Carbohydrates Polyhydroxyaldehydes

Carbohydrates Polyhydroxyaldehydes or polyhydroxy ketones or substances that yield these by hydrolysis. They are obtained from plants or animals. 

Trivial names are common in carbohydrate nomenclature. Fifteen of them form the basis of the systematic nomenclature. They are assigned to the simple aldoses (polyhydroxyaldehydes), from triose to hexoses. 

Carbohydrates (saccharides) are aliphatic polyhydroxyaldehydes (aldoses), polyhydroxyketones (ketoses), or compounds that can be hydrolyzed to them. The suffix -ose denotes this class of compounds. 

The simplest carbohydrates, sometimes referred to as monosaccharides, or sugars, are either polyhydroxyaldehydes (aldoses) or polyhydroxyketones (ketoses). They can be derived from polyalcohols (polyols) by oxidation of one carbinol group to a carbonyl group. For example, the simple three-carbon triol, glycerol, can be converted either to the aldotriose, glyceraldehyde, or to the ketotriose, dihydroxyacetone, by loss of two hydrogens (fig. 12.1). 

The term carbohydrate arose because most sugars have molecular formulas Cffi(H20),n, suggesting that carbon atoms are combined in some way with water. In fact, the empirical formula of most simple sugars is C(H20). Chemists named these compounds hydrates of carbon or carbohydrates because of these molecular formulas. Our modern definition of carbohydrates includes polyhydroxyaldehydes, poly-hydroxyketones, and compounds that are easily hydrolyzed to them. 

Monosaccharides, or simple sugars, are carbohydrates that cannot be hydrolyzed to simpler compounds. Figure 23-1 shows Fischer projections of the monosaccharides glucose and fructose. Glucose is a polyhydroxyaldehyde, and fructose is a polyhydroxy ketone. Polyhydroxyaldehydes are called aldoses (aid- is for aldehyde and -ose is the suffix for a sugar), and polyhydroxyketones are called ketoses (ket- for ketone, and -ose for sugar). 

Much of the interest in acetal chemistry is related to carbohydrates, because polyhydroxyaldehydes exist predominantly as cyclic hemiacetalsJ Digoxin, for example, possesses acetal functions subject to hydrolysis. 

Carbohydrates are either polyhydroxyaldehydes (aldoses, oses) or polyhydroxyke-tones (ketoses, uloses) there is an electron gap at their carbonyl carbon atom. Typically, aldehydes and ketones accept nucleophiles such as water to form hydrates or alcohols to form hemiketals (5.1 and 5.3) and hemiacetals (5.4 and 5.6), respectively. In pentoses, pentuloses, hexoses, hexuloses, and higher carbohydrates, one of the hydroxyl groups can play the role of internal nucleophile. Thus, open-chain structure (5.2 and 5.5) cyclizes into internal hemiacetals and ketals, all with either five- (5.1 and 5.3) or six- (5.4 and 5.6) membered cycles. 

Historically, carbohydrates were defined as substances with the empirical formula Cn(H20)ni. The common sugars such as glucose and fructose (n = m = 6), or sucrose (n=12, m=ll) fit this formula, but nowadays the convention is to regard as a carbohydrate a polyhydroxyaldehyde or polyhydroxy ketone with the classical formula, a molecule closely related to it, or oligomers or polymers of such molecules. Their study evolved as a separate sub-discipline within organic chemistry for practical reasons - they are water-soluble and difficult to crystallise - so that their manipulation demanded different sets of skills from classical natural products such as terpenes, steroids, alkaloids, etc. 

Carbohydrates, also referred to as saccharides, are polyhydroxyaldehydes or polyhydroxyketones or compounds that can be hydrolyzed to or derived from polyhydroxyaldehydes or polyhydroxyketones. Carbohydrates have many important biological functions— to store and generate energy in animals and plants (glucose, starch, glycogen), as supportive structures in plants (cellulose) and crustaceans (chitin), and as components of cell membranes and nucleic acids. 

All carbohydrates are polyhydroxyaldehydes, polyhydroxyketones, or molecules that yield polyhydroxy aldehydes or ketones on hydrolysis. Monosaccharides are the smallest carbohydrate molecules and include five- and six-carbon sugars, namely, pentoses and hexoses. Polysaccharides, such as starch and cellulose, lead to many monosaccharides upon hydrolysis. 

Carbohydrate (Section 22.1 A) A group of naturally occurring compounds that are usually defined as polyhydroxyaldehydes or polyhydroxyketones, or as substances that undergo hydrolysis to yield such compounds. In actuality, the aldehyde and ketone groups of carbohydrates are often present as hemiacetals and acetals. The name comes from the fact that many carbohydrates possess the empirical formula Cx(H20)y. 

At the molecular level, most carbohydrates are polyhydroxyaldehydes, polyhydroxy-ketones, or compounds that yield them after hydrolysis. Therefore, the chemistry of carbohydrates is essentially the chemistry of hydroxyl and carbonyl groups and of acetal bonds (Section 12.6A) formed between these two functional groups. 

Carbohydrate A polyhydroxyaldehyde or polyhydroxyk-etone or a substance that gives these compounds on hydrolysis. 

Aldoses polyhydroxyaldehydes, one of the two main subdivisions of the monosaccharides (the other is the ketoses see Carbohydrates). A. are characterized by a terminal aldehyde group -CHO, which is always given the number 1 in systematic nomenclature. The A. are formally derived from their simplest representative, glyceraldehyde, by chain extension. According to the number of carbon atoms in their chain, they are classified as trioses, tetroses, etc. The pentoses and hexoses are particularly important metaboli-cally. 

We can now define carbohydrates more precisely in terms of their organic structures. Carbohydrates are polyhydroxyaldehydes, polyhydroxyketones, or substances that give such compounds on hydrolysis. The chemistry of carbohydrates is mainly the combined chemistry of two functional groups the hydroxyl group and the carbonyl group. 

The addition of cyanide to the aldehydic carbon of polyhydroxyaldehydes (see Carbohydrates, Chapter 11, and the Kiliani-Fischer synthesis) and simple aldehydes... 

A carbohydrate may be defined as a polyhydroxyaldehyde, polyhydroxyketone or a substance that can be hydrolysed to give these compounds. The carbohydrates are usually classified as follows ... 

The chemistry of the carbohydrates will be treated extensively in Chapt. XV and XVII. Here we mention that they are polyhydroxyaldehydes or ketones, which exist in the form of cyclic hemiacetals. The OH group arising from the hemiacetal structure is particularly reactive and can react with amines with loss of water (cf. formulas in Chapt. VII-1). Compounds resulting from this reaction are called N-glycosides (cf. Chapt. VI-3, XVII-1). 

---