TMH-JEE-Chemistry- book-chapters Ch.31 Carbohydrates Study Guide/Note

JEE Syllabus

Carbohydrates:
Classification;
mono and di-saccharides (glucose and sucrose);
Oxidation, reduction,
glycoside formation and hydrolysis of sucrose.
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Main Topics Covered in the TMH Book

MONOSACCHARIDES
DISACCHARIDES
POLYSACCHARIDES
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Carbohydrates means "hydrates of carbon".
These are poly-hydroxylated-aldehydes or poly-hydroxylated-ketones. The general formula is C-x(H-2O)-y

Aldehydes are compounds containing the carbonyl group -C=O as functional groupThe general formula is C-nH-2n-O, In aldehydes, the carbonyl carbon atom carries at least one H e.g.,

H
!
C=O
!
H

Formaldehyde


CH-3
!
C=O
!
H

Acetaldehyde

Ketones also have the same general formula C-nH-2nO but, the carbonyl atom does not carry any H atoms, but it is attached to two alkyl or aryl groups

CH-3
!
C=O
!
CH-3
It is Dimethyl ketone or Acetone. Its IUPAC name is Propanone




Carbohydrates are classified as:
Monosaccharides

Oligosaccharides

Polysaccharides

Monohydrates may be represented as follows
H
!
C=O
!
(H-C-OH)-n
!
CH-2OH

A polyhdroxy aldehyde

CH-2OH
!
C=O
!
(H-C-OH)-n
!
CH-2OH

A polyhdroxy ketone

Monosaccharides are named as follows.
1. The calss name of monosaccharides begins with the prefix 'aldo' for polyhydroxy aldehydes and "keto' for polyhydroxy ketones.
2. the name ends with suffix 'ose".
3. In between prefix and suffix, the number of carbon atoms (i.e. di, tri, tetr etc.) is inserted.

Examples

H
!
C=O
!
(H-C-OH)
!
CH-2OH

Aldotriose - (A polyhdroxy aldehyde)

CH-2OH
!
C=O
!
CH-2OH

Ketotriose - (A polyhdroxy ketone )

Reducing and Nonreducing Sugars

Reducing sugars are easily oxidized to give carboxylic acid.

They reduce
(i) Tollens reagent (an ammoniacal solution of silver nitrate) to shiny silver mirror.
(ii) Fehling's solution (an acqueous solution of cupric iron and tartrate salts) to red precipitate of cuprous oxide, and
(iii) Benedict's reagent (an alkaline solution containing a cupric citrate complex) to red precipitate of curous oxide)

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JEE Question 2007 Paper II

STATEMENT.1: Glucose gives a reddish brown precipitate with Fehling’s solution.

Because

STATEMENT.2: Reaction of glucose with Fehling’s solution gives CuO and gluconic acid.


(A) Statement.1 is True, Statement.2 is True; Statement.2 is a correct explanation for Statement.1.
(B) Statement.1 is True, Statement.2 is True; Statement.2 is NOT a correct explanation for Statement.1.
(C) Statement.1 is True, Statement.2 is False.
(D) Statement.1 is False, Statement.2 is True.

Answer: C.

Cu-2O is given not CuO.
--------------------------
Statement 1 is True. But statement 2 is false because Cu-2O Cuprous oxide is formed and not CuO.
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All aldoses are reducing sugars, but some ketoses are reducing sufars as well. For example, fructose (a ketose) reduces Tollens reagent. This occurs because fructose is readily isomerized to an aldose in basic solution.









A set of questions on this chapter posted in

www.iit-jee-chemistry-ps.blogspot.com