In this notes, we shall learn about Types of Organic Reactions. They are common. We should certainly know about these reactions to make easy the study of organic chemistry. These reactions are generally used in the preparation and properties of the organic compounds. These reactions also boost my knowledge and grip over organic chemistry for the students who are preparing for the entrance exams like jee mains, IIT advance, NEET, NDA. In this simple and easy notes we have included all the important ideas about organic reactions. Hence you all are requested to read till end of this notes.
Types of Organic Reactions: Short and Easy Notes for Class 11th
In an organic reaction, the organic molecules also referred as substrate reacts with a suitable reagent. Which leads to the formation of one or more intermediate and finally products. The general reactions are depicted as follows:
What are substrate?
That reactant which supplies carbon to the new bond in a reaction is called substrate.
What are reagents?
The reactants other than substrate participate in a reaction are called reagents.
Types of reagents: There are two types of reagents
1. Nucleophiles. A reagent that brings an electron pair to the reactive site is called nucleophile. They are electron rich molecules or negative ions. For example NH3, H2O, OH–, CN– etc.
2. Electrophiles. A reagent that pull an electron pair from reactive site is called electrophile. They are electron deficient molecules or positive ions. For example BF3, AlCl3, CH3+, Cl+ etc.
Types of Intermediates in organic reactions:
During organic reactions, Intermediates are formed. They are following types:
1. Carbocation. A species having a carbon atom possessing sextet of electrons and a positive charge is called a carbocation. For example CH3+, CH3CH2+,
⇒ Carbocation has sp2 hybridised carbon. Its shape is planar and bond angle is 120o. It is diamagnetic in nature.
Stability of carbocations:
(C6H5)3C+ > (C6H5)2CH+ > (CH3)3C+ > C6H5CH2+ > (CH3)2CH+ > CH2=CH- C+H2 > R-C+ = CH2 > CH3CH2+ > R-CH=CH+ > C6H5+ > CH3+ > CH≡C+.
2. Carbanion. A species having a carbon atom possessing octet of electrons and a negative charge is called a carbanion. For example CH3–, CH3CH2–,
⇒ Carbanion has sp3 hybridised carbon. Its shape is pyramidal and bond angle is less than 109o. It is diamagnetic in nature.
Stability of carbanions: (C6H5)3C– > (C6H5)2CH– > C6H5CH2– > allylCH3– > 1o > 2o > 3o (R–)
3. Carbon free radical. During homolysis, neutral species are formed which contain an unpaired electrons in unhybridised orbitals are called carbon free radical. For example CH3., CH3CH2. etc.
Carbon free radical: it has sp2 hybridised carbon. Its shape is planar and bond angle is 120o. It is paramagnetic in nature.
Stability of carbon free radicals:
(C6H5)3C• > (C6H5)2CH• > C6H5CH2• > CH2=CH- C•H2 > (CH3)3C• > (CH3)2CH• > CH3CH2• > CH3• > CH2=CH2•.
4. Carbenes. These are the species containing divalent carbon having no charge and surrounded by sextet of electrons.
For example. :CH2, :CCl2 etc.
Types of carbenes. There are two types of carbenes:
1. Singlet carbene. In this carbene, the unshared electrons are paired and present in the same sp2- hybridised orbitals. It behaves as electrophile as well as Lewis acid.
2. Triplet carbene. In this carbene, the unshared electrons are not paired and are present in different unhybridised sp- orbitals.
Note. Singlet carbene is diamagnetic in nature, has planar shape and less stable than Triplet carbene. While Triplet carbene is paramagnetic in nature and has linear shape.
Types of Organic Reactions
Organic reactions can be classified into the following categories
- Substitution reaction
- Addition reaction
- Elimination reaction
- Rearrangement reaction
- Condensation reactions
- Isomerisation reactions
- Pericyclic reactions
- Polymerisation
Substitution reactions
That Types of Organic Reactions in which an atom or a group of atoms are replaced by another atom or group of atoms from an organic compound without any change in the remaining part of molecule are called substitution reactions. For example conversion of chloromethane into methanol.
Read more:-11th Organic Chemistry: Notes Some Basic Principles and Techniques
Types of substitution reactions
There are there types of substitution reactions.
1. Nucleophilic substitution reactions. In this reaction, weaker nucleophile is substituted by stronger nucleophile. These are typical of alkyl halides. For example conversion of chloroethane into ethanol.
2. Electrophilic substitution reactions. Those substitution reactions which are brought about by electrophiles are called electrophilic substitution reactions. These are typical of arenes and other aromatic compounds. For example conversion of nitraton, halogenation, sulponation and Friedel crafts reactions.
3. Free radical substitution reactions. Those substitution reactions which are brought about by free radicals are called Free radical substitution reactions. For example chlorination of alkanes in the presence of difused sunlight to give chloroalkane.
Addition reactions
That Types of Organic Reactions which involve combination between two reacting molecules to give a single molecule of the product are called addition reactions. These reactions are typical of compounds containing double or triple bonds. Depending upon the nature of attacking reagents, addition reactions are of the following three types:
1. Nucleophilic addition reactions. Those addition reactions which are brought about by nucleophiles are called nucleophilic addition reactions. These reactions are typical of aldehydes and ketones. For example base catalysed addition of HCN to aldehydes or ketones.
2. Electrophilic addition reactions. Those addition reactions which are brought about by electrophiles are called electrophilic addition reactions. These reactions are typical of alkenes and alkynes. For example, addition of Br2 or HBr to propylene.
3. Free radical addition reactions. Those addition reactions which are brought about by free radicals are called Free radical addition reactions. For example, addition of HBr to alkenes in presence of peroxide.
Elimination reactions
That Types of Organic Reactions which involve the loss of two atoms or groups of atoms from the same or adjacent atoms of a substance leading to the formation of a multiple bond either double or triple bond are called Elimination reactions. Depending upon the relative positions of the atoms or groups, they have been classified into three categories.
1. α- elimination reactions. In these reactions, the loss of the two atoms or groups occurs from the same atom of the substrate molecule. For example base catalysed dehydrohalogenation of Chloroform to form dichlorocarbene.
2. β- elimination reactions. In these reactions, the loss of the two atoms or groups occurs from the adjacent atoms of the substrate molecule. For example, acid catalysed dehydration of alcohols and base catalysed dehydrohalogenation of alkyl halides.
3. γ- elimination reactions. In these reactions loss of two atoms or groups occurs from α- and y- positions of the substrate molecule leading to the formation of three membered rings. For example, conversion of 1,3- Dichloropropane in presence of zinc dust into cyclo propane.
Condensation reactions
In this Types of Organic Reactions, two or more molecules of the same or different reactants combine to form a product with or without the elimination of simple molecules such as H2O, HCl, NH3 etc. For example, two molecules of ethanal condense in presence of dilute alkalies to form 3- hydroxybutanal.
This reaction in case of aldehydes and ketones is known as aldol condensation.
Rearrangement reactions
That Types of Organic Reactions which involve the migration of an atom or group from one atom to another within the same molecule are called Rearrangement reactions. For example, synthesis of urea from ammonium cyanate, Hoffmann bromamide reaction.
Isomerisation reactions
Those Types of Organic Reactions which involve interconversion of one isomer into another keeping the molecular formulae as well as the carbon skeletons of the reactants and the products intact are called Isomerisation reactions. For example, interconversion trans – but- 2- ene to cis- but -2- ene and vice versa in presence of difused light.
Pericyclic reactions
Those Types of Organic Reactions in which bond making and bond breaking take place simultaneously without any catalyst in the presence of heat or light and form cyclic compounds are called pericyclic reactions. For example formation of cyclobutane from two molecules of ethene by addition. This is also called cyclo addition reaction.
Polymerisation reactions
Those Types of Organic Reactions which involve the union of a large number of small molecules called monomers to form a molecule with high molecular masses are called polymerisation. These compounds are called polymers. For example Polythene is a polymer of monomer ethene.
Summary of the Types of Organic Reactions
In this notes of Types of Organic Reactions, we have included all the general organic reactions which are essential for study. These reactions are substitution reactions, addition reactions, elimination reactions, rearrangement reactions, condensation, polymerisation, pericyclic reactions, Isomerisation reactions. Besides these reactions, we have also discussed the Intermediates produced during the reactions like carbocations, carbanions, carbon free radicals and carbenes with their nature, stability and shapes. We hope that this notes of Types of Organic Reactions will be beneficial a lot of to you all then you all are requested to share among your friends and favourites.
FAQ in Types of Organic Reactions
Q.No 1. Why triplet carbene is more stable than singlet carbene ?
Ans:- This is because in Triplet carbene, the unshared electrons are present in different orbitals which feel less repulsion with respect to the unshared pair of electrons present in the same orbitals of singlet carbene.
Q.No 2. Why carbocations are considered as Lewis acid?
Ans:- Because they are electron deficient.
Q.No 3. Why carbon free radicals are paramagnetic in nature?
Ans:- This is because, they have unpaired electrons in their orbitals.
Q.No 4. What is reaction mechanism?
Ans:- The detailed step by step description of a chemical reaction is called reaction mechanism.