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When two or more molecules react to make a new compound. The compounds which interact or react are Reactant and the final product is known as Product. The Chemical Reaction can take place between atoms, ions or molecules.
The sequence of steps through which a reaction is supposed to take place is known as Reaction Mechanism.
There are two kinds of Reaction Mechanism:
- Concerted Mechanism – Breaking of bonds takes place simultaneously without formation of any intermediate.
- Non-concerted Mechanism – Mechanism which proceed through the formation of intermediates before the product formation.
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Key Terms: Organic compound, covalent bond, reactions, reactant, Stereoisomers, atoms, ions, molecules
Isomerism
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When the same molecular formula defines two or more compounds differing in their physical and chemical properties.
- Structural Isomers: These are the compounds having same molecular formula but different arrangement of atoms within the molecule.
- Stereoisomers: These are the compounds having same molecular formula but different spatial arrangement of atoms and groups.
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Electrons Displacements in Organic Compounds:
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- Inductive Effect – It is the permanent displacement of the shared pair of electrons in a carbon chain towards the more electronegative group.
- Electromeric Effect – It is the complete transfer of a shared pair of electrons of multiple bonds to one of the bonded atoms under the influence of attacking reagent.
- Resonance Effect – It is also known as Mesomeric Effect. It is the flow of electrons from one part of the conjugated π- system to the other caused by resonance.
Fission of a Covalent Bond:
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- Homolytic Bond Fission – This takes place when the shared pair of electrons is distributed equally between the bonded atoms.
- Heterolytic Bond Fission – This takes place if the shared pair of electrons is distributed unequally between the bonded atoms.
Types of Chemical Reactions:
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Chemical Reactions are accompanied by breaking and bonding of covalent bonds which involve of exchange of electrons. The functional groups of Organic Compound play a significant role in the process. Based on the above theory, reactions can be classified into five main groups:
- Rearrangement Reactions
- Substitution Reactions
- Addition Reactions
- Elimination Reactions
- Polymerization Reactions
Rearrangement Reactions – These are type of reactions in which products get formed simply by the rearrangement of atoms and electrons in the reactant molecules.
O
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NH4CNO → NH2 –C – NH2
Substitution Reactions – These are the reactions in which an atom or group of atoms is replaced by some other atom or group of atoms without any change in the structure of the remaining part of the molecule.
CH3Br + KOH (aqueous) → CH3OH + KBr
Electrophilic Substitution Reaction –Reactions in which atom or group of atoms is replaced by an electrophile such a s Nitration of Benzene.
C6H6 + HNO3 → C6H5NO2 + H2O
General Mechanism of Electrophilic Substitution Reaction are:
- Generation of an Electrophile
- Formation of Intermediate Carbocation
- Formation of a Substitution Product
Nucleophilic Substitution Reaction – The reactions in which an atom or group of atoms is replaced by nucleophile.
- SN1 Mechanism (Substitution Nucleophilic Unimolecular)
- SN2 Mechanism (Substitution Nucleophilic Bimolecular)
- Free Radical Substitution Reaction
Cl – Cl → 2CL*
CH4 + Cl * → CH3Cl + HCL
Addition Reactions – These are the reactions in which products get formed by the addition of some reagent to an unsaturated compound.
CH2 = CH2 + HCl → CH5Cl
- Electrophilic Addition Reactions
- Nucleophilic Addition Reactions
- Free Radical Addition Reactions
Elimination Reaction – The type of reactions in which the products get formed by the loss of simple molecule like HX from the reactant molecules.
C2H5OH → C2H4
- EN1 (Nucleophilic Elimination Unimolecular)
- EN2 (Nucleophilic Elimination Bimolecular)
Polymerisation Reactions – The union of two or more molecule of a substance to form a single molecule with higher molecular weight is known as Polymerisation Reaction.
n (CH = CH2) → (-CH2 – CH2 -) n
Condensation Reactions – These are the reactions in which different organic compounds unite to give the product with or without the elimination of another simple molecule.
O OH
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CH3 – C – H + CH3CHO → CH3 – CH – CH2CHO
OH
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CH3 – CH – CH2CHO → CH3CH = CH.CHO + H2O
Isomerisation Reactions – These are type of reactions involving interconversion of isomers where the molecular formulae and carbon skeletons of the reactant and product molecules remains the same.
Trans- But-2-ene → Cis – But -2 -ene
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Sample Questions
Ques 1. a. Out of (CH3)3C-Br and (CH3)3C-I, which one is more reactive towards SNI and why?
a) Write the product formed when p-nitro-chlorobenzene is heated with aqueous NaOH at 443 K followed by acidification.
b) Why dextro and laevorotatory isomers of Butan-2-ol are difficult to separate by fractional distillation? (2019)
Ans. a. When it comes to SNI reaction (CH3)3C-I tends to be more reactive than (CH3)3C-Br due to the fact that the C-I bond is weaker than the C-Br bond (because the size of the I- is bigger than the Br-) helping in easy formation of the tertiary carbocation.
- When we heat p-nitro chlorobenzene with NaOH at a 443 K and later on acidify it leads to the formation of 4-Nitrophenol .
- The Dextro and laevorotatory isomers of Butan-2-oI are each other’s stereoisomers and share similar physical properties. Due to their equal boiling points, it is very difficult to separate them through the process of fractional distillation.
Ques 2. Out of Chlorobenzene and Cyclohexyl chloride, which one is more reactive towards nucleophilic substitution reaction and why? (Outside Delhi 2019)
Ans. Cyclohexyl chloride tends to be more reactive incase of nucleophilic substitution reaction due to the reason being the carbo having the chlorine atom lacks in electrons and seeks for a nucleophile. In the case of Chlorobenzene the carbon that carries halogen belongs to the aromatic ring and is rich in electrons due to the presence of a large number of electrons in the ring.
Ques 3. Out of chlorobenzene and benzyl chloride, which one gets easily hydrolysed by aqueous NaOH and why? (2018)
Ans. The hydrolysis of Benzyl chloride is more easily done in comparison to chlorobenzene. In the above reactive situation hydrolysis is preceded by the nucleophilic substitution mechanism and the formation of the benzylic carbonium ion after dropping of the leaving group (-Cl) is better balanced (through the resonating of structures) therefore reacts with ease.
Ques 4. Following compounds are given to you:
2-Bromopentane, 2-Bromo-2-methyl butane, 1-Bromopentane
a) Write the compound which is most reactive towards SN2 reaction.
b) Write the compound which is optically active.
c) Write the compound which is most reactive towards β-elimination reaction. (2017)
Ans. a. 1-Bromopentane is the compound which is most reactive towards SN2 reaction since it follows the order 1º > 2º > 3º
- The compound which is optically active is 2-Bromopentane.
- The compound which is most reactive towards β-elimination reaction is 2-Bromo-2-methyl butane.
Ques 5. Write the structure of an isomer of compound C-H-Br which is most reactive towards SN1 reaction. (AI 2016)
Ans. 
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