Introduction to Mechanism of Organic Reaction

MECHANISM OF ORGANIC REACTION

A chemical equation is only a symbolic representation of chemical reaction which indicates the initial reactants and final products involved in a chemical change. Reactants generally consist of two species.

(a) Substrate: One which is being attacked in a chemical reaction

(b) Reagents: The species which attack the substrate molecule

Substrate + Reagent $\to$Products

It is important to know not only what happens in a chemical reaction but also how it happens. Most of the reactions are complex and take place via reactive intermediates which may be or may not be isolated. The reaction intermediates are generally very reactive which readily react with other species present in the environment to form the products. The detailed step by step description of chemical reaction is called its mechanism. Mechanism is only a hypothesis to explain various facts regarding a chemical reaction.

Substrate $\to$ Reactive intermediates - Products

By knowing the mechanism we can predict the product of a chemical reaction, adjust the experimental conditions to improve the yield of the products or even alter the course of reaction to get the different products.

Most of the attacking reagents carry either positive charge (an electron deficient species) or a negative charge (electron rich species). The positively charged reagents attack the substrate at points of high electron density while (-vely) charged reagents attack the point of low electron density. The organic reactions essentially involve changes in the existing covalent bonds present in the molecules. These changes may involve electronic displacements in covalent bonds breaking of some of the existing bonds (bond fission), formation of new bonds as well as energy change accompanying the bond fission and bond cleavage.

We can understand the mechanism of various organic reactions in terms of following well established basic concepts.

(i) Electronic displacement in covalent bond

(ii) Fission (cleavage) of covalent bonds

(iii) Nature of attacking reagents

Types of Organic Reactions:

All organic reactions can be broadly classified into four catagories.

(a) Substitution reactions

(b) Addition reactions

(c) Elimination reactions and

(d) Rearrangement reactions

(a) Substitution Reactions

In these an atom or a group of atoms in an organic molecule is replaced by another atom or group of atoms without any change in the remaining part of the molecule. These reactions may be initiated by free radical, electrophile or nucleophile.

(i) Free radical substitution reaction:

This substitution reaction is brought about by free radicals. For example chlorination of methane in presence of diffused sunlight. The mechanism of the reaction is as follows.

(ii) Nucleophilic substitution reoactions:

These reactions are brought about by nucleophile. The reaction can proceed either via S_N1 or S_N2 mechanism.

S_N1 mechanism:

Rate determining step involves only the species. For example the reaction.

takes place as follows

1^st step: (CH₃)₃CBr $\underset{\det er\min ing\text{ step}}{\stackrel{Rate}{\to }}$$\underset{Carbocation}{(C{H}_3)C^{+}+B{r}^{-}}$

2^nd step: Attack by nucleophile

The stability of carbocation is the controlling factor for this mechanism the formation of 3° carbocation as an intermediate proceeds via this mechanism. In an optically active compound substitution at chiral centre through S_N1 mechanism produces recemic mixture (No 100% recemization is observed?).

S_N2 mechanism: Rate determining step involves two species and reaction proceeds through transition state.

$C{H}_{3}Cl+O{H}^{-}\to C{H}_{3}OH+C{l}^{-}$

Since 1° carbocation is less stable than the transition state formed above, the reaction involving 1° alkyl halides proceed via S_N2 mechanism. During reaction configuration of carbon is inverted which is referred to as Walden inversion.

Points to Remember

$\bullet$ The higher the polarity of solvent greater the tendency for S_N1 reaction.

$\bullet$ High concentration of the nucleophile favours S_N2 reaction while low concentration favours S_N1 reaction.

$\bullet$ Rearrangement of the carbocation (formed in S_N1 reaction) leading to more stable carbocation is observed in S_N1 reaction (discussed latter).

$\bullet$ In general S_N2 mechanism is strongly inhibited by increasing steric bulk of the reagents. In such case S_N1 mechanism is favoured.

(iii) Electrophilic substitution reactions:

The reaction initiated by an electrophile is known as electrophilic substitution reaction. Aromatic substitution reactions are the examples of this type of reaction.

C₆H₆ + Cl₂ $\underset{or\text{ AlC}{\text{l}}_{\text{3}}}{\stackrel{FeC{l}_3}{\to }}$ C₆H₅Cl + HCl

The mechanism of this reaction as follows.

Formation of electrophile: Cl : Cl + AlCl₃ $\to$ Cl⁺ + $AlC{l}_4^{+}$

Electrophile attack:

Elimination of proton:

(b) Addition Reactions

Reactions which involve combination between two molecules to give a single molecule of the product are called addition reactions. Such reactions are typical of compounds containing multiple (double or tripe) bonds. Depending upon the nature of the attacking species (electrophiles, nucleophiles or free radicals) addition reactions are of the following types.

(i) Electrophilic addition reaction:

These reactions are brought about by electrophiles and are typical reactions of alkenes and alkynes.

ii) Nucleophilic addition reactions:

These reactions are brought about by nucleophiles. The characteristics reaction of aldehyde and ketone are nucleophilic addition reaction i.e., base catalysed addition of HCN to aldehydes or ketones.

HO^– + HCN $\to$ H₂O + CN^–

(iii) Free radical addition reactions:

Addition reactions brought about by free radicals are called free radical addition reactions for example addition of HBr to alkenes in presence of peroxides.

CH₃—CH=CH₂+HBr $\stackrel{Peroxide}{\to }$ $\underset{n-propyl\text{ hydride}}{C{H}_3-C{H}_2-C{H}_{2}Br}$

The reaction proceeds through following mechanism.

$R-O-O-R\stackrel{\Delta }{\to }$2RO°

RO${}^{\bullet }$ + HBr $\to$ ROH + Br${}^{\bullet }$

CH₃—CH=CH₂ + Br $\stackrel{slow}{\to }$CH₃—${}^{\bullet }$CH —CH₂—Br

CH₃—${}^{\bullet }$CH—CH₂—Br$\underset{fast}{\stackrel{HBr}{\to }}$CH₃—CH₂CH₂—Br + Br

Br${}^{\bullet }$ + Br${}^{\bullet }$ $\to$ Br₂

Illustration 1. Identify the 1, 2- and 1-4 addition products of Free Radical addition of CBrCl₃ to 1, 3-butadiene.

Solution:

Illustration 2. Addition of HCl on 1, 3 butadiene gives two products. Explain.

Solution: Mechanism of addition of HCl on 1, 3-butadiene is as follows

Step:1

Step:2

In step 1 proton adds to one of the terminal cabons of 1, 3 butadiene to form, as usual, the more stable carbonium ion, in this case a resonance stabilized allylic cation. Addition of one of the inner carbon atoms would have produced a much less stable primary cation that could not be stabilized by resonance;

In step (2), a chloride ion forms a bond to one of the carbon atoms of the allylic cation that bears a partial positive charge. Reaction at one carbon atom results in the 1, 2 – addition product, reaction at the other gives that 1,4 addition product.

(c) Elimination Reactions

An elimination reaction is one which involves the loss of two atoms or groups of atoms from the same or adjacent atoms of a substrate molecule leading to formation of multiple (double or triple) bond. These are of two types.

(i) $\beta$ - elimination reactions. In these reactions, loss of two atoms or groups occurs from the adjacent atoms of the substrate molecule e.g., acid catalysed dehydration of alcohol and base catalysed dehydrogenation of alkyl halides.

(ii) $\alpha$ - Elimination – In these reactions loss of two atoms or groups occurs from the same atom of the substrate molecule. E.g., base catalysed dehydrohalogenation of chloroform to form dichlorocarbene.

Dichlorocarbene is the reactive intermediate involved in carbylamine reaction and

Reimer – Tieman reaction.

(d) Rearrangement Reaction

These reactions involve the migration of an atom or a group of atoms from one atom to another within the same molecule.

Some reactions involving rearrangement.

$C{H}_3-C{H}_2-C{H}_2-C{H}_2-Br\underset{575K}{\stackrel{AlC{l}_3}{\to }}$ $C{H}_3-C{H}_2-\stackrel{\begin{array}{c}Br\\ |\end{array}}{C}H-C{H}_3$

1, 2 Hydride shift

1, 2 Methyl shift

Illustration 3. Can you think why the below given is order of bases reaction with conc. HCl

(in presence of anhydrous ZnCl₂, mixture is called Lucas's reagent).

Solution: Naturally the one which is the strongest base, will react faster

$\underset{base}{\mathrm{ROH}}+\underset{Conc.acid}{\mathrm{HCl}}\stackrel{ZnC{l}_2}{\to }\underset{whiteturbidity}{\mathrm{RCl}}+H_{2}O$

It is simply a reaction between acid and a base and the strongest base will be reacting fastest. This is called Luca's test of making distiction between 1⁰, 2⁰ and 3⁰ alcohol.