Alkanes and Cycloalkanes

1. Alkanes and Cycloalkanes Conformational and Geometric Isomerism

2. Hydrocarbons • Petroleum • Saturated • Single-bonded C’s • Alkanes or cycloalkanes • Unsaturated • At least 1 double- or triple-bonded pair of C’s • Alkenes—double • Alkynes—triple • Can be cyclic • Aromatic • Structures like benzene

3. Alkane Structure • Simplest • Bond angle • CnH2n + 2 • Normal v. branched • n-alkanes • Methylene group • —CH2 • Homologous series • Consecutive building with predictable chemical and physical properties

4. Nomenclature of Organic Compounds • Common • Sources, structure, or uses • Limonene • Cubane • IUPAC • Systematic method • Unique name • Write structures from names • 1-methyl-4-(1-methylethenyl)-cyclohexene • pentacyclo[4.2.0.02,5.03,8.04,7]octane

5. First 10 Unbranched Alkanes

6. IUPAC Rules for Alkanes 1. The –aneending will be used for all saturated hydrocarbons. 2. Alkanes without branches are named according to the number of C’s 3. Branched alkanes use a root or parent name for the longest continuous chain of C’s

7. IUPAC Rules for Alkanes 4. Anything not on the root chain is known as a substituent. Saturated substituents are known as alkyl groups, and names come from # of C’s. methyl ethyl propyl

8. IUPAC Rules for Alkanes n-propyl isopropyl n-butyl isobutyl sec-butyl

9. IUPAC Rules for Alkanes 5. The root chain is numbered such that the first substituent along the chain receives the lowest possible number. • Any other substituents are also located by name and location. Every substituent must be named and numbered. • If two or more of the same substituent are present, prefixes such as di, tri, tetra, penta, etc. are used • If two or more different types of substituents are used, they are listed alphabetically, ignoring prefixes such as di, tri, n, iso, sec, tert unless necessary to alphabetize

10. IUPAC Rules for Alkanes 5. Separate numbers from words with a hyphen; separate numbers from numbers with a comma. There are no spaces within an IUPAC name. Let’s use some C skeletons as our first examples:

11. IUPAC Rules for Alkanes

12. IUPAC Rules for Alkanes

13. Halogen Substituents • The letter R represents alkyl group. R-H means any alkane. • The halogens that may take the place of the H: F, Cl, Br, and I • The name of halogen loses ine suffix and becomes o: fluoromethane, chloromethane, bromomethane, iodomethane • Common names treat R as substituent and halogen as root chain

14. Alkane Sources • Petroleum • Mixture of alkanes and cycloalkanes • Gasoline v. diesel • LPG—1° propane • butane • Natural Gas • ~80% methane and 5-10% ethane • Liquefied for transportation (-160°C, 1 m3l = 600 m3g)

15. Alkane Properties and Intermolecular Forces • Insoluble in water • H-bonding v. LDF • Lower bp for MW than other organics • bp rises as chain length increases • bp falls as chain becomes more branched

16. Alkane Conformations • Stereoisomers • Atoms connected in same order but arranged differently in space • Sigma bonds • Single bonds allow rotation • Staggered • Eclipsed • Rotamers or Conformers • Dash-wedge • Sawhorse • Newman projections

17. Staggered Conformer of Ethane

18. Eclipsed Conformer of Ethane

19. Cylcoalkane Nomenclature Ring of C’s cyclo is prefix on alkane name With 1 substituent, no number needed More than 1 substituent, numbers needed Alphabetic priority gets lowest number cylcopropanecyclobutanecyclopentane cyclohexane cycloheptanecyclooctane bp = -32.7°C 12°C 49.3°C 80.7°C 118.5°C 149°C

20. Cycloalkane Nomenclature CH3 CH3 CH3 CH3 CH2CH3 methylcyclooctane NOT… 1,2-dimethylcyclooctane NOT… 1-ethyl-2-methylcyclooctane NOT…

21. Cylcoalkane Conformations • Cyclopropane • Planar with 60° C-C-C (only cycloalkane that is planar) • Cyclobutane • 88° C-C-C (predicted 90°) • Cyclopentane • 105° C-C-C (predicted 108°) • Cyclohexane • 109.5° C-C-C • Chair conformation

22. Cyclohexane Chair Conformations Axial…3 up, 3 down Equatorial, 6 in mean plane of C’s Flipping occurs easily at RT; slows as T decreases Important in biomolecules like glucose

23. Cyclohexane Boat Conformation Why is this conformation less stable than the chair conformation?

24. Cis-Trans Isomerism in Cycloalkanes Type of stereoisomers or geometric stereoisomers in which substituents are on the same side (cis) or the opposite side (trans) of a ring Different physical and chemical properties Not readily converted like conformers are

25. Isomer Review

26. Reactions of Alkanes • Relatively inert • Oxidation (Combustion) • Excess O2 produces CO2, most oxidized form of C (ox # is +4) • Insufficient O2 means lower ox # of C • CO • C • CH2O • CH3CO2H • Halogenation (substitution rxn) • Excess halogen results in more substitution…even mixtures • Cycloalkanes make pure organic products

27. Oxidation of Alkanes CH4 + 2O2 CO2 + 2H2O 2CH4+ 3O2 2CO + 4H2O CH4 + O2 C + 2H2O CH4 + O2 CH2O + H2O 2C2H6+ 3O2 2CH3CO2H + 2H2O

28. Halogenation of Alkanes CH4 + Cl2 CH3Cl + HCl Process is known as chlorination (others are fluorination, bromination, and iodination) Excess of halogen, more H’s are substituted CCl4 can be produced with enough chlorine. + F2  + HF energy light F

29. Halogenation: Free Radical Chain Mechanism • Rxn Mechanism • Most rxns have multiple steps • Chain-initiating step • Breaking of halogen molecule • Chain-propagating step • Radical is formed (odd # of unshared e-) • Each step: radical is consumed, but another radical is produced • Chain-terminating step • No new radicals are formed and all radicals are combined with some other radical

30. Halogenation: Free Radical Chain Mechanism R-H + Cl-Cl R-Cl + H-Cl Chain-initiating step Chain-propagating steps Possible chain-terminating steps light

31. Reaction Summary Combustion Halogenation

32. Reaction Mechanism Summary Halogenation