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Cyclyoalkanes , Cycloalkenes and Alkadienes Cycloalkanes

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B.Sc. - I, Sem. –II Chem-III Notes by Prof. Dr.M.M.Mane 1 Cyclyoalkanes , Cycloalkenes and Alkadienes Cycloalkanes Cycloalkanes (also called naphthenes - not to be confused with naphthalene) are types of alkanes that have one or more rings ofcarbon atoms in the chemical structure of their molecules.Cycloalkanes are cyclic saturated hydrocarbons having general formula CnH2n Cycloalkanes with a single ring are named analogously to their normal alkane counterpart of the same carbon count: cyclopropane, cyclobutane, cyclopentane, cyclohexane, etc. The larger cycloalkanes, with greater than 20 carbon atoms are typically called cycloparaffins. Cycloalkanes are classified into small, common, medium, and large cycloalkanes, where cyclopropane and cyclobutane are the small ones, cyclopentane, cyclohexane, cycloheptane are the common ones, cyclooctane through cyclotridecane are the medium ones, and the rest are the larger ones. Methods of preparations 1) From Carbene Carbenes are highly reactive carbon species and they add to alkene to form cyclopropane derivaties. Carbene being highly reactive intermediate it cannot be isolated, hence reaction is carried out by reacting alkene wih compounds generating carbene. H3C H3C h + CH2N2 -N2 H3C H3C 2-Butene 1,2-dimethyl cyclopropane 2) From dihalo alkane( Freunda method) Dihalo alkane on treatment with sodium , gives cycloalkane. Br H2C CH2 + 2 Na H2C CH2 -2NaBr Br 1,4-dibromo butane cyclo butane. 3) From Diel’s Alder reaction Cycloaddition of conjugated diene and dienophile to give cyclic compound is called Diel’s Alder reaction CH CH 2 2 473 K Reduction + CH2 CH 2 1,3-Butadiene cyclohexene cyclohexane R.B.M. College Chandgad. Dept. of Chemistry B.Sc. - I, Sem. –II Chem-III Notes by Prof. Dr.M.M.Mane 2 4) From Aromatic compounds. Aromatic compound on catalytic hydrogenation gives corresponding cyclic compound. H2 / Ni Reduction Chemical Properties: 1) Photohalogenation Cycloalkanes on halogenations in presenceof sunlight forms substitution product. Cl h + Cl Cl + HCl 2) Catalytic halogenations: Cycloalkanes on reaction with halogen in presence of catalyst forms addition product by ring opening CH FeBr3 Br 2 Br Br Br CH + 2 CH addition 2 Cyclo propane 1,3-dibromo propane 3) Catalytic hydrogenation: Cycloalkanes reacts with hydrogen in presence of catalyst to form corresponding alkane by ring opening. Ni / CH2 H H C + 2 3 CH addition 3 Cyclo propane propane 4) Effect of Heat: Cycloalkanes on heating in presence of catalysts like Pt, decomposes and form corresponding alkene by ring opening. Pt / CH2 H C 2 CH 3 Propene 5) Reaction with hydrogen halide. Cycloalkanes reacts with ydrogen halide and forms corresponding alkyl halide by ring opening. CH2 Br + HBr H C CH2 3 Propyl bromide Cycloalkene: Cyclic unsaturated hydrocarbon having general molecular formula CnH2n is called cycloalkene. They are named by attaching prefix cyclo to the name of corresponding alkene 1,5-cyclooctadiene R.B.M. College Chandgad. Dept. of Chemistry B.Sc. - I, Sem. –II Chem-III Notes by Prof. Dr.M.M.Mane 3 Preparation: 1) Hoffmann elimination : Cyclopropane can be prepared by elimination of cyclopropyl trimethyl ammonium hydroxide H O + CH3 + N(CH3)3 + 2 N - OH CH3 CH H 3 Cyclopropene 2) By dehydrohalogenation When bromo cyclopentane is treated with alcoholic KOH it forms cyclopeneten by dehydrohalogenation. Br alco. KOH -HBr Cyclopentene 3) Dehydration: Cycloalkanol on heating with Conc. H2SO4 undergo dehydration to form cycloalkaene. OH Conc.H2SO4 + H2O Cyclohexanol cyclohexene Chemical Properties 1) Hydrogenation: Cycloalkenes reacts with hydrogen in presence of catalyst and forms cycloalkanes. catalyst + H2 addition Cyclohexene cyclohexane 2) Addition of halogen: Cycloalkenes undergo addition reaction with halogen to give dihalo compound. Br Br + Br Br addition Cyclopentene 1,2-dibromo cyclopentane 3) Addition of hydrogen acid: Cycloalkenes undergo addition reaction with hydrogen halide to give halo compound. Br + HBr addition Cyclohexene bromo cyclohexane 4) Allylic halogenations Cycloalkenes on reaction with halogen in presence of sunlight and NBS( N-Bromo Succinamide) , undergo halogenations at allylic position. Br NBS + Br Br h Bromo cyclohexene Alkadiene Aliphatic unsaturated hydrocarbons containing two Carbon carbon double bonds and having general formula CnH2n-2 are called alkadiene. Nomenclature: They are named by replacing ‘ane’ of parent alkane by ‘diene’ with position of two double bonds. 2 1 CH2 4 H2C 3 1,3-Butadiene R.B.M. College Chandgad. Dept. of Chemistry B.Sc. - I, Sem. –II Chem-III Notes by Prof. Dr.M.M.Mane 4 Classification: They are calssified depending on position of two double bonds as, 1) Isolated diene: Here two double bonds are separated by more than one C-C Single bond 2 4 1 5 H C 2 CH2 3 1,4-Pentadiene 2) Conjugated diene: Here two C-C double bond are separated by one C-C single bond. e.g. 1,3-Butadiene 3) Cumulated diene: Here two double bonds are present on three succession Carbon atoms. They are also called as alllenes. e.g. 2 1 C H2C CH2 3 1,2-Propadiene 1,3-Butadiene It is conjugated diene and exist in following resonance structure. + + CH CH CH2 CH CH2 CH 2 - CH - etc H2C CH H C CH 2 H2C Thus double bond in butadiene is delocalized and actual structure is hybride of all these resonating structures H CH3 H3C H Molecular Orbital picture. In 1,3-Butadiene each Carbon is SP2 hybridise and forms three covalent bond with carbon and hydrogen atoms to form paler molecule. The angle between any two bond is 1200. Now each C contains one unhybridised P orbital, having dumbel shape and perpendicular to the hybride orbital and contains single electron. Delocalized pi H H 2 4 electon cloud H CH3 3 H 1 H3C Unhybridised p orbital H H The four P orbitals on C1,C2,C3 and C4 undergo lateral overlapping to form delocalized π electon cloud.Half of cloud is above the plane and half is below the plane of the molecule. Method of preparation : 1) From Cyclohexene: When vapors of cyclohexene are passed over heated nichrome alloy, retro Diel’s Alder takes place to form Butadiene. CH2 2 CH2 1 CH2 4 + H2C nichrome CH 3 2 CH2 Butadiene R.B.M. College Chandgad. Dept. of Chemistry B.Sc. - I, Sem. –II Chem-III Notes by Prof. Dr.M.M.Mane 5 2) From Butane: Butane or Butene on heating with chromium oxide undergo dehydrogenation to form butadiene. CH Cr2O3 2 CH3 H2C H3C CH Cr2O3 2 CH3 H C H2C 3 Butane 3) From acetylene: Acetylene and acetaldehyde undergo following series of reactions to give butadiene. OH OH NaNH2 Partial reduction HC CH H C CH HC CH +H3C CHO 3 2 3 H H CH2 Al2O3 H2C -H O 2 4) From Butan -1,3-diol Butan-1,3-diol on heating with acid catalyst undrgo dehydration to form butadiene + CH CH3 H 2 H2C -H2O OH OH 5) From Ethanol and acetaldehyde: When mix. Of ethanol and acetaldehyde is passed over heated catalyst butadiene is formed CH2 H C H C CHO 3 + 3 H2C OH Chemical properties: 1) Reaction with hydrogen halide: When 1,3-butadiene is treated with HCl, a mix. Of two products namely 3-chloro-1-butene and 1-chloro-2-butene is formed. Here addition takes place across C1 and C2 (1:2 addition ) and at C1 and C4 ( 1:4 addition) to give mix of products. Cl CH 1: 2 2 H3C CH CH2 HCl 3 chloro 1 butene H2C CH addition 1: 4 H3C Cl 1 chloro 2 butene 2) Reaction with halogens: a) With one molar equivalent of bromine butadiene forms two products due to 1:2 and 1:4 addition CH 1: 2 2 Br CH CH 2 Br Br2 H2C CH 3,4 dibromo 1 butene addition 1: 4 Br Br 1,4 dibromo 2 butene b) With two molar equivalents of bromine butadiene forms single product tetrabromo butane R.B.M. College Chandgad. Dept. of Chemistry B.Sc. - I, Sem. –II Chem-III Notes by Prof. Dr.M.M.Mane 6 CH CH2 H C CH Br Br 2 + Br Br Br Br 1,2,3,4- tera bromo butane 3) Diel’s Alder reaction Cycloaddition of conjugated diene and dienophile to form cyclic product is called Diel’s Alder reaction. CH2 CH2 + CH2 CH2 Cyclohexene 4) Reduction : Butadiene on catalytic reduction gives mixture of 1-butene and 2-butene. CH 1: 2 2 H3C CH CH2 H2 H2C CH 1 butene addition 1: 4 CH3 H3C 2 butene 5) Oxidation( ozonolysis) : Butadiene on ozonolysis forms formaldehyde and glyoxal. O O CH CH2 Zn OHC CHO 2 O H C CH HC 2 H CHO H2C CH + 3 2 CH2 + O O O O Formaldehyde glyoxal 6)Polymerization: In presence of catalyst ( Sodium) butadiene polymerise to give polybutadiene, called as BuNa rubber. Here monomers are linked by 1:4 linkage. CH CH CH CH2 2 Na CH2 CH2 H2C CH n H C CH H2C CH 2 n 2 + 1: 4 addition CH CH BUNA rubber R.B.M. College Chandgad. Dept. of Chemistry .
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