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Ch. 19-20 Lect. 2 Carboxylic Acids and Derivatives

Ch. 19-20 Lect. 2 Carboxylic Acids and Derivatives. Alkanoyl Halides Preparation Inorganic esters utilized, just as in conversion of alcohols to alkyl halides PBr 3 , SOCl 2 convert carboxylic acids to alkanoyl halides Nomenclature Pentanoic acid becomes pentanoyl chloride

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Ch. 19-20 Lect. 2 Carboxylic Acids and Derivatives

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  1. Ch. 19-20 Lect. 2 Carboxylic Acids and Derivatives • Alkanoyl Halides • Preparation • Inorganic esters utilized, just as in conversion of alcohols to alkyl halides • PBr3, SOCl2 convert carboxylic acids to alkanoyl halides • Nomenclature • Pentanoic acid becomes pentanoyl chloride • Cyclohexane carboxylic acid becomes cyclohexanecarbonyl chloride

  2. Reactivity: most reactive and useful derivative • Modified LiAlH4 reagent LiAl[OC(CH3)3]3H required to prevent over-reduction to alcohols • Organocuprates are less reactive alkyl metal reagents to prevent second addition to ketone • Amines do addition-elimination to give amides • Alcohols do addition-elimination to give esters • Acids do addition-elimination to give anhydrides • Water does addition-elimination to give acids

  3. Anhydrides • Preparation • As just seen, the reaction of an acid and an alkanoyl halide form anhydrides • The –OH group of the acid is a weak nucleophile for the reactive alkanoyl halide • Dehydration of two carboxylic acids can also work if 5-6 membered ring forms • Nomenclature • Replace “acid” with “anhydride” for the components • Acetic anhydride; Butanedioc anhydride • Unsymmetric anhydrides or mixed anhydrides are possible • Reactivity • All the alkanoyl halides reactions work for anhydrides, but are slower • Leaving group is a carboxylate anion, removed by aqueous extraction • Use anhydride as an activated substitute for reactions with carboxylic acids Acetic propanoic anhydride Acetic anhydride

  4. Esters • Preparation • As seen, alcohols do addition-elimination to alkanoyl halides to give esters • Acid catalyzed addition-elimination of alcohols to carboxylic acids is also useful • Esterification (and Ester Hydrolysis) Mechanism • Intramolecular esterifications give cyclic esters called lactones Favored for 5-6 membered rings

  5. Nomenclature: named as alkyl alkanoates • Reactivity • Ester hydrolysis forms the component alcohol and carboxylic acid • Acid catalyzed reaction is the reverse of esterification shown above • Base catalyzed ester hydrolysis is also possible • Transesterification occurs with alcohols • Acid or base catalyzed conversion of one ester to another • Control the equilibrium by adding a large excess of the new alcohol

  6. Ester + Amine + Heat gives Amides (amines are more nucleophilic than alcohols) • Grignard Reagents + Esters give Alcohols • The first reaction is an addition-elimination giving a Ketone product • The Ketone reacts with another Grignard molecule giving the alcohol • Esters can be reduced by hydrides to alcohols or aldehydes • LiAlH4 fully reduced the ester to an alcohol (similar to Grignard above) • DIBAL reduces ester only to an aldehyde DIBAL = diisobutylaluminum hydride

  7. Esters in Nature • Esters are important aroma and flavor agents in natural foods • Isopentyl acetate = banana oil • Octyl acetate = orange oil • Methyl salicylate = oil of wintergreen • Waxes are long chain esters • Beeswax • Spermaciti: sperm whale wax • Fats and Oils = triesters of glycerol (1,2,3-propanetriol)

  8. Amides • Preparation • Amines react with carboxylic acids as bases and as nucleophiles • Heating favors the thermodynamic product: amide • Amino acids cyclize to give lactams • Nomenclature: alkanamides or cycloalkanecarboxamides Kinetic product Thermodynamic product

  9. Reactivity: least reactive of the carboxylic acid derivatives • Resonance structures prevent rotation around C—N bond • Ea = 21 kcal/mol for rotation about this single bond • Two peaks seen in the proton NMR • Hydrolysis requires heat and concentrated acid or base • Proteins are held together by strong amide bonds; they don’t break easily • Reduction to Amine by LiAlH4

  10. Alkanenitriles • Preparation: SN1 or SN2 reaction of haloalkanes • Nomenclature • Alkanenitriles or cycloalkanenitriles • Reactivity: synthesis of carboxylic acids • Acid catalyzed mechanism • Similar Base catalyzed mechanism • Adds one carbon to the haloalkane, then turns it into carboxylic acid • Make derivatives from there; take advantage of all of their reactions

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