Carbohydrates — Revision Notes

⚡ 30-Second Revision

Definition: — Polyhydroxy aldehydes (aldoses) or polyhydroxy ketones (ketoses).
General Formula: — $C_x(H_2O)_y$ (not always true, e.g., deoxyribose).
Monosaccharides: — Glucose, Fructose, Galactose. Cannot be hydrolyzed.
Disaccharides: — Sucrose (Glucose + Fructose, $alpha$-1,2), Maltose (Glucose + Glucose, $alpha$-1,4), Lactose (Galactose + Glucose, $\beta$-1,4).
Polysaccharides: — Starch (Amylose $alpha$-1,4, Amylopectin $alpha$-1,4 & $alpha$-1,6), Cellulose ($\beta$-1,4), Glycogen ($alpha$-1,4 & $alpha$-1,6).
D/L Configuration: — Based on -OH on penultimate chiral carbon (right=D, left=L).
Anomers: — $alpha$ and $\beta$ forms, differ at anomeric carbon (C1 for aldoses, C2 for ketoses).
Epimers: — Diastereomers differing at one chiral center other than anomeric carbon (e.g., Glucose & Mannose at C2, Glucose & Galactose at C4).
Mutarotation: — Change in optical rotation due to interconversion of $alpha$, $\beta$ anomers and open-chain form.
Reducing Sugars: — Have free hemiacetal/hemiketal group. Reduce Tollen's/Fehling's. All monosaccharides, Maltose, Lactose.
Non-reducing Sugars: — No free hemiacetal/hemiketal. Sucrose.
Reactions:

- Oxidation: $Br_2/H_2O \rightarrow$ Aldonic acid (e.g., Gluconic acid). - Oxidation: $HNO_3 \rightarrow$ Aldaric acid (e.g., Saccharic acid). - Reduction: $NaBH_4/H_2 \rightarrow$ Alditol (e.g., Sorbitol). - Glycoside formation: Monosaccharide + Alcohol $xrightarrow{H^+}$ Glycoside (acetal/ketal).

2-Minute Revision

Carbohydrates are polyhydroxy aldehydes or ketones, serving as primary energy sources and structural components. They are classified into monosaccharides (e.g., glucose, fructose), oligosaccharides (e.

g., disaccharides like sucrose, maltose, lactose), and polysaccharides (e.g., starch, cellulose, glycogen). Monosaccharides exist in both open-chain (Fischer projection) and cyclic (Haworth projection) forms, with cyclization leading to $alpha$ and $\beta$ anomers at the anomeric carbon.

The D/L configuration is determined by the -OH group on the chiral carbon farthest from the carbonyl. Sugars with a free hemiacetal or hemiketal group are reducing sugars (all monosaccharides, maltose, lactose), capable of reducing Tollen's and Fehling's reagents.

Sucrose is a non-reducing sugar because its anomeric carbons are involved in the glycosidic bond. Key reactions include oxidation (to aldonic or aldaric acids), reduction (to alditols), and glycoside formation.

Understanding these classifications, structures, and properties is crucial for NEET.

5-Minute Revision

Carbohydrates are essential biomolecules defined as polyhydroxy aldehydes (aldoses) or polyhydroxy ketones (ketoses). Their general formula $C_x(H_2O)_y$ is a historical approximation. They are broadly classified into three types:

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Monosaccharides: — Simple sugars like glucose, fructose, and galactose. They cannot be hydrolyzed further. They exist in open-chain forms (Fischer projections) and cyclic forms (Haworth projections). Cyclization forms a hemiacetal or hemiketal, creating a new chiral center called the anomeric carbon, leading to $alpha$ and $\beta$ anomers. For example, $alpha$-D-glucopyranose and $\beta$-D-glucopyranose. The D/L configuration is determined by the -OH group on the chiral carbon farthest from the carbonyl group (right for D, left for L).

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Oligosaccharides: — Yield 2-10 monosaccharide units on hydrolysis. Disaccharides are most common:

* Sucrose: Glucose + Fructose, linked by an $alpha$-1,2-glycosidic bond. It is non-reducing because both anomeric carbons are involved in the bond. * Maltose: Glucose + Glucose, linked by an $alpha$-1,4-glycosidic bond. It is reducing due to a free hemiacetal group. * Lactose: Galactose + Glucose, linked by a $\beta$-1,4-glycosidic bond. It is also reducing.

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Polysaccharides: — Large polymers of many monosaccharide units. Examples:

* Starch: Plant storage. Composed of amylose ($alpha$-1,4 linear) and amylopectin ($alpha$-1,4 with $alpha$-1,6 branches). * Cellulose: Plant structural. Linear polymer of $\beta$-1,4 glucose units. * Glycogen: Animal storage. Highly branched, similar to amylopectin.

Key Properties & Reactions:

Reducing Sugars: — Possess a free hemiacetal/hemiketal group, allowing them to reduce Tollen's or Fehling's reagents. All monosaccharides, maltose, and lactose are reducing.
Mutarotation: — The interconversion of $alpha$ and $\beta$ anomers in solution, leading to a change in optical rotation until equilibrium.
Oxidation: — Mild agents like bromine water oxidize aldoses to aldonic acids (e.g., glucose to gluconic acid). Strong agents like $HNO_3$ oxidize both aldehyde and primary alcohol to aldaric acids (e.g., glucose to saccharic acid).
Reduction: — Carbonyl group reduces to an alcohol, forming alditols (e.g., glucose to sorbitol).
Glycoside Formation: — Reaction of the anomeric -OH with an alcohol, forming an acetal/ketal (glycoside) and a glycosidic bond. This makes the anomeric carbon non-reducing.

Mastering these structural details, classifications, and reactions is paramount for NEET success.

Prelims Revision Notes

Carbohydrates are polyhydroxy aldehydes or ketones. Their classification is crucial: Monosaccharides (e.g., Glucose, Fructose) are the simplest. Oligosaccharides yield 2-10 monosaccharides on hydrolysis; Disaccharides (e.g., Sucrose, Maltose, Lactose) are key. Polysaccharides (e.g., Starch, Cellulose, Glycogen) are large polymers.

Monosaccharide Structures:

Fischer Projection: — Linear representation. D-sugar if -OH on penultimate chiral carbon is right. L-sugar if left.
Haworth Projection: — Cyclic representation (pyranose: 6-membered, furanose: 5-membered). $alpha$-anomer: -OH on anomeric carbon is down (for D-sugars). $\beta$-anomer: -OH on anomeric carbon is up (for D-sugars).
Anomeric Carbon: — C1 for aldoses, C2 for ketoses. New chiral center formed upon cyclization.
Mutarotation: — Spontaneous change in optical rotation due to interconversion of $alpha$ and $\beta$ anomers in solution.

Isomerism:

Enantiomers: — D-Glucose and L-Glucose (mirror images).
Epimers: — Differ at one chiral center (e.g., Glucose and Mannose are C2 epimers; Glucose and Galactose are C4 epimers).
Anomers: — Differ at the anomeric carbon (e.g., $alpha$-D-glucose and $\beta$-D-glucose).

Reducing vs. Non-reducing Sugars:

Reducing: — Possess a free hemiacetal/hemiketal group. Reduce Tollen's/Fehling's. Examples: All monosaccharides, Maltose, Lactose.
Non-reducing: — Anomeric carbons involved in glycosidic bond. No free hemiacetal/hemiketal. Example: Sucrose.

Disaccharide Composition & Linkages:

Sucrose: — $alpha$-D-Glucose + $\beta$-D-Fructose ($alpha$-1,2-glycosidic bond).
Maltose: — $alpha$-D-Glucose + $alpha$-D-Glucose ($alpha$-1,4-glycosidic bond).
Lactose: — $\beta$-D-Galactose + $\beta$-D-Glucose ($\beta$-1,4-glycosidic bond).

Polysaccharide Structures & Functions:

Starch (plants): — Energy storage. Amylose (linear, $alpha$-1,4) + Amylopectin (branched, $alpha$-1,4 & $alpha$-1,6).
Cellulose (plants): — Structural. Linear, $\beta$-1,4 glucose units. Indigestible by humans.
Glycogen (animals): — Energy storage. Highly branched, similar to amylopectin.

Key Reactions:

Oxidation:

* Mild ($Br_2/H_2O$): Aldehyde $ightarrow$ Carboxylic acid (Aldonic acid, e.g., Gluconic acid). * Strong ($HNO_3$): Aldehyde + Primary alcohol $ightarrow$ Dicarboxylic acid (Aldaric acid, e.g., Saccharic acid).

Reduction: — Carbonyl $ightarrow$ Alcohol (Alditol, e.g., Sorbitol from glucose).
Glycoside Formation: — Anomeric -OH + Alcohol $ightarrow$ Glycoside (acetal/ketal).

Vyyuha Quick Recall

For Disaccharides: Sucrose is Glucose + Fructose, and is Non-reducing. Maltose is Glucose + Glucose, and is Reducing. Lactose is Galactose + Glucose, and is Reducing.

Mnemonic: SGFN, MGGR, LGGR (Sounds like 'Sugar, My Good Girl, Love Good Girl').