When ethanol reacts with potassium dichromate, which one of the following is produced ?

1. Carbon Compounds and Functional Groups (basic)

At the heart of organic chemistry lies the unique ability of carbon atoms to bond with one another, forming straight chains, branched chains, or even rings. This versatility allows carbon to create a staggering variety of molecules, from simple gases to complex biological structures Science, Class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.77. However, the true 'personality' of these molecules is often determined by a specific atom or group of atoms called a functional group. When a functional group replaces a hydrogen atom in a carbon chain, it bestows characteristic chemical properties upon the compound, regardless of how long the carbon chain is Science, Class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.66.

When you have a family of compounds where the same functional group substitutes for hydrogen in carbon chains of different lengths, it is called a homologous series. For instance, methanol (CH₃OH), ethanol (C₂H₅OH), and propanol (C₃H₇OH) all belong to the alcohol homologous series. Because they share the same -OH functional group, their chemical reactions are remarkably similar. To name these compounds, we look at the parent carbon chain and add a specific suffix or prefix. If the suffix begins with a vowel (a, e, i, o, u), we drop the final 'e' of the alkane name—so propane becomes propanol when an alcohol group is added Science, Class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.67.

Functional Group	Formula	Suffix	Example
Alcohol	-OH	-ol	Ethanol
Aldehyde	-CHO	-al	Ethanal
Carboxylic Acid	-COOH	-oic acid	Ethanoic acid

Sources: Science, Class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.66; Science, Class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.67; Science, Class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.77

2. Properties and Uses of Ethanol (basic)

Ethanol, commonly known as alcohol, is a versatile carbon compound with the chemical formula C₂H₅OH. At room temperature, it exists as a clear liquid. One of its most defining physical characteristics is its excellent solubility; it mixes with water in all proportions and serves as a powerful solvent for substances that do not dissolve in water. This makes it indispensable in the medical field for preparing tincture iodine, cough syrups, and various health tonics Science, Class X, Carbon and its Compounds, p.72. While it is the active ingredient in alcoholic beverages, it is important to distinguish between dilute ethanol and absolute alcohol (100% pure ethanol), the latter of which can be lethal even in small quantities.

Chemically, ethanol is highly reactive, especially when exposed to oxidizing agents. When ethanol is heated with acidified potassium dichromate (K₂Cr₂O₇) or alkaline potassium permanganate, it undergoes a transformation into ethanoic acid (CH₃COOH). During this specific reaction, the orange dichromate ions are reduced to green chromium(III) ions, providing a clear visual indicator that oxidation has occurred Science, Class X, Carbon and its Compounds, p.71. This ability to be converted into acetic acid (vinegar) is a fundamental concept in organic chemistry, showing how a simple alcohol can be upgraded to a carboxylic acid.

Beyond the laboratory and the pharmacy, ethanol plays a critical role in sustainable energy. It is increasingly used as an additive to petrol because it is a "cleaner" fuel that produces only carbon dioxide and water upon combustion. In India, the government has implemented policies for Ethanol Blending of Gasoline to reduce dependency on fossil fuel imports and lower carbon emissions Environment, Shankar IAS Academy, India and Climate Change, p.315.

Property Type	Description
Physical	Liquid at room temperature; soluble in water; good solvent for medicines.
Chemical	Oxidizes to ethanoic acid (acetic acid) using K₂Cr₂O₇.
Commercial	Used as a fuel additive (blending) and in alcoholic beverages.

Sources: Science, Class X, Carbon and its Compounds, p.71; Science, Class X, Carbon and its Compounds, p.72; Environment, Shankar IAS Academy, India and Climate Change, p.315

3. Carboxylic Acids: Ethanoic Acid (Acetic Acid) (basic)

Welcome back! Now that we have a grasp of hydrocarbons and alcohols, let's look at a compound that probably sits in your kitchen cupboard right now: Ethanoic Acid, more commonly known as Acetic Acid. As a member of the carboxylic acid family, it is defined by the functional group -COOH. In everyday life, a 5-8% solution of this acid in water is what we call vinegar, which we use as a preservative in pickles and to add tang to our food Science, Class X (NCERT 2025 ed.), Chapter 4, p.73.

One of the most fascinating physical properties of pure ethanoic acid is its melting point, which is 290 K (about 17°C). Because this is close to room temperature, the acid often freezes during winter in colder climates, forming ice-like crystals. This unique behavior earned it the name "Glacial Acetic Acid" Science, Class X (NCERT 2025 ed.), Chapter 4, p.73. Chemically, ethanoic acid is a weak acid. Unlike mineral acids like Hydrochloric acid (HCl) which ionize completely in water, ethanoic acid only partially ionizes, meaning it doesn't release all its hydrogen ions at once.

How do we make it? We can produce ethanoic acid by the controlled oxidation of ethanol. When we add a strong oxidizing agent, such as acidified potassium dichromate (K₂Cr₂O₇) or alkaline potassium permanganate, to ethanol, it adds oxygen to the molecule. During this reaction, you will notice a distinct color change: the orange dichromate ions are reduced to green chromium(III) ions as the ethanol transforms into ethanoic acid.

To keep your basics sharp, remember that different natural sources contain different organic acids. Here is a quick reference table:

Natural Source	Acid Present
Vinegar	Acetic Acid (Ethanoic Acid)
Lemon / Orange	Citric Acid
Ant / Nettle Sting	Methanoic Acid
Sour Milk (Curd)	Lactic Acid

Science, Class X (NCERT 2025 ed.), Chapter 2, p.28

Sources: Science, Class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.71, 73, 74; Science, Class X (NCERT 2025 ed.), Chapter 2: Acids, Bases and Salts, p.28

4. Chemical Reactions: Oxidation in Organic Chemistry (intermediate)

In organic chemistry, oxidation is more than just burning; it is a controlled transformation. While we often think of oxidation as combustion—where carbon compounds burn in oxygen to release CO₂, water, and heat Science, Class X, Carbon and its Compounds, p.69—chemists can also use specific oxidizing agents to add oxygen atoms or remove hydrogen atoms from a molecule without destroying its carbon backbone.

A classic example is the conversion of alcohols to carboxylic acids. When you heat an alcohol like ethanol (CH₃CH₂OH) with a strong oxidizing agent, it doesn't just burn; it transforms into ethanoic acid (CH₃COOH), also known as acetic acid. This process typically uses substances like alkaline potassium permanganate (KMnO₄) or acidified potassium dichromate (K₂Cr₂O₇). These chemicals are known as oxidizing agents because they are capable of adding oxygen to the starting material Science, Class X, Carbon and its Compounds, p.70.

Let’s look at the specific chemistry of Acidified Potassium Dichromate. When it reacts with ethanol, the dichromate ions (Cr₂O₇²⁻) act as the oxygen source. As the ethanol is oxidized to ethanoic acid, the chromium in the dichromate is reduced. You can visually witness this chemical change: the solution shifts from a bright orange color to a distinct green. While the reaction technically passes through an intermediate stage called ethanal (an aldehyde), the presence of excess oxidizing agent ensures the process continues until the stable ethanoic acid is formed.

Feature	Combustion	Controlled Oxidation
Reagent	Oxygen (Air)	Oxidizing agents (e.g., K₂Cr₂O₇)
Final Product	CO₂ and H₂O	Carboxylic Acids (like Ethanoic Acid)
Energy	Large heat/light release	Chemical transformation

Sources: Science, Class X, Carbon and its Compounds, p.69; Science, Class X, Carbon and its Compounds, p.70

5. Strong Oxidizing Agents: KMnO₄ and K₂Cr₂O₇ (intermediate)

In the world of organic chemistry, think of oxidizing agents as the 'oxygen-delivery couriers' of a chemical reaction. According to the fundamental principles of chemistry, oxidation occurs when a substance either gains oxygen or loses hydrogen during a reaction Science, Class X (NCERT 2025 ed.), Chapter 1, p. 12. While many substances can cause oxidation, two 'heavyweights' stand out for their ability to transform alcohols into more complex molecules: Alkaline Potassium Permanganate (KMnO₄) and Acidified Potassium Dichromate (K₂Cr₂O₇).

When these strong oxidizing agents react with a primary alcohol like ethanol, they don't just add a single oxygen atom and stop. They carry out a complete transformation. Initially, the ethanol is converted into an intermediate called ethanal (an aldehyde), but because these agents are so powerful, they immediately push the reaction further until the final stable product is formed: ethanoic acid (commonly known as acetic acid) Science, Class X (NCERT 2025 ed.), Chapter 4, p. 71. This process is essentially 'adding oxygen to the starting material,' which is why these chemicals are defined as oxidizing agents.

One of the most fascinating aspects of these reactions is the visible color change, which serves as a built-in laboratory indicator. For example, when you add purple potassium permanganate to warm ethanol, the purple color initially disappears as the reagent is 'consumed' while doing its work. Only when the ethanol is completely oxidized does the purple color persist Science, Class X (NCERT 2025 ed.), Chapter 4, p. 70. Similarly, potassium dichromate undergoes a dramatic shift from orange to green as it reduces itself while oxidizing the alcohol.

Oxidizing Agent	Initial Color	Color After Reaction
Alkaline KMnO₄	Deep Purple	Colorless (or Brown ppt)
Acidified K₂Cr₂O₇	Bright Orange	Chrome Green (Cr³⁺)

Sources: Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.12; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.70; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.71

6. Natural Organic Acids and their Sources (exam-level)

In our journey through organic chemistry, we encounter a vital class of compounds known as Carboxylic Acids. These are organic compounds characterized by the presence of a carboxyl functional group (—COOH). Unlike mineral acids such as Hydrochloric acid (HCl), which ionize completely in water, organic acids are typically weak acids, meaning they only partially ionize in solution Science, Class X, Carbon and its Compounds, p.73. In nature, these acids serve various roles—from providing the sharp tang in citrus fruits to acting as a chemical defense mechanism in insects.

One of the most commercially significant organic acids is Ethanoic acid, commonly known as Acetic acid. A 5-8% solution of acetic acid in water is what we call vinegar, a staple preservative in our kitchens Science, Class X, Carbon and its Compounds, p.73. Chemically, it can be synthesized through the oxidation of ethanol. When ethanol is treated with strong oxidizing agents like acidified potassium dichromate (K₂Cr₂O₇) or alkaline potassium permanganate (KMnO₄), it transforms into ethanoic acid. A fascinating physical property of pure ethanoic acid is its melting point of 290 K (approx 17°C); because it often freezes into ice-like crystals during winter in cold climates, it is famously referred to as Glacial Acetic Acid.

Nature is a prolific chemist, producing a variety of these acids in the foods we consume and the environment around us. Understanding these sources is crucial for both biology and chemistry:

Natural Source	Organic Acid Present
Vinegar	Acetic Acid (Ethanoic Acid)
Orange / Lemon	Citric Acid
Tamarind	Tartaric Acid
Tomato	Oxalic Acid
Sour Milk (Curd)	Lactic Acid
Ant Sting / Nettle Sting	Methanoic Acid (Formic Acid)

Science, Class X, Acids, Bases and Salts, p.28

Sources: Science, Class X, Acids, Bases and Salts, p.28; Science, Class X, Carbon and its Compounds, p.73

7. The Specific Reaction: Ethanol to Ethanoic Acid (exam-level)

In the study of organic chemistry, the transformation of ethanol (a primary alcohol) into ethanoic acid (a carboxylic acid) is a fundamental oxidation reaction. While we often think of oxidation as combustion (burning), in a laboratory setting, we can use specific chemicals called oxidizing agents to carry out a controlled conversion. As noted in Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.70, alcohols can be converted to carboxylic acids by adding oxygen to the molecule. For ethanol (CH₃CH₂OH), this means losing hydrogen and gaining oxygen to become ethanoic acid (CH₃COOH).

The two most common reagents used for this purpose are alkaline potassium permanganate (KMnO₄) or acidified potassium dichromate (K₂Cr₂O₇). These substances are termed 'oxidizing agents' because they are capable of adding oxygen to other substances. When you add alkaline KMnO₄ to warm ethanol, the deep purple color of the permanganate initially disappears because it is being consumed in the reaction. However, once all the ethanol has been converted, adding more KMnO₄ will cause the purple color to persist as there is no more alcohol left to react with Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.71.

If you use acidified potassium dichromate, you will witness a distinct color shift from orange to green. This happens because the orange dichromate ions (Cr₂O₇²⁻) are reduced to green chromium(III) ions (Cr³⁺) while they oxidize the ethanol. Although the reaction can pass through an intermediate stage called ethanal (an aldehyde), the use of strong, warm oxidizing agents typically ensures the reaction proceeds fully to the final stable product: ethanoic acid.

Sources: Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.70; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.71

8. Solving the Original PYQ (exam-level)

Now that you have mastered the basics of functional groups and chemical reactions, this question serves as the perfect application of alcohol oxidation. You have learned that ethanol is a primary alcohol and that potassium dichromate is a powerful oxidizing agent. As highlighted in Science, Class X (NCERT), this reaction is a classic example of how carbon compounds are transformed by the addition of oxygen. When these building blocks come together, the ethanol molecule undergoes a controlled "combustion" of sorts, where the oxidizing agent forces the alcohol group to evolve into a more stable acidic form.

To arrive at the correct answer, think of the reaction as a two-step progression: ethanol first oxidizes into acetaldehyde, but because the acidified potassium dichromate is such a strong reagent, it rarely stops there. It immediately pushes the reaction further to produce (A) Acetic acid (ethanoic acid). You can mentally verify this reaction by remembering the signature color change from orange to green, which signals that the dichromate has successfully done its job of transforming the alcohol. Since Acetic acid is the stable end-product of this two-carbon chain oxidation, it is the only logical choice.

UPSC often uses "biological distractors" like Citric acid, Malic acid, and Folic acid to test your depth of knowledge. These are complex, multi-carbon organic acids found in fruits and biological processes; they cannot be synthesized by simply oxidizing a basic two-carbon ethanol molecule. A common trap is to choose a familiar-sounding acid from daily life, but your logic should remain focused on the carbon-count: a two-carbon alcohol like ethanol must yield a two-carbon carboxylic acid. By sticking to this fundamental rule, you can confidently eliminate the complex decoys and identify the correct chemical transformation.