chemistry of carbohydrates and its properties

chemistry of carbohydrates

1. CARBOHYDRATES

2. Contents

3. Contents continuation…

4. Introduction Carbohydrates are polyhydroxylated aldehydes or ketones and their derivatives. The word "carbohydrate" includes polymers and other compounds synthesized from polyhydroxylated aldehydes and ketones. They can be synthesized in the laboratory or in living cells. Simple carbohydrates or the entire carbohydrate family may also be called saccharides. (Latin , saccharum = sugar ) In general carbohydrates have the empirical formula (CH 2 O) n . The term "carbohydrate" generated from carbon and hydrate; though some also contain nitrogen, phosphorus, or sulfur. Chemically, carbohydrates are molecules that are composed of carbon, along with hydrogen and oxygen - usually in the same ratio as that found in water (H2O). (2:1).

5. In carbohydrates we have an aldehyde group which combines with an alcoholic OH of the same molecule to form an internal hemiacetal. Finally, we can accurately define carbohydrates as : A polyhydroxy compound that has an aldehyde or a ketone function present, either free or as hemiacetal or acetal. Some examples of carbohydrates are : e.t.c .

6. Occurrence of Carbohydrates Carbohydrates are the most abundant biomolecules belonging to class of organic compounds found in living organisms on earth. Each year, more than 100 billion metric tons of CO 2 and H2 2 are converted into cellulose and other plant products due to photosynthesis. Living matter is largely made of biomolecules consisting of water and complex polymers of amino acids, lipids, nucleotides and carbohydrates. Carbohydrates are linked with amino acid polymers (proteins) forming glycoproteins and with lipids as glycolipids. Carbohydrates are present in DNA and RNA, which are essentially polymers of D-ribose-phosphate and 2-deoxy-D-ribose. Carbohydrates are a widely diverse group of compounds that are ubiquitous in nature. More than 75% of the dry weight of the plant world is carbohydrate in nature - particularly cellulose, hemicellulose and lignin.

7. Continuation… Existence of sugars is confirmed before life itself appeared on earth. It is highly probable that there was a relative abundance of various sugars and their phosphates in the prebiotic world. The reality or possibility that carbohydrates will be found in fossils is almost zero because they are relatively unstable, capable of being dehydrated and of combining with other molecules. At higher temperatures they caramelize and char. It has been known for over a century that many sugars formed from formaldehyde in alkaline solution. Condensation of formaldehyde, a prebiotic constituent, led to formation of glycol-aldehyde, trioses, tetroses , pentoses and hexoses in the laboratory. Carbohydrates comprise a comprehensive group of naturally occurring substances, which include innumerable sugars and sugar derivatives, as well as high-molecular weight carbohydrates (polysaccharides) like starch and cellulose in plants and glycogen in animals.

8. Classification of C arbohydrates Carbohydrates can be classified on the basis of several factors that are given in the table :- Sweetness Sugars Non- sugars Monosaccharides, Oligosaccharides polysaccharides

9. 1. Sugars Sugars are crystalline, colorless, water soluble and sweet in taste. These can be further divided in two categories : a). Monosaccharides b). Oligosaccharides Some examples of sugars are – glucose, fructose e.t.c .

10. a). Monosaccharides These are the simplest carbohydrates (monomers) that cannot be hydrolyzed further to give simpler units of polyhydroxy aldehyde or ketone. Sweet in taste and water-soluble crystalline compounds. molecules of monosaccharides polymerize to give higher carbohydrates. These are optically active and have empirical formula of (C n H 2n O n ). Monosaccharides contains 3 to 10 numbers of carbon atoms.

11. Examples of monosaccharides - e.t.c …

12. Further classifications of monosaccharides The monosaccharides are further divided on the basis of two factors - 1). On the basis of attached carbonyl group - i ). Aldose – those containing aldehyde group (-CHO) are called aldoses. ii). Ketose – those containing ketone group (>C=O) are called ketoses. 2). Based on the numbers of carbon atoms – Num. of carbon atoms in molecule designation 3 Trioses 4 Tetroses 5 Pentoses 6 Hexoses 7 Heptoses 8 octaoses

13. b). Oligosaccharides An oligosaccharide is a saccharide polymer containing a small number (typically two to ten) of monosaccharides sugars. On hydrolysis these give 2 to 10 molecules of monosaccharides. Water soluble and sweet in taste. Based on obtained numbers of monosaccharides molecules on hydrolyses these are called as disaccharide, trisaccharide, tetrasaccharide and so on.

14. Examples of oligosaccharides Disaccharide – yields two monosaccharide molecules on hydrolysis. Examples – sucrose etc. Trisaccharide -

15. Continuation… Tetrasaccharides – pentasaccharides Verbascose

16. 2.) Non-sugars or Polysaccharides Polysaccharides contain long monosaccharide units joined together by glycosidic linkage. The bulk of the annually renewable carbohydrate-biomass are polysaccharides (glycans), such as cellulose, hemicelluloses, chitin, starch, and inulin. Invariably composed of monosaccharide units, they have high molecular masses and, hence, differ significantly in their physical properties. The majority of naturally occurring polysaccharides contain 80 – 100 units, although a few are made up of considerably more. Cellulose is also one of the polysaccharides that are mostly found in plants. It is composed of β-D- glucose units joined by a glycosidic linkage between C1 of one glucose unit and C4 of the next glucose unit. Empirical formula – (C 6 H 10 O 5 ) n

17. Examples of polysaccharide

18. D-L configuration Glyceraldehyde contains a central asymmetric carbon atom therefore; it exists in two enantiomers – 1 2 The enantiomer which rotates the plane of polarized light to the right is written as (+)-glyceraldehyde. The other enantiomer that rotates the plane to the left is (-)-glyceraldehyde. The (+) and (-) signs gave no indication as to how the OH and H are arranged about the asymmetric carbon atom. In 1906 Rosanoff decided arbitrarily that the enantiomer (1) with OH to the right may be designated as D-glyceraldehyde and the enantiomer (2) with OH to the left as L-glyceraldehyde. It has been shown by the modern X-ray diffraction technique that Rosanoff's D-glyceraldehyde was actually (+)- glyceraldehyde and that its absolute configuration was the same as indicated in formula (1). +

19. Continuation… The sugars are divided into two families, the D-family and L-family. The sugars having the same configuration as of D-glyceraldehyde at the asymmetric carbon most distant from the carbonyl group are designated as D-Sugars. Those with opposite configuration (that of L-glyceraldehyde) are called L-Sugars. Thus natural glucose is D(+)- glucose and fructose is D(-)-fructose. If there are n asymmetric carbon atoms in a molecule, the number of optical isomers is 2 n . Thus we have 2 aldotrioses , 4 aldotetroses , 8 aldopentoses , and 16 aldohexoses. Although the naturally occurring sugars generally belong to the D-family, there are an equal number of compounds that have the L-configuration.

20. The configurations of the D-sugars up to hexoses as determined by experiment and reasoning are given in Figure..

21. Continuation..

22. Constitution of glucose and fructose Glucose- Systemic name: D-Glucose Molecular formula: C 6 H 12 O 6 Glucose ( Glc ), a monosaccharide (or simple sugar), is an important carbohydrate in biology. The living cell uses it as a source of energy and metabolic intermediate. Glucose is one of the main products of photosynthesis and starts cellular respiration in both prokaryotes and eukaryotes. The name comes from the Greek word ‘ glykys ’, which means "sweet", plus the suffix "- ose " which denotes a sugar. Two stereoisomers of the aldohexose sugars are known as glucose, only one of which (D-glucose) is biologically active). This form (D-glucose) is often referred to as dextrose monohydrate, or, especially in the food industry, simply dextrose (from dextrorotatory glucose). The mirror-image of the molecule, L-glucose, cannot be metabolized by cells in the biochemical process known as glycolysis.

23. Preparation of glucose Glucose can be prepared in the laboratory by boiling sucrose (cane sugar) with dilute hydrochloric acid or sulphuric acid for about two hours. This hydrolyzes sucrose to glucose and fructose. (C 12 H 22 O 11 ) n + n H 2 O C 6 H 12 O 6 + C 6 H 12 O 6 glucose fructose Separation - In order to separate glucose from fructose, alcohol is added during cooling. Glucose is almost insoluble in alcohol. It crystallizes out first, while fructose is more soluble. It remains in the solution. The solution is filtered to obtain the crystals of glucose. HCl

24. Commercially, it is prepared by the hydrolysis of starch with dil. HCL at high temperature and pressure. (C 6 H 10 O 5 ) n + nH 2 O nC 6 H 12 O 6 starch glucose Separation - An aqueous suspension of starch obtained from corn is acidified with hydrochloric acid. It is then heated with high-pressure steam in an autoclave. When the hydrolysis is complete, the liquid is neutralized with sodium carbonate to a pH of 4-5. The resulting solution is concentrated under reduced pressure to get the crystals of glucose. HCl

25. Properties of glucose - Physical properties – Glucose is a white crystalline solid, mp 146°C. When crystallized from cold water, it forms glucose monohydrate (C 6 H 12 O 6 .H 2 O). Mp 86°C. It is extremely soluble in water. Only sparingly soluble in ethanol, and insoluble in ether. It is about three-fourths as sweet as cane-sugar (sucrose). It is optically active, and the ordinary naturally occurring form is (+)-glucose.

26. Chemical properties - 1.) Oxidation – a) with weak oxidants -