Ans: – Definition of monosaccharides: –Carbohydrates are biological molecules that contain carbon (C), hydrogen (H), and oxygen (O) atoms. Carbohydrates are very important because they provide energy and fuel for our bodies so that our brains can function properly, and so that our muscles can work. Carbohydrates are our preferred source of energy. They can range from complex carbohydrates to simple carbohydrates. The simplest form of carbohydrates are monosaccharides.

Monosaccharides are the simplest units of carbohydrates and the simplest form of sugar. They are the building blocks of more complex carbohydrates such as disaccharides and polysaccharides. Physically, they are usually colorless, can dissolve in water and have the appearance of a crystal-like substance. Below are some examples of sources and food that contain monosaccharides. Molasses has a high content of the monosaccharide glucose. Cherries contain the monosaccharide fructose, and yogurt contains the monosaccharide galactose.

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Monosaccharide classification

Monosaccharides can be classified in three main ways, according to:

The number of carbon atoms – Monosaccharides containing three carbon atoms are referred to as trioses, while those with four carbons are called tetroses and those with five are called pentoses etc.
The location of the carbonyl group – If the carbonyl group is an aldehyde, then the monosaccharide is an aldose whereas if the carbonyl group is a ketone, the monosaccharide is a ketose.
The molecule’s stereochemistry or chiral handedness – This refers to the configuration of the molecule, which may exist in different structural forms or isomers. The first two classification systems above are often combined and a monosaccharide may be called an aldohexose (e.g. glucose), an aldopentose (e.g. ribose) or a ketohexose (e.g. fructose), for example.

Apart from the first and last carbon atom, each carbon atom bearing a hydroxyl group is asymmetric and may have two possible configurations in space (R or S). A number of isomers can therefore exist for any monosaccharide.

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Structure of Monosaccharides Number of Carbon Atoms

We mentioned earlier that carbohydrates are composed of carbon, hydrogen, and oxygen atoms. Since monosaccharides are the simplest units of carbohydrates, then they are also made up of carbon, hydrogen, and oxygen atoms. Carbohydrates have this general formula:

The possible values for n are positive integers equal or greater than 3. If, for instance, n = 3, that means that there are three carbon atoms and these monosaccharides are called trioses. If n = 4, there are four carbon atoms, so these

You may notice that in the table above, for arabinose and glucose, their structures ‘open up’ from a polygon-like structure to a chain. This is because biochemists usually use two types of drawings, called Fischer projections and Haworth projections, to draw sugar molecules.

The Haworth projections are sugar molecules depicted in their cyclic forms (left) and

Fischer projections are open chain forms (right), as shown below:

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The carbon atoms are numbered in the Haworth and Fischer projections so that each edge in the Haworth projection is one carbon atom. Carbon 1 in the Haworth projection is also carbon 1 in the Fischer projection, and so forth.

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INTRODUCTION

Carbohydrates are formed by green plants as a product of photosynthesis and are the most abundant compounds found on earth. Carbohydrates are organically derived compounds composed of carbon atoms combined with “hydrates” (such as water, H₂O). All carbohydrates follow the empirical formula CnH2nOn.

QNO: – What are carbohydrates?

Ans:-Carbohydrate may be defined as:

“Alcohol or aldehyde derivatives of polyhydroxy alcohols or as compounds that yield these derivatives on hydrolysis” They are the most abundant molecules on earth produced by photosynthesis.

1st Semester Notes

QNO: – What is Mutarotation?

Mutarotation of Glucose

Confirmation of Mutarotation

D- (+) Glucose forms two isomeric methyl D-glucosides. Aldehydes, react with alcohols in the presence of anhydrous HCl to form acetals. If the alcohol is, methanol, the acetal contains two methyl groups.

When D- (+) – glucose is treated with methanol and HCl, the product, methyl D- glucoside, contains only one -CH₃ group, yet it has properties resembling those of a full acetal. It does not spontaneously revert to aldehyde and alcohol on contact with water, but requires hydrolysis by aqueous acids.
Furthermore, not just one but two of those mono methyl derivatives of D- (+) glucose are known, one with melting point of 165^o C and specific rotation +158^o and the other with melting point 107^oc and specific rotation -33^o.
The isomer of higher positive rotation is called methyl α – D glycoside and the other is called methyl β – D glycoside. These glycosides do not undergo mutarotation and do not reduce Tollen’s reagent or Fehling’s solution, as Glucose.