In an organic reaction, the organic molecule reacts with an appropriate attacking reagent and leads to the formation of one or more intermediate(s) and finally product(s)
. We name one reagent as substrate and other as reagent.
A molecule whose carbon is involved in new bond formation is called substrate and the other one is called reagent.
When carbon-carbon bond is formed, the choice of naming the reactants as substrate and reagent is arbitrary and depends on molecule under observation
A sequential account of each step, describing details of electron movement, energetics during bond cleavage and bond formation, and the rates of transformation of reactants into products (kinetics) is
referred to as reaction mechanism
Lets take a look at various concepts of Organic Reaction
Fission of a Covalent Bond
A covalent bond can undergo Fission in two ways:
(a) Heterolytic cleavage
In heterolytic cleavage, the bond breaks in such a fashion that the shared pair of electrons remains with one of the fragments.
The atom which is having more electron and lone pair is negatively charged. Another atom has a sextet electronic structure and is positively charged
Example
$CH_3 Br \to CH_3^+ + Br^-$
A species having a carbon atom possessing sextext of electrons and a positive charge is called a carbocation (earlier called carbonium ion)
The $CH_3^+$ in above reaction is called methyl cation and it is sp2 hybridized
We can have primary, secondary or tertiary and depending on whether one, two or three carbons are directly attached to the positively charged carbon
Just like we have carbon having sextet pair, we can have Carbon getting the lone pair and negatively charged . Such a carbon species carrying a negative charge on carbon atom is called
carbanion and they are generally sp3 hybridized
The organic reactions which proceed through heterolytic bond cleavage are called ionic or heteropolar or just polar reactions
Carbocations and carbanion are highly unstable and reactive species.
Alkyl groups directly attached to the positively charged carbon stabilise the carbocations due to inductive and hyperconjugation effects
The observed order of carbocation stability is:
$CH_3^+ < CH_3C^+H_2 < (CH_3)_2 C^+ H < (CH_3)_3 C^+ $
(b) Homolytic Fission
In Homolytic fission the bond breaks in such a fashion that the wherein each of the atoms acquires one of the bonding electrons.
This fission results in the formation of neutra species (atom or group) which contains an unpaired electron. These species are called free radicals.
Example
Alkyl radicals are classified as primary, secondary, or tertiary. Alkyl radical stability increases as we proceed from primary to tertiary
$CH_3^. < CH_3C^.H_2 < (CH_3)_2 C^. H < (CH_3)_3 C^. $
The Organic reactions, which proceed by homolytic fission are called free radical or homopolar or nonpolar reactions.
Electrophiles and Nucleophiles
We can defined attacking reagents into two categories
Electrophiles are electron-deficient atoms or molecules that seek out electrons to complete their valence shell. They are often positively charged or have a partial positive charge, making them attracted to electron-rich areas.
Nucleophiles are electron-rich species that have a pair of electrons to donate. They are attracted to positive or partially positive centers in molecules, where they can donate their electrons to form new chemical bonds.
During a polar organic reaction, a nucleophile attacks an electrophilic centre of the substrate which is that specific atom or part of the substrate which is electron deficient.
Similarly, the electrophiles attack at nucleophilic centre, which is the electron rich centre of the substrate
