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Chapter 2: Alkanes and Cycloalkanes ; Conformational and Geometric Isomerism

Chapter 2: Alkanes and Cycloalkanes ; Conformational and Geometric Isomerism. Alkanes. Alkanes are saturated hydrocarbons , containing only carbon–carbon single bonds . Cycloalkanes contain rings.

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Chapter 2: Alkanes and Cycloalkanes ; Conformational and Geometric Isomerism

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  1. Chapter 2: Alkanesand Cycloalkanes; Conformational and Geometric Isomerism

  2. Alkanes • Alkanes are saturated hydrocarbons, containing only carbon–carbon single bonds. • Cycloalkanes contain rings. • Unsaturated hydrocarbons contain carbon–carbon double or triple bonds. Aromatic hydrocarbons are cyclic compounds structurally related to benzene.

  3. Alkane bonds

  4. Three-dimensional models of ethane, propane, and butane.

  5. Names and Formulas of the First Ten Unbranched Alkanes

  6. Alkanes • All alkanes fit the general molecular formula • CnH2n+2 • Unbranched alkanes are called normal alkanes, • or n-alkanes. • -CH2- group is called a methylene group.

  7. IUPAC Rules for naming Alkane The root name of an alkane is that of the longest continuous chain of carbon atoms. Substituents are groups attached to the main chain of a molecule. Saturated substituents containing only C and H are called alkyl groups. The one-carbon alkyl group derived from methane is called a methyl group.

  8. Alkyl and Halogen Substituents The two-carbon alkyl group is the ethyl group. The propyl group and the isopropyl group are three-carbon groups attached to the main chain by the first and second carbons, respectively.

  9. R is the general symbol for an alkyl group. • The formula R-Hherefore represents any alkane, • The formula R-Cl stands for any alkyl chloride • (methyl chloride, ethyl chloride, and so on). • Halogen substituents are named by changing the -ine ending of the element to -o.

  10. 2,2,4-trimethylpentane

  11. Examples of Use of the IUPAC Rules

  12. Rule (Cont’d) NOT NOT

  13. Rule (Cont’d) • When two substituents are present on the same carbon, use that number twice

  14. Rule (Cont’d) • For identical substituents, use prefixes di-, tri-, tetra- and so on NOT NOT

  15. Rule (Cont’d) • When two chains of equal length compete for selection as parent chain, choose the chain with the greater number of substituents NOT

  16. Rule (Cont’d) • When branching first occurs at an equal distance from either end of the longest chain, choose the name that gives the lower number at the first point of difference NOT

  17. Example 1 • Find the longest chain as parent

  18. Example 1 (Cont’d) • Use the lowest numbering for substituents • Substituents: two methyl groups • dimethyl

  19. Example 1 (Cont’d) • Complete name

  20. Example 2

  21. Example 2 (Cont’d) • Find the longest chain as parent

  22. Example 2 (Cont’d) • Find the longest chain as parent ⇒ Nonane as parent

  23. Example 2 (Cont’d) • Use the lowest numbering for substituents

  24. Example 2 (Cont’d) • Substituents • 3,7-dimethyl • 4-ethyl

  25. Example 2 (Cont’d) • Substituents in alphabetical order • Ethyl before dimethyl(recall Rule 4 – disregard “di”) • Complete name

  26. 3C. How to Name Branched AlkylGroups • For alkanes with more than two carbon atoms, more than one derived alkyl group is possible • Three-carbon groups

  27. Four-carbon groups

  28. A neopentyl group

  29. Example 1

  30. Example 1 (Cont’d) • Find the longest chain as parent 6-carbon chain 7-carbon chain 8-carbon chain 9-carbon chain

  31. Example 1 (Cont’d) • Find the longest chain as parent ⇒ Nonane as parent

  32. Example 1 (Cont’d) • Use the lowest numbering for substituents 5,6 4,5 (lower numbering) ⇒ Use 4,5

  33. Example 1 (Cont’d) • Substituents • Isopropyl • tert-butyl ⇒ 4-isopropyl and 5-tert-butyl

  34. Example 1 (Cont’d) • Alphabetical order of substituents • tert-butyl before isopropyl • Complete name

  35. Example 2

  36. Example 2 (Cont’d) • Find the longest chain as parent 8-carbon chain 9-carbon chain ⇒ Decaneas parent 10-carbon chain

  37. Example 2 (Cont’d)

  38. Example 2 (Cont’d) • Use the lowest numbering for substituents 5,6 5,6 ⇒ Determined using the next Rules

  39. Example 2 (Cont’d) • Substituents • sec-butyl • Neopentyl But is it • 5-sec-butyl and 6-neopentyl or • 5-neopentyland 6-sec-butyl ?

  40. Example 2 (Cont’d) • Since sec-butyl takes precedence over neopentyl • 5-sec-butyl and 6-neopentyl • Complete name

  41. Physical Properties of Alkanes and Nonbonding Intermolecular Interactions Hydrogen Bonding: (b) hydrogen bonding between water molecules Hydrogen Bonding: (a) polar water molecule: ball-and-stick model

  42. Molecules with partially positive and partially negative ends Van der Waals attractions. Hydrogen bonding and van derWaals attractions are nonbonding intermolecular interactions.

  43. Boiling points of the normal alkanesIn Isomers

  44. 2,2-Dimethylpropane ball-and-stick model

  45. Pentane: ball-and-stick model

  46. 2,2-Dimethylpropane space filling model

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