CHE202 - Chapter 20 and 21 Lecture Notes

CHE202 - Chapter 20 and 21 Lecture Notes - Organic...

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Chapter 20/21 Carboxylic Acids and Their Derivatives. Lecture date : April 7, 2009 Organic Chemistry , 6 th Edition L. G. Wade, Jr.
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2 Introduction ± Carbonyl (-C=O) and hydroxyl (-OH) on the same carbon is carboxyl group. ± Carboxyl group is usually written -COOH. ± Aliphatic acids have an alkyl group bonded to -COOH. ± Aromatic acids have an aryl group. ± Fatty acids are long-chain aliphatic acids. =>
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3
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4 ± Nomenclature and Physical Properties In IUPAC nomenclature, the name of a carboxylic acid is obtained by changing the -e of the corresponding parent alkane to - oic acid The carboxyl carbon is assigned position 1 and need not be explicitly numbered The common names for many carboxylic acids remain in use Methanoic and ethanoic acid are usually referred to as formic and acetic acid Cycloalkanes bonded to -COOH are named as cycloalkanecarboxylic acids. Aromatic acids are named as benzoic acids. o -hydroxybenzoic acid (salicylic acid) 2-isopropylcyclopentanecarboxylic acid
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5 z Dicarboxylic Acids Dicarboxylic acids are named as alkanedioic acids in the IUPAC system. For IUPAC name, number the chain from the end closest to a substituent. Common names are often used for simple dicarboxylic acids Two carboxyl groups on a benzene ring indicate a phthalic acid.
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6 Structure of Carboxyl Carbon is sp 2 hybridized. Bond angles are close to 120 ° . O-H eclipsed with C=O, to get overlap of π orbital with orbital of lone pair on oxygen. =>
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Boiling and Melting POINTS Higher boiling points than similar alcohols, due to dimer formation. Aliphatic acids with more than 8 carbons are solids at room temperature. Double bonds (especially cis) lower the melting point. Note these 18-C acids: Stearic acid (saturated): 72 ° C Oleic acid (one cis double bond): 16 ° C Linoleic acid (two cis double bonds): -5 ° C Solubility Water solubility decreases with the length of the carbon chain. Up to 4 carbons, acid is miscible in water. More soluble in alcohol. Also soluble in relatively nonpolar solvents like chloroform because it dissolves as a dimer. 7
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8 z Acidity of Carboxylic Acids Î The carboxyl proton of most carboxylic acids has a p K a = 4 - 5 + Carboxylic acids are readily deprotonated by sodium hydroxide or sodium bicarbonate to form carboxylate salts + Carboxylate salts are more water soluble than the corresponding carboxylic acid Î Electron-withdrawing groups near the carboxyl group increase the carboxylic acid’s acidity + They stabilize the carboxylate anion by inductive delocalization of charge
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9 Acidity
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10 Resonance Stabilization Substituent Effects on Acidity
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11 Salts of Carboxylic Acids Sodium hydroxide removes a proton to form the salt. Adding a strong acid, like HCl, regenerates the carboxylic acid.
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This note was uploaded on 10/10/2010 for the course CHE 202 taught by Professor Richard during the Spring '05 term at SUNY Buffalo.

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CHE202 - Chapter 20 and 21 Lecture Notes - Organic...

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