physical properties, industrial synthesis, and diagnostic chemical reactions of ethanol and ethanoic acid, explore esterification and saponification, analyze the physiological effects of alcohol, and examine denaturation safeguards.
Physical Properties and Molecular Architecture of Ethanol
Ethanol ($\text{C}_2\text{H}_5\text{OH}$) is the second member of the homologous alcohol series, featuring a terminal hydroxyl group ($-OH$) attached to an ethyl group.
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| PHYSICAL PROFILE OF ETHANOL (C2H5OH) |
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| Molecular Formula | C2H6O (Condensed: CH3-CH2-OH) |
| Molecular Mass | 46.07 g/mol |
| Physical State at 20 deg C | Clear, colorless, volatile liquid |
| Characteristic Odor | Pleasant, sweet spirituous aroma |
| Boiling Point | 78.37 deg C (351.5 K) |
| Freezing Point | -114.1 deg C (159 K) |
| Aqueous Solubility | Completely miscible in all proportions |
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Intermolecular Hydrogen Bonding and Boiling Point
Ethanol boils at $78.4^\circ\text{C}$, a substantially higher temperature than its structural isomer dimethyl ether ($\text{CH}_3-\text{O}-\text{CH}_3$), which boils at $-24^\circ\text{C}$, despite both compounds sharing the identical molecular mass ($46\text{ g/mol}$). This marked difference arises from the high electronegativity of oxygen relative to hydrogen ($3.44$ versus $2.20$), which polarizes the hydroxyl covalent bond ($-\text{O}^{\delta-}-\text{H}^{\delta+}$).
The partial positive charge on the hydrogen atom interacts electrostatically with the lone electron pair on the oxygen atom of an adjacent ethanol molecule, forming intermolecular hydrogen bonds:
CH3 - CH2 - O - H : : : : O - CH2 - CH3
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H H
Overcoming these intermolecular hydrogen bonds requires significant thermal kinetic energy, elevating the boiling point of ethanol.
Complete Aqueous Miscibility
Ethanol dissolves in water in all proportions. The polar hydroxyl group forms extensive hydrogen bonds with surrounding water molecules, releasing hydration enthalpy that overcomes the hydrophobic interactions of the short two-carbon ethyl tail. However, in higher alcohols with longer aliphatic carbon chains (such as hexanol or octanol), the non-polar hydrophobic hydrocarbon chain dominates, drastically reducing water solubility.
Chemical Properties and Diagnostic Reactions of Ethanol
Ethanol exhibits characteristic chemical reactions governed by its hydroxyl functional group.
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| KEY CHEMICAL REACTIONS OF ETHANOL |
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| 1. Reaction with Sodium Metal | 2C2H5OH + 2Na -----> 2C2H5ONa + H2(g) (Pop) |
| 2. Acid-Catalyzed Dehydration | C2H5OH --[conc H2SO4, 443 K]--> C2H4 + H2O |
| 3. Controlled Chemical Oxidation | C2H5OH --[K2Cr2O7/H+]--> CH3COOH (Orange->Grn)|
| 4. Esterification with Acids | C2H5OH + CH3COOH <==[H+]==> CH3COOC2H5 + H2O |
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Reaction with Sodium Metal
Ethanol behaves as a very weak acid (weaker than water), reacting with active electropositive metals like sodium to evolve hydrogen gas:
2CH3CH2OH(l) + 2Na(s) -> 2CH3CH2ONa(alc) + H2(g)
The reaction proceeds with steady effervescence, producing a solution of sodium ethoxide ($\text{CH}_3\text{CH}_2\text{ONa}$), a strong organic base. The evolved hydrogen gas can be confirmed by its characteristic popping sound upon ignition with a burning splint. This reaction is often used to safely dispose of waste sodium metal in the laboratory.
Acid-Catalyzed Dehydration to Ethene
When ethanol is heated with an excess of concentrated sulfuric acid ($\text{H}_2\text{SO}_4$) at precisely 443 kelvin (170 degrees Celsius), it undergoes intramolecular dehydration:
CH3CH2OH(l) --[Excess conc H2SO4, 443 K]--> CH2=CH2(g) + H2O(l)
Concentrated sulfuric acid acts as a powerful dehydrating agent, abstracting the elements of water from adjacent carbon atoms in the alcohol molecule to generate the unsaturated alkene ethene. If the reaction temperature is lowered to 413 kelvin (140 degrees Celsius) with excess ethanol, intermolecular dehydration occurs instead, producing diethyl ether:
2CH3CH2OH --[conc H2SO4, 413 K]--> CH3CH2-O-CH2CH3 + H2O
Physical Properties and Molecular Architecture of Ethanoic Acid
Ethanoic acid ($\text{CH}_3\text{COOH}$), commonly known as acetic acid, is the second member of the carboxylic acid family.
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| PHYSICAL PROFILE OF ETHANOIC ACID (CH3COOH) |
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| Molecular Formula | C2H4O2 (Condensed: CH3-COOH) |
| Molecular Mass | 60.05 g/mol |
| Physical State at 20 deg C | Colorless, corrosive liquid |
| Characteristic Odor | Pungent, sharp vinegar scent |
| Boiling Point | 118.1 deg C (391.2 K) |
| Melting / Freezing Point | 16.6 deg C (289.8 K) |
| Common Solution (Vinegar) | 5% to 8% aqueous solution |
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Glacial Acetic Acid and Dimerization
Ethanoic acid has a freezing point of $16.6^\circ\text{C}$ ($62^\circ\text{F}$). In cold climates or laboratory refrigerators, pure anhydrous ethanoic acid freezes into an icy crystalline solid, historically termed glacial acetic acid.
Ethanoic acid has a high boiling point ($118.1^\circ\text{C}$), well above that of alcohols of comparable molecular mass. In pure liquid phase and non-polar solvents, ethanoic acid molecules pair up via dual hydrogen bonds, forming stable cyclic dimers:
O : : : : : H - O
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CH3 - C C - CH3
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O - H : : : : : O
This dimerization doubles the effective molecular mass in solution, strengthening intermolecular attractions and elevating the boiling point.
Acidity and Comparison with Mineral Acids
Ethanoic acid is a weak monocarboxylic acid that ionizes partially in aqueous solution:
$$\text{CH}_3\text{COOH(aq)} + \text{H}_2\text{O(l)} \rightleftharpoons \text{CH}_3\text{COO}^-\text{(aq)} + \text{H}_3\text{O}^+\text{(aq)} \quad (K_a \approx 1.8 \times 10^{-5})$$
While strong mineral acids such as hydrochloric acid ($\text{HCl}$) dissociate completely ($\alpha \approx 100\%$), a $0.1\text{ M}$ solution of ethanoic acid ionizes to approximately $1.3\%$, yielding a pH around $2.9$. Consequently, ethanoic acid reacts more slowly and less vigorously with metals and carbonates than equimolar hydrochloric acid.
Chemical Properties and Diagnostic Reactions of Ethanoic Acid
Ethanoic acid exhibits characteristic carboxylic acid reactions with alkalis, metal carbonates, and alcohols.
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| KEY CHEMICAL REACTIONS OF ETHANOIC ACID |
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| 1. Neutralization with Alkalis | CH3COOH + NaOH -----> CH3COONa + H2O |
| 2. Reaction with Carbonates | 2CH3COOH + Na2CO3 -> 2CH3COONa + H2O + CO2(^) |
| 3. Reaction with Bicarbonates | CH3COOH + NaHCO3 -> CH3COONa + H2O + CO2(^) |
| 4. Esterification with Ethanol | CH3COOH + C2H5OH <==[H+]==> CH3COOC2H5 + H2O |
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Reactions with Carbonates and Bicarbonates
When ethanoic acid is added to solid sodium carbonate or sodium hydrogen carbonate, brisk effervescence occurs as carbon dioxide gas is evolved:
2CH3COOH(aq) + Na2CO3(s) -> 2CH3COONa(aq) + H2O(l) + CO2(g)
CH3COOH(aq) + NaHCO3(s) -> CH3COONa(aq) + H2O(l) + CO2(g)
The escaping carbon dioxide gas turns clear limewater milky due to calcium carbonate precipitation. This effervescence test is the standard diagnostic laboratory method for distinguishing carboxylic acids from neutral alcohols, as ethanol does not react with metal carbonates.
Esterification and Saponification Equilibrium
Esterification is the reversible condensation reaction between a carboxylic acid and an alcohol in the presence of an acid catalyst, yielding an ester and water:
CH3COOH(l) + CH3CH2OH(l) <==[conc H2SO4, Heat]==> CH3COOCH2CH3(l) + H2O(l)
When ethanoic acid is warmed with absolute ethanol and a few drops of concentrated sulfuric acid in a water bath, the mixture produces ethyl ethanoate (ethyl acetate), an ester with a pleasant, fruity aroma. Esters are used industrially in perfumes, artificial flavoring agents, and lacquers.
Saponification is the alkaline hydrolysis of an ester, reversing esterification. When ethyl ethanoate is boiled with aqueous sodium hydroxide, it cleaves into sodium ethanoate and ethanol:
CH3COOCH2CH3(l) + NaOH(aq) -> CH3COONa(aq) + CH3CH2OH(aq)
Because this reaction is used in soap manufacturing—where natural glyceryl esters of fatty acids are cleaved with lye to produce soap and glycerol—the process is called saponification.
Physiological Effects of Ethanol and Industrial Denaturation
Ethanol is a central nervous system depressant that disrupts cellular metabolism and neurological signaling.
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| METABOLIC PATHWAYS OF ETHANOL IN THE BODY |
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| Ethanol Acetaldehyde Acetic Acid |
| CH3-CH2-OH --[ADH]--> CH3-CHO --[ALDH]--> CH3-COOH |
| (Depressant) (Toxic Toxin) (Krebs Cycle / CO2) |
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| ADH: Alcohol Dehydrogenase (Liver Enzyme) |
| ALDH: Aldehyde Dehydrogenase (Liver Enzyme) |
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Physiological and Neurological Toxicity
Upon ingestion, ethanol is rapidly absorbed through the gastric lining and transported into the bloodstream. In the brain, ethanol enhances inhibitory gamma-aminobutyric acid ($\text{GABA}$) receptors and inhibits excitatory NMDA glutamate receptors, depressing the central nervous system. This causes impaired coordination, slowed reaction times, reduced inhibitions, slurred speech, and blurred vision.
In the liver, the enzyme alcohol dehydrogenase oxidizes ethanol into acetaldehyde ($\text{CH}_3\text{CHO}$), a toxic and reactive compound that damages cellular macromolecules and causes hangover symptoms. Prolonged chronic alcohol consumption overwhelms hepatic antioxidant defenses, causing fatty liver disease, fibrous tissue deposition, and irreversible hepatic cirrhosis.
Methanol ($\text{CH}_3\text{OH}$) is significantly more toxic than ethanol. When consumed even in tiny quantities ($10\text{ to }30\text{ mL}$), methanol is oxidized in the liver by alcohol dehydrogenase into methanal (formaldehyde, $\text{HCHO}$), which denatures protoplasm, coagulating proteins much like egg white when cooked. Formaldehyde also oxidizes into formic acid ($\text{HCOOH}$), causing severe metabolic acidosis and attacking the optic nerve, resulting in permanent blindness and death.
Denatured Alcohol Safeguards
Ethanol produced for industrial manufacturing, varnishes, and cosmetics is subject to minimal commercial taxation, whereas recreational alcoholic beverages carry heavy excise duties. To prevent industrial ethanol from being diverted for illicit beverage consumption without paying duties, it is denatured:
Denatured alcohol is ethanol rendered poisonous, foul-tasting, and foul-smelling by the addition of small quantities of adulterating chemical additives, typically $5\%$ to $10\%$ toxic methanol, accompanied by pungent pyridine and blue indicators like copper(II) sulfate or methylene blue. The distinctive color and unpalatable taste prevent accidental consumption.
Chapter Exercises
Exercise 1: Describe two distinct chemical tests that can be performed in the laboratory to differentiate between a sample of pure ethanol and a sample of pure ethanoic acid, providing balanced chemical equations and visible observations for each test.
Exercise 2: Write balanced chemical equations for the following transformations: first, the reaction between ethanoic acid and sodium hydroxide; second, the reaction between ethanoic acid and solid sodium hydrogen carbonate; and third, the reaction between ethanol and acidified potassium dichromate under sustained heating.
Exercise 3: Explain the chemical basis of esterification, describe the dual catalytic role played by concentrated sulfuric acid in this reaction, and explain how the pleasant odor of the product confirms that esterification has occurred.
Solutions and Explanations
Solution 1: The first chemical test is the sodium hydrogen carbonate effervescence test. Add a small spatula of solid sodium hydrogen carbonate ($\text{NaHCO}_3$) to separate test tubes containing each liquid. The test tube containing ethanoic acid exhibits vigorous effervescence as carbon dioxide gas is evolved: $\text{CH}_3\text{COOH} + \text{NaHCO}_3 \rightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O} + \text{CO}_2\text{(g)}$, and the gas turns limewater milky. The test tube containing ethanol shows no effervescence or observable reaction. The second chemical test is the blue litmus paper test. Moisten blue litmus paper with a drop of each liquid. Ethanoic acid ionizes in the moisture to produce hydronium ions ($\text{CH}_3\text{COOH} + \text{H}_2\text{O} \rightleftharpoons \text{CH}_3\text{COO}^- + \text{H}_3\text{O}^+$), turning the blue litmus paper red. Ethanol is a neutral covalent molecule that does not produce hydronium ions in water, leaving the blue litmus paper unchanged.
Solution 2: For the reaction between ethanoic acid and sodium hydroxide: $\text{CH}_3\text{COOH(aq)} + \text{NaOH(aq)} \rightarrow \text{CH}_3\text{COONa(aq)} + \text{H}_2\text{O(l)}$. For the reaction between ethanoic acid and solid sodium hydrogen carbonate: $\text{CH}_3\text{COOH(aq)} + \text{NaHCO}_3\text{(s)} \rightarrow \text{CH}_3\text{COONa(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$. For the oxidation of ethanol by acidified potassium dichromate under sustained heating: $\text{CH}_3\text{CH}_2\text{OH(l)} + 2[\text{O}] \xrightarrow{\text{K}_2\text{Cr}_2\text{O}_7 / \text{H}_2\text{SO}_4, \text{Heat}} \text{CH}_3\text{COOH(aq)} + \text{H}_2\text{O(l)}$, with the solution transitioning from bright orange to deep green as dichromate ions ($\text{Cr}_2\text{O}_7^{2-}$) reduce to chromium(III) ions ($\text{Cr}^{3+}$).
Solution 3: Esterification is an acid-catalyzed reversible condensation reaction between the carboxyl group ($-COOH$) of a carboxylic acid and the hydroxyl group ($-OH$) of an alcohol, forming an ester linkage ($-COO-$) with the elimination of water. In this reaction, concentrated sulfuric acid serves two catalytic roles: first, it acts as an acid catalyst, protonating the carbonyl oxygen of the carboxylic acid to increase the electrophilicity of the carbonyl carbon, accelerating nucleophilic attack by the alcohol oxygen; second, it functions as a strong dehydrating agent, absorbing water from the equilibrium mixture. By removing water, sulfuric acid shifts the dynamic equilibrium toward ester formation according to Le Chatelier's principle, maximizing ester yield. The reaction is confirmed when the pungent vinegar odor of ethanoic acid and the sharp spirituous scent of ethanol are replaced by the sweet, fruity aroma characteristic of ethyl ethanoate.