Chapter 12.10 - Organic Reaction Mechanism - Fundamental Concepts

In the previous section, we saw isomerism. In this section, we will see fundamental concepts in organic reaction mechanism.

• In a chemical reaction, the reactant which is being observed is known as substrate.
• In our present discussion, we consider reactions involving organic compounds. So for our present discussion, the organic compound will be the substrate.
• The other substance which reacts with the organic compound will be called the reagent.
• As an organic reaction proceeds, first we get some intermediate products.
   ♦ When the reaction is complete, all the intermediate products will have disappeared.
   ♦ They will be converted into products and byproducts.
   ♦ This is shown in the fig.12.67 below:

Fig.12.67

• We will be interested in collecting the products. But in some cases, the byproducts will also have some commercial value.

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• Let us see how an organic reaction can be described. An organic reaction can be described in a sequential order. The order is written in 3 steps below:
(i) First we write the details of electron movement.
• Electron(s) originally possessed by the substrate may get attached to the reagent or vice versa.
(ii) Secondly we write the energies absorbed or released when new bonds are formed and existing bonds are broken.
(iii) Finally, we write the rate at which reactants are transformed to products (quantity of product obtained in unit time).
• The description containing the above three items in sequential order is known as reaction mechanism.
• A good knowledge about the reaction mechanism will help to preplan a reaction so that, maximum quantity of products can be obtained.

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We will now see the basic principles which will enable us to write the reaction mechanism.

Fission of a covalent bond

• In an organic reaction, some of the covalent bonds will undergo fission.
   ♦ The word 'fission' means splitting. The dictionary meaning can be seen here.
   ♦ Another word for fission is cleave. The dictionary meaning can be seen here.
• A covalent bond can get cleaved by any one of the two methods:
(i) heterolytic cleavage (ii) homolytic cleavage.
First we will see heterolytic cleavage. It can be written in 9 steps:
1. In heterolytic cleavage, the covalent bond breaks in such away that, both the electrons in that bond stays with one of the fragments.
• This can be explained using an example. It can be written in 5 steps:
(i) Fig.12.68(a) below shows the Lewis dot structure of CH₃Br
    ♦ The green dots indicate the one valence electron of H
    ♦ The red dots indicate the four valence electrons of C
    ♦ The blue dots indicate the seven valence electrons of Br.

Fig.12.68

(ii) The cyan dashed curve indicates that, the bond between C and Br undergoes fission.
• After the fission, we get two species. They are shown in figs.12.68 (b) and (c)
(iii) Fig.b shows that, the C atom now has only six electrons around it.
• This electronic configuration is called sextet electronic configuration.
   ♦ Note that, the C atom has lost one red dot.
   ♦ So C has now a +ve charge.
   ♦ This species is written as: $\mathbf{\rm{{H_3}\overset{+}{C}}}$
(iv) Fig.c shows that, the Br atom now has eight electrons around it.
• We know that, this electronic configuration is called octet electronic configuration.
   ♦ Note that, one red dot which originally belonged to the C, is now with Br.
   ♦ So Br now has a -ve charge.
   ♦ Also there are three lone pairs.
   ♦ This species is written as: $\mathbf{\rm{\overset{-}{Br}}}$
(v) The process in fig.12.68 can be written in a condensed form as shown below:

• The curved arrow indicates that, both the electrons in the bond are transferred to the Br atom.
2. Based on the above example, we can write:
◼ After the heterolysis,
   ♦ One atom has a sextet configuration and a +ve charge.
   ♦ The other atom has an octet configuration and a -ve charge.
         ✰ Also this other atom will have at least one lone pair.
3. A species having a C atom with sextet configuration and a +ve charge is called a carbocation.
   ♦ Earlier, it was called carbonium ion.
• So what we have in fig.12.68(b) above, is a carbocation. In our present case, it can be named as methyl cation or methyl carbonium ion.   
4. Consider fig.12.69(a) below:

Fig.12.69

• The compound under went heterolysis in such a way that, there are only six electrons around the right side C atom. Also this C atom is positively charged.
   ♦ So the species as a whole is a carbocation.
• Note the right side C atom which actually lost the electron.
   ♦ It is the positively charged C atom.
   ♦ Only one other C atom is directly attached to this positively charged C atom.
   ♦ So this species is known as primary carbocation.
   ♦ It is named as ethyl cation.
   ♦ It is written as $\mathbf{\rm{CH_3\overset{+}{C}H_2}}$.
5. Consider fig.12.69(b) above.
• It has three C atoms. So it is related to propane. The middle C atom originally had two H atoms.
• But fig.b shows that, the middle C atom has lost an electron.
• Fig.c shows the structural formula. We see that, there are only six electrons around the middle C atom.
   ♦ The middle C atom is the positively charged C atom.
   ♦ Two other C atoms are directly attached to this positively charged C atom.
   ♦ So this species is known as secondary carbocation.
   ♦ It is named as isopropyl cation.
   ♦ It is written as $\mathbf{\rm{(CH_3)_2\overset{+}{C}\,H}}$.
6. Consider the species in fig.12.69(d). It also has three C atoms. The right most C atom is positively charged. But this species cannot be called a secondary carbocation. This is because, the positively charged C atom is directly attached to only one other C atom. It is a primary carbocation.
7. Consider fig.12.70(a) below:

Fig.12.70

• The compound under went heterolysis in such a way that, there are only six electrons around the central C atom.
   ♦ So the species as a whole is a carbocation.
• Note the central C atom which actually lost the electron.
   ♦ It is the positively charged C atom.
   ♦ Three other C atoms are directly attached to this positively charged C atom.
   ♦ So this species is known as tertiary carbocation.
   ♦ It is named as tert-butyl cation.
   ♦ It is written as $\mathbf{\rm{(CH_3)_3\overset{+}{C}}}$.
8. In the above steps, we counted the ‘number of C atoms’ which are directly attached to the positively charged C atom.
• Instead of the ‘number of C atoms’, we can use the ‘number of alkyl groups’ also. This can be explained in 3 steps:
(i) In fig.12.69(a), the positively charged C atom is directly attached to one C atom. This one C atom is from a methyl group. So we can write:
The positively charged C atom is directly attached to one alkyl group.
(ii) In fig.12.69(b) and (c), the positively charged C atom is directly attached to two C atoms. These two C atoms are from methyl groups. So we can write:
The positively charged C atom is directly attached to two alkyl groups.
(iii) In fig.12.70, the positively charged C atom is directly attached to three C atoms. These three C atoms are from methyl groups. So we can write:
The positively charged C atom is directly attached to three alkyl groups.
9. Carbocations are highly unstable.
• To attain stability, they tend to enter into reactions with other species. That means, carbocations are very reactive.
• However, the alkyl groups which are directly attached to the positively charged C atom, help to stabilize the carbocation. We will see the mechanism in later sections.
• Among the four carbocations that we saw above, the tertiary butyl cation has the greatest stability. Followed by isopropyl cation, followed by ethyl cation, followed by methyl cation.

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Now we will see the shape of methyl carbocation. It can be written in 3 steps:
1. Recall the shape of methane molecule. Details here.
• The electronic configuration of C is 1s²2s²2p_x¹2p_y¹2p_z⁰
• When enough energy is available, one of the two electrons in the 2s orbital jumps to the 2p_z orbital. Thus we get four half filled orbitals: 2s¹2p_x¹2p_y¹2p_z¹
• These four orbitals mix together to form the four sp³ hybrid orbitals.
    ♦ Thus the C atom is sp³ hybridized.
• The four H atoms enter into sigma bond with the four sp³ hybrid orbitals to give CH₄ molecule. It has a tetrahedral shape.
2. In our present case of methyl carbocation, the C atom has one electron less.
• So the valence electronic configuration just before hybridization will be:
2s¹2p_x¹2p_y¹2p_z⁰
• The 2s, 2p_x and 2p_y will mix together to give three sp² hybrid orbitals.
• These three hybrid orbitals will be directed towards the three corners of an equilateral triangle.
    ♦ The nucleus of the C atom will be at the centroid of the triangle.
    ♦ The p_z orbital does not participate in the hybridization.
    ♦ It will remain perpendicular to the plane of the triangle.
    ♦ This is shown in fig.12.71 below:

Fig.12.71

• The three yellow dashed lines are imaginary lines. They help to visualize the triangular shape in which the hybrid orbitals (shown in pink color) are oriented.
3. What we see in fig.12.71 above, is a 3D view. We have to represent it in 2D form. Consider fig.12.72(a) below. 

Fig.12.72

• A plane is passing through one of the H atoms. The p_z orbital lies on this plane. So the orientation of the plane is fixed.
• In this orientation, one of the remaining two H atoms is in front of the plane. So we connect it using a solid wedge. This is shown in fig.b
• The other remaining H atom is behind the plane. So we connect it using a dashed wedge.
• The H atom which lies on the plane is connected using a solid line.
• Thus fig.b gives the 2D representation of the methyl ion.

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• In the discussion so far, we saw the heterolysis in which both the electrons in the bond, were removed from the C atom.
• In another type of heterolysis, both the electrons in the bond stays with the C atom. This is shown in fig.12.73 (a) below.

Fig.12.73

• In fig.b, we see that C has an extra blue dot which originally belonged to the Z atom. So the C atom has a -ve charge.
   ♦ This species is written as: $\mathbf{\rm{{H_3}\,\overset{-}{C:}}}$
• The process can be written in a condensed form as shown below:

• The curved arrow indicates that, both the electrons in the bond are transferred to the C atom.
• A carbon species in which the C atom carries a negative charge is called carbanion. In our present case, it is the $\mathbf{\rm{{H_3}\,\overset{-}{C:}}}$. It is named as methyl anion.
• Carbanions are also unstable and hence very reactive.

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• The organic reactions which proceed through heterolytic bond cleavage are called ionic reactions.
• They are also known as heteropolar reactions.
• Some times they are just called polar reactions.

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In the next section, we will see homolytic cleavage.

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