Chapter 2

1. Chapter 2 Alkanes and Cycloalkanes

2. Hydrocarbons • Hydrocarbon: a compound composed of only carbon and hydrogen

3. Alkanes • Alkanes: hydrocarbons containing only carbon-carbon single bonds (saturated hydrocarbons) • the first two alkanes are methane and ethane

4. Alkanes • line-angle formula: • a line represents a carbon-carbon bond and an angle represents a carbon atom • a line ending in space represents a -CH3 group • hydrogen atoms are not shown in line-angle formulas

5. Alkanes • the first 10 alkanes with unbranched chains

6. Constitutional Isomerism • Constitutional isomers:compounds that have the same molecular formula but different structural formulas • CH4, C2H6, and C3H8 only have one possible structural formula so they don’t have constitutional isomers • for the molecular formula C4H10, two constitutional isomers are possible

7. IUPAC Names • 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. IUPAC Names • 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

9. IUPAC Names • 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

10. IUPAC Names 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, 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

11. IUPAC Names 4. If there is one substituent, number the parent chain from the end that gives the substituent the lower number

12. IUPAC Names 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

13. IUPAC Names 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

14. IUPAC Names 7. 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

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

16. 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

17. Cycloalkanes • 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.

18. Conformations - Alkanes • Conformation: 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

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

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

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

22. Cyclohexane • the more stable conformation of a substituted cyclohexane ring has substituent group(s) equatorial rather than axial

23. Cis-Trans Isomers • Same molecular formula • Same connectivity of atoms • Different arrangement of atoms in space due to restricted rotation

24. Cis-Trans Isomers • Cis: on the same side • Trans: across from • we can view a cyclopentane ring edge-on

25. Cis-Trans Isomers • alternatively, we can view it from above

26. Cis-Trans Isomers • to determine cis-trans isomers if cis-trans isomers are possible, we can view a cyclohexane ring as a planar hexagon • because cis-trans isomers differ in the orientation of their atoms in space, they are stereoisomers • cis-trans isomers are one type of stereoisomers

27. Physical Properties • 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

28. Physical Properties • 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

29. Physical Properties

30. Physical Properties • alkanes that are constitutional isomers are different compounds and have different physical and chemical properties

31. Physical Properties • 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 higher-molecular-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

32. 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)

33. Reactions • Reaction with halogens (halogenation) • halogenation of an alkane is a substitution reaction

34. Sources of Alkanes • 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 • Petroleum • a thick, viscous liquid mixture of thousands of compounds, most of them hydrocarbons formed from the decomposition of marine plants and animals

36. Chapter 2 End Chapter 2