Wednesday, September 19, 2007

Answers to SQ Ch. 31 Carbohydrates

1. what are carbohydrates?

Carbohydrates means hydrates of carbon. They are polyhydroxy aldehydes or polyhodroxy ketones.

2. How are carbohydrates classified?

Based on their solubility in water
i) Sugars - soluble
ii)Nonsugars - insoluble ex: starch, cellulose
Bsed on their chemical constitution
i) Monosaccharides
ii) Oligosaccharides
iii) Polysaccharides

3. How is glucose prepared from sucrose?

Sucrose is hydrolysed by concentrated alcoholic solution of hydrochloric acid. the hydrolysis is carried out at about 323 K by heating on a water bath for about two hours. Glucose and fructose will be the products, glucose being almost insoluble in alcohol crystallises on cooling while fructose being comparatively more soluble remains in solution.

4. What are monosaccharides?

These are basic units of carbohydrates and cannot be hydrolized to simpler compounds.
5. give an example of polysaccharide.

starch, cellulose, insulin

6. Give an example of oligosaccharide.

Raffinsoe, stachyose

7. How many stereo isomers a carbohydrate can have?
Carbohydrates copntain asymmetric carbon atoms. They can have stereoisomers, the number of which is equal to 2 to the power of n (2^n) where n is the number of asymmetric carbon atoms in the molecule of carbohydrate.

8. What is a cyclic hemiacetal structure?


see Page 943 of Jauhar

9. What is Haworth projection?

IN this structure, a ring is shown as a plane perpendicular to the plane of the paper. The edge of the plane that would project out of the paper toward the reader is darkened to emphasize its orientation. the hydroxyl groups are then positioned above or below the plane. The hydrogen atoms are usually not shown.

10. What is Fischer projection?

In Fischer projection formula, the carbon chains is positioned vertically with the substituents drawn to the right and left on the chain. The horizontal lines represent bonds coming towards the reader and the vetical lines represent bonds projected away from the reader. The aldehydic or ketonic group is placed at the top of the vertical line.

11. What is chair conformational formula?

Chair conformational formula give the accurate three-dimensinal picture of pyranose rings.

12. What are anomers?

When an open-chain monosaccharide cyclizes to a furanoes or pyranose form, a new chiral centre at the carbonyl carbon (known as anomeric carbon) is created. The two diasteromers produced are called anomers.

13. What is mutarotation?

When either alpha form or beta form of glucose is dissolved in water, alpha form changes to beta form (or vice versa) and a solution containing 36% of alpha form and 64% of beta form is obtained. The optical rotation will have a value of +52.6 degree. This phenomenon is known as mutarotation.

14. What reactions are supported by reducing sugars?

Reducing sugars are easily oxidized to give carboxylic acid. They reduce Tollens reagent to shiny silver mirror. They reduce Fehling's solution to red precipitate of cuprous oxide. They reduce Benedict's reagent to red precipitate of cuprous oxide.

15. Give two examples of disaccharides.
Lactose and Sucrose

16. What is invert sugar?


17. what are hydrates of carbon?


18. Give the structure of Ketopentose.
19. give the structure of ketotriose.
20. Give the structure of aldotetrose.

21. Give 5 sugars and rank them in relative order of sweetness.

Sucrose (100), Glucose (74), Fructose (73), Galactose (32), Maltose (32)

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