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Chapter CYCLOALKANES 2010 Outlines Nomenclature Isomerism Conformation of ethane Laboratory preparation Chemical reactions Nomenclature Carbocycles are organic molecules that contain one or more rings Cycloalkanes are simplest cyclic molecules which have molecular formulas CnH2n Cycloalkanes can be drawn as regular polygons using line-angle representations Cycloalkanes are named after their corresponding linear alkanes with the prefix -cyclo When more than one substituent is present, their positions on the ring are numbered in such a way that substituents receive the lowest possible numberings Nomenclature of substituted cycloalkanes CH3 CH2CH3 CH3 CH3 Ethylcyclohexane 1,1-Dimethylcyclopentane CH3 1,2-Dimethylcyclohexane Isomerism in Cycloalkanes A cycloalkane ring has two distinct faces (top and bottom) When two substituents on a ring point to the same face, they are cis When the two substituents point to opposite faces, they are trans cis-trans Isomers in cycloalkanes are stereoisomers Ring strain The carbon atoms in cycloalkanes are sp3 hybridized and a deviation from the ideal tetrahedral bond angles of 109.50 This causes an increase in potential energy and an overall destabilizing effect Eclipsing of hydrogen atoms is an important destabilizing effect, as well Conclusion: The strain energy of a cycloalkane is caused by the compound's geometry Cyclopropane In Cyclopropane the carbon atoms form a triangle having C-C-C bond angles of 600 Big deviation from the ideal tetrahedral angles of 109.50 There are also three pairs of eclipsed Hs Ring strain is highest in cyclopropane, around 120 kJ/mol Torsional strain and flagpole interaction cause the boat conformation to have considerably higher energy than the chair conformation More than 99% of the molecule are estimated to be in a chair conformation at any given moment The chair conformation of cyclohexane has two kinds of positions for hydrogens on the ring: axial positions that are perpendicular to the ring (parallel to the ring axis) and equatorial positions that are in the rough plane of the ring (around the ring equator) Each carbon atom has one axial and one equatorial hydrogen There are axial hydrogens and equatorial hydrogens in alternating arrangement axial equatorial Two hydrogens on the same side of a ring are always cis, regardless of whether they’re axial or equatorial and regardless of whether they’re adjacent Similarly, two hydrogens on opposite sides of the ring cis are always trans trans trans Different chair cyclohexane conformations readily interconvert, resulting in the exchange of axial and equatorial positions This interconversion of chair conformations usually referred to as a ring-flip show: Move this carbon down CH3 Ring-flip Move this carbon up CH3 Methylcyclohexane Axial and equatorial methylcyclohexane interconvert rapidly, but they aren’t equally stable The equatorial conformation is more stable (95% ) than the axial conformation by 7.6 kJ/mol (1.8 kcal/mol) The energy difference is due to an unfavorable steric interaction that occurs between the axial methyl group (C1) and the axial hydrogen atoms on C3 and C5 H H CH3 H For other monosubstituted cyclohexane: a substituent is always more stable in a equatorial position than in an axial position The steric strain increases as the size of the axial substituent group increases Preparation Cycloalkanes can be formed in a reaction followed by a catalytic hydrogenation Diels-Alder Industrially, cyclohexane is made by catalytic hydrogenation of benzene in the presence of a nickel catalyst at about 2000C and 20-40 bar: + + H2/ cat + 6H Ni Chemical Reactions The simple and the bigger cycloalkanes are very stable, like alkanes Like alkanes, they invove in radical chain reactions: Cl2, hν Cl The small cycloalkanes have a lower stability; they react similarly to alkenes Cyclopropane reacts with + Br2 + Br2 ring-opening: BrCH2-CH2-CH2Br BrCH2-CH2-CH2-CH2Br Bicyclic alkanes One of the most important bicyclic system is bicyclo[4.4.0]decane, a common name, decalin They can exist in cis- and trans-forms H H H cis-Decalin H trans-Decalin Bicyclo[2,2,1]heptane Key Words Eclipsed conformation Newman projection Sawhorse representation Staggered conformation Stereoisomers Steric strain Ring-flip Equatorial position [...]... the axial substituent group increases Preparation Cycloalkanes can be formed in a reaction followed by a catalytic hydrogenation Diels-Alder Industrially, cyclohexane is made by catalytic hydrogenation of benzene in the presence of a nickel catalyst at about 2000C and 20-40 bar: + + H2/ cat + 6H Ni Chemical Reactions The simple and the bigger cycloalkanes are very stable, like alkanes Like alkanes,... 20-40 bar: + + H2/ cat + 6H Ni Chemical Reactions The simple and the bigger cycloalkanes are very stable, like alkanes Like alkanes, they invove in radical chain reactions: Cl2, hν Cl The small cycloalkanes have a lower stability; they react similarly to alkenes Cyclopropane reacts with + Br2 + Br2 ring-opening: BrCH2-CH2-CH2Br BrCH2-CH2-CH2-CH2Br Bicyclic alkanes One of the most important
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