10.1 Carbohydrates
Sugars, starch and cellulose: how carbohydrates are classified, how chemists worked out the open-chain and ring structures of glucose, and how glycosidic links build disaccharides and polysaccharides.
A spoonful of table sugar, a slice of bread, a cotton shirt and the paper of a notebook are made of almost the same thing. Sucrose, starch and cellulose are all built from the six-carbon sugar glucose. Yet sugar dissolves in tea, starch is digested within an hour of a meal, and cellulose passes through the human gut untouched and can last for centuries as wood. The difference lies in how the glucose units are joined: one small change in the direction of a single bond decides whether a polymer is food for people or food only for termites and cows.
Carbohydrates are the most abundant organic compounds on Earth. Green plants make them from carbon dioxide and water by photosynthesis, and almost every living cell burns glucose for energy. In this section you will classify carbohydrates, follow the chemical evidence that reveals the structure of glucose (and why a simple open chain is not the whole story), meet the ring forms called anomers, and see how glycosidic linkages join monosaccharides into sucrose, maltose, lactose, starch, cellulose and glycogen.
Classifying Carbohydrates
The name "carbohydrate" comes from the old observation that many of these compounds have the formula $\mathrm{C_{x}(H_{2}O)_{y}}$, as if they were hydrates of carbon: glucose is $\mathrm{C_{6}H_{12}O_{6}}$, or $\mathrm{C_{6}(H_{2}O)_{6}}$. The formula is not a reliable guide, though. Rhamnose, $\mathrm{C_{6}H_{12}O_{5}}$, is a carbohydrate that does not fit it, while ethanoic acid, $\mathrm{C_{2}H_{4}O_{2}}$, and methanal, $\mathrm{CH_{2}O}$, fit it but are not carbohydrates. Chemists therefore define carbohydrates by their functional groups.
Carbohydrates are optically active polyhydroxy aldehydes or polyhydroxy ketones, or compounds that give these on hydrolysis. By their behaviour on hydrolysis they are classified as:
- Monosaccharides: cannot be hydrolysed into simpler polyhydroxy aldehydes or ketones. About 20 occur in nature; examples are glucose, fructose, galactose and ribose.
- Oligosaccharides: give two to ten monosaccharide units on hydrolysis. A disaccharide gives two (identical or different) units: sucrose gives glucose and fructose; maltose gives two glucose units. Trisaccharides give three, tetrasaccharides four.
- Polysaccharides: give a large number of monosaccharide units on hydrolysis: starch, cellulose, glycogen, gums.
Monosaccharides and oligosaccharides are crystalline, water-soluble and sweet; they are called sugars. Polysaccharides are not sweet and are called non-sugars.
A monosaccharide is an aldose if it contains an aldehyde group and a ketose if it contains a keto group. The number of carbon atoms is shown by the stem: triose (3), tetrose (4), pentose (5), hexose (6), heptose (7). Glucose is an aldohexose, fructose a ketohexose, ribose an aldopentose.
Carbohydrates that reduce Fehling's solution and Tollens' reagent are reducing sugars. All monosaccharides, aldoses and ketoses alike, are reducing sugars. A disaccharide is reducing if one of its units still has a free aldehyde or keto group (more exactly, a free ring carbon C1 or C2 that can open, see below), as in maltose and lactose. If the reducing groups of both units are used up in the bond that joins them, as in sucrose, it is a non-reducing sugar.
| Carbon atoms | General term | Aldose | Ketose |
|---|---|---|---|
| 3 | triose | aldotriose (glyceraldehyde) | ketotriose (1,3-dihydroxypropanone) |
| 4 | tetrose | aldotetrose | ketotetrose |
| 5 | pentose | aldopentose (ribose) | ketopentose |
| 6 | hexose | aldohexose (glucose, galactose) | ketohexose (fructose) |
| 7 | heptose | aldoheptose | ketoheptose |
Glucose: from an Open Chain to a Ring
Glucose occurs free in sweet fruits and honey, and combined in sucrose, starch, cellulose and glycogen. It is prepared
- from sucrose (cane sugar), by boiling with dilute $\mathrm{HCl}$ or $\mathrm{H_{2}SO_{4}}$ in alcoholic solution; glucose and fructose form in equal amounts: $$\mathrm{C_{12}H_{22}O_{11}+H_{2}O\xrightarrow{\ H^{+}\ }C_{6}H_{12}O_{6}+C_{6}H_{12}O_{6}}$$ (sucrose $\rightarrow$ glucose $+$ fructose);
- from starch, commercially, by boiling with dilute $\mathrm{H_{2}SO_{4}}$ at $393\ \mathrm{K}$ under a pressure of 2–3 atm: $$\mathrm{(C_{6}H_{10}O_{5})_{n}+n\,H_{2}O\xrightarrow{\ H^{+}\ }n\,C_{6}H_{12}O_{6}}$$
How do we know the structure of glucose? Each of the reactions below answers one question about the molecule. Write $\mathrm{R}$ for $\mathrm{HOCH_{2}(CHOH)_{4}{-}}$, so that the open-chain formula of glucose is $\mathrm{R{-}CHO}$.
- Formula $\mathrm{C_{6}H_{12}O_{6}}$ from analysis and molar mass.
- Straight chain of six carbons. On prolonged heating with $\mathrm{HI}$ glucose gives hexane, $\mathrm{CH_{3}(CH_{2})_{4}CH_{3}}$, an unbranched chain.
- A carbonyl group. Glucose forms an oxime with hydroxylamine, $\mathrm{R{-}CH{=}NOH}$, and adds $\mathrm{HCN}$ to give a cyanohydrin, $\mathrm{R{-}CH(OH)CN}$.
- The carbonyl group is an aldehyde. The mild oxidising agent bromine water converts glucose to gluconic acid, $\mathrm{R{-}COOH}$, a six-carbon carboxylic acid with no loss of carbon.
- Five $\mathrm{{-}OH}$ groups on different carbons. Acetylation with ethanoic anhydride gives a stable glucose pentaacetate. (Two $\mathrm{OH}$ groups on one carbon would lose water and could not both be acetylated.)
- A primary alcohol group. Nitric acid oxidises glucose and gluconic acid alike to saccharic (glucaric) acid, $\mathrm{HOOC(CHOH)_{4}COOH}$: the $\mathrm{{-}CH_{2}OH}$ at the other end of the chain becomes a second $\mathrm{{-}COOH}$.
Conclusion: glucose is $\mathrm{OHC{-}(CHOH)_{4}{-}CH_{2}OH}$, a 2,3,4,5,6-pentahydroxyhexanal. Its four $\mathrm{CHOH}$ carbons are stereocentres; the arrangement found by Emil Fischer is drawn below.
The prefixes D and L give the relative configuration of a sugar, compared with glyceraldehyde. In the Fischer projection (aldehyde or keto group at the top), look at the stereocentre farthest from C1, the lowest $\mathrm{CHOH}$ (C5 in glucose and fructose). If its $\mathrm{{-}OH}$ is on the right, as in (+)-glyceraldehyde, the sugar belongs to the D series; if it is on the left, as in (−)-glyceraldehyde, to the L series.
D and L have nothing to do with the sign of optical rotation, which is written separately as (+) for dextrorotatory and (−) for laevorotatory: natural glucose is D-(+)-glucose, natural fructose is D-(−)-fructose. (Mirror-image molecules and optical rotation are treated in Chapter 6, Haloalkanes and Haloarenes.)
1. Missing aldehyde reactions. Although glucose has an aldehyde group, it does not give the 2,4-DNP test or Schiff's test, and it does not form an addition product with sodium hydrogensulphite, $\mathrm{NaHSO_{3}}$.
2. The pentaacetate is not an aldehyde. Glucose pentaacetate does not react with hydroxylamine, so it has no free $\mathrm{{-}CHO}$ group.
3. Two crystalline forms. Glucose crystallised from a concentrated solution at $303\ \mathrm{K}$ is the α form (m.p. $419\ \mathrm{K}$); crystallised from a hot saturated solution at $371\ \mathrm{K}$ it is the β form (m.p. $423\ \mathrm{K}$). One open-chain structure cannot explain two different compounds.
All three facts are explained if the $\mathrm{{-}OH}$ group on C5 adds to the aldehyde group of the same molecule. An alcohol adds to an aldehyde to give a hemiacetal; here the addition is internal, so it closes a ring of five carbon atoms and one oxygen atom. Only a tiny fraction (well under 1%) of glucose in water is in the open-chain form at any moment, which is why the slow aldehyde tests fail, while fast reactions that use up the open form (oxidation, oxime formation) pull the ring open and go to completion.
In its cyclic form glucose is a six-membered ring containing one oxygen atom, like the heterocycle pyran; such a ring sugar is called a pyranose. Ring closure turns C1 into a new stereocentre, with an $\mathrm{{-}OH}$ group (the hemiacetal $\mathrm{OH}$) and an $\mathrm{{-}H}$. C1 is the anomeric carbon.
The two possible configurations at C1 give two stereoisomers, α-D-glucopyranose and β-D-glucopyranose. Isomers of a cyclic sugar that differ only in the configuration at the anomeric carbon (C1 of an aldose, C2 of a ketose) are called anomers. In the Haworth projection of a D sugar, the anomeric $\mathrm{{-}OH}$ points down (opposite side to the $\mathrm{CH_{2}OH}$ group) in the α anomer and up (same side) in the β anomer.
The two anomers are interconverted through the open chain: in water, the ring of either anomer opens to the aldehyde and closes again, with the $\mathrm{OH}$ on C1 ending up on either side. (What this does to the optical rotation of a glucose solution is beyond the board syllabus; see the 🚀 box at the end of the section.)
Fructose and the Disaccharides
Fructose, $\mathrm{C_{6}H_{12}O_{6}}$, is obtained together with glucose when sucrose is hydrolysed, and is the sweetest natural sugar (it is plentiful in fruits and honey). It is a ketohexose with the keto group at C2, and its C5 $\mathrm{OH}$ is on the right in the Fischer projection, so it is D-(−)-fructose. Like glucose it exists mainly as rings: the C5 $\mathrm{OH}$ adds to the C2 keto group to give a five-membered ring of four carbons and one oxygen, like the heterocycle furan. Such a ring sugar is a furanose, and fructose has α and β anomers that differ at C2.
Two monosaccharide units are joined when the anomeric $\mathrm{{-}OH}$ of one unit and an $\mathrm{{-}OH}$ of the other lose a molecule of water. The oxygen atom that links them, $\mathrm{C{-}O{-}C}$, is a glycosidic linkage. It is named by the anomer and the carbon numbers it joins: α(1→4) joins C1 of an α unit to C4 of the next. A disaccharide $\mathrm{C_{12}H_{22}O_{11}}$ is two hexoses minus one water: $2(180.16)-18.02=342.30\ \mathrm{g\,mol^{-1}}$. Hydrolysis by dilute acid or by an enzyme adds the water back and breaks the link.
Sucrose (cane or beet sugar) is formed from α-D-glucose and β-D-fructose joined between C1 of glucose and C2 of fructose. Both anomeric carbons are tied up in the link, neither ring can open, and sucrose is a non-reducing sugar. Sucrose is dextrorotatory, $[\alpha]=+66.5^{\circ}$. Its hydrolysis gives equal amounts of D-(+)-glucose ($+52.5^{\circ}$) and D-(−)-fructose ($-92.4^{\circ}$); fructose wins, and the mixture is laevorotatory. Because the sign of rotation is inverted, the hydrolysis is called the inversion of cane sugar (a pseudo-first-order reaction, Section 3.2) and the product mixture is called invert sugar. The enzyme invertase in bees and yeast does the same job, which is why honey is largely invert sugar.
Maltose (malt sugar, from the partial hydrolysis of starch) is made of two α-D-glucose units joined C1 to C4, an α(1→4) link. The second unit keeps a free hemiacetal $\mathrm{OH}$ at C1, which can open to an aldehyde in solution, so maltose is a reducing sugar. Lactose (milk sugar) is made of β-D-galactose and D-glucose joined C1 of galactose to C4 of glucose, a β(1→4) link; its glucose C1 is free, so lactose is also reducing. (Galactose differs from glucose only in the configuration at C4.)
Polysaccharides and Why Carbohydrates Matter
Plants store glucose as starch, animals as glycogen, and plants build their cell walls from cellulose. All three are polymers of D-glucose; they differ in the anomer used and in the pattern of links.
- Starch, the main storage carbohydrate of plants (cereals, potatoes, roots, tubers), is a polymer of α-D-glucose with two components. Amylose (about 15–20%) is water-soluble: a long unbranched chain of 200–1000 α-D-(+)-glucose units joined by C1–C4 glycosidic links. Amylopectin (about 80–85%) is insoluble in water: a branched polymer whose chains are joined C1–C4, with branches attached by C1–C6 links.
- Cellulose, the main constituent of plant cell walls and the most abundant organic compound in the plant kingdom, is a straight-chain polymer of β-D-glucose joined C1–C4. Humans have no enzyme that breaks β(1→4) links, so we cannot digest cellulose; cattle and termites rely on microbes in their gut to do so.
- Glycogen, the storage carbohydrate of animals ("animal starch"), has a structure like amylopectin but is even more highly branched. It is stored in the liver, muscles and brain and is broken down to glucose when the body needs energy; it also occurs in yeast and fungi.
Why carbohydrates matter. Carbohydrates are the main fuel of living things: glucose is oxidised in cells to carbon dioxide and water, releasing energy, and honey has long been used as an instant source of energy in traditional systems of medicine such as Ayurveda. They are the energy stores of plants (starch) and animals (glycogen), and the structural material of plants (cellulose in wood and cotton). Carbohydrates are raw materials for the textile, paper, lacquer and brewing industries, and two sugars, D-ribose and 2-deoxy-D-ribose, are part of the nucleic acids (Section 10.3).
- Monosaccharide? Then yes, aldose or ketose.
- Find every anomeric carbon: C1 of each aldose unit, C2 of each ketose unit.
- Is any of them free, carrying a ring $\mathrm{OH}$ rather than a glycosidic $\mathrm{O}$? If yes, that ring can open to $\mathrm{{-}CHO}$ (or $\mathrm{{>}C{=}O}$): reducing (maltose, lactose). If all are locked: non-reducing (sucrose).
- Polysaccharides have at most one free end per huge chain and count as non-reducing.
- After hydrolysis every product is a monosaccharide, so the test turns positive.
1. D/L is not (+)/(−). D-fructose is laevorotatory. D/L comes from a drawing; (+)/(−) from a polarimeter.
2. α/β is not D/L. α and β differ only at the anomeric carbon; D and L describe the lowest stereocentre.
3. Sucrose is non-reducing although both its units are reducing sugars: the link uses up both anomeric carbons.
4. Starch and cellulose are both glucose polymers. They differ in the anomer (α vs β), not in the monomer.
5. Furanose vs pyranose. Glucose rings are six-membered (pyranose); fructose rings in sucrose are five-membered (furanose), closed from C5 to C2.
In JEE Main, NEET and JEE Advanced 2026 (glucose; JEE Advanced names anomers); not required by the CBSE syllabus.
A fresh solution of pure α-D-glucose has $[\alpha]=+112^{\circ}$; one of pure β-D-glucose has $+18.7^{\circ}$. In each the rotation drifts to the same final value, the $+52.5^{\circ}$ quoted for glucose, as the rings open and close (Interactive 1). This change of rotation is called mutarotation. If $x$ is the fraction of α at equilibrium, $112x+18.7(1-x)=52.5$ gives $x=0.362$: about $36\%$ α and $64\%$ β. The β anomer wins because, in the real puckered (chair) shape of the ring, all its bulky groups, including the C1 $\mathrm{OH}$, can sit pointing outwards from the ring, where they are least crowded.
Exercises
- 1 Define carbohydrates. Why is the name "hydrates of carbon" misleading? Give one compound that fits $\mathrm{C_{x}(H_{2}O)_{y}}$ but is not a carbohydrate, and one carbohydrate that does not fit it.
- 2 Classify as monosaccharide, oligosaccharide or polysaccharide: glucose, sucrose, starch, maltose, raffinose (which gives three monosaccharide units on hydrolysis), glycogen.
- 3 Name the class (e.g. aldohexose) of: glyceraldehyde, 1,3-dihydroxypropanone, ribose, fructose, galactose.
- 4 What is a reducing sugar? Give two disaccharides that are reducing and one that is not.
- 5 Why are polysaccharides called non-sugars?
- 6 Which of these give a silver mirror with Tollens' reagent: glucose, fructose, sucrose, maltose, lactose, starch?
- 7 A water-soluble, sweet carbohydrate X does not reduce Fehling's solution, but after it is boiled with dilute $\mathrm{HCl}$ and neutralised it does. What is X likely to be? Explain.
- 8 Give a chemical test to distinguish glucose from fructose and one to distinguish starch from glucose.
- 1 Write the product of glucose with $\mathrm{HI}$ (prolonged heating). What does it prove?
- 2 Write the product of glucose with $\mathrm{HCN}$ and with hydroxylamine. What do they prove?
- 3 Glucose is oxidised by bromine water. Name and write the product. What does the reaction prove?
- 4 What is formed when glucose is treated with ethanoic anhydride? What does it prove?
- 5 What is formed when glucose is oxidised by nitric acid? What does it show?
- 6 State three facts that the open-chain structure of glucose cannot explain.
- 7 What are anomers? Draw the Haworth projection of β-D-glucopyranose and mark the anomeric carbon.
- 8 Explain the meaning of "D" and "(+)" in D-(+)-glucose. Is D-fructose dextrorotatory?
- 9 Glucose has an aldehyde group but does not react with sodium hydrogensulphite. How does the cyclic structure explain this?
- 1 Name the products of complete hydrolysis of sucrose, maltose and lactose.
- 2 Describe the glycosidic linkage in each of sucrose, maltose and lactose (which carbons, which anomers).
- 3 Why is sucrose a non-reducing sugar while maltose is a reducing sugar?
- 4 What is invert sugar, and why is it so called?
- 5 Give three differences between amylose and amylopectin.
- 6 Cows can live on grass, but humans cannot digest cellulose. Explain.
- 7 Compare glycogen with amylopectin. Where is glycogen stored in the body?
- 8 Starch and cellulose both give only D-glucose on complete hydrolysis. Why are their properties so different?
- 1 Calculate the molar masses of glucose and of sucrose.
- 2 What masses of glucose and of fructose are obtained by complete hydrolysis of $68.46\ \mathrm{g}$ of sucrose?
- 3 A trisaccharide is formed from three hexose units. Find its molecular formula and molar mass.
- 4 What mass of glucose pentaacetate is formed by complete acetylation of $2.70\ \mathrm{g}$ of glucose?
- 5 (Illustrative.) A glycogen molecule contains $30\,000$ glucose units. Estimate its molar mass.
- 6 An amylose chain of $600$ glucose units is formed from glucose. How many glycosidic links does it contain, and how many water molecules are released?
- 7 $34.23\ \mathrm{g}$ of sucrose is dissolved to make $100\ \mathrm{mL}$ of solution in a $1.00\ \mathrm{dm}$ tube. Find the observed rotation at the start and after complete inversion.
- 8 Food science. A cola contains $10.6\ \mathrm{g}$ of sugar per $100\ \mathrm{mL}$ (taken as sucrose, illustrative). How many moles of sucrose are in a $330\ \mathrm{mL}$ can, and what mass of glucose would complete hydrolysis give?
E1. Board · 2 marks What happens when D-glucose is treated with (a) $\mathrm{HI}$ on prolonged heating, (b) bromine water? Write the reactions and say what each shows.
E2. NEET Which of the following is a non-reducing sugar? (A) maltose (B) lactose (C) sucrose (D) fructose
E3. JEE Main Amylose and cellulose are both linear polymers of D-glucose. They differ in that (A) amylose has C1–C6 links (B) cellulose has α(1→4) links and amylose β(1→4) (C) amylose has α(1→4) links and cellulose β(1→4) (D) cellulose is branched and amylose is not.
E4. JEE Main Numerical value. $0.0100\ \mathrm{mol}$ of sucrose is completely acetylated with ethanoic anhydride. By how many grams does the mass increase? (Each acetylated $\mathrm{OH}$ adds $\mathrm{C_{2}H_{2}O}=42.04\ \mathrm{g\,mol^{-1}}$.)
E5. Board · 3 marks Give three observations which show that glucose cannot be fully described by an open-chain structure, and state how the cyclic structure accounts for them.
E6. Board · Case study Many adults produce little lactase, the intestinal enzyme that hydrolyses milk sugar. Undigested lactose passes to the large intestine, where bacteria ferment it, causing discomfort. "Lactose-free" milk is made by adding lactase to milk in the dairy; it tastes noticeably sweeter than ordinary milk. (a) Name the products of hydrolysis of lactose and the linkage broken. (2 marks) (b) Is lactose a reducing sugar? Explain. (1 mark) (c) Suggest why lactose-free milk tastes sweeter. (1 mark)
Research question. Could fuel for cars be made from straw and wood chips instead of from food crops, and what limits the yield?
Why it matters. Brazil, the USA and India blend ethanol into petrol, mostly made from sugar cane, molasses or maize starch, which competes with food. Cellulose from crop waste is far more plentiful, but its β(1→4) links are hard to break. Second-generation ("cellulosic") ethanol plants, including ones being built for rice and wheat straw in India, depend on enzymes that hydrolyse cellulose to glucose.
What to do.
- Explain, with a sketch of two linked rings, why the enzymes that digest starch cannot digest cellulose.
- (Illustrative, constructed for this exercise.) $1.00$ tonne of pure cellulose is hydrolysed completely. Calculate the mass of glucose formed (each $\mathrm{C_{6}H_{10}O_{5}}$ unit gives one $\mathrm{C_{6}H_{12}O_{6}}$).
- Yeast converts glucose to ethanol: $\mathrm{C_{6}H_{12}O_{6}\rightarrow2C_{2}H_{5}OH+2CO_{2}}$. Find the maximum mass of ethanol and its volume (density $0.789\ \mathrm{g\,mL^{-1}}$).
- Real straw is only about 35–40% cellulose, and only part of it is hydrolysed. Estimate the ethanol from one tonne of straw if 38% is cellulose and 70% of that is converted (illustrative figures).
Tools: A calculator; molar masses from this section; news or agency reports on cellulosic ethanol plants.
Hand in: Your sketch, the calculations, and a paragraph weighing straw-based against grain-based ethanol.
Going further. Find out which microbes make cellulase enzymes, and why burning crop residue in fields is a problem that cellulosic ethanol might help to solve.
🧪 Self-check quiz
Q1. α-D-glucopyranose and β-D-glucopyranose differ in the configuration at
Q2. Which carbohydrate gives a silver mirror with Tollens' reagent?
Q3. Invert sugar is
Q4. Cellulose is a polymer of
Q5. Which reaction shows that the six carbon atoms of glucose are in a straight chain?