Chapter 2 Alkanes

1. Chapter 2Alkanes

2. Hydrocarbon: a compound composed of only carbon and hydrogen. • Alkanes: hydrocarbons containing only carbon-carbon single bonds.

4. The first two alkanes are methane and ethane.

5. Table 2.1 The first 10 alkanes with unbranched chains

6. 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 saturated hydrocarbon.

8. 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

10. Alkyl group: 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.

12. If there is one substituent, number the parent chain from the end that gives the substituent the lower number.

13. 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.

15. 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.

17. Do not include the prefixes di-, tri-, tetra-, and so on, or the hyphenated prefixes sec- and tert- in alphabetizing; • alphabetize the names of substituents first, and then insert these prefixes

18. Halogens as substituent groups • Same priority as alkyl groups • Fluoro, chloro, bromo, iodo

19. Common names; in this older system, • The number of carbon atoms determines the name. • The first three alkanes are methane, ethane, and propane.

20. All alkanes of formula C4H10 are called butanes, all those of formula C5H12 are called pentanes, etc. • For alkanes beyond propane, iso shows that one end of an otherwise unbranched chain terminates in (CH3)2CH- • For more complex alkanes, use the IUPAC system.

21. Cycloalkanes • Cyclic hydrocarbon:a hydrocarbon that contains carbon atoms joined to form a ring.

22. 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.

23. Five-membered (cyclopentane) and six-membered (cyclohexane) rings are especially abundant in nature.

24. Nomenclature • To name a cycloalkane, prefix the name of the corresponding open-chain alkane with cyclo-, and name each substituent on the ring.

25. 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.

26. Conformation: any three-dimensional arrangement of atoms in a molecule that results by rotation about a single bond.

27. following are three conformations for a butane molecule.

28. Cyclopentane • The most stable conformation of a cyclopentane ring is an envelope conformation.

29. Cyclohexane • The most stable conformation of a cyclohexane ring is the chair conformation. • all bond angles are approximately 109.5°.

30. In a chair conformation, • six C-H bonds are equatorial. • six C-H bonds are axial.

31. The more stable conformation of a substituted cyclohexane ring has substituent group(s) equatorial rather than axial.

32. Cis-trans isomers • Cis: on the same side. • Trans: across from each other. Same molecular formula, same connectivity—different orientation of atoms in space. Stereoisomers.

33. Viewing a cyclopentane ring edge-on:

34. View from above:

35. View of the cyclohexane ring as a planar hexagon:

36. Physical Properties • The most important physical property of alkanes and cycloalkanes is their almost complete lack of polarity.

37. 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.

38. Melting and boiling points • 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.

40. Alkanes that are constitutional isomers are different compounds and have different physical and chemical properties.

42. 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.

43. Reactions • Oxidation (combustion) • Oxidation of hydrocarbons, including alkanes and cycloalkanes, is the basis for their use as energy sources for heat [natural gas, liquefied petroleum gas (LPG), and fuel oil] and power (gasoline, diesel fuel, and aviation fuel).

45. Reaction with halogens (halogenation) • Halogenation of an alkane is a substitution reaction.

47. Summary of Topics: Chapter 2 • Nomenclature • Cis-trans isomers • Conformations—particularly cyclohexane • Properties (mp/bp; solubility) • Reactions: • Combustion • Radical halogenation