Name the enzymes and write the reactions involved in the preparation of ethanol from sucrose by fermentation.

NCERT Exemplar · Q33

Q. Name the enzymes and write the reactions involved in the preparation of ethanol from sucrose by fermentation.

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Concept understanding — Electrophilic Addition Reactions

Electrophilic Aromatic Substitution – The First Meeting

Imagine you have a benzene ring — that perfect, flat hexagon of six carbons with alternating double bonds. It's stable, almost stubbornly so. You want to attach something new to it, say a bromine atom or a nitro group. But benzene doesn't react like an alkene. It doesn't just add across a double bond. Instead, it does something more elegant: it kicks out a hydrogen and keeps its aromatic ring intact.

That's the heart of Electrophilic Aromatic Substitution (EAS).

The Intuition: Why "Substitution" and Not "Addition"?

Benzene's stability comes from its delocalised $\pi$ electrons — a cloud above and below the ring. This cloud is electron-rich, so it attracts electrophiles (electron-loving species). But if an electrophile simply added to a double bond, the ring would break its aromaticity, losing that huge stabilisation. That would be energetically costly.

So benzene does something smarter: it lets the electrophile attack, temporarily breaks aromaticity to form a high-energy intermediate (the arenium ion), and then loses a proton to restore the aromatic ring. The net result? A hydrogen is replaced by the electrophile. The ring is back to its stable, aromatic self.

Note

The key trade-off: temporary loss of aromaticity is acceptable because the final product regains it. Addition reactions would permanently destroy aromaticity — benzene avoids that.

The Precise Statement

Electrophilic Aromatic Substitution is a reaction in which an electrophile ( $E^+$ ) replaces a hydrogen atom on an aromatic ring, proceeding through a sigma complex (arenium ion) intermediate, and restoring aromaticity after deprotonation.

The general equation:

\ce{Ar-H + E+ -> Ar-E + H+}

where Ar represents an aromatic ring.

The Mechanism in Three Steps

Generation of the electrophile – Many EAS reactions need a catalyst to create a strong enough $E^+$ . For example, bromination uses $\ce{FeBr3}$ to polarise $\ce{Br2}$ into $\ce{Br^+}$ .
Attack by the aromatic ring – The $\pi$ electrons of benzene attack the electrophile, forming a sigma complex (also called the arenium ion or Wheland intermediate). This intermediate is non-aromatic — it has four $\pi$ electrons delocalised over five carbons, and one $sp^3$ carbon bearing the electrophile and a hydrogen.

\text{Benzene} + E^+ \longrightarrow \text{Sigma complex (non-aromatic)}

Deprotonation – A base (often the counterion of the catalyst, like $\ce{FeBr4-}$ ) removes the proton from the $sp^3$ carbon. The pair of electrons from the C–H bond flows back into the ring, restoring the aromatic sextet.

\text{Sigma complex} + \text{Base} \longrightarrow \text{Product} + \ce{H+}

Watch out

The sigma complex is not aromatic. It's a high-energy intermediate. Students often mistakenly think it's still aromatic — it isn't. That's why the step is fast and the complex is short-lived.

Why This Matters for Exams

EAS is the gateway to understanding how to put groups onto benzene rings. The rate-determining step is usually the formation of the sigma complex (step 2). The regiochemistry (where the electrophile goes) depends on whether the ring already has a substituent — that's the topic of activating/deactivating groups and ortho/para vs. meta directors. …

Why this formula?

Electrophilic Addition Reactions: Why the Mechanism Works

Electrophilic addition is a cornerstone of alkene and alkyne chemistry. Instead of memorising the "arrow pushing," let's understand why the reaction proceeds the way it does — driven by electron density, stability, and charge.

1. The Core Idea: Why Alkenes React This Way

Alkenes have a π-bond — a cloud of electrons above and below the plane of the σ-bond. This π-electron cloud is:

Electron-rich (nucleophilic)
Exposed (not shielded by σ-bonds like in alkanes)

An electrophile (electron-lover) is attracted to this high electron density. The reaction is electrophilic addition because the electrophile attacks first.

Key principle: The π-bond acts as a Lewis base (electron donor). The electrophile is a Lewis acid (electron acceptor).

2. The General Mechanism (Two-Step)

Step 1: Formation of a Carbocation (or Bridged Intermediate)

The electrophile (E⁺) attacks the π-bond. The π-electrons form a new σ-bond to E⁺, leaving the other carbon with a positive charge — a carbocation.

\ce{C=C} + \ce{E+} \longrightarrow \ce{C+-C-E}

Why does this happen?

The π-bond is weaker than a σ-bond (~260 kJ/mol vs ~350 kJ/mol). Breaking the π-bond to form a σ-bond is energetically favourable because the new σ-bond is stronger. The carbocation is a high-energy intermediate, but it's stabilised by:

Hyperconjugation (alkyl groups donate electron density)
Inductive effect (alkyl groups push electrons toward the positive carbon)

Step 2: Nucleophilic Attack

A nucleophile (Nu⁻) attacks the carbocation, forming a second σ-bond.

\ce{C+-C-E} + \ce{Nu-} \longrightarrow \ce{C-Nu-C-E}

Why does this happen?

The carbocation is electron-deficient (positive charge). The nucleophile is electron-rich. Opposite charges attract — this is electrostatic and orbital overlap driven.

3. The Key "Formula" — Markovnikov's Rule

Statement: In the addition of HX to an unsymmetrical alkene, the hydrogen attaches to the carbon with more hydrogens already, and the X attaches to the carbon with fewer hydrogens.

Why does this rule hold? (The reasoning)

Consider propene: $\ce{CH3-CH=CH2}$ + HBr.

Possible carbocations:
1. Primary carbocation: $\ce{CH3-CH+-CH2Br}$ (less stable)
2. Secondary carbocation: $\ce{CH3-CHBr-CH2+}$ (more stable)

The more substituted carbocation (secondary > primary) is more stable due to:

Hyperconjugation: More alkyl groups = more C–H σ-bonds that can donate electron density into the empty p-orbital of the carbocation.
Inductive effect: Alkyl groups are electron-donating, stabilising the positive charge.

Result: The reaction proceeds via the more stable carbocation, leading to Markovnikov addition.

Markovnikov's rule is not a law — it's a consequence of carbocation stability.

4. The "Anti-Markovnikov" Exception (Why It Happens)

With HBr in the presence of peroxides (ROOR), the addition is anti-Markovnikov — Br goes to the less substituted carbon.

Why? The mechanism changes from ionic to free-radical.

Peroxide decomposes to radicals: $\ce{ROOR -> 2 RO•}$
RO• abstracts H from HBr: $\ce{RO• + HBr -> ROH + Br•}$
Br• adds to the alkene — at the less substituted carbon (because the radical formed is more stable — tertiary > secondary > primary).
The new radical abstracts H from another HBr, regenerating Br•. …

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Fermentation of sucrose to ethanol uses the enzyme invertase to hydrolyse sucrose into glucose and fructose, followed by zymase (a complex of enzymes in yeast) that converts these monosaccharides into ethanol and carbon dioxide. The overall reaction is $\ce{C12H22O11 + H2O ->[invertase] C6H12O6 + C6H12O6 ->[zymase] 4 C2H5OH + 4 CO2}$ .

This is a classic example of how biological catalysts (enzymes) are used in industrial chemistry — a process that has been known for millennia but only understood mechanistically in the last century. The key insight is that sucrose itself cannot be directly fermented by yeast; it must first be broken down into simpler sugars that the yeast can metabolise.

Let’s walk through the two distinct enzymatic stages.

1. Hydrolysis of sucrose by invertase

Sucrose is a disaccharide composed of glucose and fructose linked by an $\alpha$ ‑1,2‑glycosidic bond. Yeast cells secrete the enzyme invertase (also called sucrase), which catalyses the cleavage of this bond:

\ce{C12H22O11 (sucrose) + H2O ->[invertase] C6H12O6 (glucose) + C6H12O6 (fructose)}

Note

Both glucose and fructose have the molecular formula $\ce{C6H12O6}$ , but they are structural isomers. Fructose is a ketose, glucose is an aldose. Yeast can ferment both equally well after this step.

2. Fermentation of monosaccharides by zymase

The mixture of glucose and fructose is then acted upon by a complex of enzymes collectively called zymase, which is present in yeast cells. Zymase is not a single enzyme but a multi‑enzyme system that includes decarboxylases, dehydrogenases, and kinases. The overall net reaction for each monosaccharide is:

\ce{C6H12O6 ->[zymase] 2 C2H5OH + 2 CO2}

This is a multi‑step anaerobic pathway (glycolysis followed by alcoholic fermentation). The key intermediate is pyruvic acid, which is decarboxylated to acetaldehyde and then reduced to ethanol.

Watch out

A common mistake is to write the fermentation of sucrose directly as $\ce{C12H22O11 -> 4 C2H5OH + 4 CO2}$ without including water. The hydrolysis step consumes one molecule of water, so the correct balanced equation must show $\ce{H2O}$ on the reactant side.

3. Overall balanced reaction

Combining the two steps: …

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Here are the common mistakes students make on this question, along with how to avoid each.

Mistake 1: Confusing the two enzymes and their specific roles

The Mistake:

Students often write "yeast" as the enzyme, or they mix up invertase and zymase. They might say invertase converts sucrose to ethanol, or that zymase breaks down sucrose.

Why it happens:

The process is a two-step cascade, but students memorize it as a single "fermentation by yeast" step. They forget that yeast secretes two different enzyme complexes for the two distinct chemical transformations.

How to Avoid:

Remember the "splitting" vs. "converting" logic:

Invertase (or sucrase) is the sugar-splitter. It breaks the disaccharide sucrose into its monosaccharide building blocks (glucose and fructose). This is hydrolysis.
Zymase is the alcohol-maker. It is a complex of enzymes that takes the simple sugars (glucose/fructose) and converts them into ethanol and CO₂. This is fermentation.

Exam Tip: Write the names clearly: Invertase (first step) and Zymase (second step). Never write "yeast" as the enzyme name.

Mistake 2: Writing the wrong chemical formulas or products

The Mistake:

Students write incorrect formulas for sucrose (e.g., $C_{12}H_{22}O_{11}$ is correct, but they might write $C_{12}H_{24}O_{12}$ ). They also forget that fructose is a product of the first reaction, not just glucose.

How to Avoid:

Memorize the exact formula: Sucrose is $C_{12}H_{22}O_{11}$ .
Write both products of hydrolysis: The reaction is:

C_{12}H_{22}O_{11} + H_2O \xrightarrow{\text{Invertase}} C_6H_{12}O_6 + C_6H_{12}O_6

(Glucose and Fructose — both are $C_6H_{12}O_6$ but are structural isomers).

Exam Tip: Write "Glucose + Fructose" explicitly. Do not just write "sugars."

Mistake 3: Forgetting the co-products (CO₂ and water) in the second reaction

The Mistake:

Students write the second reaction as:

C_6H_{12}O_6 \xrightarrow{\text{Zymase}} C_2H_5OH

This is incomplete. It misses the carbon dioxide and the stoichiometric balance.

How to Avoid:

The fermentation of one glucose molecule yields two ethanol molecules and two carbon dioxide molecules. The correct balanced equation is:

C_6H_{12}O_6 \xrightarrow{\text{Zymase}} 2C_2H_5OH + 2CO_2

Exam Tip: Always check the atom balance. Count carbons: 6 on left → 4 (from ethanol) + 2 (from CO₂) = 6 on right.

Mistake 4: Writing the reactions in the wrong order or as a single step

The Mistake:

Students combine both steps into one equation, skipping the intermediate products (glucose and fructose). For example:

C_{12}H_{22}O_{11} + H_2O \xrightarrow{\text{Yeast}} 4C_2H_5OH + 4CO_2

Why it happens:

They think "fermentation" is one process, so they try to write one big equation.

How to Avoid:

Always write two separate, numbered reactions:

Hydrolysis (by invertase) …

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Q.Name the enzymes and write the reactions involved in the preparation of ethanol from sucrose by fermentation.

Concept understanding — Electrophilic Addition Reactions

Electrophilic Aromatic Substitution – The First Meeting

The Intuition: Why "Substitution" and Not "Addition"?

The Precise Statement

The Mechanism in Three Steps

Why This Matters for Exams

Why this formula?

Electrophilic Addition Reactions: Why the Mechanism Works

1. The Core Idea: Why Alkenes React This Way

2. The General Mechanism (Two-Step)

Step 1: Formation of a Carbocation (or Bridged Intermediate)

Step 2: Nucleophilic Attack

3. The Key "Formula" — Markovnikov's Rule

Why does this rule hold? (The reasoning)

4. The "Anti-Markovnikov" Exception (Why It Happens)

1. Hydrolysis of sucrose by invertase

2. Fermentation of monosaccharides by zymase

3. Overall balanced reaction

Concept: Fermentation — Anaerobic Breakdown of Sugars by Enzymes

Method: Invertase + Zymase Fermentation Pathway

Step 1: Hydrolysis of Sucrose (by Invertase)

Step 2: Fermentation of Glucose & Fructose (by Zymase)

Mistake 1: Confusing the two enzymes and their specific roles

Mistake 2: Writing the wrong chemical formulas or products

Mistake 3: Forgetting the co-products (CO₂ and water) in the second reaction

Mistake 4: Writing the reactions in the wrong order or as a single step

Q.Name the enzymes and write the reactions involved in the preparation of ethanol from sucrose by fermentation.

1. Hydrolysis of sucrose by invertase

2. Fermentation of monosaccharides by zymase

3. Overall balanced reaction

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