Ethanol and Ethanoic Acid: Important Carbon Compounds
A comprehensive guide for chemistry students
Introduction
Carbon forms the backbone of a vast array of compounds essential to life and industry. Among these, ethanol (C2H5OH) and ethanoic acid (CH3COOH) stand out as particularly significant organic compounds with diverse applications across various fields. These two substances exemplify the versatility of carbon chemistry and represent important functional group categories: alcohols and carboxylic acids, respectively.
This comprehensive guide explores the structures, properties, reactions, and applications of these compounds from an examination perspective, equipping you with the knowledge required for a thorough understanding of these foundational topics in organic chemistry.
Ethanol (C2H5OH)
Structure and Nomenclature
Molecular Formula: C2H5OH or CH3CH2OH
IUPAC Name: Ethanol
Common Names: Ethyl alcohol, Grain alcohol
H H
| |
H-C-C-O-H
| |
H H
Ethanol is the second member of the alcohol homologous series, consisting of two carbon atoms with a hydroxyl (-OH) functional group attached to one of them. The functional group (-OH) is responsible for the properties of alcohols.
Physical Properties
- Appearance: Colorless, volatile liquid
- Odor: Pleasant, characteristic alcoholic smell
- Boiling Point: 78.37°C
- Melting Point: -114.1°C
- Solubility: Miscible with water in all proportions due to hydrogen bonding
- Density: 0.789 g/cm³ at 20°C
Hydrogen Bonding in Ethanol: The hydroxyl group (-OH) in ethanol can form hydrogen bonds with water molecules, explaining its complete miscibility with water. This hydrogen bonding also explains ethanol’s relatively high boiling point compared to hydrocarbons of similar molecular weight.
Methods of Preparation
1. Fermentation
The traditional and most common method for producing ethanol is through the fermentation of sugars by yeast:
In this process, glucose is converted to ethanol and carbon dioxide by enzymes present in yeast, primarily zymase. The process occurs in the absence of oxygen (anaerobic conditions) and is typically conducted at temperatures between 25-30°C.
Exam Note: Remember that fermentation can only produce ethanol up to approximately 14-15% concentration as higher concentrations become toxic to the yeast.
2. Hydration of Ethene
Industrial production often uses the direct hydration of ethene (ethylene) with steam under high pressure in the presence of an acid catalyst:
This method is economically viable for large-scale industrial production of ethanol.
Chemical Properties of Ethanol
1. Combustion
Ethanol undergoes complete combustion in excess oxygen to produce carbon dioxide and water with the release of energy:
This property makes ethanol a valuable biofuel and an alternative to fossil fuels.
2. Oxidation
Ethanol can be oxidized to form acetaldehyde (ethanal) and then further to ethanoic acid. The reagents commonly used for this oxidation are acidified potassium dichromate or potassium permanganate:
(Ethanol) → (Acetaldehyde) → (Ethanoic acid)
The oxidation of ethanol proceeds in two steps: first to an aldehyde and then to a carboxylic acid. The characteristic color change during this reaction (orange to green when using K2Cr2O7) is used as a qualitative test for alcohols.
Exam Tip: The orange color of potassium dichromate (K2Cr2O7) changes to green (Cr3+) during the oxidation of ethanol. This color change forms the basis of the breathalyzer test used to detect alcohol consumption.
3. Dehydration
When heated with concentrated sulfuric acid at about 170°C, ethanol undergoes dehydration to form ethene:
(Ethanol) → (Ethene) + (Water)
This reaction demonstrates the elimination of water from alcohols to form alkenes.
4. Reaction with Sodium
Ethanol reacts with sodium metal to produce sodium ethoxide and hydrogen gas:
(Ethanol) + (Sodium) → (Sodium ethoxide) + (Hydrogen)
This reaction is characteristic of the acidic nature of the hydroxyl group in alcohols and can be used to test for the presence of the -OH group.
5. Esterification
Ethanol reacts with carboxylic acids in the presence of concentrated sulfuric acid (as a catalyst) to form esters:
(Ethanol) + (Ethanoic acid) → (Ethyl ethanoate) + (Water)
This reaction is reversible and follows Le Chatelier’s principle. The ester formed (ethyl ethanoate) has a characteristic fruity smell.
Applications of Ethanol
- Beverages: Used in alcoholic beverages (beers, wines, spirits)
- Solvent: Used as a solvent in the pharmaceutical industry, perfumes, and cosmetics
- Fuel: Used as a biofuel alternative to gasoline (petrol) either directly or blended (gasohol)
- Antiseptic: Used in medical wipes and hand sanitizers due to its antimicrobial properties
- Chemical Intermediate: Used in the production of various chemicals including ethanoic acid, ethyl esters, and ethyl halides
- Preservative: Used in biological specimens and medical formulations
Ethanoic Acid (CH3COOH)
Structure and Nomenclature
Molecular Formula: CH3COOH or C2H4O2
IUPAC Name: Ethanoic acid
Common Name: Acetic acid
H O
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H-C-C
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H O-H
Ethanoic acid is the second member of the carboxylic acid homologous series. It contains the carboxyl functional group (-COOH), which consists of a carbonyl group (C=O) and a hydroxyl group (-OH).
Physical Properties
- Appearance: Colorless liquid with a pungent, vinegar-like odor
- Boiling Point: 118°C
- Melting Point: 16.6°C (pure ethanoic acid forms a crystalline solid called glacial acetic acid below this temperature)
- Solubility: Highly soluble in water due to hydrogen bonding
- Density: 1.049 g/cm³ at 20°C
- pH: 2.4 for a 1M solution (weakly acidic)
Glacial Acetic Acid: Pure ethanoic acid (99.5% or higher) is called glacial acetic acid because it forms ice-like crystals at temperatures below 16.6°C.
Methods of Preparation
1. Oxidation of Ethanol
Ethanoic acid can be prepared by the oxidation of ethanol using oxidizing agents like potassium dichromate in acidic conditions:
(Ethanol) → (Ethanoic acid)
2. Methanol Carbonylation (Monsanto Process)
One of the most important industrial methods involves the reaction of methanol with carbon monoxide in the presence of a rhodium catalyst:
3. Oxidation of Acetaldehyde
Oxidation of acetaldehyde (ethanal) with appropriate oxidizing agents:
(Acetaldehyde) → (Ethanoic acid)
4. Fermentation (Vinegar Production)
Ethanoic acid in vinegar is produced by the bacterial oxidation of ethanol:
This process, known as the acetous fermentation, is used to produce vinegar commercially.
Chemical Properties of Ethanoic Acid
1. Acidic Character
Ethanoic acid is a weak acid that partially dissociates in aqueous solution:
It turns blue litmus red and reacts with bases to form salts called ethanoates (or acetates).
2. Reaction with Bases
Ethanoic acid undergoes neutralization reactions with bases to form salts and water:
(Ethanoic acid) + (Sodium hydroxide) → (Sodium ethanoate) + (Water)
This neutralization reaction is fundamental to acid-base chemistry and is used in analytical determinations.
3. Reaction with Carbonates and Bicarbonates
Ethanoic acid reacts with carbonates and bicarbonates to produce corresponding salts, water, and carbon dioxide:
CH3COOH + NaHCO3 → CH3COONa + H2O + CO2↑
The effervescence of carbon dioxide is a characteristic test for carboxylic acids.
4. Reaction with Alcohols (Esterification)
As mentioned earlier, ethanoic acid reacts with alcohols in the presence of a strong acid catalyst to form esters:
(Ethanoic acid) + (Ethanol) → (Ethyl ethanoate) + (Water)
Esters are characterized by their pleasant fruity odors and are widely used in perfumes and artificial flavors.
5. Reaction with PCl5 or SOCl2
Ethanoic acid reacts with phosphorus pentachloride or thionyl chloride to form acetyl chloride:
(Ethanoic acid) + (Phosphorus pentachloride) → (Acetyl chloride) + (Phosphoryl chloride) + (Hydrogen chloride)
Acetyl chloride is an important reagent for the introduction of the acetyl group in organic synthesis.
6. Reduction
Ethanoic acid can be reduced to ethanol using strong reducing agents like lithium aluminum hydride (LiAlH4):
(Ethanoic acid) → (Ethanol)
Applications of Ethanoic Acid
- Food Industry: Used as vinegar (5-8% solution) for flavoring and preservation
- Chemical Production: Used to manufacture vinyl acetate monomer (for adhesives and paints), acetate esters, and cellulose acetate (for films and fibers)
- Textile Industry: Used in dyeing processes and as a textile finishing agent
- Pharmaceutical Industry: Used in the production of aspirin (acetylsalicylic acid) and other medications
- Household Uses: Used as a descaling agent, cleaner, and disinfectant
- Agricultural Applications: Used in some herbicides and pesticides
Comparison Between Ethanol and Ethanoic Acid
| Property | Ethanol (C2H5OH) | Ethanoic Acid (CH3COOH) |
|---|---|---|
| Functional Group | Hydroxyl (-OH) | Carboxyl (-COOH) |
| Physical State (Room Temp) | Colorless liquid | Colorless liquid |
| Odor | Pleasant, alcoholic | Pungent, vinegar-like |
| Boiling Point | 78.37°C | 118°C |
| Acidic Character | Very weakly acidic (pKa ≈ 15.9) | Weakly acidic (pKa ≈ 4.76) |
| Reaction with Sodium | Produces hydrogen gas (H2) | Produces hydrogen gas (H2), but reaction is more vigorous |
| Reaction with Sodium Carbonate | No reaction | Effervescence due to CO2 evolution |
| Oxidation Product | Forms ethanal, then ethanoic acid | Resistant to further oxidation by common oxidizing agents |
| Esterification | Acts as a nucleophile (attacks carbonyl carbon) | Acts as an electrophile (carbonyl carbon is attacked) |
| Primary Use | Solvent, fuel, beverages | Food additive (vinegar), chemical precursor |
Key Exam Tips and Common Questions
Important Points to Remember
- Functional Groups: Always identify the functional groups (-OH in ethanol, -COOH in ethanoic acid) as they determine the chemical properties.
-
Testing for Ethanol: Ethanol can be identified by:
- The iodoform test (gives a yellow precipitate with iodine in the presence of NaOH)
- Oxidation with K2Cr2O7/H2SO4 (orange to green color change)
-
Testing for Ethanoic Acid: Ethanoic acid can be identified by:
- Reaction with NaHCO3 or Na2CO3 (effervescence of CO2)
- Ester formation test (fruity smell when heated with an alcohol and conc. H2SO4)
- Industrial Uses: Be able to explain the industrial importance of both compounds and their manufacturing processes.
- Chemical Reactions: Know the key reactions, reagents, conditions, and products for both compounds.
- Reaction Mechanisms: For advanced courses, understand the mechanisms of key reactions like esterification.
Common Exam Mistakes to Avoid
- Confusing the products of ethanol oxidation (ethanal vs. ethanoic acid)
- Forgetting to mention the catalyst in esterification reactions (conc. H2SO4)
- Incorrectly balancing chemical equations, especially for combustion reactions
- Confusing physical properties of ethanol and ethanoic acid
- Forgetting that ethanoic acid is a weak acid, not a strong one
Sample Exam Problems
Problem 1: Esterification Reaction
Question: Calculate the mass of ethyl ethanoate (C4H8O2) that can be produced from the reaction of 23 g of ethanol with excess ethanoic acid. The molecular masses are: C = 12, H = 1, O = 16.
Solution:
Step 1: Write the balanced chemical equation:
Step 2: Calculate molar masses:
- Molar mass of ethanol (C2H5OH) = (2 × 12) + (6 × 1) + 16 = 24 + 6 + 16 = 46 g/mol
- Molar mass of ethyl ethanoate (C4H8O2) = (4 × 12) + (8 × 1) + (2 × 16) = 48 + 8 + 32 = 88 g/mol
Step 3: Calculate moles of ethanol:
Moles of ethanol = Mass / Molar mass = 23 g / 46 g/mol = 0.5 mol
Step 4: Use stoichiometry to find moles of ethyl ethanoate:
From the balanced equation, 1 mol of ethanol produces 1 mol of ethyl ethanoate.
Therefore, 0.5 mol of ethanol will produce 0.5 mol of ethyl ethanoate.
Step 5: Calculate mass of ethyl ethanoate:
Mass of ethyl ethanoate = Moles × Molar mass = 0.5 mol × 88 g/mol = 44 g
Answer: 44 g of ethyl ethanoate can be produced.
Problem 2: Acidic Strength
Question: Explain why ethanoic acid (pKa ≈ 4.76) is a stronger acid than ethanol (pKa ≈ 15.9) using structural and electronic effects.
Solution:
The acidic strength of a compound is related to the stability of its conjugate base after proton loss. The more stable the conjugate base, the stronger the acid.
Ethanoic acid (CH3COOH):
- When ethanoic acid loses a proton, it forms the ethanoate ion (CH3COO–).
- In the ethanoate ion, the negative charge is delocalized (spread out) over two oxygen atoms through resonance.
- This resonance stabilization distributes the negative charge, making the ethanoate ion more stable.
- The carbonyl oxygen’s electronegative nature also helps stabilize the negative charge.
Ethanol (C2H5OH):
- When ethanol loses a proton, it forms the ethoxide ion (C2H5O–).
- In the ethoxide ion, the negative charge is localized on a single oxygen atom with no resonance stabilization.
- The lack of resonance makes the ethoxide ion less stable than the ethanoate ion.
Conclusion: The greater stability of the ethanoate ion due to resonance and inductive effects makes ethanoic acid a stronger acid than ethanol by approximately 11 orders of magnitude (pKa difference of ~11).
Properties of Ethanol and Ethanoic Acid
The next topic in our series explores in further detail the physical and chemical properties of ethanol and ethanoic acid, examining their behavior in various chemical environments and their applications in industrial and laboratory settings.
Next Topic: Properties of Ethanol and Ethanoic Acid →Preview: Soaps and Detergents
In the upcoming installment of our carbon chemistry series, we will explore the fascinating chemistry of soaps and detergents, building upon the concepts we’ve learned about ethanoic acid and other carboxylic acids. We’ll examine how these important cleaning agents function at a molecular level and their environmental implications.
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