More from Mr. S. B. KAMBLE

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at aul iv Vv, , , 24 HOMOLYTIC AND HETEROLYTIC BOND BREAKING (FISSION |, " - Comsider 2 covalent bond between atoms A and B., A:-B or A—B, The cleavaze (or breaking) of this bond can take place in three possible ways depending up, , the relative clectronegativities of A and B., ()A:B —— Ae- Be (Ae and Be of equal electronegativity), , (A-B + A + B (A more electronegative than B), , (ajA:-Bo— A + -B(B more electronegative than A), , The first type of cleavage is called homolytic fission or homolysis and leads to the formation &, very reactive species called ‘free radicals’ (atoms or groups of atoms containing odd or unpai™, electrons)., , In homolytic fission the covalent bond breaks in such a way that each carries o*, - fission d way fragment, , _ The second and third types of cleavage is called heterolytic fission and leads to the format**, Species carrying positive charge on a carbon atom are called carbonium ions or carbocalio®, Anionic species carrying negative charge on carbon atom are called carbanions. o, , In heterolytic fission the covalent bond breaks in such a way that the pair of electrons sia5

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wr e eee, , “TYPES OF ORGANIC REACTIONS, , ganic reactions may be classified into four main types, , a) Substitution reactions, , (b) Addition reactions, , (0 Elimination reactions, , @ Rearrangements., , These are separately described as under., , ) Substitution reactions. A substitution reaction is one in which a part of one molecule is, , “ed by other atom or group without causing a change in the rest of the molecule Following, gre some examples of substitution reactions., , () CH, +Cl, —+>CH,CI + HCI, , Methane Methyl!, Me chloride, , (ii) CH,Br + KOH —— C,H,OH + KBr, , cl, , i) +c, —%, + HCl, , Benzene Chlorobenzene, , The substitution reactions may be brought about by free-radicals, nucleophilic or electrophilic, reagents. Thus, there are free radical substitution reactions, nucleophilic substitution reactions and, electrophilic substitution reactions., , _4b) Addition reactions. When two molecules of same or different substances combine together, giving rising to a new product, it is an addition reaction. Examples of addition reactions are, , CH, = CH, + Br, ——~ CH,BrCH,Br, , Ethane Ethylene dibromide, , CH,CHO +HCN——» CH,CH(OH)CN, Acetaldehyde Acetaldehyde cyanohydrin, Addition reactions could be brought about by free-radical, electrophilic or nucleophilic reagents., ae in the case of substitution, there are free radical, electrophilic and nucleophilic substitution, ions,, RAs Elimination reactions. These reactions involve the removal of atoms or groups from a, 7 iis to form a new compounds containing multiple bonds. Dehydrohalogenation of alkyl halides, common example of this type reaction., , CH, — CH,CH,Cl + KOH “> CH,CH = CH, + KC]+H,0, , Eliminat; n—Propyl chloride Propene, *enomolecah reactions are of two types. Bimolecular eliminations or E, reactions and, , 7 War or E, elimination reactions., lent ab Feactions, rate determining step involves two reacting species. Dehydrohalogenation of, Step thinks halides with alc. KOH proceeds by E, mechanism. In E, reactions, the rate determining, Koy Vath, only one reacting species. Dehydrohalogenation of Primary alkyl halides with alc., 2 < through E, mechanism. °, , Sup 'gement reactions. Rearrangement reactions j roca, , fro one atom to the other within the same Tees, “Ee ti fliatioccotematore ora, , ~<a

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HH H® H H. H, Ld [|, R-E-€ <— >R-C —C4e Rett, H H i H H oH, Hyperconjugation in primary carbocation, i, HH CH,R H° fi —R . c—R, 8, R—E—¢ —— R—-E=¢ ——R fa a neh, H_ CH,R H CH,—R CH,—R, , Hyperconjugation in tertiary cation, , Greater the number of canonical forms, greater is the stability.