Fundamental Concepts in Organic Reaction Mechanism: Class 11 Revision Notes

In pure Chemistry terms, an organic reaction involves a substrate (the organic molecule supplying carbon for new bonds) and an attacking reagent.  That leads to intermediates and ultimately products. Now, when we are looking into the detailed, step-by-step account of electron movement, energetics, and reaction rates, we are basically studying the reaction mechanism.

 

Organic reactions are not easy to understand unless you learn about the mechanisms. 

They help us see why a reaction happens in a certain way and what products we'll get. 

In essence, we need to see why does ethene $\left(C_2{H}_4\right)$ react with $B{r}_2$ to form $C_2{H}_{4}B{r}_2$ .

For exams such as JEE Main, you'll have to answer questions to identify the type of reaction, predict the product, or explain how a reaction occurs.

Once you are clear with the fundamental concepts of organic reaction mechanisms, you also understand concepts, including reactivity, stability of intermediates, electron displacement effects, and reaction conditions.

 

Let's break down the main ideas you need to know for JEE Main. 

1. Bond Breaking and Bond Formation

Reactions happen when bonds in reactants break and new bonds form to make products.

So you can see that covalent bonds can break in two primary ways, clubbed under the term of bond cleavage. 

Homolytic Cleavage: A bond breaks evenly, and each atom gets one electron, forming radicals (atoms with unpaired electrons). This is shown by 'half-headed' (fish hook) curved arrows.

For instance, 

• $Cl-Cl->Cl.+Cl.$

This happens in reactions involving sunlight or heat, such as the chlorination of methane (CH₄).

The resulting species are neutral free radicals containing unpaired electrons.

Alkyl radical stability increases: tertiary > secondary > primary > methyl. Reactions proceeding via homolytic cleavage are called free radical, homopolar, or nonpolar reactions.

Heterolytic Cleavage: A bond breaks unevenly, and one atom takes both electrons, forming ions. This is shown by full curved arrows. For example:

• $\mathrm{H}-\mathrm{C}\mathrm{l}->\mathrm{H}++\mathrm{C}\mathrm{l}-$

This results in a carbocation (carbon with a sextet of electrons and positive charge, e.g., CH₃⁺, sp² hybridised, trigonal planar shape) and an anion (e.g., Br⁻, or a carbanion if carbon gains the electron pair, sp³ hybridised, distorted tetrahedron).

Carbocation stability increases: tertiary > secondary > primary > methyl due to inductive and hyperconjugation effects. Reactions proceeding via heterolytic cleavage are called ionic, heteropolar, or polar reactions.

Tip For JEE Questions

Remember the arrow types. 

Full curved arrow (→) = electron pair movement (heterolytic), half-headed arrow (↗) = single electron movement (homolytic).

2. Types of Reagents

Reagents are the species that attack the molecule during a reaction. They decide how the reaction happens. The substrate is the reactant molecule that supplies carbon to the new bond formed. There are two main types of attacking reagents:

Nucleophiles (Nu:): These are "nucleus-loving" species that donate electrons. They're electron-rich and attack electron-deficient (electrophilic) centres.

Examples include:

• Negatively charged ions with lone pairs: OH⁻, CN⁻, R₃C:⁻
• Neutral molecules with lone pairs: H₂O:, RNH₂, NH₃

Electrophiles (E⁺): These species accept electrons. They're electron-deficient and attack electron-rich (nucleophilic) centres.

Some common examples are:

• Carbocations: CH₃⁺, CH₃CH₂⁺
• Neutral molecules with electron-deficient atoms: carbon in carbonyl groups >C=O, or carbon bonded to halogens R₃C-X

One example is the reaction of ethene (C₂H₄) with HBr, where H⁺ is the electrophile. So here, the double bond in ethene acts as a nucleophile.

If you are thinking of mastering this for JEE or your annuals, remember 

• Nucleophiles are electron donors.
• Electrophiles are electron acceptors.

3. Electron Movement (Curved Arrows)

In mechanisms, we use curved arrows to show how electrons move during a reaction. 

We use a curved arrow that starts from a pair of electrons. Note that this is a lone pair or a bond. That points to where those electrons are going. 

So, the arrow must always originate from a source of electrons and point to where those electrons are moving. That goes to make a new bond, that is, an atom.

Electron displacement effects explain how electron density shifts within molecules. It influences reactivity. 

Permanent Effects: Occurring in the ground state

• Inductive Effect: Polarisation of σ-bonds due to electronegativity differences. Electron-withdrawing groups (-I effect) like halogens, -NO₂, -CN pull electron density away. Electron-donating groups (+I effect) like alkyl groups push electron density toward the attached carbon. Effect decreases rapidly after about three bonds.
• Resonance Effect: Polarity due to interaction of π-bonds or π-bond with lone pairs in conjugated systems. +R groups (like -NH₂, -OH) donate electrons to the system, while -R groups (like -NO₂, -COOH) withdraw electrons.

Temporary Effects: Occurring only with attacking reagent

• Electromeric Effect: Complete transfer of π-electrons to one atom of a multiple bond when a reagent attacks.

+E effect: electrons move to where reagent attaches. 

-E effect: electrons move away from where reagent attaches.

For example, in the reaction of HBr with ethene (C₂H₄)

• $H_{2}C=C{H}_2+HBr->H_{3}C-C{H}_{2}Br$

The double bond's electrons attack H⁺. That forms a carbocation intermediate, and then Br⁻ attacks the carbocation.

This mechanism shows arrows from the double bond to H⁺ and from Br⁻ to the carbocation.

4. Reaction Intermediates

During a reaction, some short-lived species form before the final product. These are called intermediates.

Some concepts for exams that are also in previous year question papers of JEE Mains. 

Carbocations: Positively charged carbons, such as CH₃CH₂⁺, form in reactions, that is, the addition of HBr to alkenes. These are are sp² hybridised with trigonal planar geometry. 

We can look at their stability order (3° > 2° > 1° > methyl) with the below explanation. 

• Inductive effect: More alkyl groups donate electron density
• Hyperconjugation: Delocalization of σ-electrons from C-H bonds of adjacent alkyl groups into the empty p-orbital of the carbocation (also called "no bond resonance")

Carbanions: We have negatively charged carbons, including CH₃⁻. They're sp³ hybridised. These have a distorted tetrahedral geometry. And they form in reactions with strong bases.

Free Radicals: These have unpaired electrons, like Cl•. They form in radical reactions. Chlorination of methane is one example. The stability of the free radicals also follows this order: tertiary > secondary > primary > methyl.

Resonance Hybrids: The single Lewis structures cannot always represent many molecules. They exist as resonance hybrids of multiple contributing structures. More equivalent structures also means greater stability. So, the actual molecule is an average of all resonance structures. It's not the rapidly interconverting process between them.

Key Point to Remember

Intermediates decide the reaction pathway and products. Their relative stability determines which products form preferentially.

5. Types of Organic Reactions

We have a few types of organic reactions. These are based on how they happen. 

Substitution: One group replaces another. Example: CH₄ + Cl₂ → CH₃Cl + HCl (radical substitution via homolytic cleavage)

Addition: Two molecules combine. Example: C₂H₄ + Br₂ → C₂H₄Br₂ (electrophilic addition via heterolytic cleavage)

Elimination: A small molecule (like H₂O) is removed, forming a double bond. Example: C₂H₅OH → C₂H₄ + H₂O (dehydration)

Rearrangement: Atoms or groups rearrange within the molecule to form structural isomers.

 

For Class 11 chemistry exams, take note of these points. 

• Bond breaking: Homolytic (radicals, fish-hook arrows) vs heterolytic (ions, full arrows)
• Reagent types: Nucleophiles donate electrons, while electrophiles always accept electrons
• Electron effects: They are primarily inductive (-I/+I), resonance (-R/+R), electromeric (-E/+E), hyperconjugation
• Stability orders: Always choose the 3° > 2° > 1° > methyl order for carbocations and radicals
• Reaction types: They are substitution, addition, elimination, rearrangement
• Curved arrows: They must start from electrons and point to their destination
• Resonance: Molecules exist as hybrids, not interconverting structures