Organic Chemistry III Mohammad Jafarzadeh Faculty of Chemistry, Razi University Organic Chemistry, Structure and Function (7th edition) By P. Vollhardt and N. Schore, Elsevier, 2014 1 24. Carbohydrates Take a piece of bread and place it in your mouth. After a few minutes it will begin to taste distinctly sweet, as if you had added sugar to it. The acid and enzymes in your saliva have cleaved the starch in the bread into its component units: glucose molecules. You all know glucose as dextrose or grape sugar. The polymer, starch, and its monomer, glucose, are two examples of carbohydrates. Carbohydrates are major contributors to our daily diets, in the form of sugars, fibers, and starches, such as bread, rice, and potatoes. They function as chemical energy-storage systems, being metabolized to water, carbon dioxide, and heat or other energy. They also serve as building units of fats and nucleic acids. Carbohydrates are considered to be polyfunctional, because they possess multiple functional groups. Glucose, C6(H2O)6, and many related simple members of this compound class form the building blocks of the complex carbohydrates and have the empirical formulas Cn(H2O)n, essentially hydrated carbon. As a result, they are highly water soluble. 2 24-1 NAMES AND STRUCTURES OF CARBOHYDRATES The simplest carbohydrates are the sugars, or saccharides. As chain length increases, the increasing number of stereocenters gives rise to a multitude of diastereomers. Fortunately for chemists, nature deals mainly with only one of the possible series of enantiomers. Sugars are polyhydroxycarbonyl compounds and many form stable cyclic hemiacetals, which affords additional structural and chemical variety. Carbohydrate is the general name for the monomeric (monosaccharides), dimeric (disaccharides), trimeric (trisaccharides), oligomeric (oligosaccharides), and polymeric (polysaccharides) forms of sugar (saccharum, Latin, sugar). A monosaccharide, or simple sugar, is an aldehyde or ketone containing at least two additional hydroxy groups. Thus, the two simplest members of this class of compounds are 2,3-dihydroxypropanal (glyceraldehyde) and 1,3-dihydroxyacetone. Complex sugars are those formed by the linkage of simple sugars through ether bridges. 3 1074 CHAPTER 24 Carbohydrates CHO Sugars are classifi ed as aldoses and ketoses A HOOC OH Carbohydrate is the general name for the monomeric (monosaccharides), dimeric (disac- A charides), trimeric (trisaccharides), oligomeric (oligosaccharides), and polymeric (polysac- CH2OH charides) forms of sugar (saccharum, Latin, sugar). A monosaccharide, or simple sugar, is an aldehyde or ketone containing at least two additional hydroxy groups. Thus, the two O simplest members of this class of compounds are 2,3-dihydroxypropanal (glyceraldehyde) O and 1,3-dihydroxyacetone (margin). Complex sugars (Section 24-11) are those formed by the linkage of simple sugars through ether bridges. Aldehydic sugars are classi! ed as aldoses; those with a ketone function are called ketoses. Aldehydic sugars are classified as aldoses; those with a ketone function areOn thecalled basis of their chain length, we call sugars trioses (three carbons), tetroses (four carbons), ketoses. On the basis of their chain length, we call sugars trioses (three carbons),pentoses (! ve carbons), hexoses (six carbons), and so on. Therefore, 2,3-dihydroxypropanal tetroses (four carbons), pentoses (five carbons), hexoses (six carbons),O and so on(glyceraldehyde). is an aldotriose, whereas 1,3-dihydroxyacetone is a ketotriose. 1074 CHAPTER 24 Carbohydrates2,3-Dihydroxypropanal Glucose, also known as dextrose, blood sugar, or grape sugar (glykys, Greek, sweet), is (Glyceraldehyde) Therefore, 2,3-dihydroxypropanal (glyceraldehyde) is an aldotriose, whereasa pentahydroxyhexanal1,3- and hence belongs to the class of aldohexoses. It occurs naturally in (An aldotriose) many fruits and plants and in concentrations ranging from 0.08 to 0.1% in human blood. A dihydroxyacetone is a ketotriose. corresponding isomeric ketohexose is fructose, the sweetest natural sugar (some synthetic CHO Sugars areCH classifi2OH ed as aldoses and ketoses A A sugars are sweeter), which is also present in many fruits (fructus, Latin, fruit) and in honey. HOOC OH CarbohydrateCP isO the general name forAnother the monomeric important natural(monosaccharides), sugar is the aldopentose dimeric (disac- ribose, a building block of the ribonucleic A charides), trimericA (trisaccharides), oligomericacids (Section (oligosaccharides), 26-9). and polymeric (polysac- CH2OH charides) formsCH 2ofOH sugar (saccharum, Latin, sugar). A monosaccharide, or simple sugar, is an aldehyde or ketone containing at least two additional hydroxyConstitutional groups. Thus, the two CHO CH2OH O simplest members of this class of compounds Aare 2,3-dihydroxypropanalisomers (glyceraldehyde)A O and 1,3-dihydroxyacetoneO (margin). ComplexHOO CsugarsOH (Section 24-11) are those formedCPO by CHO the linkage of simple sugarsO through ether bridges.A A A HOOOC H HOOOC H HOOC OH Aldehydic sugars are classi! ed as aldoses; thoseA with a ketone function are calledA ketoses. A On the basis of their chain length, we call sugarsHOO CtriosesOH (three carbons), tetroses (fourHOO Ccarbons),OH HOOC OH pentoses (! ve carbons), hexoses (six carbons), Aand so on. Therefore, 2,3-dihydroxypropanalA A O (glyceraldehyde) is an aldotriose, whereas 1,3-dihydroxyacetoneHOOC OH is a ketotriose. HOOC OH HOOC OH O Glucose, also known as dextrose, blood sugar,A or grape sugar (glykys, Greek, sweet),A is A 2,3-Dihydroxypropanal CH OH CH OH CH OH (Glyceraldehyde) a pentahydroxyhexanal1,3-Dihydroxyacetone and hence belongs to the class2 of aldohexoses. It occurs naturally2 in 2 (An aldotriose) many fruits(A and keto plantstriose) and in concentrations ranging fromO 0.08 to 0.1% in human blood.O A O OO4 O O corresponding isomeric ketohexose is fructose, the sweetest natural sugar (some synthetic O CH2OH O O A sugars are sweeter), which is also present in many fruits (fructus, Latin, fruit) and in honey. O O O O CPO Another important natural sugar is the aldopentoseO ribose,O a building block of the ribonucleic O A acids (Section 26-9). CH OH 2 Glucose Fructose Ribose Constitutional (An aldohexose) (A ketohexose) (An aldopentose) CHO CH2OH A isomers A O HOOC OH CPO CHO O A ExerciseA 24-1 A HOOOC H HOOOC H HOOC OH A To whichA classes of sugars do the followingA monosaccharides belong? HOOC OH HOOC OH HOOC OH CHO 2OHCH A A A A HOOC OH HOOC OH HOOC OH A O CHO HOCH CPO A AA A A A 1,3-Dihydroxyacetone CH2OH (a) HCCOHH2OH (b) HOCH C H(c)2OH HOCH A A A (A ketotriose) O HCOH O HCOH OHCOH OO O O A O A A O 2OHCH 2OHCH 2OHCH O Erythrose LyxoseO Xylulose O O O O O O Glucose A Fructosedisaccharide is derived from twoRibose monosaccharides by the formation of an ether (An aldohexose) (usually,(A keto acetal)hexose) bridge (Sections 17-7 and(An aldo 24-11).pentose Hydrolysis) regenerates the monosaccharides. Diabetics need to monitor the Ether formation between a mono- and a disaccharide results in a trisaccharide, and repetition glucose levels in their blood carefully, as with the simple of this process eventually produces a natural polymer (polysaccharide). Polysaccharides “glucometer”Exercise 24-1 shown. constitute the framework of cellulose and starch (Section 24-12). To which classes of sugars do the following monosaccharides belong? CHO 2OHCH A A CHO HOCH CPO A A A (a) HCOH (b) HOCH (c) HOCH A A A HCOH HCOH HCOH A A A 2OHCH 2OHCH 2OHCH Erythrose Lyxose Xylulose A disaccharide is derived from two monosaccharides by the formation of an ether (usually, acetal) bridge (Sections 17-7 and 24-11). Hydrolysis regenerates the monosaccharides. Diabetics need to monitor the Ether formation between a mono- and a disaccharide results in a trisaccharide, and repetition glucose levels in their blood carefully, as with the simple of this process eventually produces a natural polymer (polysaccharide). Polysaccharides “glucometer” shown. constitute the framework of cellulose and starch (Section 24-12). 1074 CHAPTER 24 Carbohydrates CHO Sugars are classifi ed as aldoses and ketoses A HOOC OH Carbohydrate is the general name for the monomeric (monosaccharides), dimeric (disac- A charides), trimeric (trisaccharides), oligomeric (oligosaccharides), and polymeric (polysac- CH2OH charides) forms of sugar (saccharum, Latin, sugar). A monosaccharide, or simple sugar, is an aldehyde or ketone containing at least two additional hydroxy groups. Thus, the two O simplest members of this class of compounds are 2,3-dihydroxypropanal (glyceraldehyde) O and 1,3-dihydroxyacetone (margin). Complex sugars (Section 24-11) are those formed by the linkage of simple sugars through ether bridges. Aldehydic sugars are classi! ed as aldoses; those with a ketone function are called ketoses. On the basis of their chain length, we call sugars trioses (three carbons), tetroses (four carbons), pentoses (! ve carbons), hexoses (six carbons), and so on. Therefore, 2,3-dihydroxypropanal O (glyceraldehyde) is an aldotriose, whereas 1,3-dihydroxyacetone is a ketotriose. 2,3-Dihydroxypropanal Glucose, also known as dextrose, blood sugar, or grape sugar (glykys, Greek, sweet), is (Glyceraldehyde)Glucose, alsoa pentahydroxyhexanalknown as dextrose, and henceblood belongssugar, to theor classgrape of aldohexoses.sugar (glykys It occurs, Greek, naturallysweet), in is a (An aldotriosepentahydroxyhexanal) many fruits and plantshence andbelongs in concentrationsto the rangingclass offromaldohexoses 0.08 to 0.1% .in human