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Chemistry

Chemistry. Session - 2. Carboxylic Acids. Session Objectives. Introduction to carboxylic acids Physical properties and structure General method of preparation By oxidation Carbonation of grignard reagent Hydrolysis of acid derivatives From nitriles

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Chemistry

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  1. Chemistry

  2. Session - 2 Carboxylic Acids

  3. Session Objectives • Introduction to carboxylic acids • Physical properties and structure • General method of preparation • By oxidation • Carbonation of grignard reagent • Hydrolysis of acid derivatives • From nitriles • 4. Reactions of the carboxylic acids

  4. Introduction The functional group of carboxylic acids consists of a C=O with -OH bonded to the same carbon. 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.

  5. -chlorobutyric acid -phenylcaproic acid => Common Names Many aliphatic acids have historical names. Positions of substituents on the chain are labeled with Greek letters.

  6. 2-chlorobutanoic acid trans-3-phenyl-2-propenoic acid (cinnamic acid) IUPAC Names • Remove -e from alkane (or alkene) name, add -oic acid. • The carbon of the carboxyl group is #1.

  7. 2-isopropylcyclopentanecarboxylic acid o-hydroxybenzoic acid (salicylic acid) Naming Cyclic Acids Cycloalkanes bonded to -COOH are named as cycloalkanecarboxylic acids. Aromatic acids are named as benzoic acids.

  8. 3-bromohexanedioic acid -bromoadipic acid Dicarboxylic Acids Aliphatic diacids are usually called by their common names (to be memorized). For IUPAC name, number the chain from the end closest to a substituent. Two carboxyl groups on a benzene ring indicate a phthalic acid.

  9. Structure of Carboxyl Carbon is sp2 hybridized. Bond angles are close to 120. O-H eclipsed with C=O, to get overlap of  orbital with orbital of lone pair on oxygen.

  10. Acetic acid, b.p. 118C Boiling Points Higher boiling points than similar alcohols, due to dimer formation.

  11. Melting Points • 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

  12. 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.

  13. Acidity

  14. Resonance Stabilization

  15. pKa = 4.46 pKa = 4.19 pKa = 3.47 pKa = 3.41 pKa = 2.16 Substituent Effects on Acidity

  16. Preparation Reactions • Oxidation of Primary Alcohols • Oxidation of Aldehydes • Oxidation of Substituted Aromatics • Carbonation of Grignard reagents • Hydrolysis of Acid derivatives and Nitriles • Haloform reaction • Periodic acid Cleavage of Vicinal Dials/Diketones • Oxidative Cleavage of Alkenes/Alkynes

  17. Carboxylic Acids via Oxidation From Primary Alcohols From Aldehydes From Substituted Aromatics

  18. Oxidative Cleavage Reactions • Alkene Cleavage • Hot Potassium Permanganate • Alkyne Cleavage • Hot Potassium Permanganate • Ozonolysis

  19. Grignard Synthesis Grignard reagent + CO2 yields a carboxylate salt.

  20. Hydrolysis of acid derivatives

  21. Hydrolysis of Nitriles Basic or acidic hydrolysis of a nitrile produces a carboxylic acid.

  22. Haloform Reaction Cleavage of methyl carbinols Cleavage of methyl carbonyls

  23. Periodic Acid Cleavage of Vicinal Dials/Diketones

  24. Reduction of Carboxylic Acids • Lithium Aluminum Hydride reduction • Diborane reduction

  25. Conversion to acid derivatives • The group bonded to the acyl carbon determines the class of compound: • -OH, carboxylic acid • -Cl, acid chloride • -OR’, ester • -NH2, amide • These interconvert via nucleophilic acyl substitution.

  26. Fischer Esterification Acid + alcohol yields ester + water. Acid catalyzed for weak nucleophile. All steps are reversible. Reaction reaches equilibrium.

  27. Conversion to Acid Chlorides An activated form of the carboxylic acid. Chloride is a good leaving group, so undergoes acyl substitution easily. To synthesize acid chlorides use thionyl chloride or oxalyl chloride with the acid.

  28. Conversion to Amides Amine (base) removes a proton from the carboxylic acid to form a salt. Heating the salt above 100C drives off steam and forms the amide.

  29. Reduction to 1 Alcohols Use strong reducing agent, LiAlH4. Borane, BH3 in THF, reduces carboxylic acid to alcohol, but does not reduce ketone.

  30. Decarboxylation of RCOOH • Thermolysis of beta-diacids • Thermolysis of beta-keto acids

  31. Substitution in the hydrocarbon part Hell-Volhard-Zelinsky reaction Acids having an a-hydrogen are halogenated at the a-position on treatment with chlorine or bromine in the presence of small amount of red phosphorus to give a-halocarboxylic acids.

  32. Ring substitution in aromatic acids —COOH group is deactivating and meta directing. Aromatic carboxylic acids do not undergo Friedel-Crafts reaction. Nitration Bromination

  33. Some commercially important carboxylic acids Methanoic acid; (HCOOH) It is colourles, pungent smelling liquid. It is powerful reducing agent. It reduces Tollen’s reagent and Fehling solution. Used in rubber, textile, dyeing, leather and electroplating industries.

  34. Ethanoic acid(acitic acid, CH3COOH) Main constituent of vinegar and is obtained by fermentation of molasses in presence of air. Industrially, it is obtained in pure form by oxidation of ethanal with air in the presence of cobalt acetate catalyst or by carbonylation of methanol in the presence of rhodium catalyst. CH3CHO + CO  CH3COOH CH3OH + CO  CH3COOH Colourless liquid with pungent odour. Freezes at 289 K forming ice like crystal. Water free acetic acid, obtained by melting of the crystals is called glacial acetic acid.

  35. Thank you

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