Organic Reactions and Preliminary Concept of Reaction Mechanism

In a chemical reaction, the reactant molecule undergoing attack is referred to as the substrate and the attacking species is the reagent. The substrate and the reagent interact to give products of the reaction.

Substrate + Reagent→ Products

The steps of an organic reaction showing the breaking and formation of new bonds of carbon atoms is the substrate leading to the formation of the final products through transitory intermediate, are often referred to as it mechanism.

Substrate→ Intermediate (Transitory)→ Products

Let us take a hypothetical reaction.

C+A-B(Reactants) → C-----A-----B (Intermediate) → C-A+B (products)

In this reaction there is a bond breaking in between A and B and a new bond is formed in between C and A. The transition state is not a true molecule, the bonds being partial. In this state, the system possesses maximum energy and is most unstable.

Concept of reaction mechanism

• Cleavage or fission of a covalent bond

A covalent bond is a strong bond, which is produced by the mutual sharing of electrons among the atoms. There is two types or bond fission:

(a) Homolytic bond fission -In this process, each of the atoms acquires one of the bonding electrons, i:e equal distribution of the electron in between two atoms to give rise to free radicals.

For example -

Free radicals are electrically neutral and have one unpaired (odd) electron associated with them. They are highly reactive because of the tendency of this electron to become paired. Homolytic fission is the most common mode of fission in the vapour phase. They are initiated by heat, light, or organic peroxides.

For example-

(b) Heterolytic bond fission -In this process, one of the atoms acquires both of the bonding electrons when the bond is broken i:e unequal division of electrons in between two atoms gives rise to cations and anions.

For example-

Because Cl atom has higher electronegative value (3.0) and carbon has low electronegative value (2.5)

• Types of reagents

Heterolytic fission of a molecule containing covalent bond results in two active reagents: one electron deficient (called electrophile) and the other electron rich (called nucleophile ).

1. Electrophiles or electrophilic reagents: A reagent which can accept an electron pair in a reaction is called as an electrophile. They may be positive ions (including carbonium ions) or neutral molecules with electron- deficient centres (lewis acid). For example- An electrophile can be represented by the general symbol E⁺.

   (i) +ve electrons: H⁺, Cl⁺, Br⁺, R⁺,NO₂⁺, R₃C⁺, RN₂⁺, H₃O⁺e.t.c.

   (ii) neutral electrophiles: AlX₃, BX₃, FeX₃.

2. Nucleophiles or nucleophilic reagents: A reagent which can donate an electron pair in a reaction is called as a nucleophile. It indicates that it attacks regions of low electron density in the substrate molecule. Nucleophiles are electron rich. They may be negative ions or neutral molecules with free electron pairs. A nucleophile can be represented by a general symbol Nu:. For example-

(i) Negative nucleophiles: F^-, Cl^-, Br^-, I^-, CN^-, OH^-, RCH₂^- e.t.c.

(ii) Neutral nucleophiles:

Reference

Adhikari, Rameshwar; Khanal, Santosh; Subba , Bimala; Adhikari, Santosh; Khatiwada, Shankar Pd. Universal Chemistry XI. First. Vol. 1st. Kathmandu: Oasis Publication, 2069.

Chaudhary, Ganga Ram; Karna, Shila Kant Lal; Sharma, Kanchan; Singh, Sanjay; Gupta, Dipak Kumar. A Textbook of Higher Secondary Chemistry XI. Ed. 2nd. Kathmandu: Vidyarthi Pustak Bhandar, 2069 (2012).