Carbohydrates (Organic Chemistry 2) PDF

# Carbohydrates ## Organic Chemistry 2 Carbohydrates (glycans) have the following basic composition: - (CH₂O)_n or H-C-OH - Carbohydrates are aldehyde or ketone compounds with multiple hydroxyl groups. - The basic molecular formula is (C_nH₂O_n...

# Carbohydrates ## Organic Chemistry 2 Carbohydrates (glycans) have the following basic composition: - (CH₂O)_n or H-C-OH - Carbohydrates are aldehyde or ketone compounds with multiple hydroxyl groups. - The basic molecular formula is (C_nH₂O_n), where n = 3 or more. - The term carbohydrate comes from the observation that when you heat sugars, you get carbon and water (hence, hydrate of carbon). ## Classification They are classified according to the number of structural units into: 1. Monosaccharides: They are the simplest carbohydrates that cannot be hydrolyzed into simpler units. 2. Disaccharides: Produce 2 molecules of monosaccharide on hydrolysis. 3. Oligosaccharides: Produce three to ten monosaccharide units on hydrolysis. 4. Polysaccharides: Produce more than 10 monosaccharide units on hydrolysis. - Monosaccharides - simple sugars with multiple OH groups. Based on the number of carbons (3, 4, 5, 6), a monosaccharide is a triose, tetrose, pentose or hexose. - Disaccharides - 2 monosaccharides covalently linked. - Oligosaccharides - a few monosaccharides covalently linked. - Polysaccharides - polymers consisting of chains of monosaccharide or disaccharide units. ## Monosaccharides - Aldoses (e.g., glucose) have an aldehyde group at one end. - Ketoses (e.g., fructose) have a keto group, usually at C2. ### D vs L Designation D & L designations are based on the configuration about the single asymmetric C in glyceraldehyde. - The lower representations are Fischer Projections. ## Sugar Nomenclature - For sugars with more than one chiral center, D or L refers to the asymmetric C farthest from the aldehyde or keto group. - Most naturally occurring sugars are D isomers. - D & L sugars are mirror images of one another. They have the same name, e.g., D-glucose & L-glucose. - Other stereoisomers have unique names, e.g., glucose, mannose, galactose, etc. - The number of stereoisomers is 2ⁿ, where n is the number of asymmetric centers. - The 6-C aldoses have 4 asymmetric centers. Thus there are 16 stereoisomers (8 D-sugars and 8 L-sugars). ## Stereoisomerism - Compound that have the same structural formula, but differ in spatial configuration are known as stereoisomers and the phenomenon is called stereoisomerism. - The number of possible isomers of a compound depends on the number of asymmetric carbon atoms (n) and is equal to 2ⁿ. - Glucose, with four asymmetric carbon atoms, has 16 isomers. The more important types of isomers include: - D and L isomers, - pyranose and furanose ring structures, - alpha and beta anomers, - epimers and - aldose-ketose isomers. ## * Nomenclature: 1. According to active group in the sugar: - If monosaccharide contains aldehyde group (CHO) → it's called aldose. - And if contain ketone group (c=o) → it's called ketose. 2. According to the number of carbon atoms (n): If sugar contains: - 3 carbons → it's called triose, 4c- tetrose - 6c- hexose - 7c- heptose 3. By combining the two methods...we find that: - 3c-Aldotriose : -ketotriose - 4c-Aldotetrose : -ketotetroSe - 5c-Aldopentise : -ketopentos - 6c-Aldohexose : -ketohexose ### Three Carbon - D-Glyceraldehyde - Dihydroxyacetone ### Four Carbon - D-Erythrose - D-Erythrulose ### Five Carbon - D-Xylose - D-Ribose - D-Arabinose - D-Xvlulose - D-Fabulose ### Six Carbon - D-Glucose - D-Fructose ## Hemiacetal & hemiketal formation - An aldehyde can react with 1 mole of alcohol to form a hemiacetal. - A ketone can react with one mole alcohol to form a hemiketal. a. Fischer projection formula where the aldehyde group reacts with an alcohol group on the same sugar to form a hemiacetal ring. b. Haworth formula: Where the cyclic structure is represented in pyranose (six-membered) and furanose (five-membered) rings resembling pyran and furan rings. - Pentoses and hexoses can cyclize as the ketone or aldehyde reacts with a distal OH. - Glucose forms an intra-molecular hemiacetal, as the C1 aldehyde & C5 OH react, to form a 6-member pyranose ring, named after pyran. These representations of the cyclic sugars are called Haworth projections. ## Anomers - The anomeric carbon is the carbon derived from the carbonyl carbon (the ketone or aldehyde functional group) ## Cyclic Structure for Glucose Glucose cyclic hemiacetal formed by reaction of -CHO with-OH on C5. ## Cyclic Structure for Fructose Cyclic hemiketal formed by reaction of C=O at C2 with -OH at C5. - Fructose forms either a 6-member pyranose ring, by reaction of the C2 keto group with the OH on C6, or a 5-member furanose ring, by reaction of the C2 keto group with the OH on C5. - Cyclization of glucose produces a new asymmetric center at C1. The 2 stereoisomers are called anomers, α & β. - Haworth projections represent the cyclic sugars as having essentially planar rings, with the OH at the anomeric C1: - α (OH below the ring) - β (OH above the ring). - Because of the tetrahedral nature of carbon bonds, pyranose sugars actually assume a "chair" or "boat" configuration, depending on the sugar. - The representation above reflects the chair configuration of the glucopyranose ring more accurately than the Haworth projection.