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Reactions of Hydrocarbons Unit 3. Review Alkanes C n H 2n+2 Alkenes C n H 2n Alkynes C n H 2n-2.

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Presentation on theme: "Reactions of Hydrocarbons Unit 3. Review Alkanes C n H 2n+2 Alkenes C n H 2n Alkynes C n H 2n-2."— Presentation transcript:

1 Reactions of Hydrocarbons Unit 3

2 Review Alkanes C n H 2n+2 Alkenes C n H 2n Alkynes C n H 2n-2

3 Combustion HC’s burn in air and produce carbon dioxide and water vapor HC + O 2 (g)  CO 2 (g) + H 2 O (g) ReactantsProducts Reactions must be balanced!

4 Example Propane: C 3 H 8 +

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 + X 2  RX + HX Alkane halogen alkyl halide hydrogen halide Ex: Form 2-iodopropane (structural/condensed) I C-C-C + I 2  C-C-C + HI CH 3 CH 2 CH 3 + I 2  CH 3 CHICH 3 + HI

8 Practice Try 3-fluoropentane (start with pentane then add F 2 )

9 Rules for Halogenation of Alkanes Only 1 X can be substituted for each X 2 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 CLabel each C as C-C-C-C1°, 2°, 3° React 2-methyl butane with bromine C C-C-C-C + Br 2  C-C-C-C + HBr Br CH 3 CH(CH 3 )CH 2 CH 3 + Br 2  CH 3 CBr(CH 3 )CH 2 CH 3 + 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 + X 2  RX + HX 1.) X 2 –(Light)  2X Chain Initiating Step – a molecule forms a free radical to start the chain rxn 2.) X + RH  R + HX 3.) R + X 2  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  X 2 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 2 nd and 3 rd 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 + H 2 –Pt & Pd  C-C Example – Hydrogenate 2-Pentene C-C=C-C-C + H 2 –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 +X 2  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) 1.Substitution of an alkane (RH + 2X 2  RX 2 + 2HX) 2.Addition to an alkane (C=C + X 2  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 CH 2 CHCH 3 + HF  CH 3 CHFCH 3  Form 2-chlorobutane by addition

31 Hydration of an Alkene Hydration – the addition of water to the C=C H 2 O = HOH (hydrogen & hydroxide) Also follows Markovnikov’s rule Hydrate 1 – Propene (str&cond) C=C –C + H 2 O  H-C-C-C OH CH 2 CHCH 3 + H 2 O  CH 3 CHOHCH 3

32 Practice Form 2 – Hydroxopentane (Str&Cond)

33 Mechanism Hydrohalogenation & Hydration occur by “Ionic Addition” (this is our 2 nd 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 2 nd 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 H 2 to the C=C H H C=C + 2H 2 –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 =2X 2  C-C 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 + 2H 2 O  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 + 2H 2 -Pt/Pd  C-C-C-C H H H H C=C-C-C=C + 2H 2 -Pt/Pd  C-C-C-C-C H H H H H C=C=C + 2H 2 -Pt/Pd  C-C-C H H H

45 2. Halogenation C=C-C=C + 2X 2  C-C-C-C X X X X C=C-C-C=C + 2X 2  C-C-C-C-C X X X X X C=C=C + 2X 2  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 + 2H 2 O  C-C-C-C H OH OH H C=C-C-C=C + 2H 2 O  C-C-C-C-C H OH OH H OH C=C=C + 2H 2 O  C-C-C H OH H

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

49 Benzene 1.Halogenation – the substitution of a halogen for a H on benxene Gen Eqn: ArH + X 2 –FeX 3  ArX + HX Benzene Aryl Halide Ar = C 6 H 5 = 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 + HNO 3 – H 2 SO 4  ArNO 2 +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 –AlCl 3  ArH + HCl Ex: Form orthodipropylbenzene (str&cond)

52 Friedel-Crafts Alkylation (Reaction Mechanism) RCl + AlCl 3  R + + AlCl 4 -1 R + + ArH  ArR + H + H + + AlCl 4 -1  AlCl 3 + HCl Write the reaction mechanism for the formation of 2-ethylbenzene (str&cond)


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