ch 19 20 lect 2 carboxylic acids and derivatives l.
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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
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
slide2
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
slide3
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

slide4
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

slide5
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
slide6
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

slide7
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)
slide8
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

slide9
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
slide10
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