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Alkanes and Cycloalkanes: Structure, Isomerism, and Conformations Explained

Learn about alkanes like ethane, propane, butane, and pentane along with cycloalkanes such as cyclopropane, cyclobutane, and cyclohexane. Explore Newman projections, nomenclature, and stereoisomerism in this comprehensive review. Understand the stability of different conformations and isomers in linear and branched alkanes. Discover the significance of optical isomerism and stereocenters in alkane chemistry.

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Alkanes and Cycloalkanes: Structure, Isomerism, and Conformations Explained

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  1. butane Review Alkanes CnH2n+2 C4H10 a) ethane b) propane c) butane d) pentane CH3 (CH2) CH3 2

  2. Newman projections butane 1 2 3 4 1 2 3 4 staggered eclipsed stable

  3. Nomenclature 1 2 1 3 7 3 1 3 5 7 4 6 4 2 4 6 5 2 5 6 7 heptane heptane heptane 3 2 branches 4 branches branches

  4. Nomenclature 1 2 2- methyl methyl 3- ethyl 3 5 7 4 6 ethyl 4- propyl propyl 5- ethyl 3,5-diethyl 3,5-diethyl -2-methyl -4-propyl heptane

  5. Cycloalkanes linear alkanes CnH2n+2 branched alkanes cycloalkanes closed rings CnH2n 2 3 cyclo propane 1 bond angle = 60o tetrahedral angle = 109o very unstable

  6. Cycloalkanes cyclobutane bond angle = 88o tetrahedral angle = 109o unstable

  7. Cycloalkanes cyclopentane bond angle = 108o tetrahedral angle = 109o stable

  8. geometricisomers Cycloalkanes - - trans- 1,3- dichloro cyclo butane cis- 1,3- dichloro cyclo butane

  9. Cycloalkanes cyclohexane predicted bond angle = 120o tetrahedral angle = 109o stable

  10. cyclohexane axial positions crowded onlyH equatorial positions larger groups boat conformation chair conformation stable unstable

  11. cyclohexane cis- 1,2- dimethyl cyclo hexane cis-1,2-dimethylcyclohexane axial-equatorial trans- 1,2-dimethylcyclohexane trans-1,2-dimethylcyclohexane axial-axial equatorial-equatorial

  12. Optical isomerism Stereocenter 4 different substituents C*= * * 3-methyl hexane bromo chloro iodo methane * * bromo cyclo pentane trans- 1,3-dibromo cyclopentane no C*

  13. Alkane Summary 1. Alkanes -sp3 hybridized 2. Relatively unreactive Substitution with halogens Combustion 3. Non-polar IMF = London Dispersion Forces size structure

  14. 4. Free rotation around C-C bonds conformations 5. Non-cyclic alkanes - structural isomers 6. Cyclic alkanes - geometric isomers cis-, trans- 7. Alkanes -optical isomers stereocenters C*

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