A carbocation is a molecule in which a carbon atom has a positive charge and three bonds. We can say that, basically, they are carbon cations. Formerly, it was known as carbonium ion. Carbocation today is defined as any even-electron cation that possesses a significant positive charge on the carbon atom.
Table of Contents
Talking about some general characteristics, the carbon cations are very reactive and unstable due to an incomplete octet. In simple words, carbocations do not have eight electrons; therefore, they do not satisfy the octet rule.
In carbocation, the hybridization of carbon will be sp2 and its shape is trigonal planar. There is also a vacant p orbital which indicates its electron-deficient nature. The carbon has 6 electrons in its valence shell. Due to this, it is an electron-deficient species, also known as an electrophile.
A carbocation is generally observed in an SN1 reaction, elimination reaction, etc.
Classification of Carbocation
The different carbocations are named on the basis of the number of carbon groups bonded to the carbon. The carbocation can be termed as methyl, primary, secondary or tertiary on the basis of how many carbon atoms are attached to it:
Methyl carbocation: If no carbon is attached to the carbon with the positive charge, it is simply called methyl carbocation.
If one, two or three carbon are attached to the carbon with the positive charge, it is called primary carbocation, secondary carbocation, and tertiary carbonation, respectively.
If there is a carbon-carbon double bond next to the carbon with the positive charge, it is termed allylic carbocation.
In the same way, if the carbon with the positive charge is attached to a double bond, the carbocation is termed vinylic carbocation. Here, the hybridization of the carbon having the positive charge is sp, and geometry is linear.
Whenever the carbon, which consists of the positive charge, is part of a benzene ring, then the carbocation is an aryl carbocation.
If the carbon having a positive charge is immediately next to a benzene ring, it is termed a benzylic carbocation.
Interestingly, in addition to these types, there is another type of carbocation, which is known as pyramidal carbocation. In this type, the ions consist of a single carbon atom that usually tends to hover over a four or five-sided polygon, which can be depicted as a pyramid. The 4 sided pyramidal ion will consist of a +1 charge, while the five-sided pyramid will have a +2 charge.
Formation of the Carbocation
The carbocations can be formed by either of the following two fundamental steps:
Cleavage of a bond of carbon
Electrophilic addition
Cleavage of Bond of Carbon
Whenever there is a cleavage of the bond of carbon and atoms attached to it, the leaving group takes away the shared electrons, thus leaving the carbon atom as electron deficient. As a result, a positive charge is developed, forming a carbocation. The more tendency of cleavage of bond or formation of a more stable carbonation, the lower the activation energy.
In many organic reactions, such as the SN1 and E1 reactions, carbocation is formed as a reaction intermediate.
Electrophilic Addition
In electrophilic addition, an electrophile attacks an unsaturated point (double or triple bond), which results in the breaking of the pi bond and in the formation of a carbocation. The more stable the carbocations, the lower the activation energy and the faster the addition. Electrophilic addition to a pi bond is illustrated by the reaction of HBr (an electrophile) with propene (CH3CH = CH2).
It can be noted that the formation of the secondary carbocation is favoured over the primary carbocation because secondary carbocation is more stabilized due to resonance. This is also in accordance with Markovnikov’s Rule. Such electrophilic addition reactions are generally seen in alkenes, alkynes and benzene rings.
We know that the carbocations are very reactive due to their electron deficiency, vacant orbital and incomplete octet. Therefore, its stability depends on the octet completion and reducing the electron deficiency.
The stability of a carbocation can be achieved by the following processes:
(a) Addition of a nucleophile
(b) Formation of a pi bond
(c) Rearrangement
Addition of a Nucleophile
A carbocation is electron-deficient and with an incomplete octet and a positive charge on it. The positive charge is stabilized by the addition of a nucleophile, thus the formation of a new covalent bond takes place. This stabilizes the carbocation. This is a very common process of stabilization of carbocation because the carbocation is very reactive, so even weak nucleophile gets attached to the carbocation.
Formation of a Pi Bond
The carbocation can receive electrons from nearby hydrogen to remove its positive charge and to complete its octet. Thus, a new pi bond can be formed. Generally, the hydrogen atom must be removed by any base. Due to the high reactivity of the carbocations, even weak bases such as water or iodide ion are able to facilitate the deprotonation. Whenever such deprotonation occurs, two types of products are formed. The more stable compound is the major product.
Rearrangement
The bonding electrons of a carbocation can be shifted between adjacent atoms so that a more stable carbocation can be formed. For instance, rearrangement will be highly favoured if there is a conversion of a secondary carbocation that can be formed from a primary carbocation. The reason is simple, the carbocation is more stabilized in secondary carbocation than in primary carbocation.
The different types of carbocation rearrangement are given below:
Hydride Shifting
Here, hydrogen is shifted from 1st carbon to 2nd carbon. So, the carbocation has changed from primary to secondary carbonation, thus forming a more stable structure.
Methyl shifting
Here, the methyl group shifts to the primary carbon to form a more stable structure. The carbocation is secondary carbocation, so more stable than primary carbocation.
Phenyl shifting
The entire phenyl group can also be shifted to obtain a more stable secondary or tertiary carbocation than a primary carbocation. It is also interesting to know that a phenyl shift is more favoured than a methyl shift.
Carbocation Stability
The stability order of carbocation is as follows:
The stability of carbocations depends on the following factors:
1. Resonance: The stability of carbocations increases with the increasing number of resonances. The more is the number of resonating structures, the more is the stability of the carbocation. The reason for this is the delocalization of the positive charge. The electron deficiency is decreased due to the delocalization, and thus it increases the stability.
When compared to substitution, the resonance effect proves to be a more dominating factor than substitution. Therefore, structures with resonance are far better stabilised than others. Cyclopropane carbocation is exceptionally very stable due to dancing resonance. Thus, tricyclo propane carbocation is the most stable carbocation.
2. Hyperconjugation and inductive effect: Increasing substitution increases the hyperconjugation, and thus it increases stability. The more is the hyperconjugation, the more is the stability.
R3C+ (3o ; most stable) > R2CH+ (2o ) > RCH2+ (1o) CH3+ (methyl; least stable)
The carbocation stability depends on the number of carbon groups attached to the carbon carrying the positive charge.
3. Electronegativity: Electronegativity indicates the capacity of an atom to attract electrons. The more is the electronegativity, the more is the attraction of the electrons towards the atom. Therefore, the electronegativity of the carbon with the positive directly affects the stability of the carbocation. So, as the electronegativity of the carbon atom increases, the stability of the carbocation decreases. sp > sp2 > sp3 (sp has maximum s character; so maximum electronegativity, sp3 has minimum s character; so minimum electronegativity).
The hybridisation of the carbon with the positive charge in the vinylic carbocation is sp, whose electronegativity is more than the sp2 hybridized carbon of the alkyl carbocation. Due to this reason, the stability of a primary vinylic carbocation is less than a primary alkyl carbocation.
In the same way, the lower stability of aryl carbocation in comparison to a secondary alkyl carbocation can be explained. Vinyl and aryl carbocations are very rare to find due to their low stability.
Solved Questions
1. Identify the most stable carbocation from the following.
Answer: b)
Explanation: It is more stable because it is a tertiary carbocation.
2. Consider the following two carbon in the following compounds.
Now, at which position will the incoming hydrogen be added?
Answer: Carbon at the primary position
Explanation: Hydrogen will be added to the primary position because it results in the formation of a more stable secondary carbocation.
3. Arrange the following carbocations in decreasing order of stability. I. CH3 —C+H—CH3 II. CH3 —C+H—OCH3 III. CH3 —C+H—CH2 —OCH3
(a) I > II > III (b) II > III > I (c) III > I > II (d) II > I > III
Answer: (d) II > I > III
Explanation: In the II compound, the lone pair of oxygen helps in the stabilization of the carbocation. Compound I has better hyperconjugation than compound III.
4. Which of the following carbocation is the most stable?
Explanation: If the positive charge on carbon is a part of aromaticity, then that carbocation is the most stable. This makes tropylium cation the most stable carbocation.
6. Arrange these carbocations on the basis of the increasing order of their stability.
I. C6H5C+H2 II. C6H5CH2C+H2 III. C6H5C+HCH3 IV. C6H5C+(CH3)2
(a) III < I < II < IV (b) II < III < I < IV (c) II < I < III < IV (d) IV < III < I < II
A carbocation is an organic reactive intermediate that is formed due to the incomplete valency of the carbon atom. The valency of carbon is four. Loss of one electron results in the formation of a carbocation. For example, CH3+ is a methyl carbocation.
Q2
Explain the stability of 3°, 2° and 1° carbocation and arrange them in decreasing order of their stability.
3° carbocation is the most stable carbocation compared to 2° and 1° carbocation. 3° carbocation is stabilized by the inductive effect of the alkyl groups. The order of stability of carbocations is 3°. > 2° > 1°. 1°. A carbocation is not attached to any alkyl groups, and thus is the least stable.
Q3
What is the geometry and bond angle of carbocation?
A carbocation is trigonal planar and has a bond angle of 120°.
Q4
The rate of which reaction mechanism depends on the formation of carbocation?
The rate of SN1 (Substitution Nucleophilic First Order) reaction depends on the formation and stability of carbocation.
Q5
Name one rearrangement reaction involving carbocation.
Wagner-Meerwein rearrangement is a rearrangement reaction in which a carbocation cation intermediate is formed.
Neeraj Anand, Param Anand
Er. Neeraj K.Anand is a freelance mentor and writer who specializes in Engineering & Science subjects. Neeraj Anand received a B.Tech degree in Electronics and Communication Engineering from N.I.T Warangal & M.Tech Post Graduation from IETE, New Delhi. He has over 30 years of teaching experience and serves as the Head of Department of ANAND CLASSES. He concentrated all his energy and experiences in academics and subsequently grew up as one of the best mentors in the country for students aspiring for success in competitive examinations.
In parallel, he started a Technical Publication "ANAND TECHNICAL PUBLISHERS" in 2002 and Educational Newspaper "NATIONAL EDUCATION NEWS" in 2014 at Jalandhar. Now he is a Director of leading publication "ANAND TECHNICAL PUBLISHERS", "ANAND CLASSES" and "NATIONAL EDUCATION NEWS".
He has published more than hundred books in the field of Physics, Mathematics, Computers and Information Technology. Besides this he has written many books to help students prepare for IIT-JEE and AIPMT entrance exams. He is an executive member of the IEEE (Institute of Electrical & Electronics Engineers. USA) and honorary member of many Indian scientific societies such as Institution of Electronics & Telecommunication Engineers, Aeronautical Society of India, Bioinformatics Institute of India, Institution of Engineers. He has got award from American Biographical Institute Board of International Research in the year 2005.
CBSE Class 11 Chemistry Syllabus is a vast which needs a clear understanding of the concepts and topics. Knowing CBSE Class 11 Chemistry syllabus helps students to understand the course structure of Chemistry.
Unit-wise CBSE Class 11 Syllabus for Chemistry
Below is a list of detailed information on each unit for Class 11 Students.
UNIT I – Some Basic Concepts of Chemistry
General Introduction: Importance and scope of Chemistry.
Nature of matter, laws of chemical combination, Dalton’s atomic theory: concept of elements, atoms and molecules.
Atomic and molecular masses, mole concept and molar mass, percentage composition, empirical and molecular formula, chemical reactions, stoichiometry and calculations based on stoichiometry.
UNIT II – Structure of Atom
Discovery of Electron, Proton and Neutron, atomic number, isotopes and isobars. Thomson’s model and its limitations. Rutherford’s model and its limitations, Bohr’s model and its limitations, concept of shells and subshells, dual nature of matter and light, de Broglie’s relationship, Heisenberg uncertainty principle, concept of orbitals, quantum numbers, shapes of s, p and d orbitals, rules for filling electrons in orbitals – Aufbau principle, Pauli’s exclusion principle and Hund’s rule, electronic configuration of atoms, stability of half-filled and completely filled orbitals.
UNIT III – Classification of Elements and Periodicity in Properties
Significance of classification, brief history of the development of periodic table, modern periodic law and the present form of periodic table, periodic trends in properties of elements -atomic radii, ionic radii, inert gas radii, Ionization enthalpy, electron gain enthalpy, electronegativity, valency. Nomenclature of elements with atomic number greater than 100.
UNIT IV – Chemical Bonding and Molecular Structure
Valence electrons, ionic bond, covalent bond, bond parameters, Lewis structure, polar character of covalent bond, covalent character of ionic bond, valence bond theory, resonance, geometry of covalent molecules, VSEPR theory, concept of hybridization, involving s, p and d orbitals and shapes of some simple molecules, molecular orbital theory of homonuclear diatomic molecules(qualitative idea only), Hydrogen bond.
UNIT V – Chemical Thermodynamics
Concepts of System and types of systems, surroundings, work, heat, energy, extensive and intensive properties, state functions. First law of thermodynamics – internal energy and enthalpy, measurement of U and H, Hess’s law of constant heat summation, enthalpy of bond dissociation, combustion, formation, atomization, sublimation, phase transition, ionization, solution and dilution. Second law of Thermodynamics (brief introduction) Introduction of entropy as a state function, Gibb’s energy change for spontaneous and nonspontaneous processes. Third law of thermodynamics (brief introduction).
UNIT VI – Equilibrium
Equilibrium in physical and chemical processes, dynamic nature of equilibrium, law of mass action, equilibrium constant, factors affecting equilibrium – Le Chatelier’s principle, ionic equilibrium- ionization of acids and bases, strong and weak electrolytes, degree of ionization, ionization of poly basic acids, acid strength, concept of pH, hydrolysis of salts (elementary idea), buffer solution, Henderson Equation, solubility product, common ion effect (with illustrative examples).
UNIT VII – Redox Reactions
Concept of oxidation and reduction, redox reactions, oxidation number, balancing redox reactions, in terms of loss and gain of electrons and change in oxidation number, applications of redox reactions.
UNIT VIII – Organic Chemistry: Some basic Principles and Techniques
General introduction, classification and IUPAC nomenclature of organic compounds. Electronic displacements in a covalent bond: inductive effect, electromeric effect, resonance and hyper conjugation. Homolytic and heterolytic fission of a covalent bond: free radicals, carbocations, carbanions, electrophiles and nucleophiles, types of organic reactions.
UNIT IX – Hydrocarbons
Classification of Hydrocarbons Aliphatic Hydrocarbons: Alkanes – Nomenclature, isomerism, conformation (ethane only), physical properties, chemical reactions. Alkenes – Nomenclature, structure of double bond (ethene), geometrical isomerism, physical properties, methods of preparation, chemical reactions: addition of hydrogen, halogen, water, hydrogen halides (Markovnikov’s addition and peroxide effect), ozonolysis, oxidation, mechanism of electrophilic addition. Alkynes – Nomenclature, structure of triple bond (ethyne), physical properties, methods of preparation, chemical reactions: acidic character of alkynes, addition reaction of – hydrogen, halogens, hydrogen halides and water.
Aromatic Hydrocarbons:
Introduction, IUPAC nomenclature, benzene: resonance, aromaticity, chemical properties: mechanism of electrophilic substitution. Nitration, sulphonation, halogenation, Friedel Craft’s alkylation and acylation, directive influence of functional group in monosubstituted benzene. Carcinogenicity and toxicity.
To know the CBSE Syllabus for all the classes from 1 to 12, visit the Syllabus page of CBSE. Meanwhile, to get the Practical Syllabus of Class 11 Chemistry, read on to find out more about the syllabus and related information in this page.
CBSE Class 11 Chemistry Practical Syllabus with Marking Scheme
In Chemistry subject, practical also plays a vital role in improving their academic scores in the subject. The overall weightage of Chemistry practical mentioned in the CBSE Class 11 Chemistry syllabus is 30 marks. So, students must try their best to score well in practicals along with theory. It will help in increasing their overall academic score.
CBSE Class 11 Chemistry Practical Syllabus
The experiments will be conducted under the supervision of subject teacher. CBSE Chemistry Practicals is for 30 marks. This contribute to the overall practical marks for the subject.
The table below consists of evaluation scheme of practical exams.
Evaluation Scheme
Marks
Volumetric Analysis
08
Salt Analysis
08
Content Based Experiment
06
Project Work
04
Class record and viva
04
Total
30
CBSE Syllabus for Class 11 Chemistry Practical
Micro-chemical methods are available for several of the practical experiments. Wherever possible such techniques should be used.
A. Basic Laboratory Techniques 1. Cutting glass tube and glass rod 2. Bending a glass tube 3. Drawing out a glass jet 4. Boring a cork
B. Characterization and Purification of Chemical Substances 1. Determination of melting point of an organic compound. 2. Determination of boiling point of an organic compound. 3. Crystallization of impure sample of any one of the following: Alum, Copper Sulphate, Benzoic Acid.
C. Experiments based on pH
1. Any one of the following experiments:
Determination of pH of some solutions obtained from fruit juices, solution of known and varied concentrations of acids, bases and salts using pH paper or universal indicator.
Comparing the pH of solutions of strong and weak acids of same concentration.
Study the pH change in the titration of a strong base using universal indicator.
2. Study the pH change by common-ion in case of weak acids and weak bases.
D. Chemical Equilibrium One of the following experiments:
1. Study the shift in equilibrium between ferric ions and thiocyanate ions by increasing/decreasing the concentration of either of the ions. 2. Study the shift in equilibrium between [Co(H2O)6] 2+ and chloride ions by changing the concentration of either of the ions.
E. Quantitative Estimation i. Using a mechanical balance/electronic balance. ii. Preparation of standard solution of Oxalic acid. iii. Determination of strength of a given solution of Sodium hydroxide by titrating it against standard solution of Oxalic acid. iv. Preparation of standard solution of Sodium carbonate. v. Determination of strength of a given solution of hydrochloric acid by titrating it against standard Sodium Carbonatesolution.
F. Qualitative Analysis 1) Determination of one anion and one cation in a given salt Cations‐ Pb2+, Cu2+, As3+, Al3+, Fe3+, Mn2+, Ni2+, Zn2+, Co2+, Ca2+, Sr2+, Ba2+, Mg2+, NH4+ Anions – (CO3)2‐ , S2‐, NO2‐ , SO32‐, SO2‐ , NO ‐ , Cl‐ , Br‐, I‐, PO43‐ , C2O2‐ ,CH3COO‐ (Note: Insoluble salts excluded)
2) Detection of ‐ Nitrogen, Sulphur, Chlorine in organic compounds.
G) PROJECTS Scientific investigations involving laboratory testing and collecting information from other sources.
A few suggested projects are as follows:
Checking the bacterial contamination in drinking water by testing sulphide ion
Study of the methods of purification of water.
Testing the hardness, presence of Iron, Fluoride, Chloride, etc., depending upon the regional variation in drinking water and study of causes of presence of these ions above permissible limit (if any).
Investigation of the foaming capacity of different washing soaps and the effect of addition of Sodium carbonate on it.
Study the acidity of different samples of tea leaves.
Determination of the rate of evaporation of different liquids Study the effect of acids and bases on the tensile strength of fibres.
Study of acidity of fruit and vegetable juices.
Note: Any other investigatory project, which involves about 10 periods of work, can be chosen with theapproval of the teacher.
Practical Examination for Visually Impaired Students of Class 11
Below is a list of practicals for the visually impaired students.
A. List of apparatus for identification for assessment in practicals (All experiments) Beaker, tripod stand, wire gauze, glass rod, funnel, filter paper, Bunsen burner, test tube, test tube stand, dropper, test tube holder, ignition tube, china dish, tongs, standard flask, pipette, burette, conical flask, clamp stand, dropper, wash bottle • Odour detection in qualitative analysis • Procedure/Setup of the apparatus
B. List of Experiments A. Characterization and Purification of Chemical Substances 1. Crystallization of an impure sample of any one of the following: copper sulphate, benzoic acid B. Experiments based on pH 1. Determination of pH of some solutions obtained from fruit juices, solutions of known and varied concentrations of acids, bases and salts using pH paper 2. Comparing the pH of solutions of strong and weak acids of same concentration.
C. Chemical Equilibrium 1. Study the shift in equilibrium between ferric ions and thiocyanate ions by increasing/decreasing the concentration of eitherions. 2. Study the shift in equilibrium between [Co(H2O)6]2+ and chloride ions by changing the concentration of either of the ions.
D. Quantitative estimation 1. Preparation of standard solution of oxalic acid. 2. Determination of molarity of a given solution of sodium hydroxide by titrating it against standard solution of oxalic acid.
E. Qualitative Analysis 1. Determination of one anion and one cation in a given salt 2. Cations – NH+4 Anions – (CO3)2-, S2-, (SO3)2-, Cl-, CH3COO- (Note: insoluble salts excluded) 3. Detection of Nitrogen in the given organic compound. 4. Detection of Halogen in the given organic compound.
Note: The above practicals may be carried out in an experiential manner rather than recording observations.
We hope students must have found this information on CBSE Syllabus useful for their studying Chemistry. Learn Maths & Science in interactive and fun loving ways with ANAND CLASSES (A School Of Competitions) App/Tablet.
Frequently Asked Questions on CBSE Class 11 Chemistry Syllabus
Q1
How many units are in the CBSE Class 11 Chemistry Syllabus?
There are 9 units in the CBSE Class 11 Chemistry Syllabus. Students can access various study materials for the chapters mentioned in this article for free at ANAND CLASSES (A School Of Competitions).
Q2
What is the total marks for practicals examination as per the CBSE Class 11 Chemistry Syllabus?
The total marks for the practicals as per the CBSE Class 11 Chemistry Syllabus is 30. It includes volumetric analysis, content-based experiment, salt analysis, class record, project work and viva.
Q3
Which chapter carries more weightage as per the CBSE Syllabus for Class 11 Chemistry?
The organic chemistry chapter carries more weightage as per the CBSE Syllabus for Class 11 Chemistry.
Anand Technical Publishers
Buy Products (Printed Books & eBooks) of Anand Classes published by Anand Technical Publishers, Visit at following link :
