CBSE Class 11 Chemistry Revision Notes Chapter 12
Class 11 Chemistry Revision Notes for Chapter 12 – Organic Chemistry – Some Basic Principles and Techniques
The Extramarks Class 11 Chemistry Chapter 12 Notes will increase students’ understanding of the chapter and the fundamental concepts of organic chemistry. Subject matter experts have drafted these concise and relevant notes according to the updated CBSE syllabus, to ensure that students perform exceptionally well in their examinations.
General Organic Chemistry
A “vital force,” according to Swedish chemist Berzelius, is what causes organic compounds to form. However, this theory was disproved in 1828 when F. Wohler, was able to synthesize urea, an organic compound from ammonium cyanate, an inorganic compound.
NH4CNO → NH2CONH2
Ammonium cyanate Urea
The ground-breaking development of acetic acid synthesised by Kolbe in 1845 and methane by Berthelot in 1856 conclusively established that organic compounds could be synthesized in a laboratory from inorganic compounds. Modern organic chemistry was shaped by the advancement of the electronic theory of covalent bonds.
The existence of carbon was shared by all the molecules created by others who then followed suit, creating organic compounds from inorganic ones. Carbon can build long chains and rings with atoms of other elements in the periodic table as well as with other carbon atoms because of a special feature called catenation.
Later, the detailed study of the fundamental concepts and factors governing the progression and outcome of such reactions came to be known as general organic chemistry.
2.1 Sigma and Pi bond
|PROPERTY||SIGMA BOND||PI BOND|
|Overlap||Axial/Head-on||Parallel or Lateral|
|Electron cloud||Along the internuclear axis||Perpendicular to the internuclear axis|
2.2 Structural Formulas
A variety of formulas are used by organic chemists to represent organic compounds.
2.3 Degrees of Carbon
It refers to how many carbons are bonded to the carbon under investigation.
Hybridisation is the process by which two or more atomic orbitals with similar energies combine to form new degenerate orbitals called hybrid orbitals in the valence shell of an atom (the core atom of a molecule or ion).
2.5 Applications of Hybridisation
The organic compounds that have two double bonds in them are called dienes. They are of three types: isolated, conjugated and cumulated.
2.7 Commonly Occurring Forms of Carbon
The commonly occurring forms of carbon are:
Breaking of Bonds:
Covalent bonds are important for the study of reactions in organic chemistry. As a result, it is important to know how a covalent bond can be broken by the following processes:
(a) Homolytic Fission
(b) Fission by Heterolysis
3.1 Homolytic Fission or Homolytic Cleavage
In Homolytic Fission or Homolytic Cleavage, each atom separates with one electron, resulting in the formation of highly reactive species known as radicals or free radicals. Bond breaking is represented by two half-headed or fish hook arrows. The movement of one electron is depicted by a half-headed arrow. The odd electron species that are neutral are radicals.
3.2 Heterolytic Fission or Homolytic Cleavage
In this type of covalent bond breaking, the shared pair of electrons are transferred to the more electronegative part. As a result, a cation and an anion are formed (ion-pair).
A full-headed arrow is used to signify such bond breaking. A full-headed arrow represents the motion of two electrons. In organic chemistry, electron movement is always depicted by curving arrows, either half-headed or full-headed.
The electron pair that creates the sigma bond when two dissimilar atoms make a covalent bond is never shared equally by the two atoms. It has a minor shift towards the more electronegative species.
There are three different kinds of groups or atoms that can be joined to carbon, as was previously stated. The bond between carbon and hydrogen is typically regarded as non-polar even though carbon has a greater electronegative than hydrogen.
4.1 Nature of Inductive Effect:
The inductive effect is directly proportional to the dipole moment and is a permanent effect. It is a minor effect as the electrons are shifted only via sigma bonds.
4.2 Effect of Branched Carbon Chain
A self-explanatory example of the operation of the inductive effect has been marked.
4.3 Electron Donating and Electron Withdrawing Groups
An inductive effect can be produced by a single atom or a group of atoms, and the relative inductive effect is determined with respect to hydrogen. Electron-donating groups (EDG) or electron-releasing groups (ERG) are those groups that are said to have a +I effect by giving electrons to the carbon chain. Electron-withdrawing groups (EWG) are those that cause the -I effect by removing electrons from the carbon chain.
4.4 Applications of Inductive Effect
- Effect on Acidic/Basic Strength – EWG decreases basic strength while raising acidic strength. ERG increases basic strength while reducing acidic strength.
- Effect of Distance – As the ERG/EWG moves away, the inductive effect decreases.
- Basicity of Amines – To find out the basic strength of amines in an aqueous solution, we must consider the solvation effect, the inductive effect, and the steric hindrance. The order of basic strength is thus experimental in the aqueous state as prioritising the stability provided by any single factor is not possible.
Although simple Lewis structures are frequently employed to depict molecules, some compounds cannot be represented by a single Lewis structure. The idea of resonance was developed as a result. Resonance is the delocalization of electrons, which are often pi electrons.
5.1 Conjugated Systems
A system of connected p-orbitals with delocalised electrons in compounds with alternating single and multiple bonds to increase stability and lower the overall energy of the molecule is known as a conjugated system. Lone pairs, radicals or carbenium ions are part of this system.
5.2 Rules for Validity of Lewis Structures
Rule 1: The Lewis octet rule applies to all Lewis structures.
Rule 2: The electrons are only in motion and the atoms in all resonant structures must be in the same location.
Rule 3: Bond pairs and lone pairs must always amount to the same number, meaning that all resonant structures must have the same number of paired and unpaired electrons.
Rule 4: Every atom in a molecule involved in resonance must be coplanar. Effective electron delocalization and p orbital overlap are needed, as seen in 3-diene, buta-1.
5.3 Criteria for Major/Minor Contributors
The criteria used to compare resonance forms are
- Since a neutral molecule is always more stable in which its octet is complete, use as many octets as you can.
- Try to form as many bonds as possible.
- Remember that the presence of a negative charge on a more electronegative atom is stable.
- The separation of charges:
(a) Similar charges – To reduce repulsion and instability, keep them as far away from each other as possible.
(b) Opposite charges must be kept as close together as possible to maximise attraction and stability.
The electron distribution in the unsaturated chain changes when a group is conjugated with a pi bond or a series of alternative bonds because permanent polarisation is transmitted through the system’s electrons. The redistribution of electrons in unsaturated compounds conjugated with electron-drawing or electron-releasing groups is known as the resonance effect or mesomeric effect (or atoms). The dipole moment in this effect suggests that it is a long-lasting effect.
6.1 Electron-Releasing and Electron-Withdrawing Groups
It refers to the groups that release or withdraw electrons via resonance.
6.2 Applications of Mesomeric Effect
- Effect of Carboxylic Acids and Phenols on Acidic Strength – The carboxylic acid’s resonant structure produces a charge-separated structure that is significantly less stable than the charge-delocalized carboxylate ion. As a result, the carboxylic acid is easily able to shed a proton and become a carboxylate ion. Similar to this, resonance in phenol speeds up ionisation by inducing charge separation, which creates the phenoxide ion, which is then stabilised by charge delocalization.
- Effect on Reactivity of Carboxylic Acid Derivatives – A typical nucleophilic reaction is represented through the symbol “:”
It becomes more difficult for a nucleophile to break a bond if the bond is stronger between C and Z and therefore, lower reactivity. Acyl Chloride > Acid Anhydride > Ester > Amide is the reactivity order of carboxylic acid derivatives towards nucleophilic acyl substitution.
Some Basic Concepts of Organic Chemistry Class 11 Notes
Students will benefit from having the Chapter 12 Chemistry Class 11 Notes to help them understand the chapter’s concepts. These notes are in-depth since they are written by specialists and NCERT-based.
A General Introduction to Organic Chemistry – Class 11 Chemistry Chapter 12 Notes
Being an important discipline of chemical science, Organic Chemistry is the branch that deals with the study of carbon compounds obtained from living organisms.
Nomenclature According to IUPAC Rules
Nomenclature of organic compounds is done following the rules laid down by the International Union of Pure and Applied Chemistry (IUPAC), which formulates rules for the naming of organic compounds.
Certain Important Concepts of Organic Chemistry
Functional Groups: An atom or a group of atoms that replaces hydrogen in an organic compound is known as a Functional Group.
Homologous Series: A sequence of organic compounds that have the same functional group and chemical properties and their member successively differ by – CH2 is known as a Homologous series.
Organic Compounds and Their Classification
Organic compounds can be classified into the following types:
It is the existence or possibility of isomers. Isomers do not always share the same chemical or physical properties. Isomerism can be further divided into the following two types: Structural Isomerism and Stereoisomerism.
They are the chemical species that bring about chemical reactions. Attacking Reagents can be further divided into the following two types: Electrophiles and Nucleophiles.
Reaction Intermediates of Class 11 Chapter 12 Chemistry Notes
Reaction Intermediates include Carbocations, Carbanions and Free Radicals.
Electron Displacement Effects in Covalent Bonds As Given in Chapter 12 Chemistry Class 11 Notes
Electron Displacement occurs in an organic compound either in the presence of a suitable attacking reagent or under the influence of an atom or substituent group in the ground state.
Methods of Purification of Organic Compounds as Mentioned in Some Basic Concepts of Organic Chemistry Class 11 Notes
Simple Crystallisation: It is the process in which a solvent is selected for the preparation of a solution. One of the elements crystallises out in the form of a compound in such a solvent.
Sublimation: It is the process used for purifying relatively volatile organic solids.
Simple Distillation: It is the process by which a liquid is first transformed into a vapour, which is then moved to another location and recovered by condensation.
Fractional Distillation: It is the process used for the purification of two substances having a lesser difference between their boiling points.
Chromatography: A mixture is passed through a solution or suspension in this process. It has different rates of moving and various types of chromatography like paper chromatography, column chromatography etc.
Qualitative Analysis in Class 11 Chemistry Chapter 12 Notes
There are many methods and tests for qualitative analysis of organic compounds. The qualitative analysis of organic compounds helps in finding out the elements present in organic compounds apart from carbon and hydrogen.
Quantitative Analysis in Chapter 12 Chemistry Class 11 Notes
The process by which the relative abundance, mass, or concentration of every single component is found is known as the quantitative analysis of organic compounds. It is important for the proper study of organic compounds.
FAQs (Frequently Asked Questions)
1. Why are organic compounds fused with sodium for testing sulphur, nitrogen and halogens?
When organic compounds are fused with sodium, the elements present in them get converted into sodium salts. Such salts are soluble in water, can be filtered, and detected using tests. This makes the testing of sulphur, nitrogen and halogens easier.
2. What does the conjugated system mean in Class 11 Chemistry Chapter 2?
A system of connected p-orbitals with delocalised electrons in compounds with alternating single and multiple bonds to increase stability and lower the overall energy of the molecule is known as a conjugated system. Lone pairs, radicals, or carbenium ions are part of this system.
3. What is the Mesomeric Effect?
When a group is conjugated to a pi bond or a series of alternative bonds, the system’s electrons become permanently polarised, changing the electron distribution in the unsaturated chain. When an unsaturated chemical compound has groups that can draw or release electrons, the Resonance Effect or Mesomeric Effect occurs (or atoms).