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Chapter 20 Carboxylic Acid Derivatives Nucleophilic Acyl Substitution

Chapter 20 Carboxylic Acid Derivatives Nucleophilic Acyl Substitution.

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Chapter 20 Carboxylic Acid Derivatives Nucleophilic Acyl Substitution

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  1. Chapter 20Carboxylic Acid DerivativesNucleophilic Acyl Substitution

  2. The key to this chapter is the next slide.It lists the various carboxylic acids in order of decreasing reactivity toward their fundamental reaction type (nucleophilic acyl substitution).The other way to read the list is in order of increasing stabilization of the carbonyl group.

  3. O RCCl O O RCOCR O RCOR' O RCNR'2 CarboxylicAcidDerivatives Acyl chloride Anhydride Ester Amide

  4. O RCCl O O RCOCR O RCOR' O RCNR'2 DecreasingReactivity(Nucleophilic AcylSubstitution) Acyl chloride Anhydride Ester Amide

  5. O RCCl O O RCOCR O RCOR' O RCNR'2 Increasingstabilizationof C=O Acyl chloride Anhydride Ester Amide

  6. 20.1Nomenclature of Carboxylic Acid Derivatives

  7. O RC X Acyl Halides • name the acyl group and add the word chloride, fluoride, bromide, or iodide as appropriate • acyl chlorides are, by far, the most frequently encountered of the acyl halides

  8. O CH3CCl O H2C CHCH2CCl O F CBr Acyl Halides acetyl chloride 3-butenoyl chloride p-fluorobenzoyl bromide

  9. O O RCOCR' Acid Anhydrides • when both acyl groups are the same, name the acid and add the word anhydride • when the groups are different, list the names of the corresponding acids in alphabetical order and add the word anhydride

  10. O O CH3COCCH3 O O C6H5COCC6H5 O O C6H5COC(CH2)5CH3 Acid Anhydrides acetic anhydride benzoic anhydride benzoicheptanoic anhydride

  11. O RCOR' Esters • name as alkyl alkanoates • cite the alkyl group attached to oxygen first (R') • name the acyl group second; substitute the suffix-ate for the -ic ending of the corresponding acid

  12. O CH3COCH2CH3 O CH3CH2COCH3 O COCH2CH2Cl Esters ethyl acetate methyl propanoate 2-chloroethyl benzoate

  13. O RCNH2 Amides having an NH2 group • identify the corresponding carboxylic acid • replace the -ic acid or -oic acid ending by -amide.

  14. O CH3CNH2 O (CH3)2CHCH2CNH2 O CNH2 Amides having an NH2 group acetamide 3-methylbutanamide benzamide

  15. O O RCNHR' RCNR'2 Amides having substituents on N • name the amide as before • precede the name of the amide with the name of the appropriate group or groups • precede the names of the groups by the letter N- (standing for nitrogen and used as a locant) and

  16. O CH3CNHCH3 O CN(CH2CH3)2 O CH3CH2CH2CNCH(CH3)2 CH3 Amides having substituents on N N-methylacetamide N,N-diethylbenzamide N-isopropyl-N-methylbutanamide

  17. RC N Nitriles • add the suffix -nitrile to the name of the parent hydrocarbon chain (including the triply bonded carbon of CN) • or: replace the -ic acid or -oic acid name of the corresponding carboxylic acid by -onitrile • or: name as an alkyl cyanide (functional class name)

  18. CH3C N C6H5C N CH3CHCH3 C N Nitriles ethanenitrileor: acetonitrileor: methyl cyanide benzonitrile 2-methylpropanenitrileor: isopropyl cyanide

  19. 20.2Structure of Carboxylic Acid Derivatives

  20. – •• •• •• O O O •• •• •• •• •• + •• •• RC X RC X RC X + Electron Delocalization and the Carbonyl Group • The main structural feature that distinguishes acyl chlorides, anhydrides, esters, and amides is the interaction of the substituent with the carbonyl group. It can be represented in resonance terms as: –

  21. – •• •• •• O O O •• •• •• •• •• + •• •• RC X RC X RC X + Electron Delocalization and the Carbonyl Group • The extent to which the lone pair on X can be delocalized into C=O depends on: • 1) the electronegativity of X • 2) how well the lone pair orbital of X interacts with the p orbital of C=O –

  22. Orbital overlaps in carboxylic acid derivatives • p orbital of carbonyl group

  23. Orbital overlaps in carboxylic acid derivatives • lone pair orbitalof substituent

  24. Orbital overlaps in carboxylic acid derivatives • electron pair of substituent delocalized into carbonyl p orbital

  25. •• O •• R C Cl •• •• •• Acyl Chlorides – •• O •• •• • acyl chlorides have the least stabilized carbonylgroup • delocalization of lone pair of Cl into C=O group isnot effective because C—Cl bond is too long R C + Cl •• ••

  26. O RCCl least stabilized C=O most stabilized C=O

  27. – •• •• •• •• O O O O •• •• •• •• •• + •• C C C C O O R R R R •• •• Acid Anhydrides • lone pair donation from oxygen stabilizes thecarbonyl group of an acid anhydride • the other carbonyl group is stabilized in anmanner by the lone pair

  28. O RCCl O O RCOCR' least stabilized C=O most stabilized C=O

  29. – •• •• O O •• •• •• + •• C C OR' OR' R R •• •• Esters • lone pair donation from oxygen stabilizes thecarbonyl group of an ester • stabilization greater than comparable stabilizationof an anhydride

  30. O RCCl O O RCOCR' O RCOR' least stabilized C=O most stabilized C=O

  31. – •• •• O O •• •• •• + •• C C NR'2 NR'2 R R Amides • lone pair donation from nitrogen stabilizes thecarbonyl group of an amide • N is less electronegative than O; therefore, amides are stabilized more than esters and anhydrides

  32. – •• •• O O •• •• •• + •• C C NR'2 NR'2 R R Amides • amide resonance imparts significant double-bondcharacter to C—N bond • activation energy for rotation about C—N bondis 75-85 kJ/mol • C—N bond distance is 135 pm in amides versusnormal single-bond distance of 147 pm in amines

  33. O RCCl O O RCOCR' O O RCOR' RCNR'2 least stabilized C=O most stabilized C=O

  34. – •• •• O O •• •• •• – •• C C O O R R •• •• •• •• Carboxylate ions • very efficient electron delocalization and dispersalof negative charge • maximum stabilization

  35. O RCCl O O RCOCR' O O RCOR' RCNR'2 O RCO– least stabilized C=O most stabilized C=O

  36. O RCCl O O RCOCR' O RCOR' O RCNR'2 Reactivity is related to structure: Table 20.1 Relative rateof hydrolysis Stabilization • The more stabilized the carbonyl group, the less reactive it is. very small 1011 small 107 moderate 1.0 large < 10-2

  37. O RCCl O O RCOCR' O O RCOR' RCNR'2 O RCO– most reactive least reactive

  38. •• •• O O •• •• C C R R X Y Nucleophilic Acyl Substitution In general: • Reaction is feasible when a less stabilized carbonyl is converted to a more stabilized one (more reactive to less reactive). + HY + HX

  39. •• OH •• O O •• •• R C C C Y R X R Y X General Mechanism for Nucleophilic Acyl Substitution involves formation and dissociationof a tetrahedral intermediate HY -HX

  40. O RCCl O O RCOCR' O O RCOR' RCNR'2 O RCO– most reactive a carboxylic acid derivative can be converted by nucleophilic acyl substitution to any other type that lies below it in this table least reactive

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