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  1. 1. Chapter 11: Alkanes K. Dunlap Chem 104
  2. 2. Hydrocarbon: a compound composed of only carbon and hydrogen.
  3. 3. Alkanes: hydrocarbons containing only carbon-carbon single bonds. – The first two alkanes are methane and ethane.
  4. 4. line-angle formula or skeletal formula: – A line represents a carbon-carbon bond and a line terminus represent a carbon atom. – Hydrogen atoms are not shown in line-angle formulas.
  5. 5. The first 10 alkanes with unbranched chains
  6. 6. Constitutional isomers: compounds that have the same molecular formula but different structural formulas (different connectivity). – For the molecular formulas CH4, C2H6, and C3H8, only one structural formula is possible; there are no constitutional isomers for these molecular formulas. – For the molecular formula C4H10, two constitutional isomers are possible.
  7. 7. Constitutional isomers Question: do the structural formulas in each set represent the same compound or constitutional isomers?
  8. 8. Solution for (a): (a) They represent the same compound. Solution for (b): (b) They represent constitutional isomers.
  9. 9. Constitutional isomers Problem: draw structural formulas for the five constitutional isomers of molecular formula C 6H14 Solution:
  10. 10. Number of Possible Isomers for Selected Alkanes
  11. 11. Naming Alkanes • The IUPAC name of an alkane with an unbranched chain of carbon atoms consists of two parts: (1) a prefix: the number of carbon atoms in the chain. (2) the suffix -ane: shows that the compound is a -ane saturated hydrocarbon.
  12. 12. Naming Alkanes • The name of an alkane with a branched chain of carbon atom consists of: – a parent name: the longest chain of carbon atoms – substituent names: the groups bonded to the parent chain
  13. 13. Alkyl groups: a substituent group derived from an alkane by removal of a hydrogen atom. – commonly represented by the symbol R-. – named by dropping the -ane from the name of the parent alkane and adding the suffix - yl. yl
  14. 14. Naming Alkanes 1. The name for an alkane with an unbranched chain of carbon atoms consists of a prefix showing the number of carbon atoms and the ending -ane. 2. For branched-chain alkanes, the longest chain of carbon atoms is the parent chain and its name is the root name. 3. Name and number each substituent on the parent chain; use a hyphen to connect the number to the name.
  15. 15. Naming Alkanes… 4. If there is one substituent, number the parent chain from the end that gives the substituent the lower number.
  16. 16. Naming Alkanes… 5. If the same substituent occurs more than once: – Number the parent chain from the end that gives the lower number to the substituent encountered first. – Indicate the number of times the substituent occurs by a prefix di-, tri-, tetra-, penta-, hexa-, and so on. – Use a comma to separate position numbers.
  17. 17. Naming Alkanes… 6. If there are two or more different substituents: – list them in alphabetical order. – number the chain from the end that gives the lower number to the substituent encountered first. – If there are different substituents in equivalent positions on opposite ends of the parent chain, give the substituent of lower alphabetical order the lower number.
  18. 18. Naming Alkanes… 7. Do not include the prefixes di-, tri-, tetra-, and so on; alphabetize the names of substituents first, and then insert these prefixes
  19. 19. Naming Cycloalkanes: • Cyclic hydrocarbon: a hydrocarbon that contains carbon atoms joined to form a ring. • Cycloalkane: a cyclic hydrocarbon in which all carbons of the ring are saturated. – Cycloalkanes of ring sizes ranging from 3 to over 30 carbon atoms are found in nature. – Five-membered (cyclopentane) and six-membered (cyclohexane) rings are especially abundant in nature. Cyclopentan e Cycloh exane
  20. 20. • Nomenclature – To name a cycloalkane, prefix the name of the corresponding open-chain alkane with cyclo-, and name each substituent on the ring. – If there is only one substituent on the ring, there is no need to give it a location number. – If there are two substituents, number the ring beginning with the substituent of lower alphabetical order. 1 Isopropylcyclopentane 4 1-tert- Butyl-4-methylcyclohexane
  21. 21. Hydrocarbon Rings
  22. 22. Name these alkanes
  23. 23. Conformation of alkanes • any three-dimensional arrangement of atoms in a molecule that results by rotation about a single bond. – following are three conformations for a butane molecule.
  24. 24. Physical properties of Alkane • The most important physical property of alkanes and cycloalkanes is their almost complete lack of polarity. – The electronegativity difference between carbon and hydrogen is 2.5 - 2.1 = 0.4 on the Pauling scale. – Given this small difference, we classify a C-H bond as nonpolar covalent. – Alkanes are nonpolar compounds and the only interaction between their molecules are the very weak London dispersion forces.
  25. 25. Melting and boiling points of Alkanes: – Boiling points of alkanes are lower than those of almost any other type of compound of the same molecular weight. – In general, both boiling and melting points of alkanes increase with increasing molecular weight.
  26. 26. Physical properties Cont… • Solubility: a case of “like dissolves like”. – Alkanes are not soluble in water; they are unable to form hydrogen bonds with water. – Alkanes are soluble in each other. – Alkanes are also soluble in other nonpolar organic compounds, such as toluene and diethyl ether. • Density – The average density of the liquid alkanes listed in Table 11.4 is about 0.7 g/mL; that of highermolecular-weight alkanes is about 0.8 g/mL. – All liquid and solid alkanes are less dense than water (1.0 g/mL) and, because they are insoluble in water, they float on water.
  27. 27. Where do hydrocarbons come from? 1) Natural gas – 90 to 95 percent methane – 5 to 10 percent ethane, and – a mixture of other relatively low-boiling alkanes, chiefly propane, butane, and 2-methylpropane. 2) Petroleum – A thick, viscous liquid mixture of thousands of compounds, most of them hydrocarbons formed from the decomposition of marine plants and animals.
  28. 28. Petroleum Distillation
  29. 29. Some Common Hydrocarbons: -fuels -paraffin -petrolatum -plastics -natural plant waxes
  30. 30. More Naming…….