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Unit 2 – Alkanes and Chemical Reactions

Unit 2 – Alkanes and Chemical Reactions. Structure and Stereochemistry of Alkanes Nomenclature of alkanes and cycloalkanes Physical Properties Conformational Analysis The Study of Chemical Reactions Kinetics and Thermodynamic Quantities Free Radical Halogenation

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Unit 2 – Alkanes and Chemical Reactions

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  1. Unit 2 – Alkanes and Chemical Reactions • Structure and Stereochemistry of Alkanes • Nomenclature of alkanes and cycloalkanes • Physical Properties • Conformational Analysis • The Study of Chemical Reactions • Kinetics and Thermodynamic Quantities • Free Radical Halogenation • Reactive Intermediates and Transition States

  2. Hydrocarbons • The simplest organic compounds are the hydrocarbons: • organic compounds that contain only carbon and hydrogen • four general types: • alkanes • alkenes • alkynes • aromatic hydrocarbons

  3. Hydrocarbons • Alkanes are often calledsaturated hydrocarbons • they contain the maximum number of hydrogen atoms per carbon atom. • Alkenes, alkynes, and aromatic hydrocarbons are calledunsaturated hydrocarbons • they contain fewer H atoms than an alkane with the same number of carbon atoms

  4. Alkanes • You must know the names and formulas for the 10 simplest alkanes: CH4 methane CH3CH3 ethane CH3CH2CH3 propane CH3CH2CH2CH3 butane CH3CH2CH2CH2CH3 pentane CH3CH2CH2CH2CH2CH3 hexane

  5. Alkanes • You must know the names and formulas for the 10 simplest alkanes: CH3CH2CH2CH2CH2CH2CH3 heptane CH3CH2CH2CH2CH2CH2CH2CH3 octane CH3CH2CH2CH2CH2CH2CH2CH2CH3 nonane CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3 decane

  6. Alkanes • The alkanes form a homologous series with a general molecular formula of CnH(2n+2) • Homologous series: • a series of compounds in which each member differs from the next member by a constant unit • Alkanes differ from each other by -CH2-

  7. Alkanes Example: Which of the following are alkanes: C2H6, C3H6, C5H12, C4H8 Example: What is the formula for an alkane with 12 carbons?

  8. Alkanes • The previous alkanes are straight-chain alkanes: • all of the carbon atoms are joined in a continuous chain • also called “normal” alkanes (n-alkanes) • Alkanes containing 4 or more carbons can also form branched alkanes • one or more of the carbon atoms form a “branch” or side-chain off of the main chain

  9. Alkanes • An example of a straight chain alkane: • C5H12pentane • Examples of branched alkanes: • C5H12 2-methylbutane 2,2-dimethylpropane

  10. Alkanes • The three structures shown previously for C5H12 are structural (constitutional) isomers: • compounds with the same molecular formula but different bonding arrangements • Structural isomers have different properties: • different melting points • different boiling points • often different chemical reactivity

  11. Alkane Nomenclature • Organic compounds can be named either usingcommon namesorIUPAC names. pentane 2-methylbutane or isopentane 2,2-dimethylpropane or neopentane

  12. Alkane Nomenclature • The common name for any alkane containing a CH3 group on the second carbon in the chain is “isoalkane.” “iso” Isobutane (4 C total) Isohexane (6 C total)

  13. Alkane Nomenclature • Most of the time, organic chemists use the IUPAC names for organic compounds. • LEARN THE RULES FOR EACH CLASS OF COMPOUNDS WE DISCUSS.

  14. Alkane Nomenclature • To name an alkane: • Find the longest continuous chain of carbon atoms and use the name of that chain as the base name of the compound: • the longest chain is often NOT written in a straight line Base name: heptane

  15. Alkane Nomenclature • Number the carbon atoms in the longest chain starting at the end of the chain closest to a substituent • a group attached to the main chain that has taken the place of a hydrogen atom on the main chain A substituent

  16. Alkane Nomenclature • Name and give the location of each substituent. • Common substituents: • Halo group • a halogen atom • “Halo” groups are named using “halo”: • Cl chloro • Br bromo • I iodo • F fluoro • Nitro group • -NO2

  17. Alkane Nomenclature • Common substituents: • alkyl group • A group that is formed by removing an H atom from an alkane • the alkyl group attaches to the main chain at the carbon that has lost its H

  18. Alkane Nomenclature • Alkyl groups are named by replacing the “ane” ending of the parent alkane with the “yl” ending.

  19. Alkane Nomenclature

  20. Alkane Nomenclature

  21. Alkane Nomenclature • Complex alkyl substituents • Use the longest alkyl chain of the substituent as the base name of the substituent • Number the substituent chain with the “head carbon” as carbon 1 • List substituents on the alkyl chain with the appropriate numbers • Use parentheses around the name of the group

  22. Alkane Nomenclature Methyl group 3-methylheptane Note: Separate numbers from letters using a hyphen. Separate groups of numbers using commas.

  23. Alkanes • Alkane Nomenclature: • When two or more substituents are present, list them in alphabetical order: • isopropyl before methyl • t-butyl or sec-butyl before chloro • When more than one of the same substituent is present (i.e. two methyl groups), use prefixes to indicate how many. Give the location of each as well. • Di = two • Tri = three • Tetra = four • Penta = five • Hexa = six Know these. Note: Ignore these prefixes when alphabetizing.

  24. Alkane Nomenclature • Additional rules: • When there are two “longest” chains of equal length, use the chain with the greater number of substituents. correct incorrect

  25. Alkane Nomenclature • Additional rules: • If each end of the longest chain has a substituent the same distance from the end, start with the end nearer to the second substituent. correct incorrect 3-chloro-2,5-dimethylhexane

  26. Alkanes Example: Name the following compounds:

  27. Alkanes Example: Name the following compound:

  28. Alkanes • You must also be able to write the structure of an alkane when given the IUPAC name. • Identify the main chain and draw the carbons in it. • Identify the substituents (type and #) and attach them to the appropriate carbon atoms on the main chain. • Add hydrogen atoms to the carbons to make a total of 4 bonds to each carbon

  29. Alkanes Example: Draw the structure for the following compounds: 3, 3-dimethylpentane 4-sec-butyl-2-methyloctane 1,2-dichloro-3-methylheptane 2-nitropropane

  30. Alkane Nomenclature Example: Draw the structural isomers of hexane (C6H14). Name each isomer. • Use a systematic approach to draw structural isomers: • Draw the unbranched isomer for the first structure. • For other structures, remove 1 or more carbons (and/or functional groups) from the unbranched isomer and reposition to make unique compounds

  31. Types of Carbon Atoms • Primary carbon (1o) • a carbon bonded to one other carbon • Secondary carbon (2o) • a carbon bonded to two other carbons • Tertiary carbon (3o) • a carbon bonded to three other carbons

  32. Physical Properties • Solubility • Alkanes are nonpolar • hydrophobic • do not dissolve in water • soluble in nonpolar or weakly polar organic solvents • Density: • varies from ~0.5 - ~0.8 g/mL • less dense than water (1.0 g/mL) • Alkanes float on water

  33. Physical Properties • Boiling Point • In general, boiling point increases as the molecular weight of the alkane increases • larger molecules have greater surface area and higher London dispersion forces of attraction • must be overcome for vaporization and boiling to occur

  34. Physical Properties • Boiling Point (cont) • Given the same total number of carbon atoms: BP (branched) < BP (n-alkane) • Branched alkanes are more compact. • less surface area • smaller London dispersion forces • lower BP

  35. Physical Properties • Melting Points: • In general, melting point increases as MW increases • irregular, sawtooth pattern

  36. Physical Properties • Melting Point: • Alkanes with odd number of carbons have lower than expected melting points (compared to the previous alkane with an even number of carbons) • Even # carbons • better packing in solid structure • higher MP • Odd # carbons • do not pack as well • lower MP

  37. Physical Properties • Melting Points: • Given the same total number of carbon atoms: MP (branched) > MP (n-alkane) • branched alkanes have more compact structure • better packing • higher MP

  38. Sources & Uses of Alkanes • Alkanes are derived primarily from petroleum and petroleum by-products: • Refining via fractional distillation gives useful mixtures of alkanes: • C2 -C4 liquified petroleum gas • C4 - C9 gasoline • C8 - C16 kerosene • C10 - C18 diesel • C16+ heavy/mineral oil

  39. Reactions of Alkanes • Catalytic Cracking: • converts alkanes into more valuable mixtures of smaller alkanes and alkenes alkane smaller alkanes + alkenes C12H26 D SiO2 or Al2O3 catalyst D SiO2 +

  40. Reactions of Alkanes • Hydrocracking: • converts higher alkanes into shorter alkanes and eliminates N and S impurities Alkane shorter alkanes C12H26 D H2, SiO2 or Al2O3 catalyst D + H2, SiO2

  41. Reactions of Alkanes • Combustion: • a rapid, exothermic redox reaction that converts hydrocarbons into carbon dioxide and water alkane + O2 CO2 + H2O 2 C6H14 + 19 O2 12 CO2 + 14 H2O (unbalanced)

  42. Reactions of Alkanes • Halogenation: • a reaction in which a halogen atom is substituted for a hydrogen atom on an alkane alkane + X2 mixture of alkyl halides CH4 + Cl2 CH3Cl + CH2Cl2 + CHCl3 + CCl4 D or hu hu unbalanced

  43. Conformations of Alkanes • The simplest alkane, CH4, is perfectly tetrahedral: • bond angle = 109.5 • C-H bond length = 1.09 A • free rotation of the C-H bond

  44. Conformations of Alkanes • Ethane: • Two carbons • overlapping sp3 hybrid orbitals form a sigma bond

  45. Conformations of Alkanes • The two methyl groups are relatively free to rotate about the sigma bond between the two carbon atoms • sigma bond maintains its overlap at all times • The different arrangements formed by rotation around a single bond are called conformations. • Conformer: • a specific conformation • a “conformational isomer”

  46. Conformations of Alkanes • Conformers are often drawn using Newman projections: • a way of drawing a molecule looking straight down the bond connecting two carbon atoms • front carbon atom is represented by three lines joined together in a Y shape • back carbon is represented by a circle with three bonds pointing out from it

  47. Conformations of Alkanes View from this end = 3-D structure of one conformer of ethane Newman projection

  48. Conformations of Alkanes • An infinite number of conformations are possible for ethane and higher alkanes. • The dihedral angle (q) can have an infinite number of values • angle between the C-H bonds on the front and back carbons q

  49. Conformations of Alkanes Important conformations for ethane: Eclipsed conformation Staggered conformation Skew conformation Molecules are constantly rotating through all possible conformations.

  50. Conformations of Alkanes • The conformation of ethane changes constantly at room temperature. • Conformations may have different energies. • Lowest energy conformer is most favored. • Highest energy conformer is least favored. • Conformational analysis: • the study of the energies of different conformations • helps predict which conformation are favored and which reaction may occur

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