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					Advance Organic Chemistry

               0202421

                   By
             Dr. Uthai Sakee

Department of Chemistry, Faculty of Science
       Mahasarakham University




                                              1
    Chapter 1 : Nucleophiles and Electrophiles
• 1.1 Definition of Nucleophiles and Electrophiles
Nucleophiles         Electron-rich molecules which take
  part in reactions.

Electrophiles          Electron-deficient molecules which
  take part in reactions.

A specific region of the molecule which is electron-rich
  called nucleophilic center.

A specific region of the molecule which is electron-
  deficient called electrophilic center.
                                                            2
• 1.2 The nucleophilic and electrophilic property of ions

  Let us first consider the following two ions.
Do you think they are electron-rich or electron-deficient?

  If so, which is which. Can you identify any electrophilic
  centers or nucleophilic centers in the molecule?



                                       H

               H    O                  C
                                   H       H



                                                             3
Answer


         nucleophilic
         center
                            H          electrophilic center

          H       O         C
                        H       H

          nucleophile   electrophile




                                                              4
Identify which molecules are nucleophiles and which are
electrophiles?

Identify the nucleophilic/electrophilic center in each case?




            O                                       O
                                         H
            N            C   N       N              C
  I
                                         H   H3 C       O
            O




                                                               5
Answer
                                                                              nucleophilic
                                                                              center

                nucleophilic                                         O
                center
      I                                                              C                                 H
                                          C    N           H 3C           O                       N
  nucleophile                          nucleophile                                                     H
                                                                nucleophile
                                                                                             nucleophile




                                   O
                                         electrophilic center
                                   N

                                   O

                               electrophile




                                                                                                           6
• 1.3 Nucleophilic and electrophilic properties of neutral
  inorganic compounds
       Let us first look at the water molecule and see if we
  can identify any nucleophile or electrophilic centers.




                             O
                         H        H

      There are no positive or negative charge in this
  molecule, but are there any atoms with a partial
  charge? Such partial charge are possible if there are
  any polar bonds.


                                                           7
     Answer
                               

                           O
                      H           H 


Taking into account the polar bond in water, can you identify
electrophilic and nucleophilic centers?

                                   O
                           H             H


                               Nucleophilic
             Answer            center


                                   O
                               H         H


                           Electrophilic
                           centers
                                                                8
• Since water has both nucleophilic and electrophilic centers, you
  may well ask whether the molecule will react as an electrophile or
  a nucleophile.

• The answer is that it can react ad either, depending on what it
  reacts with.

• As an illustration of this, consider the following reactions. Identify
  whether water is reaction with a nucleophile or an electrophile in
  each of these reactions, and then determine whether water is
  reacting an electrophile or as a nucleophile.
                                                      H

             O               +   H                    O
         H           H                        H               H




                 O           +                            O       +   H   NH
                                     NH                                        2
             H           H                2       H



                                                                                   9
Answer                                               H

              O           +        H                 O
      H               H                          H       H
                              Electrophile




          O           +                          O       +   H   NH
                              NH                                      2
  H               H                2         H
                          Nucleophile




                                                                          10
• The following molecules all contain polar covalent
  bonds. Identify these and hence identify possible
  nucleophilic and electrophilic centers in each molecule




                              Cl
             N
                                           H   F
         H       H           Al
             H
                                   Cl
                        Cl




                                                        11
Answer
                                    

                                 Cl
            N
                                                  H   F   
   H           H             Al
            H                          Cl
                            Cl
                                            
                        




                                                               12
  • Although ammonia has electrophilic and nucleophilic
    centers, it usually react as a nucleophile. By contrast,
    HF and AlCl3 preferreact as electrophiles and very
    rarely react as nucleophile. Water appears to be in
    between since it can react as a nucleophile or as an
    electrophile. What conclusions can you draw regarding
    the relative nucleophilic strengths of neutral nitrogen,
    oxygen and a halogen?

Answer
The evidence suggests that halogen atoms are only weakly nucleophilic,
 whereas nitrogen atoms are strongly nucleophilic. Oxygen appears
to be somewhere in between.
(Note that here we are discussing neutral halogen, oxygen and nitrogen
 atoms incorporated in a molecule. We have already seen
 that negative charges ion such as -OH, -NH2 or –Br are good nucleophiles.)


                                                                     13
• What conclusions can your draw regarding the relative
  electrophilic strengths of the hydrogens in ammonia,
  water and hydrogen fluoride?



  Answer
 Since HF prefers to react as an electrophile,
 the hydrogen in must be a strong electrophilic center.
 In contrast, the hydrogens in ammonia are very weakly
 electrophilic. The electrophilic strength of the hydrogens in
  water is somewhere in between.




                                                          14
• Can you see any trend which relates the nucleophilic
  strengths of the atoms N, O and F to their position in
  the periodic table?


  Answer
  As one moves to the right across the periodic table
  from nitrogen to oxygen and then to fluoride,
  the atoms become less nucleophilic.




                                                           15
• Is the relative strength of the nucleophilic centers
  explained by the relative electronagativities if the atoms
  involved?



 Answer

 It would appear not . Electronegativity increases from
 nitrogen to fluorine, so one would expect fluorine to be
  more electron-rich than oxygen or nitrogen.
 Yet it appears to be the weakest nucleophilic center.




                                                            16
• In fact, the relative nucleophilic strengths of these
  atoms can be explained if we look at the products which
  would be formed if these atoms were to act as
  nucleophiles.
• If there are to act as nucleophiles with say a proton,
  these atoms must provide the electrons for the new
  bond. As a result, they will gain a positive charge as
  shown below. Which atom is able to tolerate a positive
  charge the best? Which tolerates in the least?



                          H
         H   F                          H    F
                                                 H



                                                        17
                                          H
                           H
               O                      O
           H           H          H               H


                                      H
                           H
               N                      N
           H       H             H            H
               H                      H

• If hydrogen fluoride is to act as a nucleophile with an
  electrophile like hydrogen, then the fluoride atom will
  gain a positive charge.
• The fluorine atom is electronegative and does not
  tolerate a positive charge. Therefore, this reaction does
  not take place. Oxygen is less electronegative and is able
  to tolerate the positive charge slightly better, such that
  an equilibrium is possible between the charged and
  uncharged species. Nitrogen is the least eletronegative
  and tolerates the positive charge even better, shifting
  the equilibrium to the salt.                              18
• The same argument can be used in reverse when
  looking at the electrophilicities of the hydrogens in HF,
  H2O and NH3. In this case, reaction with a strong
  nucleophile or base would generate the following
  anions.



                             O               N
             F           H               H       H




• Which are the most stable anions and why? How does
  this affect the electrophilicities of the hydrogens in
  hydrogen fluoride, water and ammonia?


                                                           19
Answer
The most electronegative atom will tolerate the negative
charge the best. This is F atom. O is able to tolerate a
negative charge, but N is not so tolerant of a negative
charge.
If the anion is stable, it will be easily formed and hence
the H which is lost will be strongly electrophilic. This is
the case for HF. If the anion is unstable (e.g.NH2- anion)
it is not easily formed and the H (in ammonia) is very
weak electrophile. N anions are only formed with strong
base.
The relative stability of anions is also influenced by the size
of the anions. Thus , within the halogen family, stability
follows the order I- > Br- > Cl- >F-. This means that the
electrophilicity or acidity of the corresponding acid follows
the order HI>HBr>HCl>HF.
                                                             20
• In conclusion, we can identify nucleophilic and
  electrophilic centers in a molecule by identifying polar
  covalent bond.
• However, some nucleophilic and electrophilic centers
  are usually too weak to react as nucleophiles or
  electrophiles.
• Hence, halogen atoms in a molecule rarely react as
  nucleophiles, while hydrogen atoms attached to
  nitrogen rarely react as electrophiles.




                                                         21
• 1.4 Nucleophilic and electrophilic properties of alkanes
Consider the following alkane. Has it any electrophilic or
  mucleophilic centers?
                          CH   3

                          C
                      H            CH   3
                          H


Answer
There are no charge in the molecule and there are no polar
covalent bond.
There are no electrophilic or nucleophilic centers, and the
 molecule is unreactive to most chemical reagents.



                                                         22
• 1.5 Nucleophilic and electrophilic properties of amines
Amines contain a nitrogen atom which we have already
  seen is a good nucleophilic center. Amines can have N-
  H and N-C bonds. We have already looked at the N-H
  bond. Do you think N-C bond are polar, and if so to
  what extent?

    Answer
    Nitrogen is immediately to the right of carbon
    in the periodic table, so we would expect it to be
    more electronegative.

    However, since they are so close to each other in
    the table, the difference is slight and we can usually
    ignore the N-C bonds when looking for electrophilic
    and nucleophilic centers.
                                                             23
• Identify the nucleophilic or electrophilic centers in the
  following amines and determine whether these
  molecules will behave as nucleophiles or electrophiles.

                                                                       NH   2
                                                                                             N
                                              N                                                  H
            N
    H 3C                CH   3       H 3C         CH      3
            CH                                H
                    3




   Answer
                                                                                         
                                                                                                     
                                                                                       H
                                                      
                                                                                N                    N
                                                  N                                                      H
                N                                                                   H
     H 3C                   CH         H 3C                   CH
                                 3                                 3                                     
                CH                                H                                 
                        3

                                                  




                                                                                                         24
• 1.6 Nucleophilic and electrophilic centers in alcohols and
  ethers
we have already seen that oxygen with its two lone pairs
  of electrons can act as a nucleophilic center. We have
  also seen that the hydrogens attached to oxygen are
  electrophilic center. C-H and C-C bonds are not polar
  and so these do not provide any nucleophilic or
  electrophilic centers. What other type of bond is present
  in alcohols and ethers such as the following?


      H 3C   O                   H   O

Answer           CH   3
                                         CH   3




Carbon-oxygen bonds are present in both alcohols and ether
                                                          25
• Identify the electrophilic and nucleophilic centers in the
  following alcohols and ethers.

                                                      H    O
     H 3C       O              H   O                               H2
                                                           H 2C    C
                    CH 3                   CH 3
                                                                         CH 3


   Answer



                                                             
                                                      H   O
    H 3C    O              H       O                               H2
                                                        H 2C     C
                CH 3                  CH 3
                                                                       CH 3
                                                             



                                                                                26
• In alcohols, there are two possible electrophilic centers-
   the carbon atom directly attached to oxygen and the
   hydrogen directly attached to oxygen will be more
   electrophilic. Which do you think will be more
electrophilic?

  Answer


The O-H bond will be more polar than the C-O bond, so the
hydrogen atom will more electrophilic.
However, the hydrogen atom is still very weak electrophile
in alcohols and so alcohol generallyreact as nucleophiles
through their oxygen atom.


                                                          27
• 1.7 Nucleophilic and electrophilic centers in alkyl halides
Identify the nucleophilic and electrophilic centers in the
  following alkyl halides.




                                              H
                               Cl

        I   CH 3                              C
                                     H 6 Cl          Br
                                              CH 3




                                                           28
Answer                                                             

                                                            Cl
                         
                                                                             Nucleophilic
            I        CH 3
                                                                             center

                                                             Electrophilic
                                                             center

  Nucleophilic                Electrophilic
  center                      center
                 H
                             
                 C
  H 6 Cl                 Br                   Nucleophilic
                                              center
                 CH 3

                                  Electrophilic
                                  center




                                                                                            29
   • Based on what you have learnt in previous sections, do
     you think these molecules are more likely to react as
     electrophiles or as nucleophiles?



 Answer

Whe have already seen that halogen atoms within molecules
are poor nucleophiles.
Therefore , alkyl halides are more likely to react as electrophiles
at the electrophilic carbon center.




                                                               30
• 1.8 Nucleophilic and electrophilic centers in aldehydes
  and ketones
• Aldehydes and ketones contain a carbonyl (C=O) group
  where one bond is strong sigma bond and other is a
  weak pi bond. The following structures are two
  examples of alsehydes and ketones. Identify and
  nucleophilic or electrophilic center which you think
  might be present.

                  O                           O


         H 3C              H       H 3C             CH   2 CH 3


                                             
Answer                 O                  O


                H 3C           H   H 3C           CH 2 CH 3
                                         

                                                                  31
 • 1.9 Nucleophilic and electrophilic centers in carboxylic
   acids and acid derivatioves.
 • We have looked at the nucleophilic and electrophilic
   centers of aldehydes and ketones. Both these functional
   groups contain a carbonyl(C=O) group. However,
   there are other functionsl groups which include a C=O
   group. What are they?

 Answer
The other functional groups which contain C=O are the
carboxylic acid and their derivatives(i.e. acid chlorides,
acid anhydrides, esters amindes)




                                                             32
• Like ketones and aldehydes, the carboxylic acids and
  their derivatives have both electrophilic and
  nucleophilic centers. Identify these in the following
  examples.

                                         O
                     O
                                                 CH 3
                              H   H 3C       N
              H 3C       O
                                             CH 3




              O                              O          O

                         CH   3
                                                        C
       H 3C          O               H 3C           O       CH   3




                                                                     33
                                  Nucleophilic centers
Answer

                                                   O
                       O
                                                              CH      3
                                  H        H 3C          N
                H 3C          O
                                                         CH   3




                           Electrophilic centers


                           Nucleophilic centers




                  O                                O              O

                             CH   3
                                                                  C
         H 3C          O                    H 3C         O                CH   3




                           Electrophilic centers
                                                                                   34
• 1.10 Nucleophilic and electrophilic centers in alkenes,
  alkynes and aromatic compounds
Alkenes, alkynes and aromatic compound are
  nucleophilic. In other word, they are electron-rich and
  prefer to react with electrophiles (molecules which are
  electron-dificient). Suggest why these molecules are
  electron-rich and identify the nucleophilic centers.
                         R      R

        R        R

                           R      R
 Answer
All these functional groups have multiple bonds
(i.e. they have double or triple bonds between two carbons).
Consequently, the area in space between the multiple bonded
 carbons must be rich in electrons. The nucleophilic center in
these molecules is not an atom, but the bond(s).
                                                            35
• Identify the nucleophilic centers in the following
  molecules?

                                        H               CH 2 CH 3                          CH 3

H 3 CH 2 C                     CH 3

                                      H 3C              H

                                                                            CH 3
  Answer

                                        H              CH 2 CH 3           CH 3
   H 3 CH 2 C                  CH 3

                                      H 3C             H

                                                                            Nucleophilic
                Nucleophilic            Nucleophilic                CH 3    center
                center                  center



                                                                                             36
• 1.11 Nucleophilic and electrophilic centers in multi-
  functional molecules.
• Most intersting molecules in organic chemistry and
  biochemistry contain more than one functional group.
  They are called multi-functional as a result. Since they
  may have several nucleophilic and electrophilic
  centers. This makes the number of possible reactions
  quite large and it is part of an organic chemist’s skill to
  be able to identify which of several nucleophilic or
  electrophilic sites is the most likely site for a particular
  reactoin.
• Consider the following molecule and answer the
  questions below?              O
                                             H
                                C
                                        N

                                        Me


                                                            37
     ( 1) Outline and identify the functional groups in
       the moleclule.
     ( 2) Identify the sites in the molecule where a
       nucleophile might react.
     ( 3) Identify the sites in the molecule where an
       electrophiles might react.

Answer
                                 Ketone

                             O
                                               H
                             C
1)                                        N

                                          Me

                                              Amine
             Aromatic ring


                                                          38
Answer
            O
                                     H
            C
                                N

                                Me

         Electrophilic center




                            Nucleophilic
                            center
           O
                                     H
            C
                                N

                                Me



           Nucleophilic
           center

                                           39
• Repeat the above question with the following molecule.



                           H

                       O

               H 3C        CH 2


                                  O
                 H     H 2C

                                  O   CH 3




                                                       40
• 1) The functional groups are as shown.


                                   Alcohol
                            H

                        O

               H 3C         CH 2
      Alkene
                                     O       Ester
                 H     H 2C

                                    O    CH 3




                                                     41
• 2) Nucleophiles react at electrophilic centers. There are
  four such centers in this molecule.


                                 H
                                              Electrophilic centers
                             O

                  H 3C           CH 2


                                        O
                     H       H 2C

                                        O   CH 3



     Electrophilic centers


                                                                      42
3) Electrophiles react at nucleophilic centers. There are
   several such centers in the molecule.



                                      H
                                                  Nucleophilic centers
                                  O

                   H 3C               CH 2


                                             O
                       H          H 2C

                                             O   CH 3


           Nucleophilic centers




                                                                         43
• The following molecule is called procaine and is an
  important local anaesthetic. Outline and identify the
  functional groups.




                            O
       H
           N
                                H2
           H                O   C           CH   2 CH 3
                                 H 2C   N
                                            CH   2 CH 3




                                                          44
                          O
    H
           N
                                 H2
           H              O      C           CH   2 CH 3
                                 H 2C    N
Aromatic
                                             CH   2 CH 3
amime                         Aromatic
               Aromatic       ester      Amine
               ring or
               arene




                                                           45
  • Now isentify the nucleophilic and electrophilic centers


                        Electrophilic
                        centers
                                                         Nucleophilic
                                                         centers

                                                                                 Electrophilic
                                                 O                               center
                H
                    N
                                                     H
                    H                            O   C                  CH   2 CH 3

Electrophilic                                        H 2C          N
center
                                                                        CH   2 CH 3
                                  Nucleophilic
                                  centers




                                                                                          46
• Sumamary
•      Nucleophilic centers in a molecule are electron-rich
  and can react with nucleophiles.
•      Electrophilic centers in a molecule are electron-
  deficient and can react with nucleophiles
•      Negatively charged ions are nucleophiles. The
  nucleophilic centers is the atom with negative charge.
•      Positively charged ions are electrophiles. The
  electrophilic centers id the atom with positive charge.
•      Atoms with lone pairs of electrons can act as
  nucleophilic centers(e.g. N, O, Halogens)
•      Double bonds, triple bonds and aromatic ring
  sytems can act as nucleophilic centers.


                                                         47
•      Hydrogen atoms attached to electronegative atoms
  are electrophilic.
•      Carbon atom attached to oxygen or a halogen are
  electrophilec
•      A nitrogen atom is more nucleophilic than an
  oxygen atom. Halogens within a molecule are only very
  weakly nucleophilic.
•      Hydrogen atoms attached to halogens are more
  electrophilic than hydrogens attached to oxygen, which
  in turn are more electrophilic than hydrogens attached
  to nitrogen.




                                                       48

				
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