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https://docs.google.com/document/d/1jjqrxC3qu-mX5jFhfzjKRU-efsUN09lC/edit?usp=sharing&ouid=109474854956598892099&rtpof=true&sd=true Some Basic Principles of Organic Chemistry Tetravalency of carbon; Shapes of simple molecules - hybridization (s and p); Classification of organic compounds based on functional groups: -C = C - , - C = C - and those containing halogens, oxygen, nitrogen and sulphur; Homologous series; Isomerism - structural and stereoisomerism. Nomenclature (Trivial and IUPAC) Covalent bond fission - Homolytic and heterolytic: free radicals, carbocations and carbanions; stability of carbocations and free radicals, electrophiles and nucleophiles. Electronic displacement in a covalent bond - Inductive effect, electromeric effect, resonance and hyperconjugation. C H A P T E R TETRAVALENCY OF CARBON Carbon contains four electrons in outermost shell, so need to make 4 bonds to get nearest noble gas configuration. Spectroscopic studies and properties of methane have revealed that the four C-H bonds in methane are identical and symmetrically disposed at an angle of 109° 28' to each other. This may be accounted for on the basis of Hybridization. i.e. Carbon (ground state) 1s2 2s2 2p 1 2p 1 2p 0 CHAPTER INCLUDES Classification of organic compounds Tetravalency of Carbon (excited state) x y z 1s 2s 2p carbon sp3 hybridization The sp3 hybrid orbitals of carbon overlap with four 1s atomic orbitals of four H atoms. IUPAC nomenclature 1s of H 1s of H H sp3 orbitals of C C 1s of H Isomerism Electronic Effect 1s of H H H H CLASSIFICATION ON THE BASIS OF STRUCTURE The organic compounds may be classified as under: ORGANIC COMPOUNDS Open Chain (Aliphatic) Cyclic Saturated Unsaturated Homocyclic Heterocyclic Alkanes (containing single bond) Alkenes (containing double bond) Alkynes (containing triple bond) Alicyclic Aromatic Benzenoid Non benzenoid Teaching Care Online Live Classes https://www.teachingcare.com/ +91-9811000616 Homologous Series A series of organic compounds, in which all the members can be represented by one general formula is termed as Homologous series and the members are termed as Homologues. Characteristics of Homologous series: All homologues can be represented by one general formula. Two consecutive homologues differ by 1-CH2 unit. Two consecutive homologues differ by 14 amu. All homologues can be prepared by some common methods. They have different physical properties but almost similar chemical properties. e.g., Alkanes formula (CnH2n+2) Methane Ethane Propane CH4 C2H6 C3H8 CH2 unit/14 amu Isomerism Compounds having same molecular formula but differing in their physical and chemical properties are called isomers. This phenomenon is called isomerism. They are classified into two types Structural isomerism Stereoisomerism Structural Isomerism: When same molecular formula represents two or more compounds which differ in the arrangements of atoms within the molecule, then such compounds are called structural isomers and the phenomenon is called structural isomerism. It is of the following types : Chain Isomerism: When the same molecular formula represents two or more compounds which differ in the nature of carbon chain (straight or branched), the isomers are called chain isomers and the phenomenon is known as chain isomerism. Position Isomerism: Compounds having same structural formulas but differing only in the positions of the substituent atom or group on the carbon chain are called position isomers e.g. C3 H7 OH has two position isomers CH3 CH2 CH2 –OH 1- propanol and CH3–CH–CH3 | OH 2-propanol Functional Isomerism : When any two compounds have same molecular formula but possess different functional groups, they are called functional isomers and the phenomenon is called functional isomerism. Functional isomerism is exhibited by Alcohol and ether Carboxylic acid and ester, etc. Metamerism : When the same molecular formula represents two or more compounds which differ in the nature of alkyl groups attached to the same functional group, then such compounds are called metamers. This phenomenon is called metamerism. (a) Metamers of C2H5–O–C2 H5 are C2H5–O–C2H5 Diethyl ether CH3CH2CH2–O CH3 Methyl propyl ether CH3–O–CH CH3 CH3 Methyl isopropyl ether Tautomerism : (Tauto = Same, Meris = Parts) : This is a special case of functional isomerism. Here the same molecular formula represents two or more compounds which exist in dynamic equilibrium with each other. Structural requirement for Tautomerism Compound should have electronegative atom bonded with multiple bond (i.e. N and O) Compound should have atleast 1 acidic hydrogen present on α – carbon of the molecule. Stereo Isomerism : The isomers which differ only in the orientation of atoms in space are know as stereoisomers. Before discussing stereoisomerism let us discuss two terms - Configuration and Conformation. The fixed relative spatial arrangement of atoms in molecule is known as configuration. A particular orientation (or arrangement) of atoms in a molecule differing from other possible orientation by rotation around single bonds is known as conformation. Configurational Isomerism Geometrical isomerism Optical isomerism Geometrical Isomerism Cis-trans isomerism : When similar groups are on the same side, it is cis and if same groups are on the opposite side it is trans isomer. COOH C = C COOH H COOH C = C H H cis isomer COOH H trans isomer CH3 CH3 Me H cis isomer trans isomer Syn - anti : This type of isomerism is exhibited by compounds containing > C = N– bond unit. e.g. Oximes C6H5 H C N OH syn - benzaldoxime H C6H5 C N OH Anti - benzaldoxime Optical Isomerism : When two or more compounds have the same molecular formula, same structural formula but different behaviour towards plane polarised light, then they are called optical isomers. Dextrorotatory (d-form) : Molecules which rotate the plane polarised light in clockwise direction are called dextrorotatory or d-form or (+) form. Laevorotatory (l-form) : Molecules which rotate the plane polarised light in anticlockwise direction are called laevorotatory or l-form or (–) form. Optically Active Molecules Molecules which can rotate plane polarised light are called optically active molecules. Optically active molecules do not have any centre (i) or plane of symmetry (may have axis of symmetry) and they are non-superimposible over their mirror image. Type of Molecules which can show Optical Activity Molecules having one chiral carbon atom i.e. carbon atom around which all the four groups are different will be optically active. Molecules having more than one chiral carbon atoms or no chiral carbon atom but having no plane or centre of symmetry are optically active. Optical Isomerism in Compounds Containing no Chiral Carbon Atom : Allenes : Allenes are the compound with the general formula C = C = C Allenes are found to be optically active when the two groups attached to each terminal carbon atom are different, i.e., a b C = C = C (I) a a or b b x C = C = C y (II) Alkylidenes : When one of the double bonds in allenes is replaced by one cycloalkyl ring, compound is known as alkylidene cycloalkane. Such compound will be optical active if two groups attached to each terminal carbon atom are different. a a x = C or = C b b y Biphenyls : Biphenyls show optical isomerism when the following two conditions are satisfied : Each ring should be unsymmetrically substituted, i.e., neither ring should have a plane of symmetry. B B A A B B B B B Both rings have no plane of symmetry (so can show optical activity) Both rings should be substituted at o-positions (minimum number of substituents should be two, one substituent in each ring). The two substituents must have a large size. A B A A A C B B D Enantiomers : Molecules which are non superimposible mirror image of each other are called enantiomers. Enantiomers have the same physical properties except their optical rotation which is equal in magnitude but in opposite direction. Enantiomers have the same chemical behaviour (rate of reaction) towards the molecules which are optically inactive, but they have different chemical behaviour (rate of reaction) towards the molecules which are optically active. Diastereomers : Stereoisomers which are not mirror image of each other are called diastereomers. Diastereomers have different physical as well as chemical properties. Number of Possible optical isomers in Compounds having n-chiral Carbon Atom : Case - I When there is no symmetry in the structure of molecule. Number of d and l forms = 2n Number of meso forms = 0 ∴ Total number of optical isomers = 2n Case - II When the molecule structurally symmetric and n is even Then the number of d and l forms = 2n – 1 The number of meso forms = 2n/2 – 1 Total number of optical isomers = 2n – 1 + 2n/2 – 1 Case - III When the molecule can be divided into two identical parts and n is odd then n − 1 Number of d and l forms = 2n – 1 – 2 2 n − 1 Number of meso forms = 2 2 ∴ Total number of optical isomers = 2n – 1 RULES FOR IUPAC NOMENCLATURE OF ORGANIC COMPOUNDS The longest carbon chain must be taken as the parent chain of the organic compound. In case of unsaturation the carbon of the multiple bond must get the lower number. In case of different alkyl substituents in the compounds their names must be written in alphabetical order. We must remember the general preference order along different entities present in an organic compound must follow the order Functional group > unsaturation > substituent groups > (Cl, Br, F, I, NO2, NO) (= > ≡) > alkyl side chains (CH3–, C2H5–) Among alkyl groups choice must be made on the basis of alphabetical order. Substituent groups are preferred as per alphabetical order also. Among unsaturation, if they are equally spaced then double bond is given preference over a triple bond, if unequal spacing then the firstly unsaturated carbon must receive lower number. e.g. CH2 = CH− CH2 − C ≡ CH Pent–1–en–4–yne 1 2 3 4 5 CH3 − CH = CH− CH2 − C ≡ CH Hex–4–en–1–yne 6 5 4 3 2 1 Among functional groups the preference must be as given below Functional Group Prefix Suffix 1. Sulphonic group Sulpho Suphonic 2. Carboxylic group Carboxy Oic acid Acid Halides Esters Halocarbonyl Alkoxycarbonyl (Carbalkoxy) Oyl halide Oate 5. Amides Carbamoyl or Carboxamido Amide 6. Cyanide Cyano Nitrile 7. Aldehydes Aldo or Formyl or oxo Al 8. Ketone Keto or oxo One 9. Alcohols Hydroxy Ol 10. Amines Amino Amine Naming of polyfunctional group compound : We must understand that in an organic compound with polyfunctional groups a principal group must be chosen as per the above preference order. The name of this group must be declared by using its suffix name so provided. All other functional groups except for the principal group must be declared by using their prefix name in alphabetical order. CH3–CH–CH=C–—CH–CH2–SOH3 | | | NH2 OH CONH2 5–amino–2–carbamoyl–3–hydroxyhex-3-enesulphonic acid BOND FISSION In any reaction, bond between the reactant molecules is broken and between product molecules is formed. Bond fission in organic molecules can take place in two ways. Homolytic Bond Fission C Cl ⎯⎯h⎯ν → C∙ + ∙Cl result of homolytic fission is free radical. (Alkyl free radical is sp2 hybridised) Heterolytic Bond Fission e– pair can move away from carbon C X C⊕ + X− This results in formation of carbocation. (Alkyl carbocation is sp2 hybridised) e– pair can move to carbon C X C− + X⊕ This results in formation of carbanion. (Alkyl carbanion is sp3 hybridised) ELECTROPHILE Electron deficient species or electron acceptor. It can be of two types Charged electrophile X⊕, R⊕ Neutral Electrophile: e.g., - BH3, CO2 NUCLEOPHILE Electron rich species or electron donor is nucleophile. It can be of three types. Charged Nucleophile - Neutral Nucleophile - H−,OH−, RO− N&H3 , R − O&H etc. Ambident Nucleophile - : C ≡ N: , both atoms are nucleophilic centre. ELECTRONIC EFFECTS Electronic Effects Inductive Hyperconjugation Electromeric Resonance Inductive Effect : The permanent displacement of electrons in a bond towards the more electronegative element is called inductive effect. The effect provides polarity to the molecule. The property of electron withdrawal shown by an atom or group is its (–I) effect and that of donation is called (+I) effect. δδδ +++ δδ+ δ+ δ– CH3 − CH2 − CH2 − CH2 − Cl + Order of –I effect : (CH3)3 N > –NO2 > –CN> –F > –COOH > –Cl > –Br > –I > –OH > –NH2 > –OR > C6H5– > –H Order of +I effect : (CH3)3C– > (CH3)2CH– > CH3–CH2– > –CH3 > –H Uses of Inductive Effect : Stability of ions: Stability of ions can be explained by using the concept of inductive effect Stability of Carbocation e.g. (CH3)3C⊕ > (CH3)2 CH⊕ > CH3CH ⊕ > CH ⊕ Stability of Carbanions (CH3 )3 CO < (CH3 )2 CHO < CH3CH2O < CH3O Acidic Properties : It is possible to compare the acidic strength of various organic compounds using the inductive effect concept. e.g., In carboxylic acid The strength of an acid depends upon the ease with which it can ionize to give proton or the stability of the conjugate bases formed, i.e. if the conjugate base formed is more stable, then the acid is more acidic. Name of Acid Formula Ka Conjugate Base Fluoroacetic acid FCH2 COOH 217 × 10–5 FCH2COO– Chloroacetic acid ClCH2 COOH 155 × 10–5 ClCH2–COO– Bromoacetic acid BrCH2COOH 138 × 10–5 BrCH2COO– Iodoacetic acid ICH2 COOH 75 × 10–5 ICH2COO– Acetic acid CH3COOH 1.8 × 10–5 CH2COO– Increasing stability of conjugate base increases the acidity of acids. Hence the acidic strength order is FCH2COOH > ClCH2COOH > BrCH2COOH > I CH2COOH > CH3COOH Furthermore, the inductive effect in di and trihalogenated acids is still more marked with the result they are progressively more acidic than the corresponding mono halogenated acids. e.g. CHCl2 – COOH Ka = 514 × 10–5 CCl3 – COOH Ka = 12100 × 10–5 In general, dicarboxylic acids are stronger acid than monocarboxylic acid, since one of the –COOH group acts as –I effect. Hyperconjugation : Is an effect involving delocalization of σ electrons in a molecule. The effect is also called No-bond resonance or Baker-Nathan effect. For example, ⊕ H H C CH = CH2 H H ° H H ⊕ C = CH – CH2 H H H H C = CH – CH2 H H H H C = CH – CH2 H ⊕ H C C° H H H C =CH2 H H C = CH2 H H C = CH2 H ° ⊕ H H H H ⊕ ⊕ H C CH2 H H C =CH2 H H C =CH2 H H C =CH2 H ⊕ Electromeric Effect : The temporary shifting of π electrons towards the more electronegative element in the presence of a reagent is electromeric effect. If reagent is an electrophile then the effect is (+E) and if a nucleophile then (–E) effect. Reagent ⊕ > C = O > C – Resonance Effect : The effect involves permanent delocalization of conjugated π electrons in a conjugated system. Intermediate structures formed are called resonating structure. The structure that collectiv ely represents all resonating structures is believed to be most stable and is called the resonance hybrid. During resonance in a molecule all bond lengths happen to be the same and lie in between those of a single and double bond. Resonating structures must have the same position of nuclei, same number of unpaired electrons. The one having the more number of covalent bond, negative charge on electronegative and positive charge on electropositive element with less charge separation found to be more stable then others. The difference between energies of most stable resonating structure and the resonance hybrid is called the resonance energy of the molecule. +M Effect ⎯⎯→ –M Effect ⎯⎯→ H C = O O– O– O– H C H C H C ⊕ ⊕ ⊕ REACTION INTERMEDIATES Common Reaction Intermediates Carbocation Carboanion Free Radical Carbocation : The stability of different carbocation by resonance. ⊕ ⊕ ⊕ ⊕ (C6H5 )3 C > (C6H5 )2 CH > C6H5 CH2 > CH2 − CH = CH2 The stability of different carbocation by inductive and hyper conjugation. CH3 CH3 C⊕ > H CH3 CH3 C⊕ > CH3 CH3 ⊕ ⊕ CH2 > CH3 Carbanion : The stability of different carbanions by resonance (C6H5)3C > (C6H5)2CH > C6H5 – CH2 > CH2 – CH = CH2 The stability of different carbanion by inductive effect. CH3 CH3 > CH3 – CH2 > CH3 – CH > CH3 – C CH3 CH3 Free Radicals Stability of different free radicals by resonance (C6H5)3C° > (C6H5)2C° H > C6H5 – C° H2 > C° H2 – CH = CH2 Stability of different free radicals by inductive effect and hyperconjugation. (CH3)3C° > (CH3)2C° H > CH3 – C° H2 > C° H3 ❑ ❑ ❑