Carboxylic acid - PowerPoint by nasif123

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									                                    Carboxylic Acid
                         O                  O                O
                  H     C             R     C           Ar   C
                              OH                 OH              OH

  Common name: generally derived from the source rather than chemical structure
                                    δ γ βα
                  Numbering         C-C-C-C-COOH

 IUPAC: suffix -oic acid is used            5 4 321

 HCOOH                 CH3COOH       CH3(CH2)10COOH              CH2=CHCOOH
 Formic acid           Acetic acid   Lauric acid                 Acrylic acid
 Methanoic acid        Ethanoic acid Dodecanoic acid             Propeonoic acid
     COOH                    COOH
                                                Lactic acid
                             NO2                1-Hydroxypropionoic acid
Benzoic acid          p-Nitrobenzoic acid
                             Dicarboxylic Acids
   They are named as alkanedioc acids

   HOOCCH2COOH             HOOCCH2CH2COOH               HOOCCH2CH2CH(Cl)COOH
   Malonic acid            Succinic acid                α-Chloroglutaric acid
   Propanedioic acid       Butanedioic acid             2-Chloroptanedioic acid

                           Salt of carboxylic acid

Name of a salt of carboxylic acid : name of the cation followed by the name of the acid
ending with -ate

                               (CH3COO)2Ca          HCOONH4

            Sodium benzoate Calcium acetate         Ammonium formate
                            Physical properties

• Molecules are polar, can form hydrogen bonds

• Aliphatic acids: first four are miscible with water, five carbon acid is partly soluble,
  higher acids are insoluble

• Boiling points are very high, higher than alcohols because carboxylic molecules
  are held together by two hydrogen bonds

• The acids are soluble in less polar solvents like ether, alcohol, benzene etc
                                 Industrial source

Acetic acid has been prepared chiefly by
i) catalytic air oxidation of various hydrocarbons or acetaldehyde
                       O2                                     O2

      hydrocarbons              CH3COOH              CH3CHO                   CH3COOH
                     catalyst                                      catalyst

i)    Reaction between methanol and carbon monoxide in presence of iodine-rhodium
                CH3OH + CO                 CH3COOH

iii) Large amounts of acetic acid is produced as dilute aqueous solution called vinegar
      Ethyl alcohol is oxidized to yield acetic acid and the catalyst used are bacterial
      (Acetobacter) enzymes.

     Aliphatic carboxylic acids are prepared from animal and vegetable fats
Oxidation of Primary alcohol
      R-CH2OH                        R-COOH

   CH3CH2CH(CH3)CH2OH                                 CH3CH2CH(CH3)COOH
        2-Methyl-1-butanol                                   2-Methylbutanoic acid

 Hydrolysis of nitriles

      R—                               Acid or base   R—COOH
            or            + H2O                          or                  + NH3
     Ar—                                              Ar—COOH

           CH2Cl   NaCN
                                        CH2CN         70% H2SO4
                                                                                         + NH4+

Benzyl chloride               Phenylacetonitrile                     Phenylacetic acid
     Carbonation of Grignard reagents

                 Mg                    CO2                       H+
     R—X                   R—MgX              R—COOMgX                  R—COOH
      or                                                                  or
     Ar—X                                                               Ar—COOH

                          Mg                          CO2   H+
C2H5C(CH3) (CH3)Cl             C2H5C(CH3) (CH3)MgCl              C2H5C(CH3) (CH3)COOH
2-Methyl-2-chlorobutane                                          2,2-Dimethylbutanoic acid
  tert Pentyl chloride                                             Ethyldimethyl acetic acid

  Substitution in alkyl group

 Halogenation of aliphatic acids ( Hell-Volhard-Zelinsky reaction)

The presence of small amount of phosphorus causes halogenation to take place exclusively
at the alpha position

        RCH2COOH + X2             P         RCHCOOH + HX                         X2 : Cl2, Br2
                                            An α-halo acid

                    Cl2,P                   Cl2,P                     Cl2,P
     CH3COOH                ClCH2COOH               Cl2CHCOOH                 Cl3CCOOH
      Acetic acid            Chloroacetic            Dichloroacetic           Trichloroacetic
                                acid                    acid                      acid
                Substitution in alkyl or aryl group

  P + X2                    PX3                                                     X2 = Cl2, Br2

  RCH2COOH + PX3                          RCH2COX

   RCH2COX + X2                        RCHCOOH + HX

   Phosphorous converts a little of the acid into acid halide


 R—COOH                      R—CH2OH
                             Primary alcohol

4R—COOH + 3 LiAlH4                           (RCH2O)4AlLi + 4H2 + 2LiAlO2

                                                                4R—CH2OH + Al2(SO4)3 + Li2SO4
                            Carboxylic acid derivatives

    O                                                     O
    ║                                                     ║
    C                 Acyl or acid chloride               C
R           Cl                                    R           NH2
    C            R’                                       ║
            O               Ester
R                                                         C          Amide
                                                  R           NHR’
    O            O
    ║            ║
    C            C
R           O         R’    Acid anhydride            O
                                              R           NR’R”
    R—C≡N                   Nitrile
                         Conversion into functional derivatives
                 O                     O
                                                              Z: Cl2, OR`, NH2
        R       C                R     C
                      OH                    Z

    i) Conversion into esters

The names are derived from the names of the alcohol ( ending with –yl) and the acid. ( ending with –ate or -oate)
The portion of the name derived from the alcohol comes first

          (A)       O                                     O
            R     C          + R`OH               R      C              + H2O
                        OH                                    OR`
                                                   An ester
                        COOH                             COOCH3
                             + CH3OH                                + H2O

                Benzoic acid    Methanol              Methyl benzoate
                   Conversion into functional derivatives

    (B)       O                             O                                O
                           SOCl2                       R`OH
    R     C                            R   C                         R     C
                  OH                              Cl                             OR`
                                       An acid                       An ester

                       SOCl2                               C2H2OH
(CH3)3CCOOH                        (CH3)3CCOCl                           (CH3)3CCOOC2H5
 Trimethylacetic                                                           Ethyl trimethyl
     acid                                                                     acetate

ii) Conversion into acid chlorides

          O                    SOCl2                          O
    R     C            +       PCl3                    R      C
                  OH           PCl5                                 Cl

Acyl chlorides are the most reactive of the acid derivatives
              Conversion into functional derivatives

iii) Conversion into acid anhydrides

RCOOH + R’COCl +                           RCOOCOR’+         Cl-
         Acyl chloride   Pyridine

iv) Conversion into nitriles

              P4O10 or (CH3CO)2O
  RCONH2                           R—C≡N + H3PO4 or CH3CO2H

                                                       (CH3CO)2O : Acetic anhydride
                                                            P4O10 : Phosphorus pentoxide
                Conversion into functional derivatives
 v) Conversion into amides

          O                                 O                          O
                     SOCl2                           NH3
    R    C                           R    C                       R    C
              OH                                Cl                           NH2
                                     An acid                      An amide

                             SOCl2                               NH3
   C6H5CH2COOH                           C6H5CH2COCl                       C6H5CH2CONH2

      Phenylacetic acid                  Phenylacetyl chloride               Phenylacetamide

Alkyl groups on the nitrogen atoms of amides are named by adding preface N or NN

 CH3COONH2                                  CH3COON(CH2) (CH2)                     CH3COONH(C2H5)
 Acetamide or ethaneamide                   N,N-Dimethylacetamide                  N-Ethylacetamide
                      Acidity of Carboxylic acids

RCOOH + H2O             RCOO- + H3O+
                                                Acetic acid   Water                 Hydronium
                                                                       ion           ion

                                                                      HA        A− + H+
                                                                      acid      Conjugate
Acid strength is characterized in terms of acidity constant Ka                   base

 Relative acidities
                 RCOOH >H2O > ROH > HC≡CH > NH3 > RH

 Relative basicities
      RCOOH < OH- < OR- < HC≡C- < NH2- < R-

Certain substituted acids are stronger or weaker than a typical acid like acetic acid
                       Acidity of Carboxylic acids

       Carboxylic acids have greater acidity than alcohols

                  Acetic acid acting as an acid
        CH3COOH + H2O               RCOO-      +       H3O+

        Acetic acid                Acetate ion     Hydronium ion

                       Ethanol acting as an acid
        CH3CH2OH + H2O              CH3CH2O-       +     H3O+
           Ethanol                 Ethoxide ion Hydronium ion

• Focusing on the relative stability of the conjugate bases: greater stabilization of
 the negative charge takes place in the carboxylate ion in comparison to alkoxide ion

• This is due to i) delocalization of charge ii) an inductive electron withdrawing effect
                  Acidity of Carboxylic acids

                                        Carboxylate ions
                                      Resonance stabilized

      • Negative charge is distributed to both oxygen atoms

      • No resonance structure possible for alkoxide ion

                     Electrostatic potential

Acetic acid (ethanoic acid)
                                             Acetate ion (ethanoate ion)

             The more red an area is, the higher the electron density
             The more blue an area is, the lower the electron density.
                                            Acetate ion (ethanoate ion)
       Acetic acid (ethanoic acid)

• There is low electron density (blue) on H atom of the -CO2H group alcohol.
• The H atom of the RCO2H is acidic (pKa ~ 5).
• There is high electron density (red) on both O atoms in the acetate ion
  of the -CO2- group alcohol, i.e. resonance and basic or nucleophilic behaviour

                                                           Ethoxide anion

                                                             Acetate anion
                              Inductive effect

• The electron cloud in a σ-bond between two unlike atoms is not uniform
and is slightly displaced towards the more electronegative of the two atoms.

• A permanent state of bond polarization occurs where more electronegative atom
  has (δ-) and the other atom has a (δ+).

• If the electronegative atom is then joined to a chain of atoms, usually carbon, the
  positive charge is relayed to the other atoms in the chain.
           Inductive effect

• Electron withdrawing groups such as halo group( -NO2, -CN etc)
 increase the acidity

• They stabilize the carboxylate anion by dispersal of
the negative charge and increase the strength of the acid

• Electron releasing groups (like alkyl groups) cause
 concentration of negative charge.

• They destabilize the carboxylate anion and decrease the strength of
  the acid.


    CH3    C                 CH3―CH2―O―H
               < O―H
         Acetic acid               Ethanol
       (stronger acid)           (weaker acid)
               O                     O                          O
                        SOCl2                   NH3
        R    C                  R   C                     R    C
                   OH                      Cl                        NH2
                                An acid                   An amide

Ammonolysis of acid anhydrides

  (CH3CO)2O + 2NH3              CH3CONH2 + CH3COO-NH4+

   Acetic anhydride             Acetamide             Ammonium acetate
                      Reactions of Amides
1. Hydrolysis

2. Conversion into imides

3. Hofmann degradation of amides


                                    R    C         + NH4+
R   C         + H2O

                                        R—COO- + NH3
    Acidic conditions
                                                              OH                              +

R    C
                           R       C
                                            }ө    H2O         │
                                                                                       R    C

                                                                                                         + NH3
                               Protonated amide               NH2                      Protonated
                                                                                       Carboxylic acid

                                                                          R       C               + NH4+

    Alkaline conditions

                                          O-                         O-
                         OH-                │           OH-          │
         R     C                       R—C—OH                      R—C—O-         Dianion looses a molecule of
                   NH2                      │                        │            A proton is transferred from water
                                          NH2            OH―H        NH2
                                                                     R     C          + NH3 + OH-
                              Conversion into imides

Imide is a functional group containing two carbonyl groups
 bound to nitrogen

Common imides are prepared by heating dicarboxylic acids or their anhydrides and
ammonia or primary amines

                       (RCO)2O + R'NH2 → (RCO)2NR' + H2O
               Hofmann degradation of amides
                                       OBr -
       R—CONH2 or Ar—CONH2                     R—NH2 or Ar—NH2 + CO3 2-
                                                    Primary amine

      R—CONH2 + Br2 + 4NaOH                    RNH2 + 2NaBr + Na2CO3 + 2 H2O

Proposed mechanism
                     O        Halogenation of amides
 1.            R    C    ..
                         NH2 + OBr
                                                 R      C          ..    + OH-
 2.        R   C    ..
                    N—Br + OH-                  R      C       ..       +H2O
                    │                                          N—Br
                    H                                          -
3.        R   C     ..
                    N—Br                         R    C    .. + Br-

4.              R
                         C   ..

                                                                      }   Simultaneous

                         Hydrolysis of isocyanates

       ..                           H2O         ..
5.   R—N=C=O + 2OH-                           R—NH2 + CO3 2-

• Hofmann rearrangement involves a 1,2-shift

• A group migrates to the electron defecient nitrogen while in carbocation
  rearrangement the group migrates to electron – deficient carbon

• Special feature is the product contains one less carbon than the starting material

    A β-lactam      A γ-lactam        A δ-lactam

• Cyclic amides are called lactams

• Penicillin antibiotics contain a β-lactam ring

                                          R = C6H5CH2 —   Penicillin G
                                          R = C6H5CH—     Ampicillin
                                          R = C6H5OCH2— Penicillin V

• Carboxylic acids whose molecules have a OH group on a γ or δ carbon
   undergo intramolecular esterification to give cyclic esters known as
  γ- or δ- lactones                                              δ γ βα
                                                               R- C-C-C-C-COOH
• The reaction is acid catalysed                               A δ-hydroxy acid

• Many lactones occur in nature like vitamin C.

• Some antibiotics such as erythromycin and nonactin are lactones with large rings

                           Vitamin C
                        ( ascorbic acid)
Intramolecular esterification
                                    OH                      OH
  δ γ βα                              OH                       +
R- C-C-C-C-COH                       OH                      O H
   ‫׀‬       ║                         +
   OH      O
                   H―A              R                       R
A δ-hydroxy acid

           OH                           O

        O       + H2 O               O        + H3O+ + A-

       R                            R
                                A δ-lactone
Esters from acyl chloride

              RCOCl + R’OH            RCOOR

Esters from carboxylic acid anhydride

              (RCO)2O + R’OH          RCOOR’ + RCOOH
                           O                                 O
                   R      C         + R’―OH          R      C             + H2O
                               OH                                   OR’

                 CH3COOH + CH3CH2OH                  CH3COOCH2CH3 + H2O
                 Acetic acid         Ethanol                Ethyl acetate

       • Acid catalyzed esterification are called Fischer esterifications

                   O                                             O
                                      18        HA
         C6H5     C            + CH3―O―H             C6H5       C    18      + H2O
                          OH                                          OCH3
           Benzoic acid
                             Acid catalyzed esterification
                     O                                 O
             R       C            + R’―OH          R   C         + H2O
                             OH                            OR’

      • The forward reaction is acid catalysed esterification of the acid
      • The reverse reaction is acid catalyzed hydrolysis of an ester
 Acid catalyzed ester hydrolysis

                         O                                            O
                 R       C          + H2O          R’―OH + R         C
                              OR’                                           OH

•The amount of esters formed is controlled by the position of the equilibrium
• If esterification of acid is required then excess of the alcohol is used and if possible
 H2O is removed as it is formed
• If ester hydrolysis is required then excess of H2O is used
                     Acid catalyzed esterification
Reactivity in esterification
     CH3OH > 1° > 2° > 3°

                 O                                     O
            R   C         + R’―OH                R    C           + H2O
                     OH                                     OR’

                                          H                              HHO
    O                                         O                        │ |+
R   C           H―O+―H               R       C
                                                           + R’―OH
        O―H                                          O―H                   │
                                                           - R’―OH
                    H                                                      O―H
                                                  +                        HO
            O                        H           O
                             +H3O+                                             │
        R   C                            R       C                        R—C—O―R’
                O―R’         -H3O+                    O―R’                     │+
                                         Protonated ester                 H―O―H
                  Base promoted ester hydrolysis

                   O                                              O
              R   C           + NaOH             R’―OH + R        C
                       OR’                                         O-Na+
                                                        Sodium carboxylate

Mechanism involves a nucleophilic addition-elimination reaction at the acyl carbon

     O                              O-
                                    │                   O
R    C          O-―H   slow
                                R—C—OH              R   C
         O―R’                        │                       O―H       + -O―R’
                               An alkoxide ion

                                                         R    C            + H―O―R’
Acidity of Carboxylic Acids

stronger acids than alcohols
pKa ~ 5
(alcohol pKa values about 15-18 )

electron-withdrawing effects of the carbonyl group tends to further polarize the
O-H bond so it is more easily ionized
the carboxylate anion is stabilized by resonance

The larger the value of pKa, the smaller the extent of dissociation.
A weak acid has a pKa value in the approximate range −2 to 12 in water.
Acids with a pKa value of less than about −2 are said to be strong acids;
a strong acid is almost completely dissociated in aqueous solution,
to the extent that the concentration of the undissociated acid becomes

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