Bài giảng hoá học hữu cơ cycloalkanes

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