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CLASS-XI, , Organic Chemistry –chapter12, , Some Basic Principles and Techniques Chemistry, Organic chemistry is the branch of chemistry that deals with the study of, hydrocarbons and their derivatives., The Shapes of Carbon Compounds:, In organic or carbon compounds, s and p orbitals are involved in hybridisation. This, leads to three types of hybridisation which are sp3(in alkanes) – Tetrahedral in shape, sp2(in alkenes) – Planar structure sp(in alkynes) – Linear molecule, , Functional Group: The functional group are atom or group of atoms joined in a, specific manner which determines the chemical properties of the organic compound., The examples are hydroxyl group (—OH), aldehyde group (—CHO) and carboxylic acid, group (—COOH) etc., • Homologous Series, A homologous series may be defined as a family of organic compounds having the, same functional group, similar chemical properties and the successive members differ
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from each other in molecular formula by —CH2 units., The members of a homologous series can be represented by same general molecular, formula., , • Nomenclature of Organic Compounds, Common name (Common system): Before the IUPAC system of nomenclature,, organic compounds were named after the sources of origin, for example, urea was so, named because it was obtained from the urine of mammals. Formic acid was so, named since it was extracted from red ants called formica., I UP AC (International Union of Pure and Applied Chemistry) System, IUPAC NAME TEMPLATE, According to IUPAC system, the name of an organic compound contains three parts: (i), word root, (ii) suffix, (iii) prefix.
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(i) Word root: Word root represents the number of carbon atoms present in the, principal chain, which is the longest possible chain of carbon atoms., , (ii) Suffix: Suffix are of two types, primary suffix, secondary suffix., (a) Primary Suffix: It indicates the type of bond in the carbon atoms., (b) Secondary Suffix: Secondar, , y suffix is used to, represent the functional group., (iii) Prefix: Prefix is a part of IUPAC name which appears before the word root. Prefix, are of two types:, a) Primary prefix: For example, primary prefix cyclo is used to differentiate cyclic, compounds., , (b) Secondary prefix: Some functional groups are considered as substituents and, denoted by secondary prefixes., For example:, Substituted Group, Secondary prefix., —F, Fluoro, — Cl, Chloro, — Br, Bromo, — NO, Nitroso, — NO2, Nitro, — CH3, Methyl, — OCH3, Methoxy, , RULES FOR NOMENCLATURE OF SIMPLE HYDROCARBONS :, Rule-1 :, Determine the longest carbon chain in the molecule (parent chain). Where a, double/triple bond is present, choose the chain which includes these bonds. If there is, a cyclic structure present, the longest chain starts and stops within the cyclic, structure.
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Rule-2 :, Assign numbers to each carbon of the parent chain. Numbering is done to identify the, parent alkane (compound root) and to locate the positions of the carbon atoms at, which branching takes place. The numbering is done in such a way that the branched, atoms get the lowest possible number., , Rule-3 :, Determine the compound root and the unsaturation index. Compound root, corresponds to the number of carbon atoms in parent chain. Add prefix cyclo if the, parent chain is cyclic, , Rule-4 :, Determine the correct name for each branch for example, alkyl groups such as methyl,, ethyl, etc. Attach the name of the branches alphabetically along with their positions to, the parent chain as prefix. Separate numbers from letters with hyphens
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Rule-5 :, When two or more branches are identical, use prefixes di-, tri-, tetra-, etc. Numbers, are separated with commas and prefixes are ignored while determining alphabetical, order, , 3, 3-Dimethyl-7-ethylnonane INCORRECT, 7-Ethyl-3, 3-dimethylnonane CORRECT, , FUNCTIONAL GROUPS:, , A functional group is an atom or a group of atoms which characterizes the chemical, reactivity of a molecule. The longest chain of atoms containing the functional group is, numbered in such a way that the carbon to which functional group is attached is, assigned the lowest number. In case of polyfunctional compounds, one of the, functional groups is chosen as the principal functional group and the compound is, named treating other functional groups as substituents, Naming of Compounds Containing Functional Groups: The longest chain of carbon, atoms containing the functional group is numbered in such a manner that the, functional group is attached at the carbon atoms possessing lowest possible number, in the chain., In case of polyfunctional compounds, one of the functional group is selected as, principal functional group and the compound is named on that basis. The choice of, principal functional group is made on the basis of order of preference.
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ISOMERISM:, When there are two or more compounds possessing the same molecular formula but, different structural formula and different physical and chemical properties, the, phenomenon is called isomerism. Such compounds are called isomers., It is of two types:, (1) Structural Isomerism, (2) Stereoisomerism
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(1) Structural Isomerism: Structural isomerism is shown by compounds having the, same molecular formula but different structural formulae differing in the arrangement, of atoms., (2) Stereoisomerism: When isomerism is caused by the different arrangements of, atoms or groups in space, the phenomenon is called stereoisomerism. The, steroeoisomers have same structural formula but differ in arrangement of atoms in, space. Stereoisomerism is of two types:, (i) Geometrical or Cis-Trans Isomerism, (ii) Optical Isomerism, , • Fundamental Concepts in Organic Reaction Mechanism, Fission of a covalent bond: A covalent bond can undergo Fission in two ways:, (i) By Homolytic Fission or Homolysis, ii) By Heterolytic Fission or Heterolysis, Homolytic Fission: In this process each of the atoms acquires one of the bonding, electrons., , Heterolytic Fission: In this process one of atoms aquires both of the bonding, electrons when the bond is broken., If B is more electronegative than A which thereby aquires both the bonding electrons, and becomes negatively charged., , The products of heterolytic fission are ions., Reaction Intermediates: Heterolytic and homolytic bond fission results in the, formation of short-lived fragments called reaction intermediates. Among the important, reaction intermediates are carbonium ions, carbanions, carbon free radicals and, carbenes. Carbonium Ions (carbocations): Organic ions which contain a positively, charged carbon atom are called carbonium ions or carbocations. They are formed by, heterolytic bond fission.
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where Z is more electronegative than carbon., Tertiary carbonium ion is more stable than a secondary, which in turn is more stable, than a primary because of +1 effect associated with alkyl group., , Carbanion: Organic ion which contains a negatively charged carbon atom are called, carbanions. They are also formed by heterolytic bond fission., , Where Z is less electronegative than carbon. A primary carbanion is more stable than, a secondary, which in turn is more stable than a tertiary, because of +1 effect, associated with alkyl group., , Electrophile: It is positively charged or neutral species which is electron deficient,, e.g.,, He–, H20+, CH3 , NH4+, AICl3 , S03 , CHCl2 , CCI3., Nucleophile: It is negatively charged or neutral species with lone pair of electrons e.g.,, (HO–), Cyanide (C = N), H20: R3N, R2NH etc., , Electron Displacement Effects in Covalent Bonds:, Electronic displacements in covalent bonds occurs due to the presence of an atom or, group of different electronegativity or under the influence of some outside attaching, group., These lead to a number of effects which are as follows:, (i) Inductive effect (ii) Elecromeric effect, (iii) Resonance or Mesomeric effect (iv) Hyperconjugation effect.
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1. Inductive Effect:, It involues c electron. The a electrons which form a covalent bond are seldom shared, equally between the two atoms. Due to different electronegatively electrons are, displaced towards the more electronegative atom., This introduces a certain degree of polarity in the bond., The more electronegative atom acquires a small negative charge (δ –). The less, electronegative atom acquires a small positive charge (δ+)., Consider the carbon-chlorine bond, As chlorine is more electronegative, it will become negatively charged with respect to, the carbon atom., , Structure I- indicates the relative charges on the two atoms., Structure II- indicates the direction in which the electrons are drawn., Atoms or groups which lose electrons towards a carbon atom are said to have a +1, effect. Those atoms or groups which draw electrons away from a carbon atom are said, to have a -I Effect., Some common atoms or groups which cause +I or -I effects are shown below:, , The inductive effect of C3 upon C2 is significantly less than the effect of the chlorine, atom on C., Resonance Structure: A number of organic compounds cannot be accurately, represented by one structure., For example, benzene is ordinarily represented as, , Carbon-carbon double bond length = 1.34 A, Carbon-carbon single bond length = 1.54A. But it has been determined experimentally, that all carbon-carbon bonds in benzene are identical and have same bond length, (1.39A)., Thus the structure of benzene cannot be represented by single structure. It can be
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represented equally well by ‘he energetically similar structures I and II. The two, structures are called resonance structures., , Actual structure of benzene is resonance hybrid of structures I and II., Another example of resonance is provided by nitromethane (CH3N02) which can be, represented by two Lewis structures., , The actual structure of nitromethane is a resonance hybrid of the two canonical forms, I and II. Resonance energy: The difference in the energy between the most stable, contributing structure for a compound and its resonance hybrid is called as resonance, energy or resonance stabilisation energy., , 2. Resonance Effect/ Mesomeric effect :, The polarity produced in the molecule by the interaction of two π-bonds or between a, π-bond and a lone pair of electrons present on an adjacent atom. There are two types, of resonance or mesomeric effects designated as R or M effect., Positive Resonance Effect (+R effect):, Those atoms which lose electrons towards a carbon atom are said to have a +M effect, or +R effect. For example:, , —Cl, —Br, —I, —NH2, —NR2, —OH, —OCH3, Negative Resonance Effect (-R effect): Those atoms or groups which draw electrons, away from a carbon atom are said to have a -M effect or -R effect., For example:
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3. Hypercojugation :, This way of electron release by assuming no bond character in the adjacent C—H, bond is called No-Bond Resonance or Hyperconjugation., , • Orbital Concept of Hyperconjugation, It involues delocalisation of o electrons of C—H bond of an alkyl group which is, attached directly to an atom of unsaturated system or to an atom with an unshared porbital., Let us consider CH3CH2 (ethyl cation) in which the positively charged carbon atom has, an empty p-orbital. One of the C—H bonds of the methyl group can align in the plane, of this empty p-orbital and electron constituting the C—H bond in plane with this porbital can then be delocalised into the empty p-orbital as in Fig.
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In general, greater the number of alkyl groups attached to a positively charged carbon, atom, the greater is the hyperconjugation., , 4. Electromeric Effect (E Effect):, The electromeric effect refers to the polarity produced in a multiple bonded compound, when it is attacked by a reagent when a double or a triple bond is exposed to an attack, by an electrophile E+ (a reagent) the two π electrons which from the π bond are, completely transferred to one atom or the other. The electromeric effect is represented, as:, , The curved arrow shows the displacement of the electron pair. The atom A has lost its, share in the electron pair and B has gained this share. Therefore A acquires a positive, charge and B a negative charge.
