Q.What happens when D-glucose is treated with the following reagents?
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Lactose Hydrolysis Products – From Intuition to Precision
Imagine you have a glass of milk. That slightly sweet taste comes from a sugar called lactose. But lactose is a disaccharide – it's actually two smaller sugar units joined together. If you could "unstick" those two units, you'd get two simpler sugars. That unsticking process is hydrolysis (water + breaking), and the two simpler sugars you get are the hydrolysis products.
The Intuition: Breaking a Sugar Chain
Think of lactose as a train with exactly two carriages. The coupling between them is a chemical bond. When you add water and the right conditions (like an enzyme called lactase, or an acid), that bond snaps. The train splits into two separate carriages. Each carriage is now a free, smaller sugar molecule.
So the hydrolysis products are simply the two individual sugar units that were originally linked to form lactose.
The Precise Statement
Lactose (C12H22O11) is a disaccharide composed of one molecule of D-galactose and one molecule of D-glucose linked by a β(1→4) glycosidic bond. Upon hydrolysis (reaction with water), this bond is cleaved, yielding the two monosaccharides:
Lactose+H2Olactase or acidD-Galactose+D-Glucose
The two hydrolysis products are:
- D-Galactose – a monosaccharide (aldohexose, C6H12O6)
- D-Glucose – a monosaccharide (aldohexose, C6H12O6)
Both are reducing sugars, and both have the same molecular formula (C6H12O6) but differ in the arrangement of the hydroxyl group on carbon 4 (they are C-4 epimers).
In the body, the enzyme lactase (present in the small intestine) performs this hydrolysis so that the resulting glucose and galactose can be absorbed into the bloodstream. Lactose intolerance occurs when lactase activity is low, leaving lactose undigested.
Why This Matters for Exams
- Always name both products: galactose and glucose. Never just "sugars" or "monosaccharides."
- Know the bond: β(1→4) glycosidic linkage. Hydrolysis breaks this specific bond. …
Why this formula?
Lactose Hydrolysis Products — Understanding the Why
Lactose is a disaccharide composed of two monosaccharides linked by a glycosidic bond. When it undergoes hydrolysis, the bond is broken, yielding specific products. Let's build the reasoning step by step.
1. What is lactose chemically?
- Lactose = galactose β(1→4) glucose
- The bond is between:
- Carbon-1 of galactose (in β configuration)
- Carbon-4 of glucose
So the structural formula is:
Galactose−O−Glucose
2. What does hydrolysis do?
Hydrolysis means "splitting with water." The reaction is:
Lactose+H2Olactase or acidGalactose+Glucose
The water molecule adds across the glycosidic bond:
- The H from water attaches to the oxygen of the galactose (forming a free –OH on galactose)
- The OH from water attaches to the carbon-1 of glucose (forming a free –OH on glucose)
3. Why are the products exactly galactose and glucose?
Because the glycosidic bond is between specific carbons:
- Galactose contributes its anomeric carbon (C1)
- Glucose contributes its C4
When the bond breaks, each sugar regains its free anomeric carbon (in the case of galactose) or free hydroxyl at C4 (in the case of glucose). No rearrangement occurs — the monosaccharides are released as they were originally linked.
4. Key formula — the hydrolysis equation
The balanced chemical equation:
CX12HX22OX11+HX2OCX6HX12OX6+CX6HX12OX6
- Lactose: CX12HX22OX11
- Water: HX2O
- Products: two molecules of CX6HX12OX6 (one galactose, one glucose)
Why the same molecular formula?
Both galactose and glucose are aldohexoses — they have the same molecular formula CX6HX12OX6 but differ in the arrangement of –OH groups (epimers at C4).
5. The "why" behind the formula
- Mass conservation: The total number of C, H, O atoms before and after must match. …
The key idea is that each reagent targets a specific functional group in glucose, producing characteristic oxidation or reduction products.
Step 1 — Reaction with HI: Hydroiodic acid is a strong reducing agent. It reduces the aldehyde group (−CHO) to a methyl group (−CH3), and also reduces all the hydroxyl groups (−OH) to hydrogen atoms, yielding n-hexane as the final product.
Step 2 — Reaction with bromine water: Bromine water (Br2/H2O) is a mild oxidising agent that selectively oxidises the aldehyde group to a carboxylic acid group, without affecting the alcohol groups. This gives gluconic acid. …
D-glucose reacts differently with each reagent: HI reduces it to n-hexane (cleaving all C–OH bonds), bromine water oxidises only the aldehyde group to give gluconic acid, and HNO₃ oxidises both ends to yield a dicarboxylic acid (glucaric acid). The key is recognising which functional groups each reagent attacks.
Let’s understand why each reagent does what it does. D-glucose is an aldohexose — it has an aldehyde group at C1 and hydroxyl groups on every other carbon. The behaviour of these reagents depends on their oxidising or reducing power and their selectivity.
1. Reaction with HI (hydroiodic acid)
HI is a strong reducing agent. In hot, concentrated HI, all the –OH groups in glucose are replaced by –I, and then the C–I bonds are reduced to C–H bonds. This is a reductive deoxygenation — every hydroxyl group gets removed, and the aldehyde group also gets reduced to a methyl group. The entire carbon chain survives intact, but all oxygen is stripped off.
The product is n-hexane (CH₃–CH₂–CH₂–CH₂–CH₂–CH₃).
A common mistake is to think HI only reduces the aldehyde. In fact, HI under these conditions reduces every C–OH bond, not just the carbonyl.
2. Reaction with bromine water
Bromine water (Br₂ in H₂O) is a mild oxidising agent. It selectively oxidises the aldehyde group (–CHO) to a carboxylic acid (–COOH) without touching the alcohol groups. This is because bromine water specifically targets aldehydes (and not ketones or alcohols) under neutral or slightly acidic conditions.
So D-glucose gives gluconic acid (a monocarboxylic acid where C1 is –COOH, and the rest of the chain remains unchanged).
Bromine water is the classic test for an aldehyde group in sugars. It won’t oxidise secondary alcohols, so it’s perfect for distinguishing aldoses from ketoses. …
Method: Reagent-Specific Functional Group Analysis
This method uses the functional group reactivity of D-glucose (an aldohexose) to predict products with each reagent.
Steps
Step 1: Identify the functional groups in D-glucose
- One aldehyde group (–CHO) at C1
- Four secondary alcohol groups (–OH) at C2, C3, C4, C5
- One primary alcohol group (–CH₂OH) at C6
Step 2: Apply each reagent based on its known reaction with these groups
(i) With HI (hydroiodic acid) — Reductive cleavage
- HI is a strong reducing agent that cleaves C–O bonds and reduces all oxygen-containing groups.
- All –OH groups are replaced by –H, and the aldehyde is reduced to –CH₃.
- Product: n-Hexane (CH3–CH2–CH2–CH2–CH2–CH3)
(ii) With bromine water (Br2/H2O) — Oxidation of aldehyde only
- Bromine water is a mild oxidizing agent that selectively oxidizes the aldehyde group to a carboxylic acid (–COOH).
- Alcohol groups remain unchanged.
- Product: Gluconic acid (a six-carbon aldonic acid)
(iii) With HNO3 (nitric acid) — Oxidation of both ends
- HNO3 is a strong oxidizing agent that oxidizes both: …
Here are the common mistakes students make when answering questions about the reactions of D-glucose with HI, Bromine water, and HNO₃, along with how to avoid each.
Mistake 1: Confusing the reaction with HI as simple reduction
The error: Students often write that HI simply reduces the aldehyde group (−CHO) to an alcohol (−CH2OH), treating it like a mild reducing agent.
Why it’s wrong: HI is a strong reducing agent under heat. It does not stop at the aldehyde. It reduces all the hydroxyl groups (−OH) to hydrogen atoms (−H), converting the sugar into a straight-chain hydrocarbon.
Correct outcome:
D-glucose + excess HI (heat) → n-Hexane (CH3(CH2)4CH3).
The entire carbon chain is reduced, and all oxygen atoms are removed.
How to avoid:
- Remember: HI + heat = complete deoxygenation of sugars.
- Think of it as “stripping” every −OH group, leaving only a saturated alkane.
Mistake 2: Thinking bromine water oxidizes all parts of glucose
The error: Students assume bromine water (Br2/H2O) is a strong enough oxidant to break the glucose chain or oxidize all carbon atoms.
Why it’s wrong: Bromine water is a mild, selective oxidant. It only oxidizes the aldehyde group (−CHO) to a carboxylic acid group (−COOH). It does not affect the alcohol groups (−OH) on the rest of the chain.
Correct outcome:
D-glucose + Bromine water → Gluconic acid (a monocarboxylic acid).
The aldehyde at C1 becomes −COOH, while all other −OH groups remain unchanged.
How to avoid:
- Memorize: Bromine water = specific test for aldehydes in sugars.
- Do not confuse it with strong oxidants like HNO3 (see next mistake).
Mistake 3: Confusing the action of HNO3 with bromine water
The error: Students write that HNO3 also gives gluconic acid, or that it only oxidizes the aldehyde group.
Why it’s wrong: HNO3 is a strong oxidizing agent. It oxidizes both ends of the glucose chain — the aldehyde group (−CHO) at C1 and the primary alcohol group (−CH2OH) at C6 — into carboxylic acid groups (−COOH).
Correct outcome: …
Showing the 12 most recent of 16 on this concept.
- CBSE 2026Set 56/3/11 markMCQQ.On hydrolysis, which of the following carbohydrates gives only glucose ? (A) Sucrose (B) Galactose (C) Lactose (D) Maltose
›Reveal solutionSolution
Hydrolysis breaks glycosidic bonds in disaccharides to release monosaccharides. Only maltose yields glucose alone because it is composed of two glucose units linked together. The answer is (D).
Understanding Carbohydrate Hydrolysis
When a disaccharide undergoes hydrolysis, water molecules break the glycosidic bond connecting two monosaccharide units. The products depend entirely on which monosaccharides were originally joined to form that disaccharide. To answer this question, we need to know the composition of each carbohydrate listed.
The key insight: if a disaccharide is made from two identical glucose molecules, hydrolysis will give us only glucose. If it contains any other monosaccharide (fructose, galactose), those will appear in the products too.
Analyzing Each Option
- Sucrose (Option A) Sucrose is a disaccharide formed from glucose and fructose linked by an α(1→2) glycosidic bond. When hydrolyzed:
Sucrose+H2O⟶Glucose+Fructose
This gives two different monosaccharides, so sucrose does not yield only glucose.
-
Galactose (Option B)
Galactose is already a monosaccharide (a simple sugar). It cannot be hydrolyzed further because there are no glycosidic bonds to break. This option is a distractor—hydrolysis doesn't apply here.
-
Lactose (Option C)
Lactose, the sugar found in milk, is composed of glucose and galactose joined by a β(1→4) glycosidic bond. Hydrolysis produces:
Lactose+H2O⟶Glucose+Galactose
Again, two different monosaccharides result, not just glucose.
- Maltose (Option D) …
- CBSE 2025Set ANNUAL1 markQ.Fill in the blank: Sucrose on hydrolysis gives glucose and ________.
›Reveal solutionSolution
Sucrose is a disaccharide of glucose and fructose; hydrolysis breaks the glycosidic bond releasing both monosaccharides.
Sucrose + H2O --(acid or enzyme invertase)--> Glucose + Fructose
…
- CBSE 2025Set BOTANY1 markMCQQ.Fill in the blank selecting the appropriate one: The enzyme beta-galactosidase breaks lactose into ____ and glucose.(a) galactose(b) sucrose(c) glycerol(d) erythrose
›Reveal solutionSolution
beta-galactosidase splits the disaccharide lactose into its two monosaccharide units, galactose and glucose.
Lactose is a disaccharide made of one galactose unit joined to one glucose unit by a beta-1,4 glycosidic bond. The enzyme beta-galactosidase (encoded by the z gene of the lac operon) hydrolyses this bond, releasing galactose and glucose, which the cell can then metabolise. Since the questio …
- CBSE 2024Set ANNUAL1 markQ.Name the milk sugar present in milk.
›Reveal solutionSolution
Lactose ("milk sugar") is the main sugar found in milk, a disaccharide made of glucose and galactose joined by a beta-1,4-glycosidic linkage.
Milk contains lactose as its principal carbohydrate, typically making up around 4-5% of milk by mass. Lactose is a disaccharide formed from one molecule of beta-D-galactose and one molecule of beta-D-glucose linked by a beta-1,4-glycosidic bond. On hydrolysis (e.g. b …
- CBSE 2023Set F1 markMCQQ.Which of the following disaccharides is present in milk?(a) Sucrose(b) Lactose(c) Maltose(d) None of these
›Reveal solutionSolution
Lactose (glucose + galactose) is the disaccharide naturally present in milk.
Lactose, commonly called milk sugar, is made of one D-glucose and one D-galactose unit joined by a beta-1,4-glycosidic bond. It occurs in the milk of mammals. Sucrose (cane sugar) is gluco …
- CBSE 2023Set A1 markQ.Match the following. Column A item: 'Milk sugar'. Choose its correct match from Column B:(a) Ether(b) Primary amine(c) Lactose(d) C12H22O11(e) Glucose(f) Negative ions(g) C6H5SO2Cl(h) +7
›Reveal solutionSolution
'Milk sugar' is the common name for lactose, the disaccharide found in milk.
Lactose is a disaccharide composed of β-D-galactose and β-D-glucose units joined by a glycosidic linkage; it is naturally pr …
- CBSE 2023Set ANNUAL1 markMCQQ.Which of the following disaccharide is present in milk?(i) Sucrose(ii) Glucose(iii) Lactose(iv) Cellulose
›Reveal solutionSolution
Lactose ('milk sugar') is the characteristic disaccharide of milk.
Sucrose is cane/beet sugar; glucose is a monosaccharide, not a disaccharide; cellulose is a polysaccharide (plant cell walls). Lactose is a disaccharide made of one glucose and one galactose unit joined by a β-1,4 glycosidic linkage, an …
- CBSE 2022Set ANNUAL1 markQ.What are the expected products of hydrolysis of lactose?
›Reveal solutionSolution
Lactose (milk sugar) is a disaccharide of glucose + galactose; on hydrolysis it yields one molecule of glucose and one of galactose.
Lactose, or milk sugar, is a reducing disaccharide.
It is composed of one β-D-galactose unit and one β-D-glucose unit linked by a β(1→4) glycosidic bond.
On hydrolysis (acid- or enzyme-catalysed by lactase), this glycosidic linkage is cleaved:
…
- CBSE 2021Set ANNUAL1 markQ.Name the sugar present in the milk.
›Reveal solutionSolution
Milk sugar = lactose, a disaccharide of glucose + galactose.
Milk contains the disaccharide lactose ("milk sugar"), made of one molecule of β-D-galactose and one molecule of β-D-glucose joined by a β-1,4-glycosidic linkage.
…
- CBSE 2020Set 56/3/11 markQ.Name the disaccharide which on hydrolysis gives two molecules of glucose.
›Reveal solutionSolution
The disaccharide that yields two molecules of glucose upon hydrolysis is maltose, because it is composed of two α-D-glucose units linked by an α(1→4) glycosidic bond.
The key to this question lies in understanding what a disaccharide is and how hydrolysis works. A disaccharide is a carbohydrate formed when two monosaccharides join together through a glycosidic bond, with the elimination of a water molecule. Hydrolysis is the reverse process — adding water breaks that bond, releasing the two original monosaccharides.
So the question is simply: which common disaccharide is built from two glucose units? Let’s recall the major disaccharides and their monosaccharide components.
- Sucrose (table sugar) is made of one glucose and one fructose. Hydrolysis gives glucose + fructose — not two glucoses.
- Lactose (milk sugar) is made of one glucose and one galactose. Hydrolysis gives glucose + galactose — again, not two glucoses.
- Maltose (malt sugar) is made of two glucose units. Hydrolysis of maltose yields exactly two molecules of glucose.
- Cellobiose is also made of two glucose units, but it is a product of cellulose breakdown and less commonly encountered in basic exam contexts. However, it too gives two glucoses on hydrolysis. …
- CBSE 2020Set ANNUAL1 markQ.Which disaccharide on hydrolysis in presence of the catalyst invertase produces glucose and fructose?
›Reveal solutionSolution
Sucrose is the disaccharide that, on enzymatic hydrolysis by invertase, breaks the glycosidic bond to give one molecule each of glucose and fructose.
Sucrose is a non-reducing disaccharide formed by an alpha-1,2-glycosidic linkage between the anomeric carbons of alpha-D-glucose and beta-D-fructose (which is why, unlike maltose or lactose, it shows no free reducing end). On hydrolysis - either by dilute acid or by the enzyme invertase (sucrase) - this glycosidic bond is cleaved:
Sucrose + H2O --invertase--> Glucose + Fructose
…
- CBSE 2020Set ANNUAL1 markMCQQ.The disaccharides present in milk is(a) sucrose(b) maltose(c) lactose(d) cellulose
›Reveal solutionSolution
Milk's characteristic sugar is lactose, a disaccharide of galactose and glucose — option (c).
Lactose is the principal carbohydrate found in milk. It is a reducing disaccharide made of one unit of β-D-galactose and one unit of β-D-glucose, joined by a β-1→4 glycosidic linkage. Sucrose (cane sugar) comes from sug …
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