13.7 Preliminary Idea of Reaction Mechanism
🎯 Learning outcomes
After the lesson, the students will be able to:
- Give a preliminary idea of reaction mechanism (homolytic and heterolytic fission, electrophiles, nucleophiles, and free radicals, inductive effect: +I and −I effect, resonance effect: +R and −R effect, steric hindrance).
Organic reactions are generally complex and occur in multiple steps. Therefore, a detailed study of each step is required to understand a chemical reaction. The reaction mechanism is a thorough study of each step of a reaction, with special attention to the reactants, products, intermediates, energy changes involved, and rates of each step.
Some fundamental knowledge necessary to study the reaction mechanisms is discussed here.
During a chemical reaction, existing chemical bonds break and new bonds form. Organic compounds have covalent bonds. Hence, organic reactions involve the breaking of existing bonds and the formation of new covalent bonds. A covalent bond undergoes cleavage in either of the following two ways.
1. Homolysis (Homolytic bond fission / Symmetrical or non-polar bond cleavage)
It is the breaking of bonds with an equal distribution of bonding electrons among bonding atoms. It generally takes place when bonding elements have nearly equal electronegativities. Typically, it takes place in the gas phase in the presence of heat or sunlight. e.g.,
This leads to the formation of free radicals.
2. Heterolysis (Heterolytic bond fission / Unsymmetrical or polar bond fission)
It is the breaking of bonds where one of the bonding atoms takes both the electrons, and the other gets none. It generally takes place when bonding elements have a large difference in electronegativities. It generally takes place in polar solvents.
Heterolysis leads to the formation of cations and anions.
Generally, reactants of organic reactions involve two species: the main organic molecule called the substrate and a species that attacks the substrate, called the reagent or attacking species.
Nucleophile (Greek: Nucleo = Nucleus; phile = loving)
Atom or groups of atoms that are electron-rich species that attack electron-deficient centres.
Nucleophiles can be negatively charged ions like OH⁻, CN⁻, I⁻, OR⁻ or neutral molecules with lone pairs of electrons like H₂O, NH₃, ROH, ROR, RNH₂ etc.
Electrophile (Greek: Electro = Electron; phile = loving)
Atoms or group of atoms that are electron-deficient species that attack electron-rich centres.
Electrophiles may be positively charged ions like H⁺, Cl⁺, Br⁺, NO₂⁺, R⁺ etc. or neutral electron deficient species like BF₃, AlCl₃, FeCl₃, CO₂, SO₃ etc.
A free radical is an atom or group of atoms with an odd or unpaired electron. These are short-lived and very reactive reaction intermediates with no net charge. The extreme reactivity of the free radicals is due to the tendency of the unpaired electron to get paired. These are a result of homolysis. e.g.
The relative stabilities of different degrees of alkyl free radicals are:
When two atoms having different electronegativities are linked together by a covalent bond, the electron pair shifts towards the more electronegative atom resulting in a certain degree of polarity in the bond. When a carbon atom is bonded to a hydrogen atom or another carbon atom by a covalent bond as in alkane, the sharing of electron pairs is symmetrical between them, and the bond is non-polar.
Thus, the polarity produced in a molecule due to the higher electronegativity of one atom compared to another is called an inductive effect. This type of polarity migrates in the carbon chain. It is indicated as arrows in the direction of electron displacement. This type of polarity migrates in the carbon chain. It is relayed up to the third or fourth carbon atom and fades afterwards.
Depending on the nature of groups attached to a C chain, the following two inductive effects are observed.
[A] Positive Inductive effect (+I effect):
If the group attached to C-atom pulls the bonded electrons to a lesser extent than an H atom, the C atom gets a partial negative charge; the group gets a partial positive charge. Such an inductive effect is called a positive inductive effect. Such groups are called electron-releasing groups. E.g., -CH₃, -COO⁻, -CH₂R, -CHR₂, -CR₃ etc.
The + I effect of the different alkyl groups is in the following order.
[B] Negative Inductive effect (-I effect)
If a group attached to the carbon atom pulls the electrons to a more extent than an H atom, the bond pair of electrons are displaced towards the group. Such groups get a partial negative charge; the carbon atom gets a partial positive charge. Such an inductive effect is called a negative inductive effect. Such groups are called electron-withdrawing groups. e.g., -NO₂, -CN, -COOH etc.
These groups are arranged in the increasing order of their −I effect as:
The phenomenon where a molecule or ion couldn’t be represented by a single structure but rather should be represented by more than one structure to justify its properties is called resonance.
Such structures are called resonance structures and the actual structure is considered a resonance hybrid.
The increase or decrease in electron density in some parts of a molecule due to the existence of resonance is called the resonance effect. The resonance effect can also be positive or negative.
−R effect of aryl group (phenyl group) in Chlorobenzene:
−R effect of aryl group (phenyl group) in Phenol:
+R effect of aryl (phenyl) group in nitrobenzene
Test your understanding
Take a short quiz on the preliminary idea of reaction mechanism.