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aldehydes and ketones24th april

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					                          Aldehydes and Ketones

             O                                           O
             ║                                           ║
          R―C―                                         R―C―R`
          H     H                                                       R
RCHO : aldehyde   C=O                            RR`CO: ketone               C=O
                      H                                                 R`
                      H                          R R` can be aliphatic or aromatic
 RCHO : aldehyde              C=O
                          R



           C=O : the carbonyl group also known as the oxo group



                                    Structure

      δ+         δ-       The carbon atom in the carbonyl group is joined to the
      C          O        other atoms by sigma and pi bonds using sp2 orbitals
          120°
                               Nomenclature
       Aldehydes
                                                  δ γ βα
 Common name: suffix -aldehyde is used            C-C-C-C-CHO
                                                  5 4 321
 I.U.P.A.C.: suffix -al is used                   C-C-C-C-CHO



                    H                                                H
                    C=O                            H                 C=O
    H                                              C=O
   HC=O                                            OH
Methanal                                                             NO2
Formaldehyde    Benzaldehyde             o-Hydroxybenzaldehyde   p-Nitrobenzaldehyde
                                           Salicylaldehyde
                                        H
                       CH3CH2CH2CHC=O
                                     CH3
                               2-Methylpentanal
                                  Nomenclature
   Ketones


Common name: suffix -ketone is used

                                           CH3—C — CH3   CH3CH2—C — CH2 CH2




                                             =



                                                                =
IUPAC: suffix -one is used
                                                  O               O
                                             Propanone         3-Pentanone
                                              Acetone          Diethyl ketone
    CH3
    │
CH3CH2—C — CH2 CH3                     C                     CH2 C     CH3




                                                                 =
                                     =
        =




           O                           O                           O
                                   Benzophenone            1-Phenyl-2-propanone
        4-Methyl-3-pentanone
        Isopropyl methyl ketone                            Benzyl methyl ketone
                              Physical Properties
• Polar carbonyl group makes it polar thus having higher boiling points. This gives rise
  to dipole-dipole attractions                                     δ- O
                                                                         C δ+
                                                                     ║ ║
                                                                  δ+
                                                                     C Oδ-

• Lower aldehydes and ketones are soluble in water

•Many aldehydes and ketones have pleasant fragrances and are used in perfume industry

• Aldehydes and ketones are soluble in organic solvents



                            Preparation of aldehydes

    1. Oxidation of primary alcohol
    2. Reduction of acid chlorides
    3. Oxidation of methylbenzenes
    4. Hydrolysis of gem-dihalides
                                        Oxidation of primary alcohol
                H
                                                  H
                               C5H5NH+CrO3Cl-
         R C— OH                                R C=O
                H


                                                 Pyridinium chlorochromate
           CH3CH2CH2CH2OH                                                    CH3CH2CH2CHO

                1-Butanol                                                       Butanal

                                Reduction of acid chlorides
                    Pd/BaSO4
RCOCl                                       RCHO
                                                                      COCl                        CHO
Acid chloride
ArCOCl                                    ArCHO

                                                                      NO2                         NO2
                                                           p-Nitrobenzoyl chloride        p-Nitrobenzaldehyde
                            Oxidation of methyl benzenes
                      Cl2 , heat                                   H2O
       Ar—CH3                                   Ar—CHCl2                        Ar—CHO

                    CrO3 , acetic anhydride                                         H2O
        Ar—CH3                                  Ar—CH(OOCCH3)2                              Ar—CHO


               CH3                                            CHCl2                          CHO
                            Cl2 , heat, light                         CaCO3 , H2O



               Br                                             Br                             Br
         p-Bromotoulene                                                                   p-Bromobenzaldehyde


                               Hydrolysis of gem-dihalides

                                 H2O                                   -H2O
   CH3—CH(Cl)2                                  CH3—CH(OH)2                          CH3—CHO
                               NaOH
                                                 (unstable)


Ketones are produced by alkaline hydrolysis of those gem halides where the two halogens are
Attached to non- terminal carbon atom
                 Preparation of Ketones

1. Oxidation of secondary alcohols


2. Friedel-Crafts acylation


3. Reduction of acid chlorides with organocopper compounds
                           Preparation of Ketones
                      Oxidation of secondary alcohol
              R`
                       K2Cr2O7 or CrO3
            R C— OH                        R C   R`




                                           =
              H                              O

                        CrO3
CH3 CH2 CH CH2 CH3                        CH3 CH2 CCH2CH3
                      Chromic anhydride
       OH                                        O
    3-Pentanol                               3-Pentanone
                        Friedel –Crafts acylation
                                            AlCl3
                    ArH+ RCOCl                      Ar—C—R + HCl




                                                     =
                        Acid chloride
                                                       O
                                                     Ketone

        COCl

                    +               AlCl3
                                                          C          + HCl




                                                        =
                                                          O
 Benzoyl chloride
                                                      Benzophenone


RCO- is acyl group
• This is an important method for preparing ketones in which the carbonyl group is
 attached to an aromatic ring

• These ketones then can be converted into (a) secondary alcohols by reduction
(b) tertiary alcohols by reaction with Grignard reagent and many other compounds
                                   Mechanism

                                              +
    1. RCOCl + AlCl3              R        O      + AlCl4-
                                   acylium ion
                        +                 H
                                   +
    2. ArH + R         O           Ar             + AlCl4-
                                          COR
              H
        +
   3.   Ar      + AlCl4-          Ar—C—R + HCl + AlCl3
              COR                   = O
                                    Ketone



• Electrophilic aromatic substitution

• Acylium ion is more stable than ordinary carbocation
Preparation of ketones by using organocopper compounds

      Li                   CuX
RX            RLi                R2CuLi
                                 Lithium dialkylcuprate


      Li                   CuX
ArX           ArLi               Ar2CuLi
                                 Lithium dialkylcuprate


 Lithium organocuprates readily react with acid chlorides to yield ketones

       R`
       │
     R—CuLi + RCOCl                         R C
                                              =         R`

                                                 O
               Acid chloride                   ketone
                            Nucleophilic addition

The carbonyl group provides a site for nucleophilic addition and increases the
acidity of the hydrogen attached to the alpha carbon          :O:
                                                                             ║ α
                                                                           R-C-C-C-β
                                                                                     HH
Carbonyl carbon is electron deficient and carbonyl oxygen is electron rich, due to its
flat structure this part is open to attack from above or below

The tendency of oxygen to acquire electrons and its ability to carry the negative charge
Is the cause of the reactivity of the carbonyl group toward nucleophiles
                    Nucleophilic oxygen reacts     δ- O
                    with acids and electrophiles
                                                      ║
                                                   δ+     Electrophilic cvarbon reacts
                                                      C   with bases and nucleophiles




Aldehydes generally undergo nucleophilic addition more readily than ketones
                               Nucleophilic addition
General mechanism
Step 1                                                   Nucleophilic oxygen reacts     δ- O
                                       Nu                with acids and electrophiles
                                         ..                                                ║
     C=O: +     Nu:-                  -C-O:-
                                         ..                                             δ+
                                                                                           C            Electrophilic cvarbon reacts
                                                                                                        with bases and nucleophiles

 Step 2
    Nu                                Nu
       .. -                             ..
    -C-O: + H+
       ..                            -C-O -H
                                        ..
                                   Addition product

   Base-catalysed addition                                                 Acid-catalysed addition
                                                      Step 1
Nu-H + :B-             Nu:- + BH
Weak                                                                                                +                  + ..
nucleophile
                     Strong                              C=O: +          H+                C=O-H
                                                                                             ..                        C-O-H
                                                                                                                         ..
                     nucleophile
                                                      Step 2                                    Protonated carbonyl group
This strong nucleophile adds to carbonyl                  ..                                   Nu
                                                        +
group as shown above                                    C-O-H + Nu:-
                                                          ..                                   -C-O -H
                                                                                          Addition product
                        Acidity of α-Hydrogens
                                                                       :O:
• The acidity is due to the fact that the anion formed is               ║ α
  stabilized by resonance                                             R-C-C-C-β
                                                                          HH
• The resonance stabilized anion is called enolate ion




          B:-
                                                                ..-
            H : O:                   :O:                       :O:
            │ ║                  ..- ║                          │
           —C—C―                —C—C―                       ―C=C―
            │                    │                           │


         Removal of H                 Resonance -stabilized
          by base                          Enolate ion
                                   Reactions
  Oxidation
 • Aldehydes are easily oxidized to carboxylic acids
 Aldehydes are oxidized by permanganate and dichromate

 Oxidation by Tollens’ reagent is a useful method to detect aldehydes

 Tollens reagent contains silver ammonia ion Ag(NH3)2 +
 RCHO + Ag(NH3)2 +               RCOO- + Ag
          Colourless solution                 Silver
                                              mirror



  CH3CHO +2 Ag(NH3)2 + + 3OH-                      CH3COO- + 2Ag + 4NH3 + 2H2O
                                                                Silver
                                                                mirror



The reaction is valuable for the synthesis of unsaturated acids from unsaturated
 aldehydes obtained from aldol condensation; Tollens` reagent does not attack carbon
carbon double bond
                    RCH=CHCHO             RCH=CHCOOH


• Oxidation of ketones takes place only under vigorous conditions except for the
haloform reaction
                             Cannizaro reaction

In the presence of concentrated alkali, aldehydes containing no α-hydrogens
undergo self oxidation and reduction to yield a mixture of an alcohol and a salt
of carboxylic acid

                  50%NaOH
                             CH3OH     +   HCOO-Na+
   2HCHO
   Formaldehyde             Methanol       Sodium formate

Crossed Cannizaro reaction
If one of the aldehyde is formaldehyde then the reaction yields exclusively sodium
formate and alcohol corresponding to the other aldehyde

                     CHO                                    CH2OH

                             + HCHO                                 +   HCOO-Na+

                     OCH3                                   OCH3
            p-Methoxybenzaldehyde                  p-Methoxybenzyl alcohol
                                  Reduction
     1. Reduction to alcohols
     2. Reduction to hydrocarbons
     3. Reductive amination



                             Reduction to alcohols
• Aldehydes get reduced to primary alcohols
• Ketones get reduced to secondary alcohols

This is brought about by either i) the use of chemical reducing agents like lithium
Aluminium hydride or ii) catalytic hydrogenation

                    H2, Ni
CH3CH=CHCHO                   CH3CH2CH2OH
    2-Butenal
                     Reduction to hydrocarbons
                             Zn(Hg), con. HCl
                              Amalgamated zinc
                                                                   Clemmensen reduction
                                                               H
           C=O                                         H
                               NH2NH2, base
                                                           H         Wolff-Kishner reduction

                                                   H


• These are important when applied to the alkyl aryl ketones obtained from

• Friedel-Crafts acylation as the reaction sequence permits indirectly the attachment
  of straight alkyl chain to the benzene ring

     OH                                OH                          OH
             CH3(CH2)4COOH, ZnCl2                Zn(Hg), HCl

          OH                               OH                          OH
                                         CO(CH2)4CH3                 CH2(CH2)4CH3
    Resorcinol
                                                                   4-Hexylresorcinol
                                                                   Used as an antiseptic
                             Reductive amination
Reduction in the presence of ammonia, this is done either i) catalytically or ii) by use of
sodium cyanohydridoborate (NaBH3CN)
            H                                  H                            H
             │                                           H2, Ni
       R—C=O                                                          R            NH2
                    + NH3                  R—C=NH        or NaBH3CN
                                                                            H
         An aldehyde                         An imine
                                                                          A 1° amine



            R`                                 R`
             │                                                              R`
                                                         H2, Ni
       R—C=O        + NH3                  R—C=NH                     R            NH2
                                                         or NaBH3CN

         A ketone                            An imine                       H
                                                                          A 1° amine

                       NH3, H2, Ni
     CH3(CH2)5CHO                    CH3(CH2)5CH2NH2

      Heptanal                          1-Aminoheptane
                        Addition of alcohols
                                          H
                         HCl         R
     C=O       + 2ROH                              OR      + H2O
                                          OR
                                          An acetal
                                          (gem-diether)


      Alcohols add to the carbonyl group of aldehydes to yield acetals

           H                                H                           H
           │                                                 ROH
                               HCl
     R`—C=O + ROH                    R`              OH            R`        OR + H2O
                                                             HCl
                                            OR                          OR
                                            Hemiacetal
                                              (unstable)


• Excess of alcohols is used to shift the equilibrium in favour of acetal formation

• Ketones do not react with alcohols to form hemiketals and ketals
                        Addition of cyanide
                             Cyanohydrin formation


                        H+
      C=O     +   CN-                      CN

                                     OH
                                     A cyanohydrin

Addition appears to involve nucleophilic attack on carbonyl carbon by cyanide ion
subsequently oxygen accepts a hydrogen ion to form the cyanohydrin product


                                            H+
          C=O                       CN                       CN

              :CN-             O-                     OH
                                                      Cyanohydrin


 Cyanohydrins can undergo hydrolysis : the product are alpha hydroxy acids or
 unsaturated acids( eg lactic acid is formed from acetaldehyde cyanohydrin).
             Addition of derivatives of ammonia
Compounds related to of ammonia add to the carbonyl group to form derivatives
These products contain carbon nitrogen double bond



                            H+
      C=O + :NH2OH                    R—C-NHOH                 C= NOH + H2O
               Hydroxylamine              OH                   Oxime



                           H+
C=O + :NH2NHC6H5                   R—C-NHNHC6H5                   C= NNHC6H5      + H2O
         Phenylhydrazine
                                       OH                         Phenylhydrazone



                           H+
C=O + :NH2NHCONH2                R—C-NHNHCONH2                    C= NNHCONH2+ H2O
        Semicarbazide
                                     OH                           Semicarbazone
                   Addition of Grignards reagents
H                              H                  H
                                          H2O
        C=O   + R—MgX     H—   C—O¯Mg+X         H— C—OH
    H                          R                  R
    Formaldehyde



H                              H                  H
                                          H2O
      C=O     + R—MgX      —
                          R`   C—O¯Mg+X          —
                                                R` C—OH
 R                             R                  R
 Higher aldehyde




R’’                            R’’                R”
                                          H2O
      C=O     + R—MgX      —
                          R`   C—O¯Mg+X          —
                                                R` C—OH
 R`                            R                  R
 Ketone
                                 Wittig Reaction
 The syhthesis of alkenes from carbonyl compounds where the carbonyl O, =O gets
 replaced by the group =CRR` was reported by Georg Wittig


                        R`                      R`                         R`
       C=O + Ph3P=C-R                                 R                          R + Ph3PO
                                                                                Triphenylphosphene
                   An ylide                                                         oxide
                                           -O +PPh3
                                            A betaine
Nucleophilic attack on the carbonyl carbon by an ylide( a molecule with adjacent opposite charges)
to form a betaiene which undergoes elimination to yield the product



  (C6H5)2CO + Ph3P=CH2                 (C6H5)2C—CH2               (C6H5)2C=CH2
 Benzophenone Methylethenetriphenyl-         -O Ph3P                1,1-Diphenylethene
              phosphorane



           This is an excellent method of preparing alkene from aldehydes and ketones
                              Haloform Reaction
                                      base
                         -
                 CH3 + OH + X2                               CX3 + X- + H2O
            ║                                         ║
            O                                         O
       Methyl Ketone                              Α-Halo ketone


 • In the presence of a base methyl ketones undergo this reaction


       Halogenation of aldehydes and ketones

                                H+ or OH-
                       + X2                               + HX
           ║                                     ║
      H    O                                 X   O
      Ketone                                 Α-Halo ketone

• Aldehydes and ketones having α hydrogen react readily with halogens

• Rates of the reactions increase with the addition of acids or bases
                      Base Promoted Halogenation

                H          O                                O δ-                       OH




                ― ―
Step 1                         slow                    δ-           fast
         B:- + ―C―C                      B:H + C―C                             C―C         + B:-
                                                 Enolate anion
                      O-                     O                             X     O
                                  -                          fast
Step 2        C―C                ―C―C             + X-X                ―C―C           + X-




                                  ―




                                                                           ―
          Enolate anion

                       Acid Catalyzed Halogenation

                                         B:- H              +
                                                            OH                        OH
                                                 ― ―
                                 fast                           slow
Step 1   ―C―C         + H:B                  ―C―C                             C―C      + H:B
          ―




                                                                               Enol
                OH                                     X     O+H
                                      fast
Step 2    C―C         + X-X                      ―C―C                  + X-
                                                   ―




          X     O+H                               X         O
                                      fast
Step 3   ―C―C          + X-                  ―C―C                   + HX
                                                  ―
         ―
                                  Analysis
• Aldehydes and ketones react with 2,4-dinitrophenylhydrazene to form an insoluble
  yellow or red solid




• Aldehydes are characterized by their ease of oxidation, they give a positive test with
  Tollen`s reagent

• Aldehydes give positive Schiff test, by reacting with fuschin-aldehyde reagent to form
  magenta colour




• Aliphatic aldehydes and ketones having α-hydrogen react with Br2in CCl4 with the
  liberation of HBr
                                    Analysis
                Methyl ketones are characterized through idoform test

                              Idoform test

The methyl ketone is treated with iodine and sodium hydroxide a methyl ketone yields
a yellow precipitate of idoform (CH3I)

            R—C—CH3
              ║               R is H or alkyl or aryl
              O


        R—C—CH3 + 3NaOI                R—C—CI3 + 3NaOH
          ║                              ║
          O                              O

        R—C—CH3 + NaOH                    RCOO-Na+ + CHI3
          ║
          O                                                Yellow
                                                           precipitate

                                               (for details read section 18.1-18.15, 18.20-18.21 Chapter 18
                                               “Organic Chemistry”, Morrison and Boyd)

				
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