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. React 2-methyl butane with bromine C C C-C-C-C + Br2  C-C-C-C + HBr
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
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
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

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 H H H
C=C-C-C=C + 2H2 -Pt/Pd C-C-C-C-C
H H H H H
C=C=C + 2H2 -Pt/Pd C-C-C
H H H

45. 2. Halogenation C=C-C=C + 2X2  C-C-C-C C=C=C + 2X2  C-C-C
X X X X
C=C-C-C=C + 2X2  C-C-C-C-C
X X X X X
C=C=C + 2X2  C-C-C
X X X

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 X X H X
C=C=C + 2HX  C-C-C
H X H

47. 4. Hydration C=C-C=C + 2H2O  C-C-C-C C=C-C-C=C + 2H2O C-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)

51. Benzene
3. Friedel-Crafts Alkylation – the substitution of an alkyl group for an H on Benzene
GenEqn: ArH + RCl –AlCl3 ArH + HCl
Ex: Form orthodipropylbenzene (str&cond)

52. Friedel-Crafts Alkylation (Reaction Mechanism)
RCl + AlCl3  R+ + AlCl4-1
R+ + ArH  ArR + H+
H+ + AlCl4-1  AlCl3 + HCl
Write the reaction mechanism for the formation of 2-ethylbenzene (str&cond)