Alkanes

1. Alkanes

2. Hydrocarbons • hydrocarbons: a family of compounds composed only of hydrogen and carbon • alkanes • alkenes • alkynes • arenes

3. Alkanes • contain only carbon and hydrogen atoms • have only single bonds • do not have functional groups • can be linear, branched, or cyclic

4. H C H H H Methane (CH4): The Simplest Hydrocarbon • Remember - carbon has 4 valence electrons, and can form 4 covalent bonds with other atoms such as hydrogen • For 1 C and 4 H, there is only one possible way they can be arranged… H H – C – H H

5. Ethane • For a molecule that only contains 2 carbon atoms and 6 hydrogen atoms, there is only one possible combination

6. Propane • For a molecule with 3 C and 8 H atoms, there is only one possible combination H H H H – C – C – C – H H H H need ball & stick

7. C4H10 • The four carbons can either be in a row or have a branched arrangement. straight-chain alkane branched-chain alkane

8. C5H12 • there are 3 ways in which molecules can be formed H H H H H H H – C – H H – C – C – C – C – C – H H H H H H H H H – C – C – C - H H H H H H H H – C – C – C – C – H H H H H – C – H H – C – H H H

9. Isomers • structural isomers: compounds with the same number of atoms and bonds, but in different arrangements • as the number of carbon atoms increases, the number of possible alkane isomers grows rapidly.

10. Number of Possible Isomers of the Alkanes # Carbons Molecular Formula Possible Isomers 1 CH4 1 2 C2H6 1 3 C3H8 1 4 C4H10 2 5 C5H12 3 6 C6H14 5 7 C7H16 9 8 C8H18 18 9 C9H20 35 10 C10H22 75 15 C15H32 4,347 20 C20H42 366,319 40 C40H82 > 62 trillion !

11. Practice • Draw the straight-chain molecule with the formula C7H16 H H H H H H H H – C – C – C – C – C – C – C – H H H H H H H H

12. Practice • Draw 2 branched-chain isomers with the formula C7H16 CH3 CH3 CH3CH2CH2CHCH3 CH3CH2CH2CHCH2CH3 CH3 CH3 CH3 CH3CH2CH2CCH3 CH3CH2CHCHCH3 CH3CH2CCH2CH3 CH3 CH3 CH3 CH3 CH3 CH2CH3 H3C CH3 CH3CHCH2CHCH3 CH3CH2CHCH2CH3 CH3C – CHCH3 CH3

13. Drawing Organic Structures • Drawing every atom and bond in a molecule is a pain in the neck for all but the smallest molecules. • 2 shorthand ways of drawing structures • condensed structures • line structures

14. Condensed Structures • C-C and C-H single bonds aren’t shown • CH3 is a carbon with 3 hydrogens bonded to it • CH2 is a carbon with 2 hydrogens bonded to it • often a row of CH2 groups is shown in parentheses, with a subscript equal to the number of groups

15. butane H H H H H - C - C - C - C – H H H H H CH3CH2CH2CH3 CH3(CH2)2CH3 2-methylpropane H H – C – H H H H – C – C – C – H H H H CH3 CH3CHCH3 Examples note that even though we may write part of a structure as CH3CH2, the covalent bond that is shown links the 2 carbon atoms together

16. Draw the following 3 isomers of C5H12 as condensed structures. CH3 CH3CCH3 CH3 CH3CH2CH2CH2CH3 or CH3(CH2)3CH3 H H H H H H H – C – C – C – C – C – H H – C – H H H H H H H H H – C – C – C - H H H H H H H H – C – C – C – C – H H – C – H H H H H H – C – H H CH3CH2CHCH3 CH3

17. Line Structures • line structure: atoms aren’t shown; instead, a carbon atom is understood to be at every intersection of lines and hydrogens are filled in mentally • CH3CH2CH2CH2CH3 • CH3CH2CHCH3 CH3

18. Your Turn • Draw the line structure for CH3 CH3CHCH2CHCH3 CH3

19. = A Key Point • As long as two structures show identical connections between atoms, they represent the same compound, no matter how the structures are drawn. CH3 CH3CHCH3 = CH3CHCH3 CH3

20. Practice • Which of the following structures are identical? CH3 CH3 CH3 CH2CH2CHCH2CH3 CH3CH2CH2CCH3 CH3 CH3 CH3CH2CHCH2CH2CH3 (a) (b) a & c are identical, and are isomers of b (c)

21. Practice • Are the following pairs of compounds the same, isomers, or unrelated? CH3 CH3 CH3CHCH2CH2 CH3CHCH2CH2CH3 CH3 CH2CH3 CH3CH2CHCH3 CH3CHCH2 CH2CH3 CH3 CH3CH2OCH3 CH3CH2CH O identical isomers unrelated

22. Naming Alkanes • Initially organic compounds were named at the whim of the discoverer • urea because it was first isolated from urine • barbiturates in honor of the discoverer’s friend Barbara • Over time, a systematic method of naming became necessary. • IUPAC (pronounced eye-you-pack) • International Union of Pure and Applied Chemistry

23. Organic Naming System • chemical name has 3 parts: • prefix: specifies the location of functional groups • parent: tells how many carbon atoms are present in the longest continuous chain • suffix: identifies what family the molecule belongs to prefix – parent - suffix

24. Naming Straight-chain Alkanes 1. Count the number of carbon atoms 2. Use the Greek numbering prefix (except for 1-3 carbon chains) 3. Add the family suffix “-ane”

25. Straight Chain Alkanes # of CStructureName 1 CH4 methane 2 CH3CH3 ethane 3 CH3CH2CH3 propane 4 CH3CH2CH2CH3butane 5 CH3CH2CH2CH2CH3pentane 6 CH3CH2CH2CH2CH2CH3hexane 7 CH3CH2CH2CH2CH2CH2CH3heptane 8 CH3CH2CH2CH2CH2CH2CH2CH3octane 9 CH3CH2CH2CH2CH2CH2CH2CH2CH3nonane 10 CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3decane

26. Straight-Chain Alkanes Cont. 11 undecane 21 henicosane 12 dodecane 22 docosane 13 tridecane 23 tricosane 14 tetradecane 24 tetracosance 15 pentadecane 30 triacontane 16 hexadecane 31 hentriacontane 17 heptadecane 32 dotriacontane 18 octadecane 40 tetracontane 19 nonadecane 50 pentacontane 20 icosane 100 hectane

27. Practice • Name the following: CH3(CH2)5CH3 heptane CH3(CH2)8CH3 decane

28. Alkyl Groups • alkyl group: a group that contains carbon and hydrogen that branches off the main chain • these are not compounds by themselves, the are part of a structure • name by using the alkane name, but replace the “-ane” ending with “-yl”

29. Naming Branched-Chain Alkanes: General 1. Find the longest carbon chain. Use as base name with “-ane” ending. 2. Number the carbon atoms in that main chain, beginning at the end nearest the first branch point 3. Locate/number all branches on the chain. Use the base name with an “-yl” ending. 4. List multiple branches alphabetically – the di, tri, … don’t count. Write the name as a single word.

30. 1. Name the Main Chain • Find the longest continuouschain of carbons, and use the appropriate Greek chain name • The longest chain may not be obvious because it may not always be written on one line. CH3CHCH2CH2CH3 CH3 – CH2 CH3 CH3 – CH – CH2 – CH3 …pentane …pentane

31. 2. Number the carbon atoms in the Main Chain • begin at the end nearest the first branch point CH3CH2CH2CH2CH3 …pentane CH3 CH3CH2 CH3CHCH2CH3 …pentane 1 2 3 4 5 1 2 3 4 5

32. methyl group attached to 2nd carbon ethyl group attached to 3rd carbon 3. Identify the Branching Substituents and Identify its Point of Attachment • if there are two substituents on the same carbon, assign the same number to both. CH3CH2CH2CH2CH3 …pentane CH3 CH3CH2 CH3CHCH2CH3 …pentane 1 2 3 4 5 1 2 3 4 5

33. 4. Write the Name As a Single Word • use hyphens to separate the numbers from the different prefixes • use commas to separate numbers if necessary • if 2 or more different substituent groups are present, cite them in alphabetical order • if 2 or more identical substituents are present, use the prefix di, tri, tetra, etc (but don’t use these prefixes for alphabetizing purposes)

34. 1 2 3 4 5 CH3CH2CH2CH2CH32-methylpentane CH3 CH3CH2 CH3CHCH2CH33-methylpentane 1 2 3 4 5

35. Additional Examples CH2CH3 CH3CH2CCH2CH2CH33-ethyl-3-methylhexane CH3 CH3 2,2-dimethylbutane CH3CCH2CH3 CH3 CH3CH2 CH3CCH2CH2CH33,3-dimethylhexane CH3

36. Your Turn • What are the names of the following alkanes? CH3 CH3 CH3CH(CH2)2CHCH2CH3 2,5-dimethylheptane 2,3,3-triethylpentane

37. Draw the Structure Corresponding to • 3,4-dimethyloctane CH3 CH3CH2CHCHCH2CH2CH2CH3 CH3 • 3-methylhexane

38. Cycloalkanes • cycloalkane: an alkane that contains a ring of carbon atoms • well known and found in nature • ring sizes from 3 to 30 have been prepared in the laboratory

39. Examples

40. Naming Cycloalkanes 1. count the number of carbons in the ring 2. use “cyclo” + the normal alkane name 3. number the substituents on the ring -start numbering at the group that has alphabetical priority -proceed around the ring in the direction that gives the second substituent the lower possible number

41. Example 1-ethyl-3-methylcyclohexane CH3 3 2 1 CH2CH3

42. Practice • What is the name of the following? CH2CH3 ethylcyclopentane CH3 CH3 1,1,3-trimethylcyclohexane CH3 CH3CH2CHCH2CH3 3-cyclobutylpentane

43. CH3 CH3 CH3 CH3 CH3 Practice • Draw a line structure for 1,4-dimethylcyclohexane • Draw a line structure for 1,3,5-trimethylcycloheptane

44. Your Turn • What is the name of the following? propylcyclohexane

45. Alkanes • alkanes contain only nonpolar C-C and C-H bonds • only weak London dispersion intermolecular forces are present • recall London dispersion forces are a weak attraction between molecules. It increases with molecular weight.

46. Impact of London Dispersion Force on Alkane Properties • mp and bp of straight-chain alkanes increase with molecular weight • branching tends to decrease melting points

47. Table from “Organic Chemistry”, Francis Carey, McGraw-Hill, 1987, p53.

48. Properties of Alkanes • odorless or mild odor • colorless • tasteless • nontoxic • nonpolar • insoluble in water but soluble in nonpolar organic solvents • less dense than water • flammable • not very reactive

49. Reactions of Alkanes 1. Combustion (burn) 2. Halogenation (add halogen)

50. 1. Combustion • combustion: burning with oxygen alkane + oxygen gas  carbon dioxide + water + heat