Carboxylic Acids and their Derivatives

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					                                         Lok Sin Tong Leung Chik Wai Memorial School                                               1
                                                        F.7 Chemistry
                                              Carboxylic acids & their derivatives

Carboxylic Acids and their Derivatives

I.      Introduction                                                                                                  O
        Carboxylic acids are those organic compounds that contain the carboxylic group                            C
                                                                                                                      OH
        , which is the combination of the carbonyl group >C=O and the hydroxyl
        group –OH.


        Example
        (1)   Aliphatic carboxylic acids:
                  O                                                   O                                       O
        H     C                                            H3 C   C                        CH3 CH2 CH2     C
                   OH                                                 OH                                          OH

        ________________                              _________________                    _____________________

                                    O                                                                    OH               O
                                                                           O
        CH3 CH2 CH CH2 C                                                                   CH3 CH CH2 CH CH2 C
                                                    CH3 CH CH CH2 C
                                        OH                                                                                    OH
                      Br                                                       OH              Cl

        ________________                              _________________                    _____________________

          O                O                         O                     O
              C       C                                    C   CH2 CH2
        HO                 OH                       HO                         OH

        ________________                              _________________


        (2)        Aromatic carboxylic acids:

                                                                          COOH                       COOH
                                    COOH


                                                    O2 N                                            COOH

        ________________                              _________________                    _____________________


II.     Formation of Carboxylic Acids
      (A)   Hydrolysis of Nitriles
            1.     Hydrolysis of cyanohydrins or -hydroxy nitriles (prepared from aldehydes and
                        ketones) results in -hydroxy carboxylic acids.
                                                                      OH            H3O+       OH
              R
                      C     O   +        HCN                      R   C    CN              R   C    COOH
              H                                                       H                        H

              2.           Hydrolysis of nitriles (prepared from nucleophilic substitution of alkyl halides with
                           sodium cyanide) by a base, or by prolonged refluxing in acid solution produces the
                           acid salt and ammonia or the acid.
                          Lok Sin Tong Leung Chik Wai Memorial School                                  2
                                         F.7 Chemistry
                               Carboxylic acids & their derivatives
Examples:

                        NaCN                                      H3O+
BrCH2CH2CH2Br

Note: This method is limited to the use of primary alkyl halides. Use of secondary or tertiary
halide often lead to elimination to form alkenes rather than substitution because the cyanide ion is
a strong base.

(B)    Oxidation of Alkanols and Aldehydes

       Oxidizing agents used : KMnO4 / H+ or K2Cr2O7 / H+

                                 +
                 KMnO4 / H
       R-CHO

                                     +
                       KMnO4 / H
       R-CH2OH

(C)    Oxidation of Aromatic side chains

       The side chains of alkylbenzenes are almost easily oxidized by strong oxidizing agents
       such as hot potassium manganate (VII) to give the acids.

                  CH3
                                          +
                         KMnO4 /         H

                          HEAT


       Note: (1) The oxidation take place at the benzylic carbon. Alkylbenzenes with alkyl groups
       greater than methyl are also degraded to benzoic acid.

                  R
                                          +
                         KMnO4 /         H

                          HEAT


       (2)     The oxidation begins at the benzylic hydrogen. Those do not possess any benzylic
                   hydrogen are resistant to side chain oxidation.

                   CH3
                                                    +
                                     KMnO4 /        H
                               CH3
                         CH2
                                         HEAT
                   CH3
                                                        NO REACTION

       (3)     If the aromatic side chain contains an aldehyde or ketone group, the oxidation
               product is determined by the strength of the oxidizing agents.

                   CH2CHO
                                                +
                                KMnO4 /         H

                                 HEAT

             CH2=CH2
                             Lok Sin Tong Leung Chik Wai Memorial School                                   3
                                            F.7 Chemistry
                                  Carboxylic acids & their derivatives

                       CH2CHO
                                               +
                                   K2Cr2O7 /   H
                                    HEAT

III.   Acidity and Influence of substituents on acidity
       Carboxylic acids are rater strong acids when compared with alcohols but are weak acids when
       compared with mineral acids.

       The acid strength of carboxylic acid is sufficient enough to liberate carbon dioxide from
       metallic carbonate , metallic hydrogencarbonate and to liberate hydrogen from some reactive
       metals.

       CH3COOH + Na2CO3             

       CH3COOH + Mg           

       Note: The acids are displaced by stronger acids from their salts.

                                  RCOO-Na+ + HCl          RCOOH + NaCl

       Reason for acidity

            (A)    Acidity
                   The strength of an acid HA, i.e. the extent to which it is dissociated in the aqueous
                   phase, is determined by the following equilibrium:

                              H2O + HA                H3O+ + A-

                   Ka = _________________


                   Ka is the acidity constant. For weak acids, the value of Ka is very small. It is more
                   convenient to express Ka by pKa where pKa = - log10Ka .

                   The smaller the value of pKa , the ___________________ is the strength of that
                   acid.

            (B)    Factors affecting Acidity
                   1.   The strength of the H-A bond.
                   2.   The electronegativity of A
                   3.   Factors stabilizing its conjugated anion A- with respect to HA.
                   4.   The nature of the solvent.

                   Example: Compare the acidity of the following compounds:
                                                                       OH

                       O                O
                   CH3C H           CH3C OH        CH3CH2 OH
                       A                B                 C                 D
                                  Lok Sin Tong Leung Chik Wai Memorial School                                4
                                                 F.7 Chemistry
                                       Carboxylic acids & their derivatives
              Interpretation:
              (1)     A has the lowest acidity
                      It is because the carbon atom which is attached to the H atom has lower
                      electronegativity than the oxygen atom in the rest compounds. This make the O-H
                            weaker than the C-H bond and thus it is easier to break.

              (2)       C has lower acidity than B and D.
                        The anion for B ( a carboxylate ion) and D (a phenoxide ion) are more stable than
                            that of C (an alkoxide ion ) due to the stabilization of the resonance effect.
                            (aromaticity)

          OH


                        H2O
                    +


          O
                            H2O
      CH3C OH           +

          (3) B has higher acidity than D
              It is due to the fact that delocalization of the negative charge in the carboxylate anion
                    involves resonance structures of identical energy content and involves two highly
                    electronegative oxygen atoms.
              In the phenoxide anion, the resonace structure involving negative charge on the nuclear
                    carbon atoms are likely to be of higher energy content than it is on oxygen atoms,
these                     canonical forms are therefore less stable and have a small contribution to the
overall                   hybrid.

      (C) Effects of Substituents on Acidity

          The ease of dissociation of the acidic compound, and the stability of the conjugate anion, are
          both affected by the substituent groups in the molecules. Any groups which can stabilize the
               resulting anion of the acid molecule will increase its acidity whereas any groups which
can            destabilized the anion will also decrease its acidity. General speaking,

          Electron withdrawing groups increase the acidity : -F , -Br , -Cl , -NO2

          Electron donating groups decrease the acidity: -CH3 , -C2H5

          For carboxylic acids,
          <1> With electron-donating substituent
                          O
                    E C OH
                                                       electron donating substituent
                when dissociation.
                     O
                E C O
                                                  The electron donating substituent pushes electrons
                                                  towards the elecron-rich –COO- group and thus destabilize
                                                  the anion.
                              Lok Sin Tong Leung Chik Wai Memorial School                                  5
                                             F.7 Chemistry
                                   Carboxylic acids & their derivatives
          <2> With electron-withdrawing substituent
             (E : electron withdrawing substituent)
                    O                               O
              E C OH                         E C O             +       +
                                                                       H

      O-H bond weakened by                    anion stabilized by electron
      electron withdrawing effect             withdrawing effect of E
      of E

These effect. can be seen in the following table.




Interpretation

<1> The more numerous the electron-withdrawing groups on the  carbon, the stronger is the acid.
    Thus the order of acidity
    ethanoic acid < chloroethanoic acid < dichloroethanoic acid <trichloroethanoic acid

<2> The more the electronegative the -substituent , the stronger is the acid. Thus the order of acidity
    ethanoic acid < iodoethanoic acid < bromoethanoic acid < chloroethanoic acid < fluroethanoic
                                                                                                 acid
<3> The shorter the carbon chain on the carboxylic acid, the stronger is the acid. Thus the order of
    acidity
    Propanoic acid < ethanoic acid < methanoic acid

Remark : The farther away of the electron-withdrawing substituent from the carbonyl group, the less
             effect on acidity.

             O                        O                            O                         O
CH3CH2CH C OH             CH3CH CH2C OH             CH2CH2CH2C OH               CH3CH2CH2C OH
        Cl                    Cl                    Cl

Ka: 1.4 x 10-3                8.9 x 10-5                 3.0 x 10-5                   1.5 x 10-5
                                  Lok Sin Tong Leung Chik Wai Memorial School                               6
                                                 F.7 Chemistry
                                       Carboxylic acids & their derivatives
IV.         Reaction of Carboxylic Acids

1.    Decarboxylation
      This is the removal of carbon dioxide from a carboxylic acid group. The usual method is to heat
      the sodium salt of the acid with soda-lime (NaOH + CaO). The product is a hydrocarbon, e.g.

                   CH3COO-Na+ + NaOH 

                        COO-Na+
                                    +    NaOH



2.    Reduction Carboxylic acids are resistant to the ordinary reducing agents but lithium aluminium
      hydride reduces them directly to primary alcohols:

                                            LiAlH4    / dry ether
                         RCOO-Na+
                                                     H3O+


      The reagent is highly selective and the C=C , CC , or C6H5- in unsaturated acids are unaffected
      by it.

3.    Conversion to Acid Derivatives
                                                       O

      (i)       Conversion to Acid chlorides ( R C Cl )
                There are three reagents (PCl5, PCl3 and SOCl2) for the preparation of acid chlorides and
                all of them give high yield.

                                                                    O
                        RCOOH +          PCl5                   R C Cl +        POCl3 +   HCl
                (1)

                        RCOOH +         PCl3
                (2)

                        RCOOH       +   SOCl2
                (3)


      (ii)      Conversion of Acid anhydrides
                Carboxylic acids react with alkanoyl chlorides in the presence of pyridine to give acid
                anhydrides.

                                        O            Pyridine
                 RCOOH        +     R C Cl

                What is the use of Pyridine?
                The pyridine is used to remove the hydrogen chloride HCl formed, so as to shift the
                equilibrium towards the products side.
                            Lok Sin Tong Leung Chik Wai Memorial School                              7
                                           F.7 Chemistry
                                 Carboxylic acids & their derivatives
(iii)    Conversion of Acid amides
         Carboxylic acids react with aqueous ammonia to form ammonium salts. By heating this
         ammonium salt, primary amides will be obtained.

                                                           excess RCOOH
        RCOOH    +    NH3                   RCOO-NH4+                           RCONH2     +   H2O


(iv)     Conversion to Ester
         Carboxylic acids react with alcohols in the presence of an acid catalyst to form esters
         through a condensation reaction known as esterification.




         The reaction is a reversible equilibrium process and it is acid catalysed.
                                   Lok Sin Tong Leung Chik Wai Memorial School                               8
                                                  F.7 Chemistry
                                        Carboxylic acids & their derivatives

V.     Acid Derivatives
     (A)   Acid Chlorides

         Acid chlorides are also called alkanoyl or acyl chlorides. They are derived from the-COOH
         group of carboxylic acids by replacing the -OH group with -Cl group, e.g.




         ___________________                 ____________________              ______________________

         Reactions of Acid Chlorides
         The halogen readily undergoes nucleophilic substitution by other nucleophiles such as OH-,
         -
          OR' , NH2- etc. The mechanism is quite similar to condensation reaction of aldehydes and
         ketones. Aromatic alkanoyl chloride like benzoyl chloride is much less reactive, due to the
         decrease in the nucleophilicity of the carbonyl carbon caused by resonance.




         1.   Hydrolysis
              The lower aliphatic alkanoyl chlorides are hydrolysed rapidly by cold water, e.g.

                                        CH3COCl + H2O  CH3COOH + HCl

              When the stopper of a bottle of ethanoyl chloride is removed, white fumes are evolved,
              owing to interaction of the hydrogen chloride with moist air. Beware of damage to the
              naked eyes.
              Aromatic alkanoyl chlorides hydrolyse much more slowly, owing to their lower
              solubility and their carbonyl carbon atom being less susceptible to nucleophilic attack.
              The hydrolysis is faster with alkali because - OH is a stronger nucleophile than H2O.

         2.   Alcoholysis - Ester Formation
              Acid halides react with alcohols and phenols to form esters, e.g.




              Phenol behaves similarly as aliphatic alcohols but a base catalyst is required for it to
              react with aromatic alkanoyl chlorides. The basic medium provides a more powerful
                                             O


              nucleophilic ion,                  (phenoxide ion)

         3.   Ammolysis – Amide formation
                       O                                                 O
                               +                                   CH3   C         +       NH4Cl
              CH3     C                NH3
                          Cl                                                 NH2
                                                                    Ethanamide
                               O                                                       O

                           C          + CH3NH2                                         C               HCl
                                                                                                   +
                               Cl                                                          NHCH3
                                 Lok Sin Tong Leung Chik Wai Memorial School                                         9
                                                F.7 Chemistry
                                      Carboxylic acids & their derivatives

      4.     Anhydride Formation

                    O                                                                  O O
             R                    +        R'COONa                       R         C           C   R'
                   C
                        Cl                                                                 O
                                                                                                        + NaCl

      5.     Reduction to Alcohols

                    O
                                                 Pt
                             +        H2                         RCH2OH            +       H2
             R     C
                        Cl

      6.     Ketone formation

             Friedel-crafts acylation

                                                                       O               R
                                                                               C
                    O                                 AlCl3
             R     C         +
                        Cl                            catalyst



(B)        Acid Anhydride
                                                                                                             O
                                                                                                             C OH
      Acid anhydrides are derived from carboxylic acids by replacing the -OH group in                               by
             O

      a    R C O       group, e.g.




             _______________________                             ______________________

      Reactions of Acid Anhydrides
      Reactions of acid anhydrides are quite similar to that of acid chlorides, although their
      reactivity is less than that of acid chlorides.
      .
      1. Hydrolysis
           Acid anhydrides are slowly hydrolysed by water to give carboxylic acids.

                                       (RCO)2O+ H2O  RCOOH

      2.     Ester FormationAcid anhydrides react with alcohols and phenols to give esters, heating
             is usually required for the reaction.

                            (RCO)2O + R'OH         RCOOR' + RCOOH
             With phenols, alkaline medium is required to provide a powerful nucleophile, the
             phenoxide ion.
                             Lok Sin Tong Leung Chik Wai Memorial School                               10
                                            F.7 Chemistry
                                  Carboxylic acids & their derivatives

     3.   Amide Formation
          Acid anhydrides react with ammonia solution and amines to give amides.
          (RCO)2O + 2NH3  RCONH2 + RCOO-NH4+

     4.   Friedel-crafts Acylation
          Acid anhydrides react with benzene in the presence of a little Lewis acid catalyst, AlCl3,
          to form a ketone.




(C) ESTERS
                                                                                        O
                                                                                        C OH
     Esters are derived from carboxylic acids by replacing the -OH group in                    by an -OR
     group from an alcohol, e.g.




     ___________________                   ____________________             ____________________

Reactions of Esters
1. Hydrolysis of Ester
    Acid-catalysed hydrolysis is the reverse of esterification:
                             O                     H3O+ / Reflux        O
                                           + H2O                                  +   R'OH
                     R       C       OR'                            R   C    OH
     A large excess of water is used but since the reaction is reversible, the hydrolysis never goes
     to completion.

     Alkali-catalysed hydrolysis is also reversible:
                             O                       OH- / reflux       O
                                           + H2O                                  +   R'OH
                     R       C       OR'                            R   C    OH

     However, in this case the carboxylic acid formed immediately reacts with hydroxide ion to
     give the acid anion:
                                 O                                      O
                                           +                                      +   H2O
                         R       C    OH       OH-                  R   C     O
     The equilibrium in this step lies almost completely to the right, so that once the acid is
     formed in the first reaction it is immediately removed by the second reaction, until practically
     all the ester is converted into its hydrolysis product.
     Alkaline hydrolysis is therefore much faster than acid catalysed hydrolysis; and it goes to
     completion. The process is known as saponification 皂化反應 as soaps are made by this
     reaction. The resulting carboxylate product can be re-acidified to get back the carboxylic
     acid.

2.   Ammonolysis - Amide Formation
     Ester reacts with ammonia or amines to form amides.
                        Lok Sin Tong Leung Chik Wai Memorial School                                        11
                                       F.7 Chemistry
                             Carboxylic acids & their derivatives
3.   Reduction to Alcohols
     This is a good way to prepare primary alcohols as esters are reduced by lithium aluminium
     hydride (LiAlH4), e.g.,




(D) ACID AMIDES
                                                                                                O
                                                                                                C OH
     Amides are derived from carboxylic acids by dropping the -OH group in                           and
     replacing it with a -NH, group, e.g.




     ___________________                  _____________________                 _____________________

     Substitutions of the hydrogens on the nitrogen atom by alkyl groups can also occur, e.g.




     ________________________                                     __________________________

     Reactions of Acid Amides
     1. Hydrolysis
         The amides are readily hydrolysed on refluxing with dilute acid or alkali.

                        O                               H3O+ / Reflux       O
                                          +       H2O                                 +    NH4+
                    R       C       NH2                                 R   C    OH

                                O                                           O
                                              +                                       +   NH3
                        R       C     NH2          OH-                  R   C     O


          The fact that heating an amide with caustic soda liberates ammonia whilst an amine does
          not react enables the two types of compound to be distinguished.


     2.   Dehydration
          Nitrile Formation Heating amides with a dehydrating agent such as phosphorus
          pentoxide leads to formation of nitrile.
                       Lok Sin Tong Leung Chik Wai Memorial School                            12
                                      F.7 Chemistry
                            Carboxylic acids & their derivatives
   3.    Hofmann Degradation
         Amides react with KOH solution and Br2 in a rearrangement reaction to give primary
         amines containing one carbon atom less. Thus it is useful for descending a homologous
         series.
         O
                   +   KOH        Br2                                        +   KBr   +   H2O
CH3CH2   C   NH2              +                        CH3CH2NH2     K2CO3



   3.    Reduction of Amines
         Amides are reduced to primary amines by either lithium aluminium hydride or sodium
         in ethanol (dissolving metal reduction).

                                  O           LiAlH4 / dry ether
                             R    C     NH2

				
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