Ethanol (also called ethyl alcohol or grain alcohol) is an important alkanol with the molecular formula C₂H₅OH. It is the alcohol found in alcoholic beverages and is widely used as a solvent, fuel, antiseptic, and industrial raw material. Ethanol is a volatile, flammable and colorless liquid with a characteristic pleasant smell.
Ethanol consists of an ethyl group (C₂H₅–) attached to a hydroxyl group (–OH).
H H
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H – C – C – O – H
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H H
Condensed formula: CH₃–CH₂–OH
In the laboratory, ethanol is prepared by the fermentation of glucose using yeast as a source of the enzyme zymase.
Ostwald’s Fermentation Equation:
C₆H₁₂O₆ --(zymase/yeast)--> 2C₂H₅OH + 2CO₂ Glucose Ethanol Carbon dioxide
During the process, glucose obtained from sugar or starchy materials is broken down anaerobically at about 30–35°C. The carbon dioxide produced causes frothing during fermentation.
Industrially, ethanol is produced mainly by fermentation of carbohydrates from cassava, sugar cane, or maize. The preparation involves several biochemical steps.
Step 1: Starch Extraction and Milling – Cassava tubers are peeled, washed, and ground into a pulp.
Step 2: Liquefaction – The starch is heated with water to form gelatinized starch.
Step 3: Saccharification – Enzymes (amylase) convert starch into fermentable sugars.
(C₆H₁₀O₅)ₙ + nH₂O --(amylase)--> nC₆H₁₂O₆ Starch Glucose
Step 4: Fermentation – Yeast converts glucose to ethanol and carbon dioxide.
C₆H₁₂O₆ --(zymase)--> 2C₂H₅OH + 2CO₂
Step 5: Distillation – Ethanol is separated by fractional distillation (≈ 78°C).
Step 1: Sugar Extraction from sugar cane juice.
Step 2: Inversion of Sucrose – Sucrose is broken into glucose and fructose using invertase.
C₁₂H₂₂O₁₁ + H₂O --(invertase)--> C₆H₁₂O₆ + C₆H₁₂O₆ Sucrose Glucose Fructose
Step 3: Fermentation – Same fermentation as earlier producing ethanol.
Cellulosic materials (wood, straw, paper waste) are hydrolysed to glucose using cellulase enzymes, then fermented to ethanol.
Ethanol is produced from palmwine through the natural fermentation of the sugars present in the palm sap. Fresh palmwine contains high amounts of glucose and fructose, and when it is collected, natural yeast present in the air and in the container immediately begins to act on these sugars. During fermentation, the yeast converts the sugars into ethanol and carbon dioxide (CO₂) gas. As fermentation continues, more ethanol is formed and more CO₂ is released. When palmwine is kept in a tightly closed bottle, the CO₂ produced has no space to escape, so pressure begins to build up inside the bottle. Over time, the pressure becomes too high and the bottle can burst or explode. It is mainly due to the rapid production and accumulation of CO₂ gas during fermentation. Although old palmwine can gradually oxidize to ethanoic acid (vinegar), giving it a sour taste. Therefore, the explosion happens because of continued fermentation, gas buildup, and increasing internal pressure, especially if the palmwine is still very fresh and active with yeast.
Enzymes are biological catalysts that speed up biochemical reactions.
They work best within specific temperatures (30–40°C for fermentation).
They are protein in nature and can be denatured by high heat.
Specific in action (zymase only catalyzes fermentation).
Work under mild pH conditions (slightly acidic).
A colorless, volatile liquid with a pleasant alcoholic smell.
Boiling point around 78°C.
Miscible with water in all proportions due to hydrogen bonding.
Flammable; burns with a blue, smokeless flame.
Density less than water (0.79 g/cm³).
Good solvent for oils, fats, perfumes, medicines, and dyes.
Combustion – Ethanol burns readily in air to give carbon dioxide and water.
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
Reaction with Sodium Metal – Produces sodium ethoxide and hydrogen gas.
2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂↑
Dehydration (Removal of Water)
Ethanol in the presence of a strong dehydrating agent like Conc. \(H_2SO_4 \) forms Ethyl hydrogen tetraoxosulphate (VI) \(C_2H_5HSO_4 \). If the mixture is reacted with excess Conc. \(H_2SO_4 \) at 170°C, ethene is formed. If the mixture is reacted with excess ethanol at 145°C, diethyl ether is formed.
a. To ethene (using conc. H₂SO₄ at 170°C)
C₂H₅OH --> C₂H₄ + H₂O
b. To ether (using conc. H₂SO₄ at 145°C)
2C₂H₅OH --> C₂H₅–O–C₂H₅ + H₂O
(diethyl ether)
Oxidation
In the presence of a strong oxidizing agent e.g Acidified potassium heptaoxodichromate (VI), \(K_2Cr_2O_7 \) Ethanol is oxidized first to ethanal and then ethanoic acid on further oxidation. The complete oxidation of ethanol produces ethanoic acid . In this reaction, orange \(K_2Cr_2O_7 \) turns green. If \(KMnO_4\) is used, its colour changes from purple to brown due to the deposit of \(MnO_2 \).
a. Mild oxidation gives ethanal
C₂H₅OH + [O] → CH₃CHO + H₂O
b. Further oxidation gives ethanoic acid
CH₃CHO + [O] → CH₃COOH
Esterification – In the presence of concentrated sulphuric acid \(H_2SO_4 \) as catalyst, ethanol reacts reversibly with ethanoic acid to yield a colourless syrupy liquid with a fruity smell called ethyl ethanoate.
C₂H₅OH + CH₃COOH ---> CH₃COOC₂H₅ + H₂O
Ethyl ethanoate (sweet-smelling)
Used in alcoholic beverages (beer, wine, spirits).
Used as a solvent in perfumes, medicines, paints, and cosmetics.
Used as a fuel and in fuel blends (e.g., gasohol).
Used in the manufacture of esters for artificial flavors and fragrances.
Acts as an antiseptic in medical procedures.
Used in the production of chemicals such as ethanal and ethanoic acid.
Used as antifreeze in some industrial applications.
| Test | Observation | Conclusion |
|---|---|---|
| 1. Acidified Potassium Dichromate (K₂Cr₂O₇) Test | Orange solution changes to green. | Ethanol is oxidized to ethanoic acid. Presence of ethanol confirmed. |
| 2. Iodoform Test | Formation of a yellow crystalline precipitate. | Ethanol (and other CH₃–CH(OH) compounds) gives yellow iodoform. |
| 3. Sodium Metal Test | Effervescence due to hydrogen gas release. | Confirms presence of an alkanol group (–OH). Ethanol reacts vigorously. |
Methanol, also called methyl alcohol or wood alcohol, is the simplest alkanol. It is a colourless, volatile and highly poisonous liquid. Unlike ethanol, methanol is not safe for drinking and even small amounts can cause blindness or death because it is oxidized in the liver to methanal and then to methanoic acid, both of which are toxic.
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Test Id : 737373 7-5-2020 ScholarpadThese are the correct answers