Chapt 21 Hydrocarbons [Selected]

1. Chapt 21 Hydrocarbons [Selected] • 21.1 Introduction to Hydrocarbons • 21.2 Alkanes [Straight-Chain Only] • 21.4 Hydrocarbon Isomers [also with O]

2. Section 21.1 Introduction to Hydrocarbons Hydrocarbons are carbon-containing organic compounds that provide a source of energy and raw materials. • Explain the terms organic compound and organic chemistry. • Identify hydrocarbons • Recognize the different ways that hydrocarbon molecules may be represented (molecular formula, structural formula, ball-and-stick model, etc.) and convert a molecular formula into a valid structural formula and vice versa, • Distinguish between saturated and unsaturated hydrocarbons.

3. Section 21.1 Introduction to Hydrocarbons Key Concepts • Organic compounds contain the element carbon • Hydrocarbons are organic substances composed of carbon and hydrogen. • The major sources of hydrocarbons are petroleum and natural gas.

4. Organic Compounds Name used because living organisms known to contain/produce them Term applied to all carbon-containing compounds except for a small number of compounds considered to be inorganic Because huge number exist, an entire branch of chemistry – organic chemistry – devoted to their study

5. Essential Organic Chemistry • Bond – force that hold atoms together in compounds • Carbon atom (C) always forms 4 bonds with other atoms; bond represented by a line • C forms covalent bonds (a particular type of bond) • Hydrogen can only form one bond

6. Organic Compounds - Hydrocarbons Simplest organic compounds; consist of only the elements carbon (C) and hydrogen (H) In hydrocarbon, C either attached (bonded) to another C or to a hydrogen Are thousands of hydrocarbons; can be in chain, branched chain, ring, and cage-like structures Major source of hydrocarbons – petroleum and natural gas (mostly CH4 – methane)

7. Hydrocarbons Carbon atoms bond to each other by single, double, & triple bonds (always 4 total bonds) Saturated hydrocarbons contain only single bonds Unsaturated hydrocarbonscontain at least one double or triple bond Bond to some unspecified atom Triple Single Double

8. Simplest Hydrocarbon - Methane • Chemical (molecular) formula: CH4 • Structural formula: Chemical bond Carbon atom with 4 bonds H H C H H

9. Simple Hydrocarbons - Methane One carbon atom attached to 4 hydrogens Shape of carbon bonded to 4 other atoms is a tetrahedron – bond angles of 109.5 Hydrogens occupy corners of tetrahedron 109.5

10. Ways of Representing Compounds • Compounds may be represented by various types of formulas and graphical presentations • Variety of these shown on following slides • Chemists use form that best shows information they wish to highlight • Molecular formula most compact but no information about connections & geometry • Structural show connections but no 3D info • Most graphical forms can be generated and/or manipulated using online chemical software

11. Ways of Representing Methane Chemical (molecular) formula CH4 Structural formula with additional geometry information – solid wedge coming toward you, dashed one away Structural formula Ball & stick model H Space-filling model H C H H

12. Chapt 21 Hydrocarbons [Selected] • 21.1 Introduction to Hydrocarbons • 21.2 Alkanes • 21.4 Hydrocarbon Isomers [also with O]

13. Section 21.2 Alkanes Alkanes are hydrocarbons that contain only single bonds. • Namea straight-chain alkane from its molecular formula or by examining its structure (up to octane). • Draw the structural formula or write the molecular formula of a straight-chain alkane when given its name (up to octane).

14. Section 21.2 Alkanes Key Concepts • Alkanes contain only single bonds between carbon atoms. • Alkanes and other organic compounds are best represented by structural formulas and can be named using systematic rules determined by the International Union of Pure and Applied Chemistry (IUPAC). • Alkanes that contain hydrocarbon rings are called cyclic alkanes.

15. Alkanes • Simple Alkanes – hydrocarbons with only single bonds and no ring structures • All have formula CnH2n+2 n = integer • All have names ending in “ane” • Simplest possible hydrocarbon = methane • Chemical (molecular) formula: CH4 • Structural formula: Chemical bond Carbon atom with 4 bonds H H C H H

16. Alkanes • All have formula CnH2n+2 n = integer • n = 2 ethane • Chemical (molecular) formula: C2H6 • Structural formula: • n = 3 propane • Chemical (molecular) formula: C3H8 • Structural formula: H H H C C H H H H H H H C C C H H H H

17. “Constructing” Alkanes Stepwise • Can think of alkanes larger than methane as being built from smaller molecules by adding a methyl group: CH3 • Process: 1) Remove H atom (leave bond) • 2) Replace removed atom with CH3 • If start with CH4, four possible choices for H to remove, but all choices result in exactly the same molecule, ethane = C2H6

18. Making Ethane (C2H6) From CH4 methaneCH4 methyl group: CH3 ethaneC2H6

19. Ethane As represented by skeletal formula • Can write formula as C2H6 or as CH3CH3 • Latter method allows one to visualize and draw structure more easily • Additional very compact representation possible –skeletal (aka line-angle or bond line) formula • Bonds are lines (as before) • Carbon atoms present where line begins or ends or where 2 lines meet • H not shown unless attached to drawn atom

20. Ethane • Most alkanes rotate freely about the single bond between carbon atoms

21. Single Bond Free Rotation • Free rotation occurs about single bonds • Consequence of free rotation: 2 molecules that may appear different when drawn may in fact be identical because one molecule may be twisted about its single bonds to have the exact same shape as the 2nd molecule

22. Two Equivalent Butane Molecules Right hand structure is twisted version of left hand structure

23. Single Bond Free Rotation • Free rotation occurs about single bonds • Because of free rotation all six hydrogen atoms in ethane are equivalent • If making new compound from ethane by replacing a hydrogen, doesn’t matter which one is chosen – result will be the same

24. Making Propane (C3H8) From Ethane Ethane: C2H6 Note: “straight” chain shown in structural formula isn’t methyl group: CH3 Propane: C3H8 or CH3CH2CH3 As represented by skeletal formula

25. Straight-Chain Alkanes • n = 3 propane • Propane: Molecular formula: C3H8 • Structural formula: • For n > 3, it makes a difference which carbon the next methyl group is added • For straight-chainalkanes, next methyl always added to an end carbon – structural formula (untwisted) has all carbons in a line H H H H C C C H H H H

26. Simple Hydrocarbons - Alkanes • Condensed formula helps to see structure • Butane C4 shown as straight-chain isomer Type of Formula Molecular Structural Ball-and-Stick Space Fill Condensed CH3CH3 CH3CH2CH3 CH3CH2CH2CH3

27. Hydrocarbons – Straight-Chain Alkanes Type of Formula Name Molecular Condensed

28. Chapt 21 Hydrocarbons [Selected] • 21.1 Introduction to Hydrocarbons • 21.2 Alkanes [Straight-Chain Only] • 21.4 Hydrocarbon Isomers [also with O]

29. Section 21.4 Hydrocarbon Isomers Some hydrocarbons [and other compounds] have the same molecular formula but have different molecular structures. • Define the terms isomer, structural isomer, and stereoisomer. • Categorize molecular structures as being structural isomers, stereoisomers or as not being isomers. • Distinguish between geometric (diastereomers) and optical isomers (enantiomers) • Differentiate between geometric isomers with cis- and trans prefixes. • Describedescribe the structural characteristics that are associated with optical isomers

30. Section 21.4 Hydrocarbon Isomers Some hydrocarbons [and other compounds] have the same molecular formula but have different molecular structures. • Generate isomers of compounds containing oxygen in addition to carbon and hydrogen

31. Section 21.4 Hydrocarbon Isomers Key Concepts • Isomers are two or more compounds with the same molecular formula but different molecular structures. • Structural isomers differ in the order in which atoms are bonded to each other. • Stereoisomers have all atoms bonded in the same order but arranged differently in space; stereoisomers can either be optical isomers (enantiomers) or not (diastereomers) • Stereoisomers which are non-superimposable mirror images of each other are called optical isomers • Some diastereomers are geometric isomers; these are associated with carbon compounds containing double bonds

32. Section 21.4 Hydrocarbon Isomers Key Concepts • When oxygen is present in a compound with carbon and hydrogen, isomers can involve hydroxy (-OH), ether (-O-) and carbonyl (C=O) groupings

33. Formulas for Compounds - Isomers • Isomers – different compounds which have the same chemical formula • 2 main categories: structural (aka constitutional) isomers and stereoisomers (aka configurational) • Structural isomer - atoms bonded in different order • Stereoisomer – atoms bonded in same order but differ in spatial orientation • # of isomers increase as # of atoms increase • Good web resources at: • http://www.chemguide.co.uk/basicorg/isomermenu.html#top • http://www.brightstorm.com/science/chemistry/organic-chemistry/isomers-stereoisomers/

34. Types of Isomers All Isomers Stereoisomers (Configurational) Structural (Constitutional) Enantiomers (optical) Diastereomers Geometric (Cis-trans) Other diastereomers (>1 chiralcenters)

35. Butane – Structural Isomers • Butane, C4H10 - smallest alkane to have isomers (has two) • Unlike construction of ethane and propane, choice of which H in propane to replace with a methyl group makes a difference • Two possible choices generate two structural isomers – carbons connected to each other differently • Structural isomers differ in physical and chemical properties

36. Making Butane (C4H10) From Propane or propane (C3H8)

37. Two Equivalent n-Butane Molecules Right hand structure is twisted version of left hand molecule

38. Butane - Bond Rotations (not isomers)

39. Butane C4H10. = Not isomers – carbons connected in same way and forms can convert from one to the other by rotating around a bond = Structural isomers – carbons connected in different way; bond must be broken to convert one form into the other

40. Structural Isomers of C4H10 Isobutane, BP = -12°C Butane, BP = 0°C

41. Structural Formula Other Skeletal Formula n-butane straight chain CH3CH2CH2CH3 Butane (C4H10) iso-butane branched CH3CH(CH3)CH3

42. n-Butane CH3CH2CH2CH3

43. Making Pentane (C5H12) From Butane At first glance, appear to be 4 isomers - but 3 isomers remain

44. The 3 Structural Isomers of Pentane

45. Structural Isomers of C5H12 (Pentane) n neopentane isopentane n-pentane Longest continuous carbon chain: pentane 5isopentane4neopentane3

46. Alkane Isomers – Alternate Strategy • Rather than building new isomers by adding a methyl group to known isomers of a particular alkane, the following slides illustrate an alternative approach • Draw straight chain version of alkane of interest • Break one or more carbon-carbon bonds and rearrange the pieces • Check that new molecules don’t repeat existing ones (reflection, rotation)

47. Structural Isomers of Hexane (C6H14) • Start: connect carbons in a line • Break bonds & rearrange to get other isomers

48. Hexane (C6H14)Isomers Showing H Atoms

49. Hexane (C6H14)Isomers In Skeleton Form

50. Structural Isomers of Hexane (C6H14) 1 2 3 1 4 5 2 3 1 2 3 5 4 4 5