& All Indian Univ.

CHE-05

ORGANIC

CHEMISTRY

CHE-05

For

Bachelor of Science [B.Sc.] By

Mishra Chandan Ajit

Useful For Delhi University (DU), IGNOU, Berhampur University (Odisha), University of Kashmir, Sambalpur University (Odisha), University of Kalyani (West Bengal), Gurukula Kangri Vishwavidyalaya (Uttarakhand), Himachal Pradesh University, Cooch Behar Panchanan Barma University (West Bengal), Ranchi University, University of Culcutta, Pune University, University of Mumbai, Andhra University, School of Open Learning (DU), Gondwana University (Maharashra), Babasaheb Bhimrao Ambedkar University (Lucknow), Dr. Babasaheb Ambedkar Marathwada University (Aurangabad), University of Madras, Netaji Subhas Open University (Kolkata), Odisha State Open University, all other Indian Universities.

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Preface

O

rganic chemistry is a highly creative science in which chemists create new molecules and explore the properties of existing compounds. However, organic chemistry is the study of the structure, properties, composition, reactions, and preparation of carbon-containing compounds, which include not only hydrocarbons but also compounds with any number of other elements, including hydrogen (most compounds contain at least one carbon–hydrogen bond), nitrogen, oxygen, halogens, phosphorus, silicon, and sulfur. This branch of chemistry was originally limited to compounds produced by living organisms but has been broadened to include human-made substances such as plastics. The range of application of organic compounds is enormous and also includes, but is not limited to, pharmaceuticals, petrochemicals, food, explosives, paints, and cosmetics. The organic chemistry course is a pivotal class taken by science students. In addition to the inherent importance of organic reactions in chemistry and biology, organic chemistry introduces and develops a type of reasoning and logic that is new to many students. Solid understanding of this subject is often critical to subsequent success in a science career. The goals of this GPH book ‘Organic chemistry (CHE-05)’ are to help students be successful in organising this vast amount of material, to stimulate their interest by making organic chemistry understandable and relevant, to demonstrate the logic and beauty of the field, and to provide a method to remember all of those many reactions. The book is written specially in question & answer format to provide students the instant gratification of a correct answer. Solved Practical Problems are also given for practice. In this book, I have tried to solve all possible questions from the exams’ point of view. Solutions of previous years question papers have also been included to help students to understand the unique examination structure. An attempt has been carefully made to present this book more useful and meet the requirement and challenges of the course prescribed by IGNOU University. I wish you a successful and rewarding career ahead. Feedback in this regard is solicited. – Mishra Chandan Ajit

Acknowledgement My compliments go to the GullyBaba Publishing House (P) Ltd., and its meticulous team who have been enthusiastically working towards the perfection of the book. Their teamwork, initiative and research have been very encouraging. Had it not been for their unflagging support, this work wouldn’t have been possible. The creative freedom provided by them along with their aim of presenting the best to the reader has been a major source of inspiration in this work. Hope that this book would be successful. – Mishra Chandan Ajit

Publisher’s Note The present book CHE-05 is targeted for examination purpose as well as enrichment. With the advent of technology and the Internet, there has been no dearth of information available to all; however, finding the relevant and qualitative information, which is focused, is an uphill task. We at GullyBaba Publishing House (P) Ltd., have taken this step to provide quality material which can accentuate in-depth knowledge about the subject. GPH books are a pioneer in the effort of providing unique and quality material to its readers. With our books, you are sure to attain success by making use of this powerful study material. Provided book is just a reference book based on the syllabus of particular University/Board. For a profound information, see the textbooks recommended by the University/Board. Our site gullybaba.com is a vital resource for your examination. The publisher wishes to acknowledge the significant contribution of the Team Members and our experts in bringing out this publication and highly thankful to Almighty God, without His blessings, this endeavor wouldn’t have been successful. – Publisher

Topics Covered Block-1

Fundamental Concepts

Unit-1 Unit-2 Unit-3 Unit-4 Unit-5

Bonding, Functional Group Classification and Nomenclature Stereochemistry – I Stereochemistry – II Effect of Molecular Architecture on Physical Properties Structure – Reactivity Relationships

Block-2

Basic Skeleton : Hydrocarbons and Heterocycles

Unit-6 Unit-7 Unit-8 Unit-9 Unit-10

Alkanes Alkenes Alkynes Aromatic Hydrocarbons and Polynuclear Aromatics Heterocyclic Compounds

Block-3

Derivatives of Hydrocarbons – I

Unit-11 Unit-12 Unit-13 Unit-14

Halogen Derivatives Alcohols and Phenols Ethers and Sulphur Analogues of Alcohols and Ethers Aldehydes and Ketones

Block-4

Derivatives of Hydrocarbons – II

Unit-15 Unit-16 Unit-17 Unit-18 Unit-19 Unit-20

Monocarboxylic and Sulphonic Acids Substituted Carboxylic Acids Functional Derivatives of Monocarboxylic Acids Nitro Compounds Amino Compounds and Diazonium Salts Natural Products

Contents Chapter-1 Chapter-2 Chapter-3 Chapter-4 Chapter-5 Chapter-6 Chapter-7 Chapter-8 Chapter-9 Chapter-10 Chapter-11 Chapter-12 Chapter-13 Chapter-14 Chapter-15 Chapter-16 Chapter-17 Chapter-18 Chapter-19 Chapter-20

Bonding, Functional Group Classification and Nomenclature .................................1-23 Stereochemistry – I ........................................................25-34 Stereochemistry – II ......................................................35-60 Effect of Molecular Architecture on Physical Properties .........................................................61-73 Structure – Reactivity Relationships .......................75-83 Alkanes .............................................................................85-95 Alkenes ..........................................................................97-110 Alkynes .........................................................................111-118 Aromatic Hydrocarbons and Polynuclear Aromatics .................................................................119-139 Heterocyclic Compounds ............................................141-151 Halogen Derivatives ....................................................153-170 Alcohols and Phenols ...............................................171-188 Ethers and Sulphur Analogues of Alcohols and Ethers ..............................................189-196 Aldehydes and Ketones ............................................197-210 Monocarboxylic and Sulphonic Acids .................211-220 Substituted Carboxylic Acids .................................221-234 Functional Derivatives of Monocarboxylic Acids .................................................................................235-246 Nitro Compounds .........................................................247-251 Amino Compounds and Diazonium Salts ...........253-263 Natural Products ........................................................265-277

Question Papers (1) June: 2013 (Solved).................................................................................281-285 (2) December: 2013 (Solved)........................................................................286-291 (3) June: 2014 (Solved)...............................................................................292-296 (4) December: 2014 (Solved)........................................................................297-299 (5) June: 2015 (Solved)...............................................................................300-304 (6) December: 2015..........................................................................................305-307 (7) June: 2016 (Solved)...............................................................................308-312 (8) December: 2016 (Solved).........................................................................313-320 (9) June: 2017 (Solved)...............................................................................321-328 (10) December: 2017 (Solved).......................................................................329-335 (11) June: 2018 (Solved)...............................................................................336-346 (12) December: 2018 ..................................................................................347-350 (13) June: 2019 (Solved)...............................................................................351-359 (14) December: 2019 (Solved)....................................................................360-368

r te p a Ch

1

Bonding, Functional Group Classification and Nomenclature

A N OVERVIEW

T

he joining of two or more atoms forms chemical compounds. A stable compound occurs when the total energy of the combination has lower energy than the separated atoms. The bound state implies a net attractive force between the atoms in a chemical bond. The two extreme cases of chemical bonds are covalent bonds and ionic bonds. The increasingly large number of organic compounds identified with each passing day, together with the fact that many of these compounds are isomers of other compounds, requires that a systematic nomenclature system be developed. Just as each distinct compound has a unique molecular structure, which can be designated by a structural formula, each compound must be given a characteristic and unique name. The IUPAC nomenclature system is a set of logical rules devised and used by organic chemists to circumvent problems caused by arbitrary nomenclature.

Organic Chemistry [CHE-05]

2 GPH Book

Q1. Define the terms covalent bond and covalent bonding. Explain the formation of a covalent bond by giving suitable example. Ans. A covalent bond is formed between two atoms by equal sharing of electrons. Whenever two atoms are held by a pair of shaired electrons a covalent bond exists between them. Therefore, a covalent bond is a chemical bond that involves the sharing of electron pairs between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding. For example, we consider the formation of nitrogen molecule. Electronic configuration of nitrogen (Z=7) is (1s2, 2s2, 2p3). It has only three electrons in its valence shell. Hence, three pairs of electrons are shared between two nitrogen atoms to complete their respective octets. Note: Electron is represented by a dot (.). Q2. Explain the different types of bond parameters in chemical bonding. Ans. Different bond parameters used in chemical bonding are as follows: (1) Bond Length: Bond length is defined as the average distance between the centers of the nuclei of the two bonded atoms in a molecule.

d

Fig. 1.1: Bond length

The bond length between the two covalently bonded atoms is equal to the sum of the covalent radii of the two atoms. Bond length is denoted by ‘d’. For example, bond length of HCl = rH + rCl

Factors affecting bond length: These factors are as follows: (i) Size of the atoms: Bond length increases with the increase in size of the atom. For example, bond lengths of H-X bond are in the order HI > HBr > HCl > HF (ii) Multiplicity of bond: As multiplicity increases bond length decreases because with the increase in multiplicity, atoms get closer to each other. Bond lengths of carboncarbon bonds are in the following order: C≡ C < C= C < C−C

3

Bonding, functional group classification and nomenclature

(2)

Bond Dissociation Energy and Bond Energy: The amount of energy required to break a particular bond is called its bond dissociation energy. It is expressed in kJ mol–1. The average value of bond dissociation energy for a particular bond is called bond energy. Thus, for methane, the bond dissociation energy for the first C—H bond is the energy change for the process: Δ H = + 432 kJ mol –1 CH4(g ) → CH3( g ) + H(g ) This bond dissociation energy is specific to this reaction, i.e. the energy for this C—H bond-breaking process depends on factors such as the other atoms attached to the carbon atom and the geometry of CH 4 and CH 3 . For example, if the remaining C— H bonds in methane are broken: CH3( g ) → CH2( g ) + H(g )

Δ H = + 470 kJ mol –1

CH2 (g ) → CH(g ) + H(g )

Δ H = + 416 kJ mol –1

CH(g ) → C( g ) + H(g )

Δ H = + 335 kJ mol –1

While bond dissociation energies are important in terms of the general understanding of physical chemistry, they are of little general use in predicting the stability of inorganic compounds, as they cannot be transferred between compounds. Bond enthalpy terms, on the other hand, are quantities assigned to each bond in a molecule such that the sum over all bonds is equal to the enthalpy change associated with the conversion of the molecule into separate atoms. Bond enthalpy terms are assumed to be constant, and therefore transferable from molecule to molecule. So for methane: CH4(g) → C(g ) + 4H( g ) Δ H = 1663 kJ mol –1 Therefore, the bond enthalpy term B(C—H) is –1 1 4 Δ H = 416 kJ mol , i.e. the average for the four C—H bonds in CH 4 . Note: If the molecule is diatomic, then bond dissociation energy is equal to the bond energy. Factors affecting Bond dissociation energy: Factors affecting bond energy are (i) Size of the atoms (ii) Multiplicity of bonds (iii) Number of lone pairs of electrons present. Order of bond energies of some bonds: • H — F > H — Cl > H — Br > H — I (Shorter the bond length, greater is the bond dissociation energy).

Organic Chemistry [CHE-05]

4 GPH Book

• C ≡ C < C = C > C — C (Greater the multiplicity, greater is the bond dissociation energy). • C — C > N — N > O — O > F— F (Greater the number of lone pairs of electrons, greater is the bond energy). • There is a definite trend in bond energy among successive members in a group. The heavier analogs generally show lower value of bond energy. Fluorine ( F2 ) is an exception because of high repulsion between non-bonding electrons. (iii) Bond Angle: The angle between the lines representing the directions of the bonds is called the bond angle. It is expressed in degree, minutes and seconds. For example, H – O – H bond angle in H 2 O is 104.5°. O

104.5° H

H

Fig. 1.2

(iv) Bond order: The number of bonds present between the two atoms is called bond order. For example, in H–H, bond order is 1, in O=O, bond order is 2 and in N ≡ N, bond order is 3. Q3. Define hybridisation. Explain the types of hybridisation. Ans. Hybridisation may be defined as the phenomenon of mixing of atomic orbitals of nearly equivalent energy, involving redistribution of energy, to form new orbitals of equal energy known as hybrid orbitals. The number of hybrid orbitals is equal to the number of the orbitals hybridised. The properties of the hybrid orbitals are in between the properties of the orbitals, which are hybridised. Types of Hybridisation: There are of three types of hibridisation: (1) sp3-Hybridisation: The process of mixing and recasting of one s-orbital and three p-orbitals of the same atom having nearly the same energy to form four equivalent sp3-hybrid orbitals of equal energy with maximum symmetry and definite orientation in space is called sp3-hybridisation. Thus, whenever carbon is bonded to four other atoms or groups, it uses sp3-hybrid orbitals. 109°28’ + 2s

+

+

2px 2py

2pz

Fig. 1.3: Formation of sp3-hybridised orbital

5

Bonding, functional group classification and nomenclature

The angle between the two orbitals is 109°28’. This hybridisation is also called tetrahedral hybridisation. For example, carbon in methane molecule, nitrogen atom in ammonia molecule, etc. are sp3-hybridised. Note: (a) Each of the sp3–hybrid orbitals has one – fourth (25%) s-character and three-fourth (75%) p-character. (b) These types of orbitals are formed only when carbon atoms form single bond with other atoms. (2) sp2 –Hybridisation: The process of mixing and recasting of one s– orbital and two p–orbitals of the same atom to form three equivalent sp2 hybrid orbitals of equal energy, maximum symmetry and definite orientation in space is called sp2–hybridisation. Thus, whenever carbon is bonded to three other atoms or groups, it always uses sp2-hybrid orbitals and a pz - orbital to form its bonds. 120° +

+

2s

2px 2py Fig. 1.4: Formation of sp2-hybridised orbital

Carbon atom in an ethene molecule is the example of sp2hybridisation. The angle between the two orbitals is 120°. This hybridisation is also called trigonal hybridisation. Note: (a) Each of the sp2 – hybrid orbitals has 33% s–character and 67% p– character. (b) These types of orbitals are formed only when carbon atoms form double bond with other atoms. (3) sp-Hybridisation: The process of mixing and recasting of one s-orbital and one p-orbital of the same atom to form two equivalent sp-hybrid orbitals of equal energy with maximum symmetry and definite orientation in space is called sp–hybridisation. Thus, whenever a carbon atom is bonded to two other atoms or groups, it always uses sp-hybrid orbitals and two 2p-unhybridised orbitals to form its bonds. 180° + s

p

sp hybrid orbital

sp hybridisation Fig. 1.5: Formation of sp-hybridised orbital

Organic Chemistry [CHE-05]

6 GPH Book

Carbon atom in acetylene is the example of sp-hybridisation. The angle between the two orbitals is 180°. This hybridisation is also called diagonal hybridisation. Note: (a) Each of sp-hybrid orbitals has 50% s–character and 50% p– character. (b) A carbon atom forming a triple bond with another atom is always sphybridised. Q4. Explain the formation of methane, ethene (ethylene) and ethyne (acetylene) molecules based on hybridisation of carbon. Ans. Formation of Methane: In methane, four hybrid orbitals of carbon overlap with 1s-orbital of each of the four hydrogen atoms. The bonds formed are sigma bonds. The bond angles in methane are also 109°28′ as the C—H bonds are also directed towards the corners of a tetrahedron with carbon at the centre. H

σ – bonds

C H

H σ – bonds

H Fig. 1.6: Bonding in methane

Formation of Ethene (Ethylene): In ethene molecule, each carbon atom is attached to three other atoms. Each carbon atom is sp 2 -hybridised. One sp 2 -hybrid orbital of one carbon atom overlaps coaxially with that of

another carbon atom to form a sigma bond. The remaining sp 2 -hybrid orbitals of two carbon atoms overlap with 1s-orbital of different hydrogen atoms and form four sigma bonds. The two carbon atoms and four hydrogen atoms lie in the same plane. The H—C—H bond angle is 120°. The unhybridized 2p z -orbitals of the two carbon atoms overlap laterally — C), to form a π -bond. The double bond between two carbon atoms (C—

thus, consists of one sigma and one π -bond.

7

Bonding, functional group classification and nomenclature

PZ

π – bonds

PZ

H

H σ – bond

σ – bonds

σ – bonds π – bonds

H

H

Fig. 1.7: Bonding in ethene

Formation of Ethyne (Acetylene): In ethyne, one sp-hybrid orbital of a carbon atom overlaps coaxially with that of another carbon atom to form a sigma bond. The remaining sp-hybrid orbitals of two carbon atoms overlap with 1s-orbital of two different hydrogen atoms to form two sigma bonds. The unhybridised p-orbitals of two carbon atoms overlap laterally to form two π -bonds. Thus, the triple bond between two carbon atoms (C — —C) — consists of one sigma and two π -bonds. Two carbon atoms and two hydrogen atoms of acetylene molecule lie in a straight line. pY

π – bonds

pY pZ

pZ sp

sp sp

H

C

sp H

C σ – bond

σ – bonds π – bonds Fig. 1.8: Bonding in ethyne

Q5. Indicate the type of hybridisation of each carbon atom in the following molecules. — CH 2 — (ii) H 2C — C— (i) CH3 —C—C—CH — 3

— (iii) CH3 —CH—CH—CH 3 1

2

3

(iv) CH 3 —CH 2 —CH 3

4

—C—C H 3 Ans. (i) C H 3 —C —

Carbon-1 is linked with four atoms = sp 3 -hybridisation Carbon-2 is linked with two atoms = sp-hybridisation Carbon-3 is linked with two atoms = sp-hybridisation Carbon-4 is linked with four atoms = sp 3 -hybridisation 1

2

3

— C— — C H2 (ii) H 2 C—

C1 = sp2 ,C 2 = sp and C 3 = sp2

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