Cycloalkanes: Common trimethylene tetramethylene Pentamethylene hexamethylene heptamethylene IUPAC name:

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1 UNIT 3 YLALKANES, YLALKENES AND ALKADIENES: - - [5] Introduction: arbocyclic or homocyclic compounds resembling aliphatic compounds in their properties are called alicyclic compounds or cycloalkanes. ycloalkanes with one double bond are called cycloalkenes and aliphatic compounds with two double bonds are called alkadienes. Nomenclature: Structure ycloalkanes: ommon trimethylene tetramethylene Pentamethylene hexamethylene heptamethylene IUPA name: cyclopropane cyclobutane cyclopentane cyclohexane cycloheptane Methods of formation: 1. Addition of carbene to alkene:carbenes are highly reactive species which add to alkenes to give cyclopropane derivatives. N 2 diazomethane 2-butene hv 1,2 dimethyl cyclopropane 2. Action of metallic sodium on α, ω- dihaloalkane (Freund s method/ Internal Wurtz reaction) 1,4 dibromo butane Na heat cyclobutane 2Na 3. By Diel s Alder Reaction: hv 473K ethylene reduction cyclohexane nline notes on

2 4. By reduction of aromatic compounds; reduction Ni/ heat Benzene cyclohexane hemical properties: 1. Photoalogenation cyclobutane l 2 hv l l 1,2-dichloro cyclobutane 2 Fe3 1,3 dibromo propane Ni catalytic 350 o Pt heat - propane bromopropane -= Propene nline notes on

3 ycloalkenes: Alicyclic hydrocarbons containing one or more double bonds are called cycloalkenes. Nomenclature: named as cyclolakenes with minimum possible numbers given to the position of double bonds. Structure IUPA name: yclopropene cyclobutene yclopenta 1,3 diene yclohexa 1,4 diene (2) Method of formation from cyclic compounds: 1. offman elimination of quaternary ammonium salt of cyclopropane NMe 3 - trimethyl cyclopropyl ammonium hydroxide 2. Dehydrohalogenation of cycloalkane halide ofmann elimination NMe 3 alcoholic K - cyclopropene cyclopentane bromide 3. Dehydration of cycloalkanol onc S 4 420K cyclopentene - cyclohexanol (3) hemical Properties a) ydrogenation cycloalkenes react with hydrogen in presence of catalyst to form cycloalkane. catalyst cyclohexane b) Addition of alogens and halogen acids. ycloalkenes on addition of halogen give dihalo cycloalkanes 2 cyclopentene 1,2 dibromo cyclopentane nline notes on

4 And on addition of haloacid give monohalo cycloalkanes bromo cyclohexane c) Allylic halogenations: under suitable conditions sunlight and N-bromosuccinamide cycloalkenes undergo substitution of from allylic position. 2 sunlight, NBS bromo 3.3 Alkadienes: Aliphatic hydrocarbons containing two double bonds in their molecules are called alkadienes. Nomenclature: The alkadienes are named by selecting the longest possible chain of carbons with both the double bonds and then naming the compound as a derivative of the parent hydrocarbon and adding the number indicating the position of the double bonds with lowest possible number and then adding the suffix as diene. =-= -=--= 1,3, 3,5 dimethyl 1,4 hexadiene (2) lassification: Based on the position of the double bonds, dienes are classified into three classes: 1. Isolated diene: dienes in which the two double bonds are separated by more than one single bonds. Eg. Penta-1,4,diene =- -= 2. onjugated diene: dienes in which the two double bonds are separated by only one single bond. Eg. Buta-1,3 diene Penta-1,3,diene =-= =-=- 3. umulative diene: dienes in which the two double bonds are in succession. Eg. Allene or propadiene == nline notes on

5 (3) Buta-1,3-diene : 4 6 : =-= a) Structure: The actual structure of the is resonance hybrid of the following resonating structures =-= -=- - - =-=. -=-. This explains the bond length of 1-2 and 3-4 as 1.35A 0 than expected 1.33 A 0 and bond length of 2-3 as 1.46 A 0 than expected 1.54 A 0. This is further explained by Molecular orbital theory. Every carbon in is sp2 hybridised and has one unhybridised Pz orbital on every carbon atom which is laterally overlapped to have delocalization of pi electrons over all the four carbon atoms. b) Methods of formation: i) From cyclohexane: when vapours of are passed over nichrome alloy is formed. Nichrome ethylene ii) From Butane by dehydrogenation: butane or butene when passed over chromium oxide catalyst, undergo dehydrogenation to give. n-butane 1 butene r 3 2 butene nline notes on

6 iii) From acetylene: acetylene and acetaldehyde undergo reations to give NaN acetylene acetaldehyde methyl ethynyl carbinol /Pd Al methyl ethenyl carbinol iv) From Butane-1,3-diol: butane-1,3 diol on heating with acid catalyst, undergo dehydration to give butane-1,3-diol v) From ethanol and acetaldehyde (Industrial method) when mixture of ethanol and acetaldehyde passed over the heated catalyst, give. (4) hemical Properties a) Reaction with hydrogen halide: - catalyst ethanol acetaldehyde 3 l addition l 3-hloro-1-butene 3 l b) Reaction with halogens i) With one molar equivalent of halogens(l 2 or 2 ) 1-chloro 2 butene 2 2 Addition 3,4-dibromo 1-butene 1,4=dibromo, 2-butene nline notes on

7 ii) With two molar equivalent of halogens(l 2 or 2 ) 2 2 Addition --- c) Diels- Alder reaction 1,2,3,4-tetrabromo-butane 575K d) Reduction hydrogenation ethylene Addition catalyst 1-butene e) xidation zonolysis: 3 2 diozonide f) Polymerization 2-butene Zn 2 - formaldehyde glyoxal n Na n Buna or poly polymerisation n Buna or poly nline notes on