Explanation of  Groups  Participation  in Type  of Products

 

Dr. Nagham  Mahmood Aljamali

Lecturer of Organic Chemistry, Chemistry Department, College of  Education,

*Corresponding Author E-mail: dr.nagham_mj@yahoo.com

The aim  of  this  survey, explanation  from various  references about  participation of  many  groups  in normal compounds and  cyclic compounds, information about  donating  and drawing   groups , changing of  membered  ring.   

 

 

 

INTRODUCTION:

Participation  of  groups gave  several  types of products. There is a bit of terminology dealing with the leaving group important to substitution and elimination. The α -carbon is the carbon atom bonded to the leaving group. β -carbons are attached to the α -carbon. The hydrogen    attached to the β -carbon are called β -hydrogen. This terminology is vitally important for our discussion of substitution and elimination reactions.

Halides and the tosyl  group (-OTs) are examples of commonly used leaving groups. In general, if the group is relatively stable after leaving the molecule with the C-LG bond's electrons, it's a good candidate for a leaving group.

 

Single atom (such as Br^-) it is still called a leaving group.

 

Bromide ion  is the leaving  group in this  S_N2 reaction.

 

Hydroxyl  ion  is the  leaving  group in this    S_N1   reaction.

 

 

Examples  for   Leaving   groups  in  Reactions :

 

Here is a table classifying some common leaving groups. 

Excellent	TsO-  ,  NH3
Very Good	I-, H2O
Good	Br-
Fair	Cl-
Poor	F-
Very Poor	HO-, NH2-, RO-

 

 

The Nature of the Leaving Group

In order to understand the nature of the leaving group, it is important to first discuss factors that help determine whether a species will be a strong base or weak base. If you remember from general chemistry, a Lewis base is defined as a species that donates a pair of electrons to form a covalent bond. The factors that will determine whether a species wants to share its electrons or not include electronegativity, size, and resonance.

 

As Electronegativity  Increases, Basicity Decreases: In general, if we move from the left of the periodic table to the right of the periodic table as shown in the diagram below, electronegativity increases. As electronegativity increases, basicity will decrease, meaning a species will be less likely to act as base; that is, the species will be less likely to share its electrons.

 

As Size Increases, Basicity Decreases: In general, if we move from the top of the periodic table to the bottom of the periodic table as shown in the diagram below, the size of an atom will increase. As size increases, basicity will decrease, meaning a species will be less likely to act as a base; that is, the species will be less likely to share its electrons.

 

Resonance Decreases Basicity: The third factor to consider in determining whether or not a species will be a strong or weak base is resonance. As you may remember from general chemistry, the formation of a resonance stabilized structure results in a species that is less willing to share its electrons. Since strong bases, by definition, want to share their electrons, resonance stabilized structures are weak bases.

 

Weak Bases are the Best Leaving Groups

Now that we understand how electronegativity, size, and resonance affect basicity, we can combine these concepts with the fact that weak bases make the best leaving groups. Think about why this might be true. In order for a leaving group to leave, it must be able to accept electrons. A strong bases wants to donate electrons; therefore, the leaving group must be a weak base. We will now revisit electronegativity, size, and resonance, moving our focus to the leaving group, as well providing actual examples.

 

Note :

1-     As Electronegativity Increases, The Ability of the Leaving Group to Leave Increases.

2-     Good leaving groups are weak bases.

3-     the weaker the base, the better the leaving group.

4-     A nucleophile donates a pair of electrons

5-     A leaving group accepts a pair of electrons

 

 

As mentioned previously, if we move from left to right on the periodic table, electronegativity increases. With an increase in electronegativity, basisity decreases, and the ability of the leaving group to leave increases. This is because an increase in electronegativity results in a species that wants to hold onto its electrons rather than donate them. The following diagram illustrates this concept, showing ^-CH₃ to be the worst leaving group and F^- to be the best leaving group. This particular example should only be used to facilitate your understanding of this concept. In real reaction mechanisms, these groups are not good leaving groups at all. For example, fluoride is such a poor leaving group that S_N2 reactions of fluoroalkanes are rarely observed.

 

As Size Increases, The Ability of the Leaving Group to Leave Increases: Here we revisit the effect size has on basicity. If we move down the periodic table, size increases. With an increase in size, basicity decreases, and the ability of the leaving group to leave increases. The relationship among the following halogens, unlike the previous example, is true to what we will see in upcoming reaction mechanisms.

 

The order of the halide leaving groups is    I− >Br− > Cl− _F−. This order is opposite to that of electronegativity and is dominated by the strength of the bond to carbon . Sulfonate esters are especially useful reactants in nucleophilic substitution reactions in synthesis. They have a high level of reactivity and can be prepared from alcohols by reactions that do not directly involve the carbon atom at which substitution  is to be effected. The latter feature is particularly important in cases where the stereo chemical and structural integrity of the reactant must be maintained.

 

1-       Note  :  NH₂  amine  group  is not  good  leaving  group  ,  but  we  can  make  it  good  leaving  group  by  this  method  :

 

2-       Note  :  OH  hydroxyl   group  is not  good  leaving  group  ,  but  we  can  make  it  good  leaving  group  by  this  method ( protonation ) :

 

Some   Reactions  are  Containing   Leaving  Groups in  their  steps :

 

 

Resonance Increases the Ability of the Leaving Group to Leave: As we learned previously, resonance stabilized structures are weak bases. Therefore, leaving groups that form resonance structures upon leaving are considered to be excellent leaving groups. The following diagram shows sulfur derivatives of the type ROSO₃^- and RSO₃^-. Alkyl sulfates and sulfonates like the ones shown make excellent leaving groups. This is due to the formation of a resonance stabilized structure upon leaving.

 

 

Neighboring  Group  Participation (NGP) :

A classic example of   NGP is the reaction of :

1-              Sulfur or Nitrogen mustard with a nucleophile, the rate of reaction is much higher for the sulfur mustard and a nucleophile than it would be for a primary alkyl chloride without a heteroatom.

 

2-       An aromatic ring can assist in the formation of a carbocationic intermediate called a phenonium  ion by delocalising the positive charge.

 

 

3-       Participation  via  Halide :

 

4-       Participation  via  Alkene :

 

5-       Participation   via  Alkyl  group :

 

Note : The Neighboring Group ( NFG )  Effect describes the acceleration of a reaction due to the influence of other functional groups in the substrate.

 

 

Ring  Expansion (  Change  the  Size  of  Cycles ):

Expansion  the  size  of  cycles  (  increase  or  decrease )  via  departing  of  leaving  group  or  by  rearrangement  or  by  stereo  or  by  participation  of  neighboring groups  or   carbine  ion  …..etc :  

 

 

The CH₃ could potentially migrate in this case, it’s favorable to shift one of the alkyl groups in the ring, which leads to ring expansion and the formation of a less strained, five-membered ring. Example of an SN1 where an alkyl shift leads to ring expansion.

 

 

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Received on 05.08.2016         Modified on 12.08.2016

Accepted on 29.08.2016         © AJRC All right reserved