12.1
(a) Free radical substitution
Substitution reaction is defined as the replacement of one atom (or group of atoms) in a molecule by another atom ( or group ).  E.g. Reaction between methane and chlorine to form hydrogen chloride & chloromethane.  The reaction is exothermic but energy in the form of U.V. light must be supplied to initiate the reaction. Chlorine absorbs the U.V light, the energy which is equal to approx. 400 kJ/mol. This is greater than the bond strength of the Chlorine molecule which then splits into Cl atoms.  This is homolytic fission as the same number of electrons from the bond are passed to each atom.  Free radical substitution has 3 steps.

step 1: Initiation Cl-Cl ----> 2Cl^.
Each atom retains one electron from the covalent bond between the atoms. An atom or group of atoms which posses an unpaired electron is called a free radical.

step 2: Propagation
Each chlorine atom then reacts with a molecule of methane by abstracting a hydrogen atom to form hydrogen chloride and a methyl radical.
H₃C-H + Cl^. -----> CH₃^. + HCl
The methyl radical reacts with a molecule of Chlorine to form chloromethane and a Cl atom:
H₃C^. + Cl-Cl ----> CH₃Cl + Cl^.

step 3: Termination
Radicals combine
H₃C ^. + Cl^. ---> CH₃Cl

In reactions 1 and 2 covalent bonds are broken so that one electron of the pair in each bond becomes associated with each of the atoms or groups. These are examples of homolysis or homolytic fission.

(b) Electrophilic addition
Heterolytic fission- In this type of fission the two shared electrons in the bond are split unequally between the two atoms. One of the atoms keeps both electrons. As a result ions are formed (there are three types of reactions-substitution, addition & elimination.

An electrophile is a species which attacks a carbon atom by accepting an electron pair. It is thus a Lewis acid
Ethene reacts with bromine to form 1,2-dibromoethane:  This is an addition reaction

The cation C₂H₅ ⁺, and other cations in which a carbon atom bears the positive charge, are given the general name carbonium ions. In the first step the two electrons which form the new bond are both provided by the alkene: thus, the reagent ( Br or Cl) is described as electrophilic ( i.e `electron-seeking').

CH2                         CH₂⁺
||   + Br^d+-Br^d- --------->      ||             + Br ^-
CH2                                         CH₂Br

CH₂⁺                 CH₂
||          + Br ^----------> ||
CH₂Br                        CH₂Br

means the movement of a pair of electrons.

Ethene reacts with hydrogen chloride to give chloroethane:
This is also an addition reaction. The first step is the formation of two ions, the ethyl cation and the chloride anion:
The two ions then rapidly combine to form the product. In this reaction, the partially charged atom in hydrogen chloride is the electrophilic reagent. H^d+ -Cl^d-
Since organic reactions involving heterolytic fission of bonds produce ions, these reactions tend to take place in polar solvents.

(c) Nucleophilic substitution

A nucleophile is a species which attacks a carbon atom with a partial positive charge by donating an electron pair.  An example of a nucleophilic substitution is provided by the hydrolysis of 2-methyl-2-bromopropane, to form 2-methylpropan-2-ol. The hydroxide ion is a nucleophile. This is known as nucleophilic substitution. The bromide ion is known as the leaving group.
(CH₃)₃CBr + NaOH ------> (CH₃)₃COH + NaBr
Rate studies of this type show that;     Rate = k[R-Br]     R=the alkyl group
This is an S_N1 mechanism with the following steps:

(CH₃)₃C-Br        ------> (CH₃)₃C⁺ + Br^-           slow
(CH₃)₃C⁺ + OH ^- ------> (CH₃)₃COH                   fast

The reaction is thus first order with respect to R-Br but zero order with respect to OH^-.  First order kinetics is good evidence that the rate determining step is unimolecular. The reaction is therefore given the symbol S_N1. Since the nucleophile is not involved in the rate determining step, the mechanism must involve at least two steps.

The reaction between bromomethane and hydroxide ion is another nucleophilic substitution reaction.
CH₃Br + OH ^- -----> CH₃OH + NaBr
The rate equation is  Rate = k[CH₃Br][OH^-]
In this case the reaction is first order with respect to both [CH₃Br] and [OH^-].  It is thus second order overall. The reaction is thus given the symbol S_N2.  It is thought to proceed in a single step involving a transition state.

HO ^- + CH₃------Br --------> [ HO - - - CH₃ - - - Br] ^-

[ HO - - - CH₃ - - - Br] ^-----------> HO-----CH₃ + Br ^-
The cyanide ion CN- can also take part in nucleophilic substitution

(d) Electrophilic substitution
Electrophilic substitution is also possible on benzene rings. In this type of substitution two of the delocalised [pi] electrons on the benzene ring are donated to the electrophile. An unstable [pi] complex containing both an electrophile and a leaving group is formed as an intermediate. The nitration is carried out under reflux at 55-60^oC using a nitrating mixture. This contains equal amounts of concentrated nitric acid and sulphuric acid. The two acids react to generate the nitryl cation NO²⁺.

HNO₃ + H₂SO₄ -------> NO₂⁺ + H₂O + HSO₄^-

  + NO₂⁺ ------->          step 1
 

     ----->      +     H⁺     step 2

12.2 Interpret simple allied reactions in terms of mechanisms
e.g. What is the mechanism for the following reaction if  rate = k[CH₃CH₂I]
CH₃CH₂I  + NH₃ -----> CH₃CH₂NH₂ + HI
CH₃CH₂I is a halogenoalkane so likely to take part in a nucleophilic substitution.  NH₃ is a nucleophile because of its lone pair of electrons on the nitrogen atom.  The rate equation shows that it is first order so the slow step in the mechanism is;

CH₃CH₂-I     -----> CH₃CH₂⁺  +  I^-
A nuclephilic attack by ammonia is now possible in a fast step;

NH₃ + CH₃CH₂⁺   ----->  CH₃CH₂NH₃⁺
a final fast step might be loss of hydrogen ion;

CH₃CH₂H₂N-H⁺ -----> CH₃CH₂NH₂  +  H⁺
 

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Updated 2 July 1999.