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Reactions of Hydrocarbons

Reactions of Hydrocarbons. Unit 3. Review. Alkanes C n H 2n+2 Alkenes C n H 2n Alkynes C n H 2n-2. Combustion. HC’s burn in air and produce carbon dioxide and water vapor HC + O 2 (g)  CO 2 (g) + H 2 O (g) Reactants Products Reactions must be balanced!. Example. Propane:

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Reactions of Hydrocarbons

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  1. Reactions of Hydrocarbons Unit 3

  2. Review • Alkanes CnH2n+2 • Alkenes CnH2n • Alkynes CnH2n-2

  3. Combustion • HC’s burn in air and produce carbon dioxide and water vapor • HC + O2 (g) CO2 (g) + H2O (g) • Reactants Products • Reactions must be balanced!

  4. Example • Propane: • C3H8 +

  5. Practice • Write and balance the following… • Pentane, 1-hexyne, octane, 2 decene, 2,5-heptadiene

  6. Substitution Reactions • Alkanes – they are saturated HC’s so they undergo substitution rxns where an atom or group replaces a H atom • Halogenation – a substitution of a halogen for a H in an alkane

  7. Substitution Reaction • General Equation: R=Alkyl Group RH + X2  RX + HX Alkane halogen alkyl halide hydrogen halide Ex: Form 2-iodopropane (structural/condensed) I C-C-C + I2 C-C-C + HI CH3CH2CH3 + I2  CH3CHICH3 + HI

  8. Practice • Try 3-fluoropentane • (start with pentane then add F2)

  9. Rules for Halogenation of Alkanes • Only 1 X can be substituted for each X2 molecule used b/c the other has to bond with the H that was replaced • Form 2,3-diiodopentane (start with pentane) • Try 1,2,4 - trichlorohexane

  10. Rules for Halogenation of Alkanes • If you aren’t given the location for the X to substitute, then the X had to bond to the carbon that is bonded to the most other carbons. • 1° = Primary C = Bonded to 1 other C • 2° = Secondary C = Bnd to 2 other C • 3° = Tertiary C = Bnd to 3 other C • Reactavity Order - 3°>2°>1°

  11. C Label each C as C-C-C-C 1°, 2°, 3° • React 2-methyl butane with bromine C C C-C-C-C + Br2 C-C-C-C + HBr Br CH3CH(CH3)CH2CH3 + Br2 CH3CBr(CH3)CH2CH3 + HBr

  12. Practice • Try to react 3-ethylheptane with fluorine

  13. Rxns • Orgo Rxns are slow so they are easily studied – there are steps that happen b/t rcts and products. • Reaction Mechanism – the step by step sequence of how a chemcial rxn occurs

  14. Examples • Halogenation of alkanes occurs by free radical substitution • Free Radical – a highly reactive species due to an unpaired e- as a result of a homolytic bond clevage • Ex: Cl-Cl  2Cl• The • represents the free radical

  15. Free Radical Substitutions • All alkyl groups are free radicals • Free radical substitution is a “chain reaction” • Chain Reaction – a reaction that occurs through a series of steps

  16. Free Radical Substitution • RH + X2 RX + HX 1.) X2 –(Light) 2X• Chain Initiating Step – a molecule forms a free radical to start the chain rxn 2.) X• + RH  R• + HX 3.) R• + X2 RX + X•

  17. Free Radical Substitution 2& 3 are Chain Propagating Steps – one free radical forms a different free radical to initiate the next step and to sustain the rxn. 4.) 2X•  X2 5.) X• + R•  RX 6.) 2R•  RR (rarely happens)

  18. Free Radical Substitution 4, 5, and 6 are… Chain Terminating Steps – two free radicals form a molecule to stop the reaction

  19. Practice • Write the reaction mechanism for the formation of 2-fluoropropane • 1 – draw the molecule • 2 – write the reaction (structural or condensed) • 3 – start the free radical substitution with the F

  20. Alkenes • Sigma (σ) bonds – have orbiral overlap directly between the nuclei of 2 atoms • More [ ] e- density = stronger bonds • All single covalent bonds are σ bonds

  21. Alkenes • Pi bonds (π) have orbital overlap above and below the nuclei of the two atoms • Less [ ] e- density and weaker bonds • The 2nd and 3rd pairs of multiple bonds are pi bonds.

  22. Alkenes • Pi Bonds

  23. Alkenes • Functional Group – an atom or group of atoms that defines a family or organic compounds and determines their properties • Alkanes FG – C-C single covalent bond • Alkenes FG – C=C - undergo addition rxns where 2 atoms or groups are added to the double bond (π weaker than σ)

  24. Reactions of Alkenes • Hydrogenation – the addition of hydrogen to the C=C bond • General Equation – • C=C + H2 –Pt & Pd C-C Example – Hydrogenate 2-Pentene C-C=C-C-C + H2 –Pt&Pd  C-C-C-C-C ~YOU FORMED PENTANE 

  25. Reactions of Alkenes • Halogenation – the addition of a halogen to the C=C bond • General equation - C=C +X2  X-C-C-X • Ex: Chlorinate 2-heptene • Ex: Form 2,3-dibromopentane by addition (also name the reactant)

  26. Reactions of Alkanes • Now you know 2 ways of forming an alkyl halide (with 2 consecutive X’s attached) • Substitution of an alkane • (RH + 2X2 RX2 + 2HX) • Addition to an alkane • (C=C + X2X-C-C-X)

  27. Examples • Form 2,3 diiodopentane by additon • Form 2,3 diiodopentane by substitution • Form 3,4 difluoroheptane by addition and substitution (name reactants)

  28. Hydrohalogenation • Hydrohalogenation – the addition of a hydrogen halide to the C=C • General Equation- C=C + HX  H-C-C-X • How do you know where the H or X will bond?

  29. Hydrohalogenation • Markovnikov’s Rule – in addition reactions of unsymmetrical reagents, the H bonds to the C bonded to the most other H atoms (only look at the two atoms making the C=C)

  30. Example • Hydrofluorinate 1-propene (str&cond) C=C-C + HF  H – C-C-C F CH2CHCH3 + HF  CH3CHFCH3 • Form 2-chlorobutane by addition

  31. Hydration of an Alkene • Hydration – the addition of water to the C=C • H2O = HOH (hydrogen & hydroxide) • Also follows Markovnikov’s rule • Hydrate 1 – Propene (str&cond) • C=C –C + H2O  H-C-C-C OH CH2CHCH3 + H2O  CH3CHOHCH3

  32. Practice • Form 2 – Hydroxopentane (Str&Cond)

  33. Mechanism • Hydrohalogenation & Hydration occur by “Ionic Addition” (this is our 2nd mechanism) C=C + HX  + C-C-H + X- X-C-C-H (Think of water as H+ and OH-)

  34. Hydrohalogenation • The X takes the e- from H so H becomes H+1 which makes a coordinate bond to C [for this to happen, the 2nd pair of e-s in the C=C bond move to one of the C’s]. This leaves the other C as a C+1. Then the X-1 bonds to the C+1.

  35. Hydrohalogenation • The + C-C-H is a carbocation (a group of atoms that has a C with only 6e-s, giving the group a + charge)

  36. Example • Write the reaction mechanism for the reaction of 3-ethyl – 3-hexene and water

  37. Alkynes • Alkynes undergo addition reactions where 4 atoms or groups are added to the C= C • Hydrogenation  addition of H2 to the C=C H H • C=C + 2H2 –Pt/Pd C-C H H Hydrogenate 4-decyne (str, cond, name)

  38. Halogenation of Alkynes • Halogenation – addition of a halogen to the C=C bond X X • C=C =2X2 C-C X X Chlorinate 2-hexyne (str,cond,name prod) Form 1,1,2,2-tetrafluoropentane by addition (str,cond,name prod)

  39. Hydrohalogenation of Alkynes H X • C=C + 2X  C-C H X • Follows Markovnikov’s rule

  40. Examples • Hydrobrominate 1-butyne (str, cond, name product) • Form 2,2 – diiodopentane by addition

  41. Hydration of an alkyne • Hydration – the addition of water to the C=C • Follows the Markovnikov’s rule H OH • C=C + 2H2O  C-C H OH

  42. Examples • Hydrate 1-propyne (str, cond, name product) • Form 2,2 - dihydrobutane

  43. Alkadienes • Undergo addition reactions where 4 atoms or groups are added to the two C=C • There are 3 different general equations for each of the 4 types because of the conjugated, isolated, and allenes orientations.

  44. 1. Hydrogenation • C=C-C=C + 2H2 -Pt/Pd C-C-C-C H HHH • C=C-C-C=C + 2H2 -Pt/Pd C-C-C-C-C H HHH H • C=C=C + 2H2 -Pt/PdC-C-C H HH

  45. 2. Halogenation • C=C-C=C + 2X2 C-C-C-C XXXX • C=C-C-C=C + 2X2 C-C-C-C-C XXXX X • C=C=C + 2X2C-C-C X XX

  46. 3. Hydrohalogenation • C=C-C=C + 2HX  C-C-C-C H X X H • C=C-C-C=C + 2HX  C-C-C-C-C H XX H X • C=C=C + 2HX C-C-C H X H

  47. 4. Hydration • C=C-C=C + 2H2O  C-C-C-C H OH OH H • C=C-C-C=C + 2H2O C-C-C-C-C H OH OH H OH • C=C=C + 2H2O C-C-C H OH H

  48. Benzene • Undergoes substitution reactions where an atom or a group of atoms replaces a H

  49. Benzene • Halogenation – the substitution of a halogen for a H on benxene Gen Eqn: ArH + X2 –FeX3 ArX + HX Benzene Aryl Halide Ar = C6H5 = Aryl Group EX:Form 1,3,5tribromobenzene(str,cond)

  50. Benzene 2. Nitration – the substitution of a nitro group for a H on Benzene Gen Eqn = ArH + HNO3 –H2SO4ArNO2 +HX Ex: Form 1,2,4,5 Tetranitrobenzene (str,cond)

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