Chapter 17 Carboxylic Acid

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Chapter 17 Carboxylic Acid Powered By Docstoc
					Chapter 17: Carboxylic Acid

         Chem 30B
         Lecture 21
                    Structure
• The functional group of a carboxylic acid is a
  carboxyl group.

    O              O
   C                         COOH          CO2 H
    O H            O H
 Alternative rep res entation s for a carboxyl grou p


  – The general formula for an aliphatic carboxylic
    acid is RCOOH; that for an aromatic
    carboxylic acid is ArCOOH.
           Nomenclature - IUPAC
• IUPAC names: drop the -e from the parent
  alkane and add the suffix -oic acid.
                                                      O
     HCOOH              CH3 COOH                          OH
  Methan oic acid      Eth anoic acid      3-Methylbutanoic acid
   (Formic acid )      (Acetic acid)          (Isovaleric acid )

   – If the compound contains a carbon-carbon double
     bond, change the infix -an- to -en-.
                                                               O
       O                      O
                                                                   OH
           OH                     OH

Propen oic acid   t rans-2-Butenoic acid    t rans-3-Phen ylp ropen oic acid
(Acrylic acid)        (Crotonic acid )              (Cinnamic acid )
             Nomenclature - IUPAC

    • The carboxyl group takes precedence
      over most other functional groups.
    OH        O            O         O                      O
                                                H2 N
                OH                      OH                     OH
(R)-5-Hydroxyhexan oic   5-Oxohexan oic acid   4-Aminobutan oic acid
         acid
         Nomenclature - IUPAC
– Dicarboxylic acids: add the suffix -dioic acid to
  the name of the parent alkane containing both
  carboxyl groups.
                    O
                                     O   O
               HO
                         OH
                                HO            OH
                 O
            Ethanedioic acid   Propanedioic acid
              (Oxalic acid)      (Malonic acid)

           O             O      O                        O
HO                                       HO
               OH   HO              OH                         OH
    O                                       O
Butanedioic acid    Pentanedioic acid      Hexan edioic acid
 (S uccinic acid)    (Glutaric acid)        (Adipic acid)
             Nomenclature - IUPAC
  – If the carboxyl group is bonded to a ring, name the ring
    compound and add the suffix -carboxylic acid.
                  2
             3
                      1   COOH
                                          HOOC            COOH

       2-Cyclohexenecarboxylic    t rans-1,3-Cyclopentan edicarboxylic
                acid                              acid

  – Benzoic acid is the simplest aromatic carboxylic acid.
  – Use numbers to show the location of substituents.
                                                              COOH
 COOH            COOH             COOH
                    OH               COOH


                                                             COOH
Benzoic 2-Hydroxyben zoic 1,2-Benzenedicarb oxylic 1,4-Ben zenedicarboxylic
 acid           acid                 acid                     acid
          (Salicylic acid )    (Ph thalic acid )      (Tereph thalic acid )
           Nomenclature-Common

      – When common names are used, the letters
        etc. are often used to locate
        substituents.
                                                     O
       O                    O
  
               H2 N                                     OH
5       1 OH                    OH
  4 3 2                                            NH2
               4-A min ob utanoic acid     (S)-2-Aminopropanoic acid
           (-A min obu tyric acid, GABA) [(S)--Aminop ropionic acid;
                                                  L-alanine]
           Physical Properties
• In the liquid and solid states, carboxylic
  acids are associated by hydrogen bonding
  into dimeric structures.
            Physical Properties
• Carboxylic acids have significantly higher boiling
  points than other types of organic compounds of
  comparable molecular weight.
   – They are polar compounds and form very strong
     intermolecular hydrogen bonds.
• Carboxylic acids are more soluble in water than
  alcohols, ethers, aldehydes, and ketones of
  comparable molecular weight.
   – They form hydrogen bonds with water molecules
     through both their C=O and OH groups.
                  Physical Properties
• Table 17.2
                                        Molecu lar Boiling
                                         Weight     Point       Solu bility
     Structu re           Name           (g/mol)     (°C)    (g/100 g H 2 O)
     CH3 COOH             Acetic acid    60.1       118        Infinite
     CH3 CH2 CH2 OH       1-Propan ol     60.1       97        Infinite
     CH3 CH2 CHO          Propan al       58.1       48        16

     CH3 ( CH2 ) 2 COOH Butanoic acid     88.1      163        Infinite
     CH3 (CH2 ) 3 CH2 OH 1-Pentanol       88.1      137        2.3
     CH3 ( CH 2 ) 3 CHO   Pentanal        86.1      103        Slight

     CH3 ( CH2 ) 4 COOH Hexan oic acid 116.2        205        1.0
     CH3 (CH2 ) 5 CH2 OH 1-Hep tanol   116.2        176        0.2
     CH3 ( CH 2 ) 5 CHO  Hep tanal     114.1        153        0.1
       Physical Properties

– Water solubility decreases as the relative
  size of the hydrophobic portion of the
  molecule increases.
                              Acidity
• Carboxylic acids are weak acids.
   – Values of pKa for most aliphatic and aromatic
     carboxylic acids fall within the range 4 to 5.
• The greater acidity of carboxylic acids relative to
  alcohols is due to resonance stabilization of the
  carboxylate anion.
                     ••                         ••
                     O
                    ••




                                                  O




                                               ••
                                               ••
            CH3    C                  CH3     C
                         O                        O
                     ••
                     ••




                                               ••
                         ••                       ••
         Thes e contributing s tructures are equivalent;
               the carboxylate anion is s tabilized
           by deloc alization of the negative charge
                         Acidity

      – Electron-withdrawing substituents near the
        carboxyl group increase acidity through
        their inductive effect.

Formula: CH3 COOH ClCH2 COOH    Cl2 CHCOOH            Cl3 CCOOH
  Name: Acetic    Chloroacetic D ich loroacetic     Trich loroacetic
           acid      acid             acid                 acid
    pKa :  4.76       2.86            1.48                 0.70
                        Increasin g acid strength
                          Acidity
      – The form of a carboxylic acid present in
        aqueous solution depends on the pH of the
        solution.
        O           -      O          O     OH-      O
                OH
     R- C-OH           R- C-OH + R- C-O -          R- C-O -
                 H+                         H+
  predominant           present in equal        predominant
species w hen the     concentrations w hen     species w hen
    pH of the             the pH of the        the pH of the
    solution is        solution is equal to   solution is 7.0
    2.0 or less        the p Ka of the acid      or greater