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Carboxylic Acids and their Derivatives

Chapter 35. Carboxylic Acids and their Derivatives. 35.1 Introduction 35.2 Nomenclature of Carboxylic Acids and their Derivatives 35.3 Physical Properties of Carboxylic Acids 35.4 Preparation of Carboxylic Acids 35.5 Reactions of Carboxylic Acids

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Carboxylic Acids and their Derivatives

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  1. Chapter 35 Carboxylic Acidsand their Derivatives 35.1Introduction 35.2Nomenclature of Carboxylic Acids and their Derivatives 35.3Physical Properties of Carboxylic Acids 35.4Preparation of Carboxylic Acids 35.5Reactions of Carboxylic Acids 35.6Reactions of the Derivatives of Carboxylic Acids 35.7Uses of Carboxylic Acids and their Derivatives

  2. 35.1 Introduction (SB p.29) Carboxylic acids refer to the class of organic compounds with the carboxyl group

  3. 35.1 Introduction (SB p.29) General formula of carboxylic acid: RCOOH Examples:

  4. 35.1 Introduction (SB p.30) Carboxylic acid derivatives

  5. 35.2 Nomenclature of Carboxylic Acids and their Derivatives (SB p.30) Carboxylic Acids Carboxylic acids are named by replacing the final “-e” of the name of the corresponding alkane with “-oic acid” When other substituents are present, the carboxyl carbon is assigned position 1 Examples:

  6. 35.2 Nomenclature of Carboxylic Acids and their Derivatives (SB p.31) Acyl Chlorides Acyl chlorides are named by replacing the final “-ic acid” of the name of the parent carboxylic acid with “-yl chloride” Examples:

  7. 35.2 Nomenclature of Carboxylic Acids and their Derivatives (SB p.31) Acid Anhydrides Acid anhydrides are named by dropping the word “acid” from the name of the parent carboxylic acid and then adding the word “anhydride” Examples:

  8. 35.2 Nomenclature of Carboxylic Acids and their Derivatives (SB p.31) Amides • Amides that have no substituents on the nitrogen atom are named by replacing “-oic acid” from the parent carboxylic acid with “amide” • Substituents on the nitrogen atom of amides are named and the named substituent is preceded by N-, or N,N- • Examples:

  9. 35.2 Nomenclature of Carboxylic Acids and their Derivatives (SB p.31) Esters • The names of esters are derived from the names of the alcohol (with the ending “-yl”) and the carboxylic acid (with the ending “-oate”) • The portion of the name derived from the alcohol comes first • Examples:

  10. 35.2 Nomenclature of Carboxylic Acids and their Derivatives (SB p.32) Example 35-1 Give the IUPAC names of the following compounds: (a) (b) (c) (d) Solution: (a) 3-Methylbutanoic acid (b)N-methylethanamide (c) Ethyl benzoate (d) Benzoic anhydride Answer

  11. 35.2 Nomenclature of Carboxylic Acids and their Derivatives (SB p.33) Solution: (a) The structural formula of methyl ethanoate is: It is formed from the reaction of ethanoic acid and methanol. Example 35-2 An ester is formed by reacting an alcohol with a carboxylic acid. Draw the structural formula of the following ester molecule and give the name of the alcohol and the carboxylic acid that form the ester. (a) Methyl ethanoate Answer

  12. 35.2 Nomenclature of Carboxylic Acids and their Derivatives (SB p.33) Solution: (b) The structural formula of ethyl methanoate is: It is formed from the reaction of methanoic acid and ethanol. Example 35-2 An ester is formed by reacting an alcohol with a carboxylic acid. Draw the structural formula of the following ester molecule and give the name of the alcohol and the carboxylic acid that form the ester. (b) Ethyl methanoate Answer

  13. 35.2 Nomenclature of Carboxylic Acids and their Derivatives (SB p.33) (a) (b) Butanoic acid (c) CH3CH(CH3)CH2COOH (d) 3-Methylbutanoic acid (e) C6H4ClCOOH (f) 2-Chlorobenzoic acid (g) CCl3COOH (h) Check Point 35-1 Complete the following table by filling in the molecular formulae, structural formulae or IUPAC names of the carboxylic acids. Answer

  14. 35.3 Physical Properties of Carboxylic Acids (SB p.34) • Carboxylic acids are colourless liquids at room conditions • They have characteristic pungent smell and sour taste

  15. 35.3 Physical Properties of Carboxylic Acids (SB p.34)

  16. 35.3 Physical Properties of Carboxylic Acids (SB p.35) Boiling Point and Melting Point Due to formation of intermolecular hydrogen bonds high b.p. and m.p. The hydrogen bonds are more extensive than those in alcohols higher b.p. and m.p. than alcohols

  17. 35.3 Physical Properties of Carboxylic Acids (SB p.36) Density • The densities of carboxylic acids decrease with increasing relative molecular masses • Only methanoic acid and ethanoic acid are denser than water at 20°C∵ closer packing of the smaller molecules in the liquid phase

  18. 35.3 Physical Properties of Carboxylic Acids (SB p.36) Solubility • Carboxylic acids of low molecular masses show appreciable solubilities in water∵ carboxylic acids are polar and can form strong hydrogen bonds with water molecules • First four carboxylic acids are miscible with water • The length of the hydrocarbon portion   solubilities in water 

  19. 35.3 Physical Properties of Carboxylic Acids (SB p.36) Example 35-3 (a) Propanoic acid has a boiling point of 141°C which is considerably higher than that of butan-1-ol (117°C), although they have the same molecular mass. Explain why. Answer Solution: (a) Each propanoic acid molecule forms two intermolecular hydrogen bonds with another propanoic acid molecule. However, two butan-1-ol molecules form only one hydrogen bond. As molecules of propanoic acid form more extensive intermolecular hydrogen bonds than those of butan-1-ol, the boiling point of propanoic acid is higher than that of butan-1-ol.

  20. 35.3 Physical Properties of Carboxylic Acids (SB p.36) Example 35-3 (b) Rank the following compounds in decreasing order of solubility in water: CH3CH2CH2COOH, CH3CH2COOCH3, CH3COOH Answer Solution: (b) The solubility of the compounds in water decreases in the order: CH3COOH > CH3CH2CH2COOH > CH3CH2COOCH3

  21. 35.3 Physical Properties of Carboxylic Acids (SB p.36) Solution: (c) Example 35-3 (c) Propanedioic acid forms intramolecular hydrogen bonds. Draw its structural formula showing clearly the formation of intramolecular hydrogen bonds. Answer

  22. 35.4 Preparation of Carboxylic Acids (SB p.37) Hydrolysis of Nitriles • 2-hydroxyalkanenitriles is formed by nucleophilic addition reaction of aldehydes or ketones with hydrogen cyanide • Acid hydrolysis or alkaline hydrolysis of 2-hydroxyalkanenitriles produce carboxylic acids or carboxylate ions respectively

  23. 35.4 Preparation of Carboxylic Acids (SB p.37) Nitriles can be formed by nucleophilic substitution reaction of haloalkanes with sodium cyanide Acid hydrolysis of the nitrile produces a carboxylic acid with one more carbon atom e.g.

  24. 35.4 Preparation of Carboxylic Acids (SB p.38) Hydrolysis of Esters • Hydrolysis of esters give parent carboxylic acids and alcohols by boiling under reflux with the dilute HCl or dilute H2SO4 • Acyl chlorides, acid anhydrides, esters and amides can be hydrolyzed to give the corresponding carboxylic acids • Reactive derivatives react vigorously with water, e.g. acyl chlorides • Less reactive derivatives require more severe conditions to bring about the reaction

  25. 35.4 Preparation of Carboxylic Acids (SB p.38) Oxidation of Alcohols and Aldehydes Aldehydes and 1° alcohols can be oxidized to carboxylic acids by KMnO4/H+

  26. 35.4 Preparation of Carboxylic Acids (SB p.38) Examples:

  27. 35.4 Preparation of Carboxylic Acids (SB p.39) Oxidation of Alkybenzenes 1° and 2° alkyl groups (but not 3°) directly attached to a benzene ring are oxidized by hot KMnO4/OH– to carboxyl groupe.g. This oxidation takes place initially at the benzylic carbon alkyl groups longer than methyl group are ultimately degraded to benzoic acid

  28. 35.4 Preparation of Carboxylic Acids (SB p.39) (a) Check Point 35-2 Write the equations for the acid-catalyzed and base-catalyzed hydrolyses of each of the following substances: (a) Ethyl butanoate Answer

  29. 35.4 Preparation of Carboxylic Acids (SB p.39) (b) Check Point 35-2 Write the equations for the acid-catalyzed and base-catalyzed hydrolyses of each of the following substances: (b) Propanamide Answer

  30. 35.4 Preparation of Carboxylic Acids (SB p.39) (c) Check Point 35-2 Write the equations for the acid-catalyzed and base-catalyzed hydrolyses of each of the following substances: (c) Benzoyl chloride Answer

  31. 35.5 Reactions of Carboxylic Acids (SB p.39) Acidity of Carboxylic Acids • Carboxylic acids are weak acids • Their acidic properties are due to the presence of ionizable hydrogen atom

  32. 35.5 Reactions of Carboxylic Acids (SB p.39) e.g. ethanoic acid The acid strength of ethanoic acid is shown by the value of its acid dissociation constant (Ka) pKais used for convenience as Ka is small for weak acidspKa = –log Ka , the smaller the pKa value, the stronger is the acid

  33. 35.5 Reactions of Carboxylic Acids (SB p.40) • Unsubstituted carboxylic acidshave pKa values in the range of 3 to 5 • Alcohols have pKa values in 15 to 18 carboxylic acids are much more acidic than alcohols

  34. 35.5 Reactions of Carboxylic Acids (SB p.40) Resonance Effect The greater stability of carboxylic acids is attributed to resonance stabilization of the carboxylate ions Example:

  35. 35.5 Reactions of Carboxylic Acids (SB p.41) • By the resonance effect, the ethanoate ion can be stabilized by spreading the negative charge over two oxygen atoms • No stabilizing resonance structures for alkoxide ionse.g. • Carboxylic acids are much acidic than alcohols∵ formation of more stable conjugate base

  36. 35.5 Reactions of Carboxylic Acids (SB p.41) Inductive Effect • Both compounds contain the highly polarized O – H bond due to the difference in electronegativity • The greater acidity of ethanoic acid is due to the presence of the powerful electron-withdrawing carbonyl group

  37. 35.5 Reactions of Carboxylic Acids (SB p.41) Two resonance structures for the carbonyl group: • The carbonyl atom of ethanoic acid bears a large partial positive charge and exerts a powerful electron-withdrawing inductive effect to hydroxyl oxygen atom, making the hydroxyl hydrogen atom much more positive proton dissociate more readily • The electron-withdrawing inductive effect of the carbonyl group stabilizes the carboxylate ion carboxylic acids are stronger acid than alcohols

  38. 35.5 Reactions of Carboxylic Acids (SB p.42) Inductive Effects of Other Groups • 1. Electron-withdrawing Groups • The presence of electron-withdrawing groups increase the acidity of a carboxylic acide.g.

  39. 35.5 Reactions of Carboxylic Acids (SB p.42) • The chlorine atom withdraws electron from the carbonyl group and oxygen making the hydroxyl hydrogen more positive than that of ethanoic acid • The chlorine atom also stabilizes the chloroethanoate ion to greater extent

  40. 35.5 Reactions of Carboxylic Acids (SB p.41) Effect of electron-withdrawing substituents on the acidity of carboxylic acids Acidity pKa CH3COOH < < < CCl3COOH Ethanoic Chloroethanoic Dichloroethanoic Trichloroethanoic acid acid acid acid 4.76 2.86 1.29 0.65 Acidity pKa CH3COOH < < < < Ethanoic Iodoethanoic Bromoethanoic Chloroethanoic Fluoroethanoic acid acid acid acid acid 4.76 3.17 2.90 2.86 2.66 • The greater the number of electron-withdrawing groups, the stronger is the acid • The more electronegative of halogen substituents (F > Cl > Br > I), the stronger is the acid

  41. 35.5 Reactions of Carboxylic Acids (SB p.43) Acidity pKa < < 4-Chlorobutanoic 3-Chlorobutanoic 2-Chlorobutanoic acid acid acid 4.52 4.06 2.84 • The further away of the substituents from the carboxyl group, the weaker is the acid

  42. 35.5 Reactions of Carboxylic Acids (SB p.43) • 2. Electron-releasing Groups • The presence of electron-releasing groups reduce the acidity of a carboxylic acide.g.

  43. 35.5 Reactions of Carboxylic Acids (SB p.43) • Electron-releasing alkyl groups release electrons towards the electron-deficient carbonyl carbon, thus reducing its charge • Reduce the positive character of the hydroxyl hydrogen atom  dissociate less readily • Intensify the negative charge on the carboxyl group  destablizes the ethanoate ion

  44. 35.5 Reactions of Carboxylic Acids (SB p.44) (a) The greater the Ka value, the stronger is the acid. CH2FCOOH is a stronger acid than CH2BrCOOH. This –F substituent exerts a stronger inductive effect than the –Br substituent, as fluorine is more electronegative than bromine. The –F substituent stabilizes the conjugate base (i.e. CH2FCOOH–) to a greater extent than the –Br substituent. Therefore, CH2FCOOH has a Ka value of 2.19  10–3 M, and CH2BrCOOH has a Ka value of 1.26  10–3 M. Check Point 35-3 (a) Match the Ka values: 2.19  10–3 M and 1.26  10–3 M, to the carboxylic acids: CH2FCOOH and CH2BrCOOH. Explain your answer briefly. Answer

  45. 35.5 Reactions of Carboxylic Acids (SB p.44) (b) CH3COOH < CH2ClCOOH < CHCl2COOH < CCl3COOH The –Cl substituent is an electron-withdrawing group. An increasing number of the –Cl substituent brings about a greater negative inductive effect. Thus, the conjugate base will be stabilized more, and the acidity of the acid increases. Check Point 35-3 (b) Arrange the following acids in ascending order of acidity: CHCl2COOH, CCl3COOH, CH2ClCOOH, CH3COOH Explain the order briefly. Answer

  46. 35.5 Reactions of Carboxylic Acids (SB p.44) Check Point 35-3 (c) Arrange the following aryl carboxylic acids in descending order of acidity: Explain the order briefly.

  47. 35.5 Reactions of Carboxylic Acids (SB p.44) (c) is more acidic than ,because contains the –Cl substituent which is an electron-withdrawing group. Thus, it exerts a negative inductive effect to the conjugate base, and hence stabilizes the carboxylate ion formed. is less acidic than , because contains a methyl group which is an electron-releasing group. Thus, it exerts a positive inductive effect to the conjugate base, and hence destabilizes the carboxylate ion formed.

  48. 35.5 Reactions of Carboxylic Acids (SB p.44) (d) CH2ClCOOH is a stronger acid as CH3CHClCOOH contains a methyl group which is an electron-releasing group. It exhibits a positive inductive effect and thus destabilizes the conjugate base of the acid. Therefore, CH3CHClCOOH is less acidic. Check Point 35-3 (d) Which is a stronger acid, CH2ClCOOH or CH3CHClCOOH? Explain your answer briefly. Answer

  49. 35.5 Reactions of Carboxylic Acids (SB p.44) Formation of Salts Reaction with Active Metals • Carboxylic acids react with reactive metals such as Na or Mg to give the corresponding metal carboxylates and hydrogen gas

  50. Example:

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