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10 introduction-to-organic-chemistry-alkanes


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Chapter 10 of General, Organic, and Biochemistry, Denniston, 7th edition

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10 introduction-to-organic-chemistry-alkanes

  1. 1. Chapter 10An Introduction to Organic Chemistry 1
  2. 2. 10.1 The chemistry of carbonWhy are there so many carbon-based compounds? Carbon atoms form stable, covalent bonds with each other. Carbon atoms form stable bonds with other elements, such as oxygen, nitrogen, sulfur, and halogens. Carbon atoms form multiple bonds with: other carbon atoms (double & triple); oxygen (double); nitrogen (double & triple). Carbon atoms can be arranged in linear chains, branched chains, and cyclic structures. 2
  3. 3. 10.1 Organic/inorganic compound differences• Bond type – Organics have covalent bonds • Electron sharing – Inorganics usually have ionic bonds • Electron transfer• Structure – Organics • Molecules • Nonelectrolytes – Inorganics • Three-dimensional crystal structures • Often water-soluble, dissociating into ions -electrolytes 3
  4. 4. 10.1 Organic/inorganic compound differences• Melting point & boiling point – Organics have covalent bonds • Intermolecular forces broken fairly easily – Inorganics usually have ionic bonds • Ionic bonds require more energy to break• Water solubility – Organics • Nonpolar, water insoluble – Inorganics • Water-soluble, readily dissociate 4
  5. 5. 10.1 Functional groupsGroups of atoms that cause organic compounds toexhibit particular properties are called functional groups. e.g., alcohols always have an –OH group.We’ll be studying organic chemistry by functional group. 5
  6. 6. 10.1 Functional groupsAlkenes contain adouble bond betweenadjacent carbonatoms.Alkynes contain atriple bond. 6
  7. 7. 10.1 Functional groupsAn alcohol contains thehydroxyl (-OH) functionalgroup.In an ether, an oxygen atomis bonded to two carbonatoms. 7
  8. 8. 10.1 Functional groupsAn aldehyde contains acarbonyl group (C=O), which isa carbon atom with a doublebond to an oxygen atom.In a ketone, the carbon of thecarbonyl group is attached totwo other carbon atoms. 8
  9. 9. 10.1 Functional groupsCarboxylic acids contain thecarboxyl group, which is acarbonyl group attached toa hydroxyl group.An ester contains thecarboxyl group betweencarbon atoms. 9
  10. 10. 10.1 Functional groupsIn amines, thefunctional group is anitrogen atom.In amides, the hydroxylgroup of a carboxylicacid is replaced by anitrogen group. 10
  11. 11. 10.2 AlkanesHydrocarbons are compounds composed only of carbonand hydrogen. Saturated hydrocarbons have only single bonds. Unsaturated hydrocarbons have one or more double and/or triple bonds.Saturated hydrocarbons are called alkanes if they are acyclic. cycloalkanes if they are cyclic. 11
  12. 12. 10.2 Sources of alkanesPetroleum, a naturally occurring mixture ofhydrocarbons, is the main source of alkanes. Liquid petroleum is called crude oil. hydrocarbons with 5 or more carbons Gaseous petroleum is called natural gas. mostly methane, with ethane, propane, and butane 12
  13. 13. 10.2 Sources of alkanes 13
  14. 14. 10.2 Physical properties of alkanesAlkanes are insoluble in water. Water molecules are polar and can take part in hydrogen bonding. Alkanes are nonpolar. “Like dissolves like.” 14
  15. 15. 10.2 Physical properties of alkanesAlkanes have lower boiling points for a given molecularweight than most other organic compounds. Alkanes are nonpolar. Molecules are only weakly held together by van der Waals attractions. Because the attractions are weak, less energy is needed to separate molecules from each other into the gas phase. Therefore, boiling points are lower. 15
  16. 16. 10.2 Physical properties of alkanesThe boiling points of alkanes rise as the chain lengthincreases and fall as the chains become branched andmore nearly spherical. 16
  17. 17. 10.2 Physical properties of alkanes 17
  18. 18. 10.2 Physical properties of alkanesThis is a good place to answer your first Journal questionfor this module.[Use tag “water”]Table salt (NaCl) dissolves in water, but iodine (I2) doesnot to any appreciable extent. Explain why this is, interms of the molecular properties of salt, water, andiodine. 18
  19. 19. 10.2 Writing structural formulasSome definitions: Atoms bonded in a “straight” line are in a continuous chain. A branched chain has some carbons branching off the longest continuous chain. 19
  20. 20. 10.2 Writing structural formulasSome hints: First, attach the carbon atoms to each other in the correct configuration. Add hydrogen atoms until each carbon atom has four bonds.Structural formula of C3H8 pencast 20
  21. 21. 10.2 Abbreviated structural formulasWriting out every atom and bond individually isinconvenient.Abbreviated structural formulas list each carbon with itshydrogens, adjacent to other carbons it is bonded to. ethanol and dimethyl ether pencast n-pentane, 2,2-dimethylpropane pencast 21
  22. 22. 10.2 Abbreviated structural formulasLine structures use no element symbols. show only carbon-carbon bonds.Line structures for n-pentane 2-methyl-butane 2,2-dimethylpropane pencast 22
  23. 23. 10.2 Structures of alkanesAll alkanes fit the same general molecular formula. CnH2n+2 where n = number of carbonsNormal alkanes (n-alkanes) have carbon chains that areunbranched.A group of alkanes that each has one more –CH2– groupis called a homologous series. Properties of the molecules are similar. Properties change gradually as carbon atoms are added. 23
  24. 24. 10.2 Structures of alkanes 24
  25. 25. 10.2 Structures of alkanesWhich of the following are alkanes? C7H18 C7H16 C8H16 C27H56 25
  26. 26. 10.2 Alkyl groupsAn alkyl group is an alkane with one hydrogen atomremovedIt is named by replacing the -ane of the alkane name with–ylMethane becomes a methyl group. CH4 – H = -CH3Ethane becomes an ethyl group. CH3CH3 – H = -CH2CH3 26
  27. 27. 10.2 Alkyl groups 27
  28. 28. 10.2 Alkyl groupsAlkyl groups are classified according to the number ofcarbons attached to the carbon atom that joins the alkylgroup to a molecule. A primary carbon (1o) is directly bonded to one other carbon. A secondary carbon (2o) is directly bonded to two other carbons. A tertiary carbon (3o) is directly bonded to three other carbons. A quaternary carbon (4o) is directly bonded to four other carbons. 28
  29. 29. 10.2 Alkyl groups 29
  30. 30. 10.2 NomenclatureOrganic compounds were initially named after theirsource, use, and properties.There are now over 15 million organic compoundsknown!Clearly, it is necessary to have a systematic method fornaming. Every compound must have a unique name. Structure must be determinable from the name. The name must be determinable from the structure.The system we use is called IUPAC. It was devised by the International Union of Pure and Applied Chemistry. 30
  31. 31. 10.2 IUPAC rules for naming alkanesThe –ane ending is used for all alkanes.Alkanes without branching are named by the number ofcarbon atoms.For branched alkanes, the root name is the one for thelongest continuous chain of carbon atoms.Groups attached to the main chain are calledsubstituents. Saturated groups composed of carbon and hydrogen are called alkyl groups. Alkyl groups are named after the corresponding alkane, with –yl replacing –ane. 31
  32. 32. 10.2 IUPAC rules for naming alkanesThe main chain is numbered so the first substituent hasthe lowest possible number. All substituent names are preceded by the number of the carbon they are attached to. If there are two or more identical substituents, there are two or more numbers preceding the name, and a prefix (di-, tri-, tetra-, . . .) on the substituent name.If two or more different substituents are present, theyare listed alphabetically (ignoring any number prefix).The only punctuation used is commas to separate numbers from each other. hyphens to separate letters from numbers. 32
  33. 33. 10.2 IUPAC rules for naming alkanesGive the IUPAC names for the following compounds: pencast pencast 33
  34. 34. 10.2 Alkyl and halogen substituentsAlkyl groups are named by changing the –ane ending ofalkanes to –yl. methyl CH3- ethyl CH3CH2- n-propyl CH3CH2CH2- isopropyl (1-methylethyl) CH3CHCH3 butyl CH3CH2CH2CH2- sec-butyl (1-methylpropyl) CH3CH2CH2CH2 isobutyl (2-methylpropyl) (CH3)2CHCH2- tert-butyl (1,1-dimethylethyl) (CH3)3C- 34
  35. 35. 10.2 Alkyl and halogen substituentsHalogen substituents are named by changing the –ineending of the element to –o. fluoro- F- chloro- Cl- bromo- Br- iodo- I-R is a general symbol for an alkyl group. R-H is any alkane. R-Cl or R-F or R-Br or R-I stand for alkyl halides. 35
  36. 36. 10.2 Alkyl and halogen substituentsGive a IUPAC name for CH2ClF.Write a formula for each compound. 1,1,1,-trichlorodecane pencast 3,3,5-trimethylpentaneName the following compounds. CH3CH2CH(CH2CH3)CH2CH2CH3 pencast CH3C(CH3)2CH(CH2CH3)CH2CH2CH3 pencast 36
  37. 37. 10.2 Use of the IUPAC rules 2-methylpentane (not 4-methylpentane) 3-methylhexane (not 2-ethylpentane or 4-methylhexane) 2,2-dimethylbutane (not 2,2-methylbutane or 2-dimethylbutane) 3-bromo-1-chlorobutane (not 1-chloro-3-bromobutane or 2-bromo-4-chlorobutane) 37
  38. 38. 10.2 IsomerismIf more than one arrangement is possible for a givenmolecular formula, the different arrangements are calledisomers.Structural isomers have the same molecular formula butdifferent structural formulas.Consider the molecular formula C3H7Cl. isomer 1 isomer 2 38
  39. 39. 10.2 IsomerismThere are 5 structural isomers of C6H14. pencast Draw the 5 isomers and name them. 39
  40. 40. 10.2 IsomerismAlthough all 5 isomers have the same molecularformula, their physical properties vary. 40
  41. 41. 10.2 IsomerismHow about another Journal question?[Use tag “isomers”]In this power point, you’ve seen data indicating that thefive isomers of C6H14 have different physical properties.With reference to the slides on properties of alkanes,explain the relative melting and boiling points of theseisomers. That is, look at the shapes of the molecules (theamount of branching) and explain how the shapescorrelate with the melting and boiling points. 41
  42. 42. 10.3 CycloalkanesCycloalkanes have two fewer hydrogens than thecorresponding chain alkane. hexane, C6H14 cyclohexane, C6H12The general formula for a cycloalkane is CnH2n. 42
  43. 43. 10.3 CycloalkanesTo name cycloalkanes, add the prefix cyclo- to the nameof the corresponding alkane.Place substituents in alphabetical order before the basename as for alkanes.For multiple substituents, use the lowest possible set ofnumbers; a single substituent requires no number. 43
  44. 44. 10.3 CycloalkanesCyclopropaneCyclobutaneCyclohexane 44
  45. 45. 10.3 Naming cycloalkanesName the two cycloalkanes shown. Parent chain 6 carbon ring, cyclohexane 5 carbon ring, cyclopentane Substituent 1 chlorine atom, chloro- 1 methyl group, methyl- chlorocyclohexane methylcyclopentane 45
  46. 46. 10.3 cis-trans isomers in cycloalkanesAtoms of an alkane can rotate freely around the carbon-carbon single bond having an unlimited number ofarrangements.Rotation around the bonds in a cyclic structure is limitedby the fact that all carbons in the ring are interlocked.Formation of cis-trans (geometric) isomers is aconsequence of the lack of free rotation around bonds.Stereoisomers are molecules that have the samestructural formulas and bonding patterns, but differentarrangements of atoms in space. See “Videos with Models” in “Powerpoints and Related Materials” folder for Module 1 46
  47. 47. 10.3 cis-trans isomers in cycloalkanesTwo groups may be on the same side (cis) of the imagined plane ofthe cyclo-ring or they may be on the opposite side (trans).Geometric isomers do not readily interconvert, because this wouldinvolve bond breaking and re-formation. 47
  48. 48. 10.3 Practice with cycloalkanesName the following compounds: pencast 48
  49. 49. 10.3 Practice with cycloalkanesDraw structures for the following compounds. trans-1,4-dimethylcyclooctane cis-1,3-dichlorocyclohexane pencast cis-1-bromo-2-fluoro-cyclobutane 49
  50. 50. 10.4 Conformations of alkanesGeometric isomers have the same number of atoms ofeach type, but are bonded together differently. Bonds must be broken to change one geometric isomer into another.Conformers are identical molecules that are arrangeddifferently in space (rotation around single bonds). An intact molecule can be changed from one conformation to another. 50
  51. 51. 10.4 Ethane and butaneIn an eclipsed conformation, groups on the front andback carbons are aligned when we look down the bond. 51
  52. 52. 10.4 Ethane and butaneIn a staggered conformation, groups on the front andback carbons are not aligned when we look down thebond. See “Videos with Models” in “Powerpoints and Related Materials” folder for Module 1 52
  53. 53. 10.4 JournalLast Journal question for this Module[Use tag “butanes”]Consider the molecules butane and cyclobutane. If two substituents are added to cyclobutane, they can be cis- or trans-. If two substituents are added to butane, these is no possibility of cis- and trans- isomers; however, butane has conformers, and cyclobutane does not.Explain these two observations in terms of the structuresof butane and cyclobutane, and the definitions of cis- andtrans- isomers and conformers. 53
  54. 54. 10.4 CyclohexaneBecause of the 109.5o carbon-carbon bonds, a cyclohexane ringdoesn’t lie flat, but is puckered. “Boat” conformation: both ends puckered up. “Chair” conformation: one end up, one end down. 54
  55. 55. 10.4 CyclohexaneThe chair conformation of cyclohexane is more stablethan the boat conformation because the hydrogens areless crowded. 55
  56. 56. 10.4 CyclohexaneOn each carbon in thecyclohexane ring, onehydrogen is vertical to aplane through the ring. See “Videos with Models” in “PowerpointsThese are the axial and Related Materials” folderhydrogens. for Module 1The second hydrogen oneach carbon is theequatorial hydrogen. 56
  57. 57. 10.5 Reactions: CombustionAlkanes and cycloalkanes react with oxygen to producecarbon dioxide and water. This is an oxidation reaction… …but we usually call it combustion.What’s wrong with this equation for the combustion ofmethane? CH4 + O2  CO2 + H2O 57
  58. 58. 10.5 Reactions: CombustionGuidelines for balancing combustion reactions: Balance carbon using CO2 product molecules. Balance hydrogen using H2O product molecules. Balance oxygen using O2 reactant molecules.Write a balanced chemical equation for the combustionof the following compounds: pentane cyclopropane 2-methylhexane 58
  59. 59. 10.5 Reactions: HalogenationIn a substitution reaction, one atom substitutes foranother on a molecule.Halogenation is the replacement of a hydrogen on analkane with a halogen atom.Since alkanes and cycloalkanes aren’t very reactive, heator light must be present for halogenation to take place. 59
  60. 60. 10.5 Reactions: Halogenation Terminology: If one halogen is added per molecule: monochlorination monobromination If all the hydrogens in the molecule are not equivalent, a mixture of products will form.3-D, rotatable model 3-D, rotatable model 1-chloropropane 2-chloropropane What are all the possible monochlorination products of pentane? pencast 60
  61. 61. 10.5 Reactions: HalogenationIf the halogenation reaction is allowed to continue, morethan one halogen may add to each molecule. CH4 + ½ Cl2  CH3Cl + HCl CH4 + Cl2  CH2Cl2 + 2 HCl CH4 + 3/2 Cl2  CHCl3 + 3 HCl CH4 + 2 Cl2  CCl4 + 4 HCl 61
  62. 62. End-of-chapter notesSummary of reactions for the class of compounds studied inthe chapter Know what reactions these compounds undergo. Know what the reactants and products are. Know what the conditions for reaction are. Be able to write a balanced equation for each reaction.Key terms These should all be familiar to you by the time we finish the chapter. Know definitions for these terms—you’ll need them in later chapters.Summary for each section of the chapter If you don’t understand everything in the chapter summaries, go back and figure out what it means! 62